What’s a constellation? What’s an asterism?

Sometimes several stars in different constellations join together to form a large asterism. That’s true of the Summer Triangle asterism, which is made of 3 bright stars – Vega, Deneb and Altair – in 3 different constellations. Image via our friend Susan Gies Jensen in Odessa, Washington.

What are constellations and asterisms?

A constellation is a recognized pattern of stars in the night sky. The word is from the Latin constellacio, meaning a set of stars. There are 88 official constellations. Many are very old. They’re a link between us and our ancestors, a projection of human imagination into the cosmos: ancient people looked at the stars and thought they saw mythical beings, beasts and cultural touchstones among the stars. On the other hand, most asterisms are relatively new. Most are small patterns within a constellation, although some are large patterns made of bright stars from several constellations. There’s nothing official about asterisms, but people on all parts of Earth still love them and enjoy them.

Stars in a constellation all lie at different distances from the Earth. For example, the three stars comprising the constellation of Triangulum are between 35 and 127 light-years away. While a constellation may look as if all of its stars are the same distance away, in reality that is only because, as we now know, stars vary in size and brightness, so two stars which appear to be the same brightness in the sky may actually be separated by vast distances. This means that an alien astronomer on a planet a hundred light years from Earth would know very different constellations, because they would see the night sky from a completely different perspective.

The Plough, for example, (also known as the Big Dipper or King Charles’ Wain) is a pattern of seven stars within the constellation of Ursa Major, the Great Bear. It is undoubtedly the most famous asterism in the sky, and not least because of its usefulness as a signpost for other stars and constellations. In the southern hemisphere, five stars comprise the Southern Cross, an asterism within the constellation of Crux. Sometimes, asterisms comprise stars of more than one constellation: for example, the glorious Summer Triangle, so prominent in the northern hemisphere sky between June and September, comprises stars in Cygnus, Lyra and Aquila. In Sagittarius there is the famous “teapot” asterism, inside which lies the location of the centre of our Milky Way galaxy.

There is no hard and fast rule for what constitutes an asterism: usually it’s a group of prominent stars in a simple pattern that are among the first that people recognise when they are learning their way around the sky.

Many constellations are well-known: Orion, Ursa Major, Cassiopeia, Cygnus , the famous star patterns you learn first when bitten by the bug of astronomy. But perhaps the most recognized are those that comprise the star signs of the zodiac: Aries, Libra, Pisces, Virgo and the eight others which had a special significance for astrologers, more than two thousand years ago when the first astrological charts were drawn by the Babylonians (although the history of the zodiac may go back further). The twelve constellations of the zodiac had a special significance because, together, they comprise the path through the heavens that the sun appears to follow during one year.

Of course, we now know that the sun does not follow this path, that it is the Earth which is moving and not the sun. We also know that since the first astrological charts were created, a gradual tilting of the Earth’s axis, causing an effect known as the precession of the equinoxes, means that the sun now appears to pass though a thirteenth constellation of the zodiac: that of Ophiuchus, the serpent-bearer. This has had the knock-on effect of changing the dates when the sun “passes through” each zodiacal constellation, so that, for example, Ophiuchus occupies most of the days in the calendar where the astrological sign of Sagittarius resides, and Aquarius largely occupies the space where Pisces is.  Although this does of course invalidate the dates of the astrological star-signs seen in the horoscopes of tabloid newspapers, as well as the dates of the supposed star sign which people are “born under,” it should be remembered that the astrological zodiac has little resemblance to the actual constellations which the star signs represent: astrology simply divides the 360-degree heavens into twelve equal segments, without regard for how many degrees each constellation actually spans in the sky. This means that an astrological star sign can encompass more than one constellation, and therefore the astrological zodiac should be seen as largely symbolic rather than factual. It has nothing to do with the real universe.

It was the Greeks and Romans who, between them, first recognized and named the constellations of the Northern Hemisphere, listed around the second century A.D., although doubtless prehistoric humans had created their own constellations long before them. Indeed, each human culture has seen its own mythology and creation stories in the stars since time immemorial. Not surprisingly, the Greeks and Romans saw the heroes, heroines and beasts from their mythologies in the sky: Pegasus, Orion, Taurus, Cassiopeia and many others.

The first list of constellations we know of appears in Ptolemy’s second-century Almagest, which was his treatise on the apparent motions and stars and planets, and which established a geocentric view of the universe which was to persist for 1200 years. While the Greeks and Romans bequeathed us the names of the Northern Hemisphere constellations, it was Arabs who were the first to name the individual stars composing each: Islamic scholars were the first to systematically map the skies.  Many of these Arabic star names have survived until today: Aldebaran, Alcor, Altair, Algol. The prefix “Al-” is a sure indication of an Islamic name: it simply means “the.” Hence, for example, Aldebaran is “the follower,” because it appears to follow the Hyades star cluster that makes up the head of the constellation of Taurus the Bull.

Certain constellations have acquired special significance over the millennia because of their appearance marking the onset of seasons, telling ancient peoples when to sow or reap their crops, when to collect food or animal skins. Because of the Earth’s orbit around the sun, different constellations become visible at different times of the year. For example, in the Northern Hemisphere the appearance of Orion in the early morning sky warns of the onset of autumn, that temperatures will shortly start to drop. The rising of the Summer Triangle to prominence in the northern sky is a harbinger of summer. Thus, to ancient cultures constellations were more than just patterns: they marked the passing of the seasons, of the years, of life itself.

The 48 constellations of the Northern Hemisphere, and their boundaries, were formally recognized by the International Astronomical Union in 1928 and the official list published in 1930. The story of the constellations of the Southern Hemisphere, however, is a little more complicated. Many of these were named by Italian, Dutch and Portuguese explorers of the 14th to 16th centuries. So as constellations there are objects and beasts associated with the great seafaring voyages of that epoch: Telescopium, the telescope; Octans, the octant; Dorado, the swordfish; Vela, the ship’s sails; Hydrus, the sea serpent. But explorers and observers often proposed different constellations with conflicting names, often to please their patrons. It was not until the 19th century that the current list of southern constellations was agreed upon and adopted.

