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The February birthstone is the purple amethyst

February birthstone: Four large, pointed purple crystals growing from a rock among similar smaller purple crystals.
An amethyst cluster – the February birthstone – from Magaliesburg, South Africa. Image via J.J. Harrison/ Wikipedia (CC BY-SA 3.0).

The February birthstone: the amethyst

February babies have amethyst – a lovely purple gemstone – as their birthstone. Amethysts contain the second most abundant mineral found in Earth’s crust: quartz. And quartz often forms the lining inside geodes, which form near sites of volcanic activity. So it’s no wonder that geodes sometimes contain amethysts, and some amethyst geodes are amazingly large.

Like quartz, amethysts are a transparent form of silicon dioxide (SiO2). An amethyst’s color can range from a faint mauve to a rich purple. But where does the color come from? Some scientists believe the purple color arises from the amethysts’ iron oxide content, while others attribute the color to manganese or hydrocarbons.

Also, amethysts are very sensitive to heat. When heated to about 750 to 900 degrees Fahrenheit (400 or 500 degrees Celsius), an amethyst’s color changes to brownish-yellow or red. Then, under some circumstances, the stones turn green when heated. In fact, heat may even transform an amethyst into a naturally rare yellow mineral called citrine. And even without heating, the violet color of amethyst may fade over time.

The 2025 EarthSky Lunar Calendar is now available! A unique and beautiful poster-sized calendar. Get yours today!

Broken-open spherical rock with thick brown outer layer closely lined with small purple crystals within.
An amethyst geode that formed when large crystals grew in an open space inside a rock. Image via Wikipedia (CC BY 3.0).

Commercial sources of amethyst are Brazil and Uruguay, and Arizona and North Carolina are the source of gem quality amethyst.

Rectangular faceted purple stone with light refracted from within and gleaming on the surface.
An emerald-cut amethyst. Image via Wikimedia (CC BY-SA 3.0).

February birthstone lore

The amethyst has a rich history of lore and legend, traceable back to 25,000 years ago in France, where it was a decorative stone used by prehistoric humans. In fact, it appears among the remains from the Neolithic era.

According to legend, the signet ring worn by Cleopatra was an amethyst engraved with the figure of Mithra, a Persian deity symbolizing the Divine Idea, Source of Light and Life.

Saint Valentine supposedly wore an amethyst engraved with the figure of his assistant, Cupid. Also, Saint Valentine’s Day is in February.

Man's head in profile with curly hair and beard carved in low relief on an oval purple stone.
Roman intaglio engraved gem of Caracalla in amethyst, once in the Treasury of Sainte-Chapelle. Image via Marie-Lan Nguyen/ Wikipedia (public domain).

The early Egyptians believed that the amethyst possessed good powers and placed the stones in the tombs of pharaohs. During the Middle Ages, people believed that an amethyst amulet would dispel sleep, sharpen intellect, and protect the wearer from sorcery. And it was thought to bring victory in battle. In Arabian mythology, amethyst supposedly protected the wearer from bad dreams and gout.

Amethyst means not drunk

As a matter of fact, the word amethyst comes from the Greek word “amethystos,” meaning “not drunk,” and so myths say it prevents its wearers from becoming intoxicated. According to the following story from Greco-Roman mythology, as quoted from Birthstones by Willard Heaps:

Bacchus, the god of wine in classical mythology, was offended by Diana the Huntress. Determined on revenge, he declared that the first person he met as he went through the forest would be eaten by his tigers. As it happened, the first person to cross his path was the beautiful maiden Amethyst on her way to worship at the shrine of Diana. In terror, she called upon the goddess to save her, and before his eyes, Bacchus observed the maiden changed to a pure white, sparkling image of stone.

Realizing his guilt and repenting his cruelty, Bacchus poured grape wine over her, thus giving the stone the exquisite violet hue of the amethyst. The carryover to non-intoxication was quite logical, and in ancient Rome, amethyst cups were used for wine, so drinkers would have no fear of overindulgence.

See the birthstones for the rest of the year

January birthstone
February birthstone
March birthstone
April birthstone
May birthstone
June birthstone
July birthstone
August birthstone
September birthstone
October birthstone
November birthstone
December birthstone

Bottom line: The purple February birthstone, the amethyst, has a colorful and interesting history.

Read from Gem Rock Auctions: Amethyst Stone Information

The post The February birthstone is the purple amethyst first appeared on EarthSky.



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February birthstone: Four large, pointed purple crystals growing from a rock among similar smaller purple crystals.
An amethyst cluster – the February birthstone – from Magaliesburg, South Africa. Image via J.J. Harrison/ Wikipedia (CC BY-SA 3.0).

The February birthstone: the amethyst

February babies have amethyst – a lovely purple gemstone – as their birthstone. Amethysts contain the second most abundant mineral found in Earth’s crust: quartz. And quartz often forms the lining inside geodes, which form near sites of volcanic activity. So it’s no wonder that geodes sometimes contain amethysts, and some amethyst geodes are amazingly large.

Like quartz, amethysts are a transparent form of silicon dioxide (SiO2). An amethyst’s color can range from a faint mauve to a rich purple. But where does the color come from? Some scientists believe the purple color arises from the amethysts’ iron oxide content, while others attribute the color to manganese or hydrocarbons.

Also, amethysts are very sensitive to heat. When heated to about 750 to 900 degrees Fahrenheit (400 or 500 degrees Celsius), an amethyst’s color changes to brownish-yellow or red. Then, under some circumstances, the stones turn green when heated. In fact, heat may even transform an amethyst into a naturally rare yellow mineral called citrine. And even without heating, the violet color of amethyst may fade over time.

The 2025 EarthSky Lunar Calendar is now available! A unique and beautiful poster-sized calendar. Get yours today!

Broken-open spherical rock with thick brown outer layer closely lined with small purple crystals within.
An amethyst geode that formed when large crystals grew in an open space inside a rock. Image via Wikipedia (CC BY 3.0).

Commercial sources of amethyst are Brazil and Uruguay, and Arizona and North Carolina are the source of gem quality amethyst.

Rectangular faceted purple stone with light refracted from within and gleaming on the surface.
An emerald-cut amethyst. Image via Wikimedia (CC BY-SA 3.0).

February birthstone lore

The amethyst has a rich history of lore and legend, traceable back to 25,000 years ago in France, where it was a decorative stone used by prehistoric humans. In fact, it appears among the remains from the Neolithic era.

According to legend, the signet ring worn by Cleopatra was an amethyst engraved with the figure of Mithra, a Persian deity symbolizing the Divine Idea, Source of Light and Life.

Saint Valentine supposedly wore an amethyst engraved with the figure of his assistant, Cupid. Also, Saint Valentine’s Day is in February.

Man's head in profile with curly hair and beard carved in low relief on an oval purple stone.
Roman intaglio engraved gem of Caracalla in amethyst, once in the Treasury of Sainte-Chapelle. Image via Marie-Lan Nguyen/ Wikipedia (public domain).

The early Egyptians believed that the amethyst possessed good powers and placed the stones in the tombs of pharaohs. During the Middle Ages, people believed that an amethyst amulet would dispel sleep, sharpen intellect, and protect the wearer from sorcery. And it was thought to bring victory in battle. In Arabian mythology, amethyst supposedly protected the wearer from bad dreams and gout.

Amethyst means not drunk

As a matter of fact, the word amethyst comes from the Greek word “amethystos,” meaning “not drunk,” and so myths say it prevents its wearers from becoming intoxicated. According to the following story from Greco-Roman mythology, as quoted from Birthstones by Willard Heaps:

Bacchus, the god of wine in classical mythology, was offended by Diana the Huntress. Determined on revenge, he declared that the first person he met as he went through the forest would be eaten by his tigers. As it happened, the first person to cross his path was the beautiful maiden Amethyst on her way to worship at the shrine of Diana. In terror, she called upon the goddess to save her, and before his eyes, Bacchus observed the maiden changed to a pure white, sparkling image of stone.

Realizing his guilt and repenting his cruelty, Bacchus poured grape wine over her, thus giving the stone the exquisite violet hue of the amethyst. The carryover to non-intoxication was quite logical, and in ancient Rome, amethyst cups were used for wine, so drinkers would have no fear of overindulgence.

See the birthstones for the rest of the year

January birthstone
February birthstone
March birthstone
April birthstone
May birthstone
June birthstone
July birthstone
August birthstone
September birthstone
October birthstone
November birthstone
December birthstone

Bottom line: The purple February birthstone, the amethyst, has a colorful and interesting history.