From an observer’s perspective, from sunset to dawn the sky appears to revolve around one fixed point in the sky. This location in the heavens is what the Earth’s axis points at and is called the celestial pole. In the Northern Hemisphere, Polaris (the pole star) lies very close to the celestial pole, whereas in the Southern Hemisphere there is no bright star marking the location. Those constellations which revolve around the celestial pole yet do not dip below the horizon during the night, due to their proximity to it, are known as circumpolar constellations. In other words, for an observer these constellations will never set. There are five of these in the Northern Hemisphere: Ursa Major, Ursa Minor, Draco, Cassiopeia and Cepheus. The Southern Hemisphere has three: Crux, Centaurus and Carina.

The constellations are not difficult for a budding astronomer to learn. There are many excellent resources and planetarium-type programs available free online. It is certainly worth learning to recognize the constellations, even if sometimes we strain to see what the ancients did!

Bottom line: Constellations and asterisms are patterns of stars. Some asterisms consist of stars from different constellations, and some asterisms are part of one constellation.

from EarthSky https://ift.tt/2YyXaI6

Sometimes several stars in different constellations join together to form a large asterism. That’s true of the Summer Triangle asterism, which is made of 3 bright stars – Vega, Deneb and Altair – in 3 different constellations. Image via our friend Susan Gies Jensen in Odessa, Washington.

What are constellations and asterisms?

A constellation is a recognized pattern of stars in the night sky. The word is from the Latin constellacio, meaning a set of stars. There are 88 official constellations. Many are very old. They’re a link between us and our ancestors, a projection of human imagination into the cosmos: ancient people looked at the stars and thought they saw mythical beings, beasts and cultural touchstones among the stars. On the other hand, most asterisms are relatively new. Most are small patterns within a constellation, although some are large patterns made of bright stars from several constellations. There’s nothing official about asterisms, but people on all parts of Earth still love them and enjoy them.

Stars in a constellation all lie at different distances from the Earth. For example, the three stars comprising the constellation of Triangulum are between 35 and 127 light-years away. While a constellation may look as if all of its stars are the same distance away, in reality that is only because, as we now know, stars vary in size and brightness, so two stars which appear to be the same brightness in the sky may actually be separated by vast distances. This means that an alien astronomer on a planet a hundred light years from Earth would know very different constellations, because they would see the night sky from a completely different perspective.

The Plough, for example, (also known as the Big Dipper or King Charles’ Wain) is a pattern of seven stars within the constellation of Ursa Major, the Great Bear. It is undoubtedly the most famous asterism in the sky, and not least because of its usefulness as a signpost for other stars and constellations. In the southern hemisphere, five stars comprise the Southern Cross, an asterism within the constellation of Crux. Sometimes, asterisms comprise stars of more than one constellation: for example, the glorious Summer Triangle, so prominent in the northern hemisphere sky between June and September, comprises stars in Cygnus, Lyra and Aquila. In Sagittarius there is the famous “teapot” asterism, inside which lies the location of the centre of our Milky Way galaxy.

There is no hard and fast rule for what constitutes an asterism: usually it’s a group of prominent stars in a simple pattern that are among the first that people recognise when they are learning their way around the sky.

Many constellations are well-known: Orion, Ursa Major, Cassiopeia, Cygnus , the famous star patterns you learn first when bitten by the bug of astronomy. But perhaps the most recognized are those that comprise the star signs of the zodiac: Aries, Libra, Pisces, Virgo and the eight others which had a special significance for astrologers, more than two thousand years ago when the first astrological charts were drawn by the Babylonians (although the history of the zodiac may go back further). The twelve constellations of the zodiac had a special significance because, together, they comprise the path through the heavens that the sun appears to follow during one year.

Of course, we now know that the sun does not follow this path, that it is the Earth which is moving and not the sun. We also know that since the first astrological charts were created, a gradual tilting of the Earth’s axis, causing an effect known as the precession of the equinoxes, means that the sun now appears to pass though a thirteenth constellation of the zodiac: that of Ophiuchus, the serpent-bearer. This has had the knock-on effect of changing the dates when the sun “passes through” each zodiacal constellation, so that, for example, Ophiuchus occupies most of the days in the calendar where the astrological sign of Sagittarius resides, and Aquarius largely occupies the space where Pisces is.  Although this does of course invalidate the dates of the astrological star-signs seen in the horoscopes of tabloid newspapers, as well as the dates of the supposed star sign which people are “born under,” it should be remembered that the astrological zodiac has little resemblance to the actual constellations which the star signs represent: astrology simply divides the 360-degree heavens into twelve equal segments, without regard for how many degrees each constellation actually spans in the sky. This means that an astrological star sign can encompass more than one constellation, and therefore the astrological zodiac should be seen as largely symbolic rather than factual. It has nothing to do with the real universe.

It was the Greeks and Romans who, between them, first recognized and named the constellations of the Northern Hemisphere, listed around the second century A.D., although doubtless prehistoric humans had created their own constellations long before them. Indeed, each human culture has seen its own mythology and creation stories in the stars since time immemorial. Not surprisingly, the Greeks and Romans saw the heroes, heroines and beasts from their mythologies in the sky: Pegasus, Orion, Taurus, Cassiopeia and many others.