Read from Gem Rock Auctions: Amethyst Stone Information

The post The February birthstone is the purple amethyst first appeared on EarthSky.



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Polar bears have unique ice-repelling fur

Polar bears have unique ice-repelling fur

If you worry about polar bears staying warm in the frozen Arctic, don’t. These animals not only survive cold temperatures, but they need them to thrive. Polar bears have the necessary adaptations to live comfortably in frigid environments. How do they do this? The University of Dublin said on January 29, 2025, that an international team of scientists found polar bears’ greasy hair prevents ice from sticking to their fur.

This is the first study that has been carried out on the composition of polar bears’ hair. The team of scientists published its results in the peer-reviewed journal Science Advances on January 29, 2025.

Big white bear lying on the edge of the ice, facing the camera. Its nose and mouth are dripping water.
By learning more about animals’ unique traits, such as the ice-repelling fur of polar bears, we can apply those findings to our daily life and improve our tools. Image via Hans-Jurgen Mager/ Unsplash.

Get an EarthSky Lunar Calendar for 2025 today! A unique and beautiful poster-sized calendar. Available now!

The secret to ice-free hair is greasy hair!

The team of scientists collected the hair of six polar bears in the wild. Scientists focused on studying the bears’ hair grease, technically known as sebum. They found ice has a harder time sticking to hair when it’s greasy than when it’s clean.

Julian Carolan is the first author of the study. Carolan is a Ph.D. student at Trinity College Dublin’s School of Chemistry and the AMBER Research Ireland Centre for Advanced Materials and BioEngineering. Carolan said:

The sebum quickly jumped out as being the key component giving this anti-icing effect as we discovered the adhesion strength was greatly impacted when the hair was washed. Unwashed, greasy hair made it much harder for ice to stick. In contrast when the polar bear hair was washed and the grease largely removed it performed similarly to human hair, to which ice sticks easily whether it is washed or greasy.

Polar bears: A big white bear standing on an icy surface on its hind legs, with its right paw up, like saying hi.
An international team of scientists discovered that ice sticks more easily to clean polar bear hair than greasy hair. Image via Hans-Jurgen Mager/ Unsplash.

Polar bears have unique fur

Scientists wondered why ice doesn’t stick to greasy hair on polar bears, but it does stick to humans’ greasy hair. The answer lies in the composition of sebum. The team performed a detailed chemical analysis of polar bear sebum. They found sebum’s key components are cholesterol, diacylglycerols and fatty acids. What they didn’t find was something called squalene.

Squalene is a fatty molecule present in human hair and other aquatic animals, such as sea otters. The absence of this element in the polar bear’s fur is important from an anti-ice perspective. Richard Hobbs of Trinity’s School of Chemistry and AMBER is one of the study’s senior authors. Hobbs said:

Despite having thick layers of insulating blubber and fur, and spending extensive periods in water at sub-zero temperatures, it seems that the fur grease provides a natural route for polar bears to easily shed ice when it forms due to the low ice adhesion on their fur.

Two bears, one older and bigger than the other, walking on their 4 legs on ice, and looking at the camera.
Polar bears’ hair does not contain a fatty molecule called squalene, which is present in our hair and in other marine animals. This makes their fur unique. Image via Hans-Jurgen Mager/ Unsplash.

Various strategies for staying warm

Animals that live near the poles have varying methods to stay warm. Hobbs added:

Anne Kietzig’s group at McGill recently found that the hierarchical structure of Gentoo penguin feathers afforded them anti-icing properties that relied on the feather structure rather than the preen oil coating. Our work shows that polar bear fur provides an alternative strategy to produce an anti-icing surface based on the characteristic blend of lipids present in their fur sebum or hair grease.

A young bear lying on its back next to its alert mother also lying on the ice.They look comfortable.
Thick layers of blubber and anti-icing fur insulate polar bears and keep them warm in the freezing Arctic. Image via Hans-Jurgen Mager/ Unsplash.

How does this finding affect us?

Discovering more about the animals that inhabit the planet with us is fascinating. But we can also apply what we discover about their capabilities to our daily lives.

For example, the scientists said we could create an artificial sebum similar to what polar bears have. We could use this as an anti-icing surface coating or in cutting-edge ski skins for skiers and snowboarders, among other things. Hobbs said:

Animals living in polar habitats have emerged as a source of inspiration for the development of new anti-icing materials.

Applying the knowledge gained from the polar bears could also help us eliminate harmful technologies. For example, PFAS are chemicals used in a wide variety of industries around the world. These chemicals degrade extremely slowly and can accumulate over time in humans, animals, water, food and the environment. Hobbs said:

We expect that these natural lipid coatings produced by the bear will help us to develop new more sustainable anti-icing coatings that may replace problematic ‘forever chemicals’ like PFAS that have been used as anti-icing coatings.

Two young polar bears playing, one standing and one sitting. They are surrounded by ice and snow.
For a long time, humans have used damaging chemicals that accumulate on Earth. Polar bears, among other animals, possess unique and natural traits that could serve as an example on how to produce nature-friendly products. Image via Hans-Jurgen Mager/ Unsplash.

Bottom line: How do polar bears stay warm in the frigid Arctic? Scientists have found their greasy hair keeps ice from forming on their coats. This knowledge could be used to create more nature-friendly products without dangerous “forever chemicals.”

Via The University of Dublin

Source: Anti-icing properties of polar bear fur

Read more: Polar bears’ biggest threat is ice loss

Read more: Otters are cute! They’re our lifeform of the week

The post Polar bears have unique ice-repelling fur first appeared on EarthSky.



from EarthSky https://ift.tt/h0NcTEq

Polar bears have unique ice-repelling fur

If you worry about polar bears staying warm in the frozen Arctic, don’t. These animals not only survive cold temperatures, but they need them to thrive. Polar bears have the necessary adaptations to live comfortably in frigid environments. How do they do this? The University of Dublin said on January 29, 2025, that an international team of scientists found polar bears’ greasy hair prevents ice from sticking to their fur.

This is the first study that has been carried out on the composition of polar bears’ hair. The team of scientists published its results in the peer-reviewed journal Science Advances on January 29, 2025.

Big white bear lying on the edge of the ice, facing the camera. Its nose and mouth are dripping water.
By learning more about animals’ unique traits, such as the ice-repelling fur of polar bears, we can apply those findings to our daily life and improve our tools. Image via Hans-Jurgen Mager/ Unsplash.

Get an EarthSky Lunar Calendar for 2025 today! A unique and beautiful poster-sized calendar. Available now!

The secret to ice-free hair is greasy hair!

The team of scientists collected the hair of six polar bears in the wild. Scientists focused on studying the bears’ hair grease, technically known as sebum. They found ice has a harder time sticking to hair when it’s greasy than when it’s clean.

Julian Carolan is the first author of the study. Carolan is a Ph.D. student at Trinity College Dublin’s School of Chemistry and the AMBER Research Ireland Centre for Advanced Materials and BioEngineering. Carolan said:

The sebum quickly jumped out as being the key component giving this anti-icing effect as we discovered the adhesion strength was greatly impacted when the hair was washed. Unwashed, greasy hair made it much harder for ice to stick. In contrast when the polar bear hair was washed and the grease largely removed it performed similarly to human hair, to which ice sticks easily whether it is washed or greasy.

Polar bears: A big white bear standing on an icy surface on its hind legs, with its right paw up, like saying hi.
An international team of scientists discovered that ice sticks more easily to clean polar bear hair than greasy hair. Image via Hans-Jurgen Mager/ Unsplash.

Polar bears have unique fur

Scientists wondered why ice doesn’t stick to greasy hair on polar bears, but it does stick to humans’ greasy hair. The answer lies in the composition of sebum. The team performed a detailed chemical analysis of polar bear sebum. They found sebum’s key components are cholesterol, diacylglycerols and fatty acids. What they didn’t find was something called squalene.

Squalene is a fatty molecule present in human hair and other aquatic animals, such as sea otters. The absence of this element in the polar bear’s fur is important from an anti-ice perspective. Richard Hobbs of Trinity’s School of Chemistry and AMBER is one of the study’s senior authors. Hobbs said:

Despite having thick layers of insulating blubber and fur, and spending extensive periods in water at sub-zero temperatures, it seems that the fur grease provides a natural route for polar bears to easily shed ice when it forms due to the low ice adhesion on their fur.

Two bears, one older and bigger than the other, walking on their 4 legs on ice, and looking at the camera.
Polar bears’ hair does not contain a fatty molecule called squalene, which is present in our hair and in other marine animals. This makes their fur unique. Image via Hans-Jurgen Mager/ Unsplash.

Various strategies for staying warm

Animals that live near the poles have varying methods to stay warm. Hobbs added:

Anne Kietzig’s group at McGill recently found that the hierarchical structure of Gentoo penguin feathers afforded them anti-icing properties that relied on the feather structure rather than the preen oil coating. Our work shows that polar bear fur provides an alternative strategy to produce an anti-icing surface based on the characteristic blend of lipids present in their fur sebum or hair grease.

A young bear lying on its back next to its alert mother also lying on the ice.They look comfortable.
Thick layers of blubber and anti-icing fur insulate polar bears and keep them warm in the freezing Arctic. Image via Hans-Jurgen Mager/ Unsplash.

How does this finding affect us?

Discovering more about the animals that inhabit the planet with us is fascinating. But we can also apply what we discover about their capabilities to our daily lives.

For example, the scientists said we could create an artificial sebum similar to what polar bears have. We could use this as an anti-icing surface coating or in cutting-edge ski skins for skiers and snowboarders, among other things. Hobbs said:

Animals living in polar habitats have emerged as a source of inspiration for the development of new anti-icing materials.

Applying the knowledge gained from the polar bears could also help us eliminate harmful technologies. For example, PFAS are chemicals used in a wide variety of industries around the world. These chemicals degrade extremely slowly and can accumulate over time in humans, animals, water, food and the environment. Hobbs said:

We expect that these natural lipid coatings produced by the bear will help us to develop new more sustainable anti-icing coatings that may replace problematic ‘forever chemicals’ like PFAS that have been used as anti-icing coatings.

Two young polar bears playing, one standing and one sitting. They are surrounded by ice and snow.
For a long time, humans have used damaging chemicals that accumulate on Earth. Polar bears, among other animals, possess unique and natural traits that could serve as an example on how to produce nature-friendly products. Image via Hans-Jurgen Mager/ Unsplash.

Bottom line: How do polar bears stay warm in the frigid Arctic? Scientists have found their greasy hair keeps ice from forming on their coats. This knowledge could be used to create more nature-friendly products without dangerous “forever chemicals.”

Via The University of Dublin

Source: Anti-icing properties of polar bear fur

Read more: Polar bears’ biggest threat is ice loss

Read more: Otters are cute! They’re our lifeform of the week

The post Polar bears have unique ice-repelling fur first appeared on EarthSky.



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Can you ID these cities at night from space? Take the quiz!

Cities at night: A city filled with lights seen from space, with a river running through it.
View larger. | Here’s a practice image to start! Astronauts aboard the International Space Station took this photograph of a glowing city on April 8, 2015. Can you identify which of the world’s cities this is? … If you guessed the City of Lights – Paris – you’re right! The Seine flows through the city, dividing it into the left and right banks. The brightest road is the Champs-Élysées, leading to the Arc de Triomphe, where other roads extend from it like spokes. Now try our quiz below to see how many cities at night from space you can name. Image via NASA.

Can you identify these cities at night from space?

We put together a fun little quiz of images of cities from space. Astronauts aboard the International Space Station captured all these images. We’ve separated them into easy, medium and difficult categories. Only true geography buffs will get all 10. Good luck, and if you get stuck, scroll down for the answers.

The 2025 EarthSky Lunar Calendar is now available! A unique and beautiful poster-sized calendar. Get yours today!

Cities at night: Easy level

A strip of lights with some lights fanned out into the black darkness of water.
1) View larger. | Here’s a city famous for its futuristic developments and construction. The biggest hint is at the top left of this photo. Can you name this city? Image via NASA.
A city from above with a squiggly river from west to east and the lights spread way out.
2) View larger. | Can you guess this city? Note the many dark spots, which indicate parks. Also, the meandering river has many bright bridges that cross it. Image via NASA.
A city with many streets and especially bright center and blackness on the edges.
3) View larger. | Many cities have sprawling suburbs. But this city looks a bit like an island oasis in the dark. Note its particularly bright city core. This area has been called the brightest spot on Earth. Can you name this city? Image via NASA.

Cities at night: Medium level

A city glowing golden at night from space with a dark slice near the bottom.
4) View larger. | Can you identify this city? Here are a couple of hints. Besides the city, there’s an entire other country within this view! And one might say that all roads lead into the city. Image via NASA.
A city at night with a fan shape, narrower at the bottom and wider at the top.
5) View larger. | Do you recognize this city from space? Note the city has a shape a bit like a fan. More lights are spread out at the top, and the city is narrower at bottom, where it hugs the river. Image via NASA.
Lights of a city along a curving body of water.
6) View larger. | The lights of this city hug the water and curve around the bay. You can also see lights from fishing boats. What city is it? Image via NASA.

Cities at night: Difficult level

A bright city bordering some darkness at right but sprawling far to the left.
7) View larger. | This city has seen stupendous growth over the past decades. The large dark area at upper right is a river that empties into a sea that the city borders. Can you name the city? Image via NASA.
A bright central area marking the city with a dark river on the east side making a large bend.
8) View larger. | The bulk of this city is on an island between 2 rivers. Can you name the city? Image via NASA.
A city with a river that is darker on one side and has a radial spread on the other.
9) View larger. | This city is fairly challenging. A famous river runs through the center, and the topography means fewer buildings on one side with growth spreading radially on the other. Can you name this city? Image via NASA.

I can see my house from here!

A green auroral glow near top with many dots of lights scattered across the landscape.
10) View larger. | Here’s a fun one! How many cities can you name in this image? There are hundreds of possibilities. You may even see your own city here. And note the green glow of the aurora. Image via NASA.

Answers for easy level

1) This city is Dubai in the United Arab Emirates. The most notable feature is the humanmade set of islands called Palm Jumeirah. They’re shaped like a palm tree and contain resorts and housing.

2) The second city is London, and the Thames River flows through its center. Hyde Park is the dark space near left center. The brightest bridge is Tower Bridge.

3) This oasis is Las Vegas in Nevada. The Las Vegas Strip is the bright area near center, which is reported to be the brightest spot on Earth.

Answers for medium level

4) “All roads lead to” … Rome! The other country in this view is Vatican City, which is to the upper left of Rome’s center.

5) This city is Cairo in Egypt. The top part of the city fans out, following the delta of the river Nile. And the southern portion of the city is narrower, staying close to the river’s banks.

6) This bright, bustling city is Bangkok, Thailand. Thailand has a big fishing industry, and the boats use lights to attract certain kinds of marine life.

Answers for difficult level

7) This is one of the most populous cities on Earth: Shanghai, China. Shanghai has a population around 30 million. The wide Yangtze river empties into the Yellow Sea just east of this image.

8) This is Montreal, the largest city in Canada’s province of Quebec. Montreal is surrounded by the St. Lawrence and Prairies Rivers.

9) This is Budapest. The Danube River runs through the city. The part of the city that spreads out was the old city of Pest, while the other side of the river, which is steep with fewer buildings, was the old city of Buda.

Answers for the bonus round

There are hundreds of cities in this view! The city at lower right is Omaha. East of it is Des Moines, with Minneapolis/St. Paul to the north of that. Toward the lower right is St. Louis. You can trace out the southwestern shore of Lake Michigan thanks to the Milwaukee/Chicago corridor. The white light in the direction of the northeastern U.S. is lightning.

Share the quiz!

How did you enjoy the quiz? Ready to stump your friends? Share this post or send this video to them. Challenge them to get at least six right!

Bottom line: Take this fun quiz to see how many cities at night you can identify from images astronauts took aboard the International Space Station.

Read more: See Mars as ISS astronauts would see it

The post Can you ID these cities at night from space? Take the quiz! first appeared on EarthSky.



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Cities at night: A city filled with lights seen from space, with a river running through it.
View larger. | Here’s a practice image to start! Astronauts aboard the International Space Station took this photograph of a glowing city on April 8, 2015. Can you identify which of the world’s cities this is? … If you guessed the City of Lights – Paris – you’re right! The Seine flows through the city, dividing it into the left and right banks. The brightest road is the Champs-Élysées, leading to the Arc de Triomphe, where other roads extend from it like spokes. Now try our quiz below to see how many cities at night from space you can name. Image via NASA.

Can you identify these cities at night from space?

We put together a fun little quiz of images of cities from space. Astronauts aboard the International Space Station captured all these images. We’ve separated them into easy, medium and difficult categories. Only true geography buffs will get all 10. Good luck, and if you get stuck, scroll down for the answers.

The 2025 EarthSky Lunar Calendar is now available! A unique and beautiful poster-sized calendar. Get yours today!