The first list of constellations we know of appears in Ptolemy’s second-century Almagest, which was his treatise on the apparent motions and stars and planets, and which established a geocentric view of the universe which was to persist for 1200 years. While the Greeks and Romans bequeathed us the names of the Northern Hemisphere constellations, it was Arabs who were the first to name the individual stars composing each: Islamic scholars were the first to systematically map the skies.  Many of these Arabic star names have survived until today: Aldebaran, Alcor, Altair, Algol. The prefix “Al-” is a sure indication of an Islamic name: it simply means “the.” Hence, for example, Aldebaran is “the follower,” because it appears to follow the Hyades star cluster that makes up the head of the constellation of Taurus the Bull.

Certain constellations have acquired special significance over the millennia because of their appearance marking the onset of seasons, telling ancient peoples when to sow or reap their crops, when to collect food or animal skins. Because of the Earth’s orbit around the sun, different constellations become visible at different times of the year. For example, in the Northern Hemisphere the appearance of Orion in the early morning sky warns of the onset of autumn, that temperatures will shortly start to drop. The rising of the Summer Triangle to prominence in the northern sky is a harbinger of summer. Thus, to ancient cultures constellations were more than just patterns: they marked the passing of the seasons, of the years, of life itself.

The 48 constellations of the Northern Hemisphere, and their boundaries, were formally recognized by the International Astronomical Union in 1928 and the official list published in 1930. The story of the constellations of the Southern Hemisphere, however, is a little more complicated. Many of these were named by Italian, Dutch and Portuguese explorers of the 14th to 16th centuries. So as constellations there are objects and beasts associated with the great seafaring voyages of that epoch: Telescopium, the telescope; Octans, the octant; Dorado, the swordfish; Vela, the ship’s sails; Hydrus, the sea serpent. But explorers and observers often proposed different constellations with conflicting names, often to please their patrons. It was not until the 19th century that the current list of southern constellations was agreed upon and adopted.

From an observer’s perspective, from sunset to dawn the sky appears to revolve around one fixed point in the sky. This location in the heavens is what the Earth’s axis points at and is called the celestial pole. In the Northern Hemisphere, Polaris (the pole star) lies very close to the celestial pole, whereas in the Southern Hemisphere there is no bright star marking the location. Those constellations which revolve around the celestial pole yet do not dip below the horizon during the night, due to their proximity to it, are known as circumpolar constellations. In other words, for an observer these constellations will never set. There are five of these in the Northern Hemisphere: Ursa Major, Ursa Minor, Draco, Cassiopeia and Cepheus. The Southern Hemisphere has three: Crux, Centaurus and Carina.

The constellations are not difficult for a budding astronomer to learn. There are many excellent resources and planetarium-type programs available free online. It is certainly worth learning to recognize the constellations, even if sometimes we strain to see what the ancients did!

Bottom line: Constellations and asterisms are patterns of stars. Some asterisms consist of stars from different constellations, and some asterisms are part of one constellation.

from EarthSky https://ift.tt/2YyXaI6

Venus’ final month in the evening sky

May will be the planet Venus’ last full month in the western twilight sky for the year 2020. On June 3, Venus will swing between the Earth and sun in its orbit, reaching the point called inferior conjunction by astronomers. Afterwards it’ll transition over to the eastern sky before sunup. But what an exciting month for Venus this will be! We’ll all be able to watch it sink closer to the western horizon day by day, finally disappearing in the sunset glare.

Those with telescopes will see that – as it gets closer to the sunset, preparing to go between us and the sun – more and more of its day side will be turned away from our direction in space: we’ll see Venus as a waning crescent. What’s more, the sun’s other inner planet – Mercury – will join the scene in the west after sunset beginning around mid-May. All in all, a fascinating month to watch the western sky after sunset!

Given clear skies, you can’t miss seeing Venus in your western sky after sunset. After all, dazzling Venus ranks as the third-brightest celestial body to light up the heavens, after the sun and moon. Some people can actually see Venus in a daytime sky, but we mere mortals can expect to see this bright beauty some 15 to 30 minutes after sundown.

Although Venus plunges closer and closer to the sunset each day, this world will be rather easy to view for the next few weeks. Venus stays out longer after dark at more northerly latitudes, however. At present – May 5, 2020 – Venus stays out for over 3 hours after sunset at mid-northern latitudes, 2 1/2 hours after the sun at the equator, and 1 3/4 hours after sunset at temperate latitudes in the Southern Hemisphere.

Have a telescope? This is by far the most exciting time to view Venus through the telescope. That’s because Venus in its faster orbit around the sun is rapidly catching up with our planet Earth. Therefore, Venus’ disk size is increasing while its phase is waning (shrinking). On May 5, 2020, Venus is 21% illuminated in sunshine. A week from now – May 12, 2020 – Venus will be 14% illuminated; and two weeks from now – May 19, 2020 – Venus will be 8% illuminated. Three weeks later – May 26, 2020 – Venus will be 3% illuminated. All the while, the disk size will have increased by 80%.

Keep in mind that you get a crisper view of Venus’ phase in a twilight or daytime sky than after dark. That’s because Venus’ glare is so overwhelming at nighttime.

When Galileo (1564-1642) first saw the phases of Venus through the telescope, he came to the realization that Venus must orbit the sun instead of orbiting Earth. This observation countered the widely-held notion at the time that Venus orbits Earth.

By the way, the planet Mercury entered the evening sky on May 4, 2020. However, the innermost planet probably won’t climb high enough from the sunset glare to become visible for another week or two. Fortunately, we can use Venus to help us locate Mercury in May 2020. Throughout the month, Venus will be falling toward the sunset while Mercury will be climbing away. On or near May 21, 2020, watch for these two worlds to be within one degree of one another on the sky’s dome. If you can see Venus with the eye alone, then aim binoculars at Venus to see Mercury sharing a single binocular field with Venus.

Depending on where you live worldwide, the planets Mercury and Venus will couple up most closely on the sky’s dome on May 21 or May 22, 2020. If you can see Venus, but not Mercury, aim binoculars at Venus to see Mercury and Venus taking stage in a single binocular field.