Cities at night: Easy level

A strip of lights with some lights fanned out into the black darkness of water.
1) View larger. | Here’s a city famous for its futuristic developments and construction. The biggest hint is at the top left of this photo. Can you name this city? Image via NASA.
A city from above with a squiggly river from west to east and the lights spread way out.
2) View larger. | Can you guess this city? Note the many dark spots, which indicate parks. Also, the meandering river has many bright bridges that cross it. Image via NASA.
A city with many streets and especially bright center and blackness on the edges.
3) View larger. | Many cities have sprawling suburbs. But this city looks a bit like an island oasis in the dark. Note its particularly bright city core. This area has been called the brightest spot on Earth. Can you name this city? Image via NASA.

Cities at night: Medium level

A city glowing golden at night from space with a dark slice near the bottom.
4) View larger. | Can you identify this city? Here are a couple of hints. Besides the city, there’s an entire other country within this view! And one might say that all roads lead into the city. Image via NASA.
A city at night with a fan shape, narrower at the bottom and wider at the top.
5) View larger. | Do you recognize this city from space? Note the city has a shape a bit like a fan. More lights are spread out at the top, and the city is narrower at bottom, where it hugs the river. Image via NASA.
Lights of a city along a curving body of water.
6) View larger. | The lights of this city hug the water and curve around the bay. You can also see lights from fishing boats. What city is it? Image via NASA.

Cities at night: Difficult level

A bright city bordering some darkness at right but sprawling far to the left.
7) View larger. | This city has seen stupendous growth over the past decades. The large dark area at upper right is a river that empties into a sea that the city borders. Can you name the city? Image via NASA.
A bright central area marking the city with a dark river on the east side making a large bend.
8) View larger. | The bulk of this city is on an island between 2 rivers. Can you name the city? Image via NASA.
A city with a river that is darker on one side and has a radial spread on the other.
9) View larger. | This city is fairly challenging. A famous river runs through the center, and the topography means fewer buildings on one side with growth spreading radially on the other. Can you name this city? Image via NASA.

I can see my house from here!

A green auroral glow near top with many dots of lights scattered across the landscape.
10) View larger. | Here’s a fun one! How many cities can you name in this image? There are hundreds of possibilities. You may even see your own city here. And note the green glow of the aurora. Image via NASA.

Answers for easy level

1) This city is Dubai in the United Arab Emirates. The most notable feature is the humanmade set of islands called Palm Jumeirah. They’re shaped like a palm tree and contain resorts and housing.

2) The second city is London, and the Thames River flows through its center. Hyde Park is the dark space near left center. The brightest bridge is Tower Bridge.

3) This oasis is Las Vegas in Nevada. The Las Vegas Strip is the bright area near center, which is reported to be the brightest spot on Earth.

Answers for medium level

4) “All roads lead to” … Rome! The other country in this view is Vatican City, which is to the upper left of Rome’s center.

5) This city is Cairo in Egypt. The top part of the city fans out, following the delta of the river Nile. And the southern portion of the city is narrower, staying close to the river’s banks.

6) This bright, bustling city is Bangkok, Thailand. Thailand has a big fishing industry, and the boats use lights to attract certain kinds of marine life.

Answers for difficult level

7) This is one of the most populous cities on Earth: Shanghai, China. Shanghai has a population around 30 million. The wide Yangtze river empties into the Yellow Sea just east of this image.

8) This is Montreal, the largest city in Canada’s province of Quebec. Montreal is surrounded by the St. Lawrence and Prairies Rivers.

9) This is Budapest. The Danube River runs through the city. The part of the city that spreads out was the old city of Pest, while the other side of the river, which is steep with fewer buildings, was the old city of Buda.

Answers for the bonus round

There are hundreds of cities in this view! The city at lower right is Omaha. East of it is Des Moines, with Minneapolis/St. Paul to the north of that. Toward the lower right is St. Louis. You can trace out the southwestern shore of Lake Michigan thanks to the Milwaukee/Chicago corridor. The white light in the direction of the northeastern U.S. is lightning.

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Bottom line: Take this fun quiz to see how many cities at night you can identify from images astronauts took aboard the International Space Station.

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Deep-sky photos for January 2025: Editor’s picks


Enjoy a video of some of January’s deep-sky photos and see more below.

Stunning deep-sky photos from our community

The EarthSky community has many talented astrophotographers who capture stunning images of the deep sky. We gathered some of our favorite deep-sky photos we received in January 2025 for you to enjoy. Do you have some of your own deep-sky images to share? You can submit them to us here. We love to see them!

The 2025 EarthSky Lunar Calendar is now available. Get yours today!

Deep-sky photos of diffuse nebulae

Deep-sky photos: Large nebulous patch with bright red and blue swirls, dark spots and numerous foreground stars.
View at EarthSky Community Photos. | Steven Bellavia in Smithfield, Virginia, captured the Tadpole Nebula on January 4, 2025. Steven wrote: “IC 410, the Tadpole Nebula, is a region of ionized hydrogen gas spanning over 100 light-years across. It’s formed by the stellar winds of the open star cluster NGC 1893 embedded within. It is 12,400 light-years away, located in the constellation Auriga.” Beautiful capture. Thank you, Steven!
Two red patches of nebulosity, one large and elongated, one round and small, with numerous foreground stars.
View at EarthSky Community Photos. | Andy Dungan near Cotopaxi, Colorado, captured IC 2177, the Seagull Nebula, on January 6, 2025. Andy wrote: “The Seagull Nebula in Monoceros (wings below and head above). Two weeks ago I did not realize this rather large nebula even existed. Where I live I have had a lot of clouds. It took 3 or 4 days to get my 41 exposures. It sure was worth it!” Thank you, Andy!
Half the image covered with wispy red clouds with a bright, electric blue nucleus, and thousands of foreground stars.
View at EarthSky Community Photos. | George Tzellos in Athens, Greece, captured reflection nebula CED 51 on January 18, 2025. George wrote: “CED 51 is a blue reflection nebula embedded in ionized hydrogen gas, which is part of the Lambda Orionis ring in the Orion constellation. I captured the exposures between January 3 and 18.” Thank you, George!
Green and yellow cloud of gas with a dark, horsehead-shaped indentation and a few foreground stars.
View at EarthSky Community Photos. | Jeremy Likness in Newport, Oregon, captured the Horsehead Nebula in the constellation Orion on January 18, 2024. Jeremy wrote: “I mapped the data for each band based on the popular Hubble color palette.” Thank you, Jeremy!

The Monkey Head Nebula

Large nebulous patch with bright red and blue swirls, dark lanes and numerous foreground stars.
View at EarthSky Community Photos. | Egidio Vergani in Milan, Italy, captured NGC 2174, the Monkey Head Nebula in Orion, on January 2, 2025. Egidio wrote: “The NGC 2174 nebula has a very particular shape. It shows the vague appearance of a monkey’s head seen in profile. In reality this celestial object, located about 6,400 light-years from us, is an area of star formation. The cloud of gas – mainly hydrogen – and dust contain some young, very bright stars. It is precisely their ultraviolet radiation that illuminates the nebula. I took the photo from the polluted sky of Milan with approximately 15 hours of exposure.” Thank you, Egidio!
Large nebulous patch with bright orange and blue swirls, dark lanes and numerous foreground stars.
View at EarthSky Community Photos. | Emmanuel Delgadillo in Jalisco, Mexico, captured the Monkey Head Nebula in Orion on January 1, 2025. Thank you, Emmanuel!

Deep-sky photos of the wonderful Rosette Nebula

Three donut-shaped swirls - the same object- side-by-side, one red, one orange and another blue, in starry space.
View at EarthSky Community Photos. | Mohammed Abdallah in Suez, Egypt, captured these 3 filtered views of the Rosette Nebula in Monoceros on January 4, 2025. Mohammed wrote: “It was fun to create the color mapping for hydrogen-alpha and oxygen III. Taken from my backyard in Suez, Egypt.” Thank you, Mohammed!
Large donut-shaped swirl of red and pink gas with thin dark lanes and numerous stars immersed within.
View at EarthSky Community Photos. | Andy Dungan near Cotopaxi, Colorado, captured the Rosette Nebula on January 25, 2025. Andy wrote: “I have taken 82 10-minute photos over the last 2 nights. Rosette is big and bright. And the results show it.” Thank you, Andy!
Large donut-shaped swirl of blue and green gas with thin dark lanes and numerous stars immersed within.
View at EarthSky Community Photos. | Steven Bellavia in Smithfield, Virginia, captured the Rosette Nebula on January 24, 2025. Steven wrote: “The Rosette Nebula, Caldwell 49, is a large spherical HII region located near one end of a giant molecular cloud in the Monoceros region of the Milky Way galaxy. The open cluster NGC 2244 (Caldwell 50) is closely associated with the nebulosity. The stars of the cluster were formed from the nebula’s matter. The cluster and nebula are 5,000 light-years from Earth and measure roughly 130 light-years in diameter.” Thank you, Steven!