Then, only a few to several days after the Venus/Mercury conjunction, watch for the slender waxing lunar crescent to join up with Mercury and Venus at dusk. Think photo opportunity! Thereafter, Venus will continue to fall downward while Mercury will soar upward. Mercury will reach its greatest eastern elongation (greatest angular distance) from the setting sun on June 4, 2020.

Have a telescope? You’ll see the phases of the moon and Venus nearly match on or around May 24, 2020.

Bottom line: Before Venus transits to the morning sky, it will be fun to watch this upcoming month, with or without a telescope. Find an unobstructed horizon in the direction of sunset, and you might be able to view Venus in the western evening dusk until the month’s end.

from EarthSky https://ift.tt/2W5KUNK

May will be the planet Venus’ last full month in the western twilight sky for the year 2020. On June 3, Venus will swing between the Earth and sun in its orbit, reaching the point called inferior conjunction by astronomers. Afterwards it’ll transition over to the eastern sky before sunup. But what an exciting month for Venus this will be! We’ll all be able to watch it sink closer to the western horizon day by day, finally disappearing in the sunset glare.

Those with telescopes will see that – as it gets closer to the sunset, preparing to go between us and the sun – more and more of its day side will be turned away from our direction in space: we’ll see Venus as a waning crescent. What’s more, the sun’s other inner planet – Mercury – will join the scene in the west after sunset beginning around mid-May. All in all, a fascinating month to watch the western sky after sunset!

Given clear skies, you can’t miss seeing Venus in your western sky after sunset. After all, dazzling Venus ranks as the third-brightest celestial body to light up the heavens, after the sun and moon. Some people can actually see Venus in a daytime sky, but we mere mortals can expect to see this bright beauty some 15 to 30 minutes after sundown.

Although Venus plunges closer and closer to the sunset each day, this world will be rather easy to view for the next few weeks. Venus stays out longer after dark at more northerly latitudes, however. At present – May 5, 2020 – Venus stays out for over 3 hours after sunset at mid-northern latitudes, 2 1/2 hours after the sun at the equator, and 1 3/4 hours after sunset at temperate latitudes in the Southern Hemisphere.

Have a telescope? This is by far the most exciting time to view Venus through the telescope. That’s because Venus in its faster orbit around the sun is rapidly catching up with our planet Earth. Therefore, Venus’ disk size is increasing while its phase is waning (shrinking). On May 5, 2020, Venus is 21% illuminated in sunshine. A week from now – May 12, 2020 – Venus will be 14% illuminated; and two weeks from now – May 19, 2020 – Venus will be 8% illuminated. Three weeks later – May 26, 2020 – Venus will be 3% illuminated. All the while, the disk size will have increased by 80%.

Keep in mind that you get a crisper view of Venus’ phase in a twilight or daytime sky than after dark. That’s because Venus’ glare is so overwhelming at nighttime.

When Galileo (1564-1642) first saw the phases of Venus through the telescope, he came to the realization that Venus must orbit the sun instead of orbiting Earth. This observation countered the widely-held notion at the time that Venus orbits Earth.

By the way, the planet Mercury entered the evening sky on May 4, 2020. However, the innermost planet probably won’t climb high enough from the sunset glare to become visible for another week or two. Fortunately, we can use Venus to help us locate Mercury in May 2020. Throughout the month, Venus will be falling toward the sunset while Mercury will be climbing away. On or near May 21, 2020, watch for these two worlds to be within one degree of one another on the sky’s dome. If you can see Venus with the eye alone, then aim binoculars at Venus to see Mercury sharing a single binocular field with Venus.

Depending on where you live worldwide, the planets Mercury and Venus will couple up most closely on the sky’s dome on May 21 or May 22, 2020. If you can see Venus, but not Mercury, aim binoculars at Venus to see Mercury and Venus taking stage in a single binocular field.

Then, only a few to several days after the Venus/Mercury conjunction, watch for the slender waxing lunar crescent to join up with Mercury and Venus at dusk. Think photo opportunity! Thereafter, Venus will continue to fall downward while Mercury will soar upward. Mercury will reach its greatest eastern elongation (greatest angular distance) from the setting sun on June 4, 2020.

Have a telescope? You’ll see the phases of the moon and Venus nearly match on or around May 24, 2020.

Bottom line: Before Venus transits to the morning sky, it will be fun to watch this upcoming month, with or without a telescope. Find an unobstructed horizon in the direction of sunset, and you might be able to view Venus in the western evening dusk until the month’s end.

from EarthSky https://ift.tt/2W5KUNK

4 amazing astronomical discoveries from ancient Greece

Earth and moon as seen by the Galileo spacecraft. Image via NASA/ The Conversation.

By Gareth Dorrian, University of Birmingham and Ian Whittaker, Nottingham Trent University

“The Histories” by Herodotus (484 B.C. to 425 B.C.) offers a remarkable window into the world as it was known to the ancient Greeks in the mid fifth century B.C. Almost as interesting as what they knew, however, is what they did not know. This sets the baseline for the remarkable advances in their understanding over the next few centuries – simply relying on what they could observe with their own eyes.

Herodotus claimed that Africa was surrounded almost entirely by sea. How did he know this? He recounts the story of Phoenician sailors who were dispatched by King Neco II of Egypt (about 600 B.C.), to sail around continental Africa, in a clockwise fashion, starting in the Red Sea. This story, if true, recounts the earliest known circumnavigation of Africa, but also contains an interesting insight into the astronomical knowledge of the ancient world.

The voyage took several years. Having rounded the southern tip of Africa, and following a westerly course, the sailors observed the sun as being on their right hand side, above the northern horizon. This observation simply did not make sense at the time because they didn’t yet know that the Earth has a spherical shape, and that there is a southern hemisphere.

1. The planets orbit the sun

A few centuries later, there had been a lot of progress. Aristarchus of Samos (310 B.C. to 230 B.C.) argued that the sun was the “central fire” of the cosmos and he placed all of the then known planets in their correct order of distance around it. This is the earliest known heliocentric theory of the solar system.

Unfortunately, the original text in which he makes this argument has been lost to history, so we cannot know for certain how he worked it out. Aristarchus knew the sun was much bigger than the Earth or the moon, and he may have surmised that it should therefore have the central position in the solar system.

Nevertheless it is a jaw-dropping finding, especially when you consider that it wasn’t rediscovered until the 16th century, by Nicolaus Copernicus, who even acknowledged Aristarchus during the development of his own work.

2. The size of the moon

One of Aristarchus’ books that did survive is about the sizes and distances of the sun and moon. In this remarkable treatise, Aristarchus laid out the earliest known attempted calculations of the relative sizes and distances to the sun and moon.

It had long been observed that the sun and moon appeared to be of the same apparent size in the sky, and that the sun was further away. They realized this from solar eclipses, caused by the moon passing in front of the sun at a certain distance from Earth.

Also, at the instant when the moon is at first or third quarter, Aristarchus reasoned that the sun, Earth, and moon would form a right-angled triangle.

As Pythagoras had determined how the lengths of a triangle’s sides were related a couple of centuries earlier, Aristarchus used the triangle to estimate that the distance to the sun was between 18 and 20 times the distance to the moon. He also estimated that the size of the moon was approximately one-third that of Earth, based on careful timing of lunar eclipses.

A 10th century reproduction of a diagram by Aristarchus showing some of the geometry he used in his calculations. Image via Wikipedia.

While his estimated distance to the sun was too low (the actual ratio is 390), on account of the lack of telescopic precision available at the time, the value for the ratio of the size of the Earth to the moon is surprisingly accurate (the moon has a diameter 0.27 times that of Earth).

Today, we know the size and distance to the moon accurately by a variety of means, including precise telescopes, radar observations and laser reflectors left on the surface by Apollo astronauts.

3. The Earth’s circumference

Eratosthenes (276BC to 195 B.C.) was chief librarian at the Great Library of Alexandria, and a keen experimentalist. Among his many achievements was the earliest known calculation of the circumference of the Earth. Pythagoras is generally regarded as the earliest proponent of a spherical Earth, although apparently not its size. Eratosthenes’ famous and yet simple method relied on measuring the different lengths of shadows cast by poles stuck vertically into the ground, at midday on the summer solstice, at different latitudes.

The sun is sufficiently far away that, wherever its rays arrive at Earth, they are effectively parallel, as had previously been shown by Aristarchus. So the difference in the shadows demonstrated how much the Earth’s surface curved. Eratosthenes used this to estimate the Earth’s circumference as approximately 25,000 miles (40,000 km). This is within a couple of percent of the actual value, as established by modern geodesy (the science of the Earth’s shape).

Later, another scientist called Posidonius (135 B.C. to 51 B.C.) used a slightly different method and arrived at almost exactly the same answer. Posidonius lived on the island of Rhodes for much of his life. There he observed the bright star Canopus would lie very close to the horizon. However, when in Alexandria, in Egypt, he noted Canopus would ascend to some 7.5 degrees above the horizon.

Given that 7.5 degrees is 1/48th of a circle, he multiplied the distance from Rhodes to Alexandria by 48, and arrived at a value also of approximately 25,000 miles (40,000 km).

4. The first astronomical calculator

The world’s oldest surviving mechanical calculator is the Antikythera Mechanism. The amazing device was discovered in an ancient shipwreck off the Greek island of Antikythera in 1900.

The device is now fragmented by the passage of time, but when intact it would have appeared as a box housing dozens of finely machined bronze gear wheels. When manually rotated by a handle, the gears spun dials on the exterior showing the phases of the moon, the timing of lunar eclipses, and the positions of the five planets then known (Mercury, Venus, Mars, Jupiter, and Saturn) at different times of the year. This even accounted for their retrograde motion – an illusionary change in the movement of planets through the sky.

We don’t know who built it, but it dates to some time between the third and first centuries B.C., and may even have been the work of Archimedes. Gearing technology with the sophistication of the Antikythera mechanism was not seen again for a thousand years.

Sadly, the vast majority of these works were lost to history and our scientific awakening was delayed by millennia. As a tool for introducing scientific measurement, the techniques of Eratosthenes are relatively easy to perform and require no special equipment, allowing those just beginning their interest in science to understand by doing, experimenting and, ultimately, following in the footsteps of some of the first scientists.

One can but speculate where our civilization might be now if this ancient science had continued unabated.

Gareth Dorrian, Post Doctoral Research Fellow in Space Science, University of Birmingham and Ian Whittaker, Lecturer in Physics, Nottingham Trent University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Bottom line: Four 4 astronomical discoveries from ancient Greece.

from EarthSky https://ift.tt/2WqJv3q

Earth and moon as seen by the Galileo spacecraft. Image via NASA/ The Conversation.

By Gareth Dorrian, University of Birmingham and Ian Whittaker, Nottingham Trent University

“The Histories” by Herodotus (484 B.C. to 425 B.C.) offers a remarkable window into the world as it was known to the ancient Greeks in the mid fifth century B.C. Almost as interesting as what they knew, however, is what they did not know. This sets the baseline for the remarkable advances in their understanding over the next few centuries – simply relying on what they could observe with their own eyes.

Herodotus claimed that Africa was surrounded almost entirely by sea. How did he know this? He recounts the story of Phoenician sailors who were dispatched by King Neco II of Egypt (about 600 B.C.), to sail around continental Africa, in a clockwise fashion, starting in the Red Sea. This story, if true, recounts the earliest known circumnavigation of Africa, but also contains an interesting insight into the astronomical knowledge of the ancient world.