A supernova remnant

Multicolored egg-shaped cloud, like an explosion frozen in space, with scattered stars.
View at EarthSky Community Photos. | Jeremy Likness in Newport, Oregon, captured Messier 1, a supernova remnant in the constellation Taurus, on January 18, 2025. Jeremy wrote: “First nebula of the year! M1, the Crab Nebula.” Thank you, Jeremy!

A star cluster

A sparse grouping of stars roughly resembling the number 37, among scattered stars.
View at EarthSky Community Photos. | Scott Smith in Palmetto, Florida, captured a star cluster on January 5, 2025. Scott wrote: “NGC 2169 is an open cluster in Orion. This asterism is nicknamed ‘The 37 Cluster’ due to its striking resemblance to the numerals ’37.’ Can you see it? Once you do, the 37 really stands out. Hint: Tilt your head to the left.” Thank you, Scott!

And a distant galaxy

A bright yellow galaxy, a spiral seen head-on, in dusty, starry space.
View at EarthSky Community Photos. | Kris Hazelbaker in Grangeville, Idaho, captured galaxy IC 342, in the constellation Camelopardalis, on January 26, 2025. Kris wrote: “The Hidden Galaxy, IC342. The dust clouds between this galaxy and us make it less obvious in the sky. Without them, it would be visible to the unaided eye. This is 9.5 hours of exposures.” Thank you, Kris!

Bottom line: Enjoy this gallery of deep-sky photos for January 2025 from our EarthSky community. If you have a great photo to share, send it in, too. We love to see them!

Share your recent Earth or sky photo at EarthSky Community Photos.

The post Deep-sky photos for January 2025: Editor’s picks first appeared on EarthSky.



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Enjoy a video of some of January’s deep-sky photos and see more below.

Stunning deep-sky photos from our community

The EarthSky community has many talented astrophotographers who capture stunning images of the deep sky. We gathered some of our favorite deep-sky photos we received in January 2025 for you to enjoy. Do you have some of your own deep-sky images to share? You can submit them to us here. We love to see them!

The 2025 EarthSky Lunar Calendar is now available. Get yours today!

Deep-sky photos of diffuse nebulae

Deep-sky photos: Large nebulous patch with bright red and blue swirls, dark spots and numerous foreground stars.
View at EarthSky Community Photos. | Steven Bellavia in Smithfield, Virginia, captured the Tadpole Nebula on January 4, 2025. Steven wrote: “IC 410, the Tadpole Nebula, is a region of ionized hydrogen gas spanning over 100 light-years across. It’s formed by the stellar winds of the open star cluster NGC 1893 embedded within. It is 12,400 light-years away, located in the constellation Auriga.” Beautiful capture. Thank you, Steven!
Two red patches of nebulosity, one large and elongated, one round and small, with numerous foreground stars.
View at EarthSky Community Photos. | Andy Dungan near Cotopaxi, Colorado, captured IC 2177, the Seagull Nebula, on January 6, 2025. Andy wrote: “The Seagull Nebula in Monoceros (wings below and head above). Two weeks ago I did not realize this rather large nebula even existed. Where I live I have had a lot of clouds. It took 3 or 4 days to get my 41 exposures. It sure was worth it!” Thank you, Andy!
Half the image covered with wispy red clouds with a bright, electric blue nucleus, and thousands of foreground stars.
View at EarthSky Community Photos. | George Tzellos in Athens, Greece, captured reflection nebula CED 51 on January 18, 2025. George wrote: “CED 51 is a blue reflection nebula embedded in ionized hydrogen gas, which is part of the Lambda Orionis ring in the Orion constellation. I captured the exposures between January 3 and 18.” Thank you, George!
Green and yellow cloud of gas with a dark, horsehead-shaped indentation and a few foreground stars.
View at EarthSky Community Photos. | Jeremy Likness in Newport, Oregon, captured the Horsehead Nebula in the constellation Orion on January 18, 2024. Jeremy wrote: “I mapped the data for each band based on the popular Hubble color palette.” Thank you, Jeremy!

The Monkey Head Nebula

Large nebulous patch with bright red and blue swirls, dark lanes and numerous foreground stars.
View at EarthSky Community Photos. | Egidio Vergani in Milan, Italy, captured NGC 2174, the Monkey Head Nebula in Orion, on January 2, 2025. Egidio wrote: “The NGC 2174 nebula has a very particular shape. It shows the vague appearance of a monkey’s head seen in profile. In reality this celestial object, located about 6,400 light-years from us, is an area of star formation. The cloud of gas – mainly hydrogen – and dust contain some young, very bright stars. It is precisely their ultraviolet radiation that illuminates the nebula. I took the photo from the polluted sky of Milan with approximately 15 hours of exposure.” Thank you, Egidio!
Large nebulous patch with bright orange and blue swirls, dark lanes and numerous foreground stars.
View at EarthSky Community Photos. | Emmanuel Delgadillo in Jalisco, Mexico, captured the Monkey Head Nebula in Orion on January 1, 2025. Thank you, Emmanuel!

Deep-sky photos of the wonderful Rosette Nebula

Three donut-shaped swirls - the same object- side-by-side, one red, one orange and another blue, in starry space.
View at EarthSky Community Photos. | Mohammed Abdallah in Suez, Egypt, captured these 3 filtered views of the Rosette Nebula in Monoceros on January 4, 2025. Mohammed wrote: “It was fun to create the color mapping for hydrogen-alpha and oxygen III. Taken from my backyard in Suez, Egypt.” Thank you, Mohammed!
Large donut-shaped swirl of red and pink gas with thin dark lanes and numerous stars immersed within.
View at EarthSky Community Photos. | Andy Dungan near Cotopaxi, Colorado, captured the Rosette Nebula on January 25, 2025. Andy wrote: “I have taken 82 10-minute photos over the last 2 nights. Rosette is big and bright. And the results show it.” Thank you, Andy!
Large donut-shaped swirl of blue and green gas with thin dark lanes and numerous stars immersed within.
View at EarthSky Community Photos. | Steven Bellavia in Smithfield, Virginia, captured the Rosette Nebula on January 24, 2025. Steven wrote: “The Rosette Nebula, Caldwell 49, is a large spherical HII region located near one end of a giant molecular cloud in the Monoceros region of the Milky Way galaxy. The open cluster NGC 2244 (Caldwell 50) is closely associated with the nebulosity. The stars of the cluster were formed from the nebula’s matter. The cluster and nebula are 5,000 light-years from Earth and measure roughly 130 light-years in diameter.” Thank you, Steven!

A supernova remnant

Multicolored egg-shaped cloud, like an explosion frozen in space, with scattered stars.
View at EarthSky Community Photos. | Jeremy Likness in Newport, Oregon, captured Messier 1, a supernova remnant in the constellation Taurus, on January 18, 2025. Jeremy wrote: “First nebula of the year! M1, the Crab Nebula.” Thank you, Jeremy!

A star cluster

A sparse grouping of stars roughly resembling the number 37, among scattered stars.
View at EarthSky Community Photos. | Scott Smith in Palmetto, Florida, captured a star cluster on January 5, 2025. Scott wrote: “NGC 2169 is an open cluster in Orion. This asterism is nicknamed ‘The 37 Cluster’ due to its striking resemblance to the numerals ’37.’ Can you see it? Once you do, the 37 really stands out. Hint: Tilt your head to the left.” Thank you, Scott!

And a distant galaxy

A bright yellow galaxy, a spiral seen head-on, in dusty, starry space.
View at EarthSky Community Photos. | Kris Hazelbaker in Grangeville, Idaho, captured galaxy IC 342, in the constellation Camelopardalis, on January 26, 2025. Kris wrote: “The Hidden Galaxy, IC342. The dust clouds between this galaxy and us make it less obvious in the sky. Without them, it would be visible to the unaided eye. This is 9.5 hours of exposures.” Thank you, Kris!

Bottom line: Enjoy this gallery of deep-sky photos for January 2025 from our EarthSky community. If you have a great photo to share, send it in, too. We love to see them!

Share your recent Earth or sky photo at EarthSky Community Photos.

The post Deep-sky photos for January 2025: Editor’s picks first appeared on EarthSky.