The voyage took several years. Having rounded the southern tip of Africa, and following a westerly course, the sailors observed the sun as being on their right hand side, above the northern horizon. This observation simply did not make sense at the time because they didn’t yet know that the Earth has a spherical shape, and that there is a southern hemisphere.

1. The planets orbit the sun

A few centuries later, there had been a lot of progress. Aristarchus of Samos (310 B.C. to 230 B.C.) argued that the sun was the “central fire” of the cosmos and he placed all of the then known planets in their correct order of distance around it. This is the earliest known heliocentric theory of the solar system.

Unfortunately, the original text in which he makes this argument has been lost to history, so we cannot know for certain how he worked it out. Aristarchus knew the sun was much bigger than the Earth or the moon, and he may have surmised that it should therefore have the central position in the solar system.

Nevertheless it is a jaw-dropping finding, especially when you consider that it wasn’t rediscovered until the 16th century, by Nicolaus Copernicus, who even acknowledged Aristarchus during the development of his own work.

2. The size of the moon

One of Aristarchus’ books that did survive is about the sizes and distances of the sun and moon. In this remarkable treatise, Aristarchus laid out the earliest known attempted calculations of the relative sizes and distances to the sun and moon.

It had long been observed that the sun and moon appeared to be of the same apparent size in the sky, and that the sun was further away. They realized this from solar eclipses, caused by the moon passing in front of the sun at a certain distance from Earth.

Also, at the instant when the moon is at first or third quarter, Aristarchus reasoned that the sun, Earth, and moon would form a right-angled triangle.

As Pythagoras had determined how the lengths of a triangle’s sides were related a couple of centuries earlier, Aristarchus used the triangle to estimate that the distance to the sun was between 18 and 20 times the distance to the moon. He also estimated that the size of the moon was approximately one-third that of Earth, based on careful timing of lunar eclipses.

A 10th century reproduction of a diagram by Aristarchus showing some of the geometry he used in his calculations. Image via Wikipedia.

While his estimated distance to the sun was too low (the actual ratio is 390), on account of the lack of telescopic precision available at the time, the value for the ratio of the size of the Earth to the moon is surprisingly accurate (the moon has a diameter 0.27 times that of Earth).

Today, we know the size and distance to the moon accurately by a variety of means, including precise telescopes, radar observations and laser reflectors left on the surface by Apollo astronauts.

3. The Earth’s circumference

Eratosthenes (276BC to 195 B.C.) was chief librarian at the Great Library of Alexandria, and a keen experimentalist. Among his many achievements was the earliest known calculation of the circumference of the Earth. Pythagoras is generally regarded as the earliest proponent of a spherical Earth, although apparently not its size. Eratosthenes’ famous and yet simple method relied on measuring the different lengths of shadows cast by poles stuck vertically into the ground, at midday on the summer solstice, at different latitudes.

The sun is sufficiently far away that, wherever its rays arrive at Earth, they are effectively parallel, as had previously been shown by Aristarchus. So the difference in the shadows demonstrated how much the Earth’s surface curved. Eratosthenes used this to estimate the Earth’s circumference as approximately 25,000 miles (40,000 km). This is within a couple of percent of the actual value, as established by modern geodesy (the science of the Earth’s shape).

Later, another scientist called Posidonius (135 B.C. to 51 B.C.) used a slightly different method and arrived at almost exactly the same answer. Posidonius lived on the island of Rhodes for much of his life. There he observed the bright star Canopus would lie very close to the horizon. However, when in Alexandria, in Egypt, he noted Canopus would ascend to some 7.5 degrees above the horizon.

Given that 7.5 degrees is 1/48th of a circle, he multiplied the distance from Rhodes to Alexandria by 48, and arrived at a value also of approximately 25,000 miles (40,000 km).

4. The first astronomical calculator

The world’s oldest surviving mechanical calculator is the Antikythera Mechanism. The amazing device was discovered in an ancient shipwreck off the Greek island of Antikythera in 1900.

The device is now fragmented by the passage of time, but when intact it would have appeared as a box housing dozens of finely machined bronze gear wheels. When manually rotated by a handle, the gears spun dials on the exterior showing the phases of the moon, the timing of lunar eclipses, and the positions of the five planets then known (Mercury, Venus, Mars, Jupiter, and Saturn) at different times of the year. This even accounted for their retrograde motion – an illusionary change in the movement of planets through the sky.

We don’t know who built it, but it dates to some time between the third and first centuries B.C., and may even have been the work of Archimedes. Gearing technology with the sophistication of the Antikythera mechanism was not seen again for a thousand years.

Sadly, the vast majority of these works were lost to history and our scientific awakening was delayed by millennia. As a tool for introducing scientific measurement, the techniques of Eratosthenes are relatively easy to perform and require no special equipment, allowing those just beginning their interest in science to understand by doing, experimenting and, ultimately, following in the footsteps of some of the first scientists.

One can but speculate where our civilization might be now if this ancient science had continued unabated.

Gareth Dorrian, Post Doctoral Research Fellow in Space Science, University of Birmingham and Ian Whittaker, Lecturer in Physics, Nottingham Trent University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Bottom line: Four 4 astronomical discoveries from ancient Greece.

from EarthSky https://ift.tt/2WqJv3q

A mystery solved? Fast Radio Burst detected within Milky Way

Not the Fast Radio Burst. Radio waves aren’t visible to the eye. This is something else, from the Hubble Space Telescope. See a spectrum of the burst below. Image via NASA/ ESA/ Hubble/ ScienceAlert.