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The mass of a star is determined from binary star systems

Mass of a star: Large white star to the left, tiny blue star to the right, against a star-strewn background.
Artist’s concept of the binary star system of Sirius A and its small blue companion, Sirius B, a hot white dwarf. The 2 stars revolve around each other every 50 years. Binary stars are useful to determine the mass of a star. Image via ESA/ G. Bacon.

To measure the mass of a star, use 2 stars

There are lots of binary stars – two stars revolving around a common center of mass – populating the starry sky. In fact, a large majority of all stars we see (around 80%) are thought to be part of multiple star systems of two or more stars! This is lucky for astronomers, because two stars together provide an easy way to measure star masses.

To find the masses of stars in double systems, you need to know only two things. First, the semi-major axis or mean distance between the two stars (often expressed in astronomical units, which is the average distance between the Earth and sun).

And second, you need to know the time it takes for the two stars to revolve around one another (aka the orbital period, often expressed in Earth years).

With those two observations alone, astronomers can calculate the stars’ masses. They typically do that in units of solar masses (that is, a measure of how many of our suns the star “weighs.” One solar mass is 1.989 x 1030 kilograms or about 333,000 times the mass of our planet Earth.)

The 2025 EarthSky lunar calendar makes a great gift. Get yours today!

Sirius is a great example

We’ll use Sirius, the brightest star of the nighttime sky, as an example. It looks like a single star to the unaided eye, but it, too, is a binary star. By the way, you can see it yourself, if you have a small telescope.

The two stars orbit each other with a period of about 50.1 Earth-years, at an average distance of about 19.8 astronomical units (AU). The brighter of the two is called Sirius A, while its fainter companion is known as Sirius B (The Pup).

Black background with one central white spot with spikes, and a tiny white dot on its left side.
View at EarthSky Community Photos. | Michael Teoh at Heng Ee Observatory in Penang, Malaysia, captured this photo of Sirius A and Sirius B (a white dwarf) on January 26, 2021. He used 30 1-second exposures and stacked them together to make faint Sirius B appear. Thank you, Michael!

Finding the mass of Sirius A and B

So how would astronomers find the masses of Sirius A and B? They would simply plug in the mean distance between the two stars (19.8 AU) and their orbital period (50.1 Earth-years) into the easy-to-use formula below, first derived by Johannes Kepler in 1618, and known as Kepler’s Third law:

Total mass = distance3/period2
Total mass = 19.83/50.12
So total mass = 7762.39/2510.01 = 3.09 times the sun’s mass

Here, the distance is the mean distance between the stars (or, more precisely, the semi-major axis) in astronomical units, so 19.8, and the orbital period is 50.1 years.

The resulting total mass is about three solar masses. Note that this is not the mass of one star but of both stars added together. So, we know that the whole binary system equals three solar masses.

Two overlapping elliptical orbits in red with white circles moving around the orbits.
An example of a binary star system, whose component stars orbit around a common center of mass (the red cross). In this depiction, the two stars have similar masses. In the case of the Sirius binary star system, Sirius A has about twice the mass of Sirius B. Image via Wikimedia Commons.

Then finding the mass of each star

To find out the mass of each individual star, astronomers need to know the mean distance of each star from the barycenter: their common center of mass. To learn this, once again they rely on their observations.

It turns out that Sirius B, the less massive star, is about twice as far from the barycenter than is Sirius A. That means Sirius B has about half the mass of Sirius A.

Thus, you know the whole system is about three solar masses by using Kepler’s Third Law. So now you can deduce that the mass of Sirius A is about two solar masses. And then Sirius B pretty much equals our sun in mass.

What about the mass of a star not in a binary system?

But what about stars that are alone in their star systems, like the sun? The binary star systems are once again the key: Once we have calculated the masses for a whole lot of stars in binary systems, and also know how luminous they are, we notice that there is a relationship between their luminosity and their mass. In other words, for single stars we only need to measure its luminosity and then use the mass-luminosity relation to figure out their mass. Thank you, binaries!

Read more: What is stellar luminosity?

Read more: What is stellar magnitude?

Bottom line: For astronomers, binary star systems are a quite useful tool to figure out the mass of stars.

The post The mass of a star is determined from binary star systems first appeared on EarthSky.



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Mass of a star: Large white star to the left, tiny blue star to the right, against a star-strewn background.
Artist’s concept of the binary star system of Sirius A and its small blue companion, Sirius B, a hot white dwarf. The 2 stars revolve around each other every 50 years. Binary stars are useful to determine the mass of a star. Image via ESA/ G. Bacon.

To measure the mass of a star, use 2 stars

There are lots of binary stars – two stars revolving around a common center of mass – populating the starry sky. In fact, a large majority of all stars we see (around 80%) are thought to be part of multiple star systems of two or more stars! This is lucky for astronomers, because two stars together provide an easy way to measure star masses.

To find the masses of stars in double systems, you need to know only two things. First, the semi-major axis or mean distance between the two stars (often expressed in astronomical units, which is the average distance between the Earth and sun).

And second, you need to know the time it takes for the two stars to revolve around one another (aka the orbital period, often expressed in Earth years).

With those two observations alone, astronomers can calculate the stars’ masses. They typically do that in units of solar masses (that is, a measure of how many of our suns the star “weighs.” One solar mass is 1.989 x 1030 kilograms or about 333,000 times the mass of our planet Earth.)

The 2025 EarthSky lunar calendar makes a great gift. Get yours today!

Sirius is a great example

We’ll use Sirius, the brightest star of the nighttime sky, as an example. It looks like a single star to the unaided eye, but it, too, is a binary star. By the way, you can see it yourself, if you have a small telescope.

The two stars orbit each other with a period of about 50.1 Earth-years, at an average distance of about 19.8 astronomical units (AU). The brighter of the two is called Sirius A, while its fainter companion is known as Sirius B (The Pup).

Black background with one central white spot with spikes, and a tiny white dot on its left side.
View at EarthSky Community Photos. | Michael Teoh at Heng Ee Observatory in Penang, Malaysia, captured this photo of Sirius A and Sirius B (a white dwarf) on January 26, 2021. He used 30 1-second exposures and stacked them together to make faint Sirius B appear. Thank you, Michael!

Finding the mass of Sirius A and B

So how would astronomers find the masses of Sirius A and B? They would simply plug in the mean distance between the two stars (19.8 AU) and their orbital period (50.1 Earth-years) into the easy-to-use formula below, first derived by Johannes Kepler in 1618, and known as Kepler’s Third law:

Total mass = distance3/period2
Total mass = 19.83/50.12
So total mass = 7762.39/2510.01 = 3.09 times the sun’s mass

Here, the distance is the mean distance between the stars (or, more precisely, the semi-major axis) in astronomical units, so 19.8, and the orbital period is 50.1 years.

The resulting total mass is about three solar masses. Note that this is not the mass of one star but of both stars added together. So, we know that the whole binary system equals three solar masses.

Two overlapping elliptical orbits in red with white circles moving around the orbits.
An example of a binary star system, whose component stars orbit around a common center of mass (the red cross). In this depiction, the two stars have similar masses. In the case of the Sirius binary star system, Sirius A has about twice the mass of Sirius B. Image via Wikimedia Commons.

Then finding the mass of each star

To find out the mass of each individual star, astronomers need to know the mean distance of each star from the barycenter: their common center of mass. To learn this, once again they rely on their observations.

It turns out that Sirius B, the less massive star, is about twice as far from the barycenter than is Sirius A. That means Sirius B has about half the mass of Sirius A.

Thus, you know the whole system is about three solar masses by using Kepler’s Third Law. So now you can deduce that the mass of Sirius A is about two solar masses. And then Sirius B pretty much equals our sun in mass.

What about the mass of a star not in a binary system?

But what about stars that are alone in their star systems, like the sun? The binary star systems are once again the key: Once we have calculated the masses for a whole lot of stars in binary systems, and also know how luminous they are, we notice that there is a relationship between their luminosity and their mass. In other words, for single stars we only need to measure its luminosity and then use the mass-luminosity relation to figure out their mass. Thank you, binaries!

Read more: What is stellar luminosity?

Read more: What is stellar magnitude?

Bottom line: For astronomers, binary star systems are a quite useful tool to figure out the mass of stars.

The post The mass of a star is determined from binary star systems first appeared on EarthSky.



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Using exploding white dwarfs to measure the universe

Exploding white dwarfs: Illustration of a telescope sending out a beam into space, with an octagon-shaped set of small squares at the end. A bright band of stars goes across the sky from left to right.
Artist’s illustration of the Rubin Observatory scanning the night sky for white dwarf supernovae. These are exploding white dwarfs, known as Type Ia supernovae. Astronomers expect Rubin to find millions of them. Image via RubinObs/ NOIRLab/ SLAC/ NSF/ DOE/ AURA/ P. Marenfeld.
  • Exploding white dwarf stars are a type of supernova. Astronomers have found thousands of them so far.
  • The upcoming new Rubin Observatory should find millions more, scientists say. It will scan the night sky faster than ever before.
  • Astronomers can use these white dwarf supernovae to measure the expansion of the universe and how dark energy might influence it.

Using exploding white dwarfs to measure the universe

When the new Vera C. Rubin Observatory becomes operational later this year, it will begin to rapidly scan the night sky. One of its objectives is to search for Type Ia supernovae. Those are a type of supernova that occur in binary star systems (where two stars orbit one another) in which one of the stars is a white dwarf. Astronomers expect the telescope to find millions more of these exploding dying stars, researchers with Rubin Observatory said on January 22, 2025. Scientists can analyze the light from these supernovae to measure distances in the universe and the universe’s expansion rate. They can also better understand how dark energy affects that expansion.

The 2025 EarthSky lunar calendar makes a great gift. Get yours today!

Millions of exploding white dwarfs

Right now, the Vera C. Rubin Observatory is under construction on Cerro Pachón in Chile. Later this year, the construction should be complete and science operations should begin. The observatory will then begin its 10-year-long Legacy Survey of Space and Time. During that survey, astronomers expect Rubin to find millions of Type Ia supernovae. Those are a type of supernova where white dwarf stars explode. And they’re some of the most powerful and brightest known.

Astronomers have already found thousands of them, but the detections are fleeting. That’s where Rubin comes in. It will scan the sky every night for 10 years and cover the entire hemisphere every few nights. When it detects a new Type Ia supernova, it will immediately send an alert to astronomers. In fact, Rubin will produce about 20 terabytes of data and generate up to 10 million alerts every night.

Seven community software systems will be needed to manage the deluge of alerts. They will process the alerts and then pass them on to astronomers. The software systems will also merge Rubin data with other datasets. Then, they will use machine-learning to classify the supernovae according to their type, such as kilonovae, variable stars or Type Ia supernovae.

Scientist Anais Möller, a member of the Rubin/LSST Dark Energy Science Collaboration, said:

The large volume of data from Rubin will give us a sample of all kinds of Type Ia supernovae at a range of distances and in many different types of galaxies.


Artist’s visualization of a white dwarf star exploding as a Type Ia supernovae. The white dwarf sucks matter from its binary star companion. When it becomes heavier than a certain limit, it becomes unstable and explodes. Video via ESO.

Expansion of the universe and dark energy

Astronomers can use the detections of Type Ia supernovae to better measure the expansion of the universe. By analyzing their brightness and color, along with information about their host galaxies, scientists can calculate their distance and how much the universe expanded while their light was traveling to us. Once they have enough observations, astronomers can measure the expansion rate of the universe and determine whether it changes over time.

These measurements can also give astronomers a better idea of how dark energy influences the universe’s expansion. Möller said:

The universe expanding is like a rubber band being stretched. If dark energy is not constant, that would be like stretching the rubber band by different amounts at different points. I think in the next decade we will be able to constrain whether dark energy is constant or evolving with cosmic time. Rubin will allow us to do that with Type Ia supernovae.

Ultimately, Rubin will change how this kind of science is done in the future. As Möller noted:

Because of the large volumes of data, we can’t do science the same way we did before. Rubin is a generational shift. And our responsibility is developing the methods that will be used by the next generation.

Bottom line: The Rubin Observatory will find millions of exploding white dwarfs, or Type Ia supernovae. Astronomers can use them to measure the universe and dark energy.

Via Vera C. Rubin Observatory

Via NOIRLab

Read more: A buddy for Betelgeuse? Does this mean no supernova?

Read more: See Supernova 1987A in a new light from Webb

The post Using exploding white dwarfs to measure the universe first appeared on EarthSky.



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Exploding white dwarfs: Illustration of a telescope sending out a beam into space, with an octagon-shaped set of small squares at the end. A bright band of stars goes across the sky from left to right.
Artist’s illustration of the Rubin Observatory scanning the night sky for white dwarf supernovae. These are exploding white dwarfs, known as Type Ia supernovae. Astronomers expect Rubin to find millions of them. Image via RubinObs/ NOIRLab/ SLAC/ NSF/ DOE/ AURA/ P. Marenfeld.
  • Exploding white dwarf stars are a type of supernova. Astronomers have found thousands of them so far.
  • The upcoming new Rubin Observatory should find millions more, scientists say. It will scan the night sky faster than ever before.
  • Astronomers can use these white dwarf supernovae to measure the expansion of the universe and how dark energy might influence it.

Using exploding white dwarfs to measure the universe

When the new Vera C. Rubin Observatory becomes operational later this year, it will begin to rapidly scan the night sky. One of its objectives is to search for Type Ia supernovae. Those are a type of supernova that occur in binary star systems (where two stars orbit one another) in which one of the stars is a white dwarf. Astronomers expect the telescope to find millions more of these exploding dying stars, researchers with Rubin Observatory said on January 22, 2025. Scientists can analyze the light from these supernovae to measure distances in the universe and the universe’s expansion rate. They can also better understand how dark energy affects that expansion.

The 2025 EarthSky lunar calendar makes a great gift. Get yours today!

Millions of exploding white dwarfs

Right now, the Vera C. Rubin Observatory is under construction on Cerro Pachón in Chile. Later this year, the construction should be complete and science operations should begin. The observatory will then begin its 10-year-long Legacy Survey of Space and Time. During that survey, astronomers expect Rubin to find millions of Type Ia supernovae. Those are a type of supernova where white dwarf stars explode. And they’re some of the most powerful and brightest known.

Astronomers have already found thousands of them, but the detections are fleeting. That’s where Rubin comes in. It will scan the sky every night for 10 years and cover the entire hemisphere every few nights. When it detects a new Type Ia supernova, it will immediately send an alert to astronomers. In fact, Rubin will produce about 20 terabytes of data and generate up to 10 million alerts every night.

Seven community software systems will be needed to manage the deluge of alerts. They will process the alerts and then pass them on to astronomers. The software systems will also merge Rubin data with other datasets. Then, they will use machine-learning to classify the supernovae according to their type, such as kilonovae, variable stars or Type Ia supernovae.

Scientist Anais Möller, a member of the Rubin/LSST Dark Energy Science Collaboration, said:

The large volume of data from Rubin will give us a sample of all kinds of Type Ia supernovae at a range of distances and in many different types of galaxies.


Artist’s visualization of a white dwarf star exploding as a Type Ia supernovae. The white dwarf sucks matter from its binary star companion. When it becomes heavier than a certain limit, it becomes unstable and explodes. Video via ESO.

Expansion of the universe and dark energy

Astronomers can use the detections of Type Ia supernovae to better measure the expansion of the universe. By analyzing their brightness and color, along with information about their host galaxies, scientists can calculate their distance and how much the universe expanded while their light was traveling to us. Once they have enough observations, astronomers can measure the expansion rate of the universe and determine whether it changes over time.

These measurements can also give astronomers a better idea of how dark energy influences the universe’s expansion. Möller said:

The universe expanding is like a rubber band being stretched. If dark energy is not constant, that would be like stretching the rubber band by different amounts at different points. I think in the next decade we will be able to constrain whether dark energy is constant or evolving with cosmic time. Rubin will allow us to do that with Type Ia supernovae.

Ultimately, Rubin will change how this kind of science is done in the future. As Möller noted:

Because of the large volumes of data, we can’t do science the same way we did before. Rubin is a generational shift. And our responsibility is developing the methods that will be used by the next generation.

Bottom line: The Rubin Observatory will find millions of exploding white dwarfs, or Type Ia supernovae. Astronomers can use them to measure the universe and dark energy.

Via Vera C. Rubin Observatory

Via NOIRLab

Read more: A buddy for Betelgeuse? Does this mean no supernova?

Read more: See Supernova 1987A in a new light from Webb

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Sirius is a future southern Pole Star

Sirius: Chart of Orion with a red arrow leading from Orion's Belt to Sirius.
Sirius is not only the brightest star in the constellation Canis Major the Greater Dog, it’s the brightest star in the sky. Plus, Sirius is a future pole star for the Southern Hemisphere. You can be sure you’re looking at the correct bright star by drawing a line from Orion’s Belt to Sirius. From the Southern Hemisphere, turn the chart upside down.

Brilliant Sirius as a southern Pole Star

On January evenings, use the Belt of Orion to star-hop to Sirius, a future southern Pole Star.