Fast Radio Bursts (FRBs) are short, intense bursts of radio waves lasting perhaps a thousandth of a second, coming from all over the sky and of unknown origin. In a shock discovery that could help to solve one of astronomy’s greatest mysteries – on April 28, 2020 – astronomers used an Astronomer’s Telegram to announce a Fast Radio Burst originating from inside our Milky Way galaxy. That’s a first. All other FRBs have been extragalactic, that is to say outside our galaxy. Even more importantly, the astronomers think they’ve also identified the source of the burst.

Explanations have ranged from neutron stars to supernovae to the inevitable aliens.

Dynamic spectrum – a range frequencies over time – from the April 28, 2020, Fast Radio Burst, via Astronomer’s Telegram.

FRBs were first detected in 2007. This new detection of an FRB is, in astronomical terms, very close to home. Astronomers found it using the CHIME (Canadian Hydrogen Intensity Mapping Experiment) radio telescope in Canada, an instrument designed specifically to study phenomena such as FRBs in order to answer major questions in astrophysics. This particular telescope has greatly increased the bursts’ detection rate since its first light in September 2017.

At the time of the April 28 signal, the telescope was not pointing straight at the source. But the signal was so strong the telescope captured it, so to speak, out of the corner of its eye. The signal was of sufficient strength to be detected from another galaxy (indicating it is the same phenomenon as those earlier extragalactic bursts detected from our galaxy), and it had the typical duration of a Fast Radio Burst.

The day before, on April 27, 2020, the Swift Burst Alert Telescope had detected a series of gamma-ray bursts originating from the same point in the sky as the FRB. Those gamma rays are associated with a known object, labeled SGR 1935+2154, a so-called Soft Gamma Repeater. This object is a type of stellar remnant known for periodically generating bursts of gamma rays. The distance to this object has been estimated at about 30,000 light-years. For comparison, the Milky Way galaxy is over 150,000 light-years across.

Excitingly, at the same time there was a burst of high-energy X-rays from the same point in the sky. The X-ray burst was observed by ground- and space-based X-ray telescopes. No FRB had ever been associated with gamma- or X-rays before, making this observation, if indeed it was of a FRB, something completely new.

Now you need to know that X-ray and gamma-ray bursts are not unusual in observations of magnetars.

Artist’s concept of an eruption on a magnetar. The Fast Radio Burst detected in our galaxy may be associated with these sorts of eruptions. Image via NASA Goddard Visualization Studio.

SGR 1935+2154 is believed to be a magnetar, a type of neutron star with a hypermagnetic field strong enough to pull the keys from your pocket from as far away as the moon!

While the reason for this ultra-strong magnetic field – a thousand times stronger than that of a normal neutron star – is unknown, astronomers theorize that FRBs might be produced when the crust of the neutron star suffers a starquake as a result of tension between the neutron star’s intense gravity and its magnetic field. This tension may be suddenly, and incomprehensibly violently, released in the starquake.

This may mean that the neutron star’s crust, thought to be a million times stronger than steel, slips by just a millimeter; however, this tiny shift may be sufficient to generate a brief burst of radio energy so powerful it can be detected from other galaxies, which we detect as an FRB.

Maybe! It seems possible, anyway, and, in astrophysics, what’s possible is the name of the game.

However, this detection does not mean that astronomers are ready to confirm that all FRBs originate from magnetars. The burst received by CHIME was at the low end of the signal strength historically associated with FRBs, which may or may not be of significance. As yet, astronomers have not analyzed the waveform of the signal to see if it matches that from FRBs. However, if this analysis and ongoing observations of magnetar SGR 1935+2154 do demonstrate conclusively that magnetars are the origin of Fast Radio Bursts, one of astronomy’s greatest mysteries will have been solved.

The CHIME radio telescope in Canada. It’s specifically designed to study objects such as Fast Radio Bursts. Image via CHIME.

Bottom line: Fast Radio Bursts are mysterious, short, intense bursts of radio waves coming from locations all over the sky. Before April 28, all the FRBs we knew were thought to come from outside our galaxy. The April 28 FRB, which apparently originated within our galaxy, will help astronomers unravel thorny questions in astrophysics.

from EarthSky https://ift.tt/3b2m5q0

Not the Fast Radio Burst. Radio waves aren’t visible to the eye. This is something else, from the Hubble Space Telescope. See a spectrum of the burst below. Image via NASA/ ESA/ Hubble/ ScienceAlert.

Fast Radio Bursts (FRBs) are short, intense bursts of radio waves lasting perhaps a thousandth of a second, coming from all over the sky and of unknown origin. In a shock discovery that could help to solve one of astronomy’s greatest mysteries – on April 28, 2020 – astronomers used an Astronomer’s Telegram to announce a Fast Radio Burst originating from inside our Milky Way galaxy. That’s a first. All other FRBs have been extragalactic, that is to say outside our galaxy. Even more importantly, the astronomers think they’ve also identified the source of the burst.

Explanations have ranged from neutron stars to supernovae to the inevitable aliens.

Dynamic spectrum – a range frequencies over time – from the April 28, 2020, Fast Radio Burst, via Astronomer’s Telegram.

FRBs were first detected in 2007. This new detection of an FRB is, in astronomical terms, very close to home. Astronomers found it using the CHIME (Canadian Hydrogen Intensity Mapping Experiment) radio telescope in Canada, an instrument designed specifically to study phenomena such as FRBs in order to answer major questions in astrophysics. This particular telescope has greatly increased the bursts’ detection rate since its first light in September 2017.

At the time of the April 28 signal, the telescope was not pointing straight at the source. But the signal was so strong the telescope captured it, so to speak, out of the corner of its eye. The signal was of sufficient strength to be detected from another galaxy (indicating it is the same phenomenon as those earlier extragalactic bursts detected from our galaxy), and it had the typical duration of a Fast Radio Burst.

The day before, on April 27, 2020, the Swift Burst Alert Telescope had detected a series of gamma-ray bursts originating from the same point in the sky as the FRB. Those gamma rays are associated with a known object, labeled SGR 1935+2154, a so-called Soft Gamma Repeater. This object is a type of stellar remnant known for periodically generating bursts of gamma rays. The distance to this object has been estimated at about 30,000 light-years. For comparison, the Milky Way galaxy is over 150,000 light-years across.

Excitingly, at the same time there was a burst of high-energy X-rays from the same point in the sky. The X-ray burst was observed by ground- and space-based X-ray telescopes. No FRB had ever been associated with gamma- or X-rays before, making this observation, if indeed it was of a FRB, something completely new.

Now you need to know that X-ray and gamma-ray bursts are not unusual in observations of magnetars.

Artist’s concept of an eruption on a magnetar. The Fast Radio Burst detected in our galaxy may be associated with these sorts of eruptions. Image via NASA Goddard Visualization Studio.

SGR 1935+2154 is believed to be a magnetar, a type of neutron star with a hypermagnetic field strong enough to pull the keys from your pocket from as far away as the moon!

While the reason for this ultra-strong magnetic field – a thousand times stronger than that of a normal neutron star – is unknown, astronomers theorize that FRBs might be produced when the crust of the neutron star suffers a starquake as a result of tension between the neutron star’s intense gravity and its magnetic field. This tension may be suddenly, and incomprehensibly violently, released in the starquake.

This may mean that the neutron star’s crust, thought to be a million times stronger than steel, slips by just a millimeter; however, this tiny shift may be sufficient to generate a brief burst of radio energy so powerful it can be detected from other galaxies, which we detect as an FRB.

Maybe! It seems possible, anyway, and, in astrophysics, what’s possible is the name of the game.

However, this detection does not mean that astronomers are ready to confirm that all FRBs originate from magnetars. The burst received by CHIME was at the low end of the signal strength historically associated with FRBs, which may or may not be of significance. As yet, astronomers have not analyzed the waveform of the signal to see if it matches that from FRBs. However, if this analysis and ongoing observations of magnetar SGR 1935+2154 do demonstrate conclusively that magnetars are the origin of Fast Radio Bursts, one of astronomy’s greatest mysteries will have been solved.

The CHIME radio telescope in Canada. It’s specifically designed to study objects such as Fast Radio Bursts. Image via CHIME.

Bottom line: Fast Radio Bursts are mysterious, short, intense bursts of radio waves coming from locations all over the sky. Before April 28, all the FRBs we knew were thought to come from outside our galaxy. The April 28 FRB, which apparently originated within our galaxy, will help astronomers unravel thorny questions in astrophysics.

from EarthSky https://ift.tt/3b2m5q0

Red rainbow over Bellingham, Washington

View at EarthSky Community Photos. | Lawrence Wong caught this photo on May 2, 2020 and wrote: “Just minutes before sunset and after raining, this double rainbow showed up on the eastern side of the sky. I gripped my camera and went outside and took several photos. This is one of them showing orange and red color inside the rainbow.”

Lawrence, you’ve caught a double red rainbow – a cousin to an ordinary multi-colored rainbow – that happens when the sun is low in the sky.

See how tall your rainbows are? The height of a rainbow corresponds (inversely) to the height of the sun in your sky. High sun, low rainbow. Low sun, high rainbow. So rainbow-watchers would know, without your having mentioned it, that the sun was low.

Now think about low suns for a moment. They typically appear reddish. That’s because – around sunset – you’re looking at the sun through a greater thickness of atmosphere than when the sun is high in the sky. At such times, the blue and green components of multi-colored sunrays are weakened by scattering during their long journey through the atmosphere to your eyes.

So red sunsets and red rainbows go hand-in-hand.

Thank you, Lawrence!

Read more and see more photos: What makes a red rainbow?

Bottom line: Photo of a red rainbow over Bellingham, Washington on May 5, 2020.

from EarthSky https://ift.tt/2z3RGum

View at EarthSky Community Photos. | Lawrence Wong caught this photo on May 2, 2020 and wrote: “Just minutes before sunset and after raining, this double rainbow showed up on the eastern side of the sky. I gripped my camera and went outside and took several photos. This is one of them showing orange and red color inside the rainbow.”

Lawrence, you’ve caught a double red rainbow – a cousin to an ordinary multi-colored rainbow – that happens when the sun is low in the sky.

See how tall your rainbows are? The height of a rainbow corresponds (inversely) to the height of the sun in your sky. High sun, low rainbow. Low sun, high rainbow. So rainbow-watchers would know, without your having mentioned it, that the sun was low.

Now think about low suns for a moment. They typically appear reddish. That’s because – around sunset – you’re looking at the sun through a greater thickness of atmosphere than when the sun is high in the sky. At such times, the blue and green components of multi-colored sunrays are weakened by scattering during their long journey through the atmosphere to your eyes.

So red sunsets and red rainbows go hand-in-hand.

Thank you, Lawrence!

Read more and see more photos: What makes a red rainbow?

Bottom line: Photo of a red rainbow over Bellingham, Washington on May 5, 2020.

from EarthSky https://ift.tt/2z3RGum

Coronavirus and cancer – May updates

• 29 April – NHS England announces second phase of NHS response to COVID-19
• 27 April – NHS campaign urges people to get help if they need it
• 21 April – Urgent cancer referrals fall across the UK
• 17 April – Cancer care needs mass COVID-19 testing, says charity.
• 21 March – Shielding measures introduced to protect people at high risk of COVID-19
• For coronavirus and cancer updates from March and April, please see our previous blog post.

With news about the coronavirus pandemic developing daily, we want to make sure everyone affected by cancer gets the information they need during this time.

We’ll be monitoring the latest government and NHS health updates from across the UK and updating this blog post regularly as new guidance emerges. But