Believe it or not, the brightest star in the sky – our sparkling night beacon, Sirius in the constellation Canis Major the Greater Dog – will someday serve as a very reasonable, and certainly very noticeable, South Star.

Unlike the Northern Hemisphere, the Southern Hemisphere currently doesn’t have a bright pole star. There’s not even a moderately bright star – like our North Star Polaris – to mark the south celestial pole, the point in the sky directly over the Earth’s South Pole.

The pole star – either north or south – is the point around which the entire sky turns. It’s like the hub of a great wheel.

Available now! 2025 EarthSky lunar calendar. A unique and beautiful poster-sized calendar showing phases of the moon every night of the year. Makes a great gift!

But not for a long, long time

We found the information about Sirius as a southern pole star in the book “Mathematical Astronomy Morsels V” by the great celestial mechanist Jean Meeus. See pages 353 to 363. Meeus wrote that Sirius will take its turn as the southern Pole Star some 60,000 years from now, around the year 66270. In that year, Sirius will come to within 1.6 degrees of the south celestial pole.

One precessional cycle later, in the year 93830, Sirius will miss aligning with the south celestial pole by only 2.3 degrees.

Stars close to the celestial poles

Our current North Star, Polaris, comes closer than that to the north celestial pole. Polaris will be within 0.5 degrees of the north celestial pole in the year 2100. For reference, the moon’s angular diameter spans 0.5 degrees of sky.

But the Southern Hemisphere will have its close pole star, too. A moderately bright star, not very different in brightness from Polaris, will take its place more or less over the south celestial pole about 7,000 years from now. Because of precession, the star Delta Velorum in the constellation Vela the Sail will come to within 0.2 degrees of the south celestial pole in the year 9250. That’s closer to marking the celestial pole than Polaris or Sirius ever do during their reigns as pole stars!

Was Sirius a pole star in the past?

A reader asked us:

Sirius will be the southern pole star around 66,270 AD. Was Sirius the southern pole star in the past as well?

Because of the 26,000-year precession of our planet, it would seem like Sirius gets to be the south celestial pole star every 26,000 years. But the answer is actually no, Sirius was not a pole star in the past, and it won’t be a pole star anytime between now and 66,270 AD.

The reason is because Sirius is so close to us. Sirius, the brightest star seen from Earth, is bright because of how close it is. And being close to us, it appears to move faster than other stars in the sky. Scientists say it has a high rate of proper motion. At Tony Dunn’s website, you can scroll down to Canis Major, Sirius’s home constellation, and click to watch an animation of how Sirius’s fast motion (relatively speaking) changes the look of the Greater Dog.

So, Sirius is not currently close to the south celestial pole or even the circle that the precession draws out on the southern sky. But it is heading that way and will close in on the south celestial pole by 66,270 AD. Orbital simulation expert Tony Dunn also has a great video showing the movement of the south celestial pole for the next 150,000 years or so. Watch how many years elapse before Sirius finally enters the scene, nearly halfway through the video as it crosses the southern sky.

Pole stars do come in handy

A pole star is a handy star to have in the sky. Some people mistakenly think Polaris is the sky’s brightest star, because it really is such an important star. It’s not all that bright, though. It’s a modestly bright star in the constellation Ursa Minor the Lesser Bear. In fact, Polaris marks the end of the handle of the Little Dipper asterism, which lies within the Bear constellation. As our North Star, Polaris stays fixed (relatively speaking), while all the stars of the northern sky wheel around it. That means that – if you know Polaris, and you get lost – this star can help you regain your bearings.

Star chart with stars in black on white. The constellation Vela the Sail.
View a larger chart. | The constellation Vela the Sail. Delta Velorum will become a moderately bright southern Pole Star in about 7,000 years. Image via Wikipedia. Used with permission.

Bottom line: Nighttime’s brightest star, Sirius aka the Dog Star, will come to within 1.6 degrees of the south celestial pole in the year 66270.

Read more: Does Mars have a North Star?

Read more: Does the North Star ever move?

The post Sirius is a future southern Pole Star first appeared on EarthSky.



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Sirius: Chart of Orion with a red arrow leading from Orion's Belt to Sirius.
Sirius is not only the brightest star in the constellation Canis Major the Greater Dog, it’s the brightest star in the sky. Plus, Sirius is a future pole star for the Southern Hemisphere. You can be sure you’re looking at the correct bright star by drawing a line from Orion’s Belt to Sirius. From the Southern Hemisphere, turn the chart upside down.

Brilliant Sirius as a southern Pole Star

On January evenings, use the Belt of Orion to star-hop to Sirius, a future southern Pole Star.

Believe it or not, the brightest star in the sky – our sparkling night beacon, Sirius in the constellation Canis Major the Greater Dog – will someday serve as a very reasonable, and certainly very noticeable, South Star.

Unlike the Northern Hemisphere, the Southern Hemisphere currently doesn’t have a bright pole star. There’s not even a moderately bright star – like our North Star Polaris – to mark the south celestial pole, the point in the sky directly over the Earth’s South Pole.

The pole star – either north or south – is the point around which the entire sky turns. It’s like the hub of a great wheel.

Available now! 2025 EarthSky lunar calendar. A unique and beautiful poster-sized calendar showing phases of the moon every night of the year. Makes a great gift!

But not for a long, long time

We found the information about Sirius as a southern pole star in the book “Mathematical Astronomy Morsels V” by the great celestial mechanist Jean Meeus. See pages 353 to 363. Meeus wrote that Sirius will take its turn as the southern Pole Star some 60,000 years from now, around the year 66270. In that year, Sirius will come to within 1.6 degrees of the south celestial pole.

One precessional cycle later, in the year 93830, Sirius will miss aligning with the south celestial pole by only 2.3 degrees.

Stars close to the celestial poles

Our current North Star, Polaris, comes closer than that to the north celestial pole. Polaris will be within 0.5 degrees of the north celestial pole in the year 2100. For reference, the moon’s angular diameter spans 0.5 degrees of sky.

But the Southern Hemisphere will have its close pole star, too. A moderately bright star, not very different in brightness from Polaris, will take its place more or less over the south celestial pole about 7,000 years from now. Because of precession, the star Delta Velorum in the constellation Vela the Sail will come to within 0.2 degrees of the south celestial pole in the year 9250. That’s closer to marking the celestial pole than Polaris or Sirius ever do during their reigns as pole stars!

Was Sirius a pole star in the past?

A reader asked us:

Sirius will be the southern pole star around 66,270 AD. Was Sirius the southern pole star in the past as well?

Because of the 26,000-year precession of our planet, it would seem like Sirius gets to be the south celestial pole star every 26,000 years. But the answer is actually no, Sirius was not a pole star in the past, and it won’t be a pole star anytime between now and 66,270 AD.

The reason is because Sirius is so close to us. Sirius, the brightest star seen from Earth, is bright because of how close it is. And being close to us, it appears to move faster than other stars in the sky. Scientists say it has a high rate of proper motion. At Tony Dunn’s website, you can scroll down to Canis Major, Sirius’s home constellation, and click to watch an animation of how Sirius’s fast motion (relatively speaking) changes the look of the Greater Dog.

So, Sirius is not currently close to the south celestial pole or even the circle that the precession draws out on the southern sky. But it is heading that way and will close in on the south celestial pole by 66,270 AD. Orbital simulation expert Tony Dunn also has a great video showing the movement of the south celestial pole for the next 150,000 years or so. Watch how many years elapse before Sirius finally enters the scene, nearly halfway through the video as it crosses the southern sky.

Pole stars do come in handy

A pole star is a handy star to have in the sky. Some people mistakenly think Polaris is the sky’s brightest star, because it really is such an important star. It’s not all that bright, though. It’s a modestly bright star in the constellation Ursa Minor the Lesser Bear. In fact, Polaris marks the end of the handle of the Little Dipper asterism, which lies within the Bear constellation. As our North Star, Polaris stays fixed (relatively speaking), while all the stars of the northern sky wheel around it. That means that – if you know Polaris, and you get lost – this star can help you regain your bearings.

Star chart with stars in black on white. The constellation Vela the Sail.
View a larger chart. | The constellation Vela the Sail. Delta Velorum will become a moderately bright southern Pole Star in about 7,000 years. Image via Wikipedia. Used with permission.

Bottom line: Nighttime’s brightest star, Sirius aka the Dog Star, will come to within 1.6 degrees of the south celestial pole in the year 66270.

Read more: Does Mars have a North Star?

Read more: Does the North Star ever move?

The post Sirius is a future southern Pole Star first appeared on EarthSky.



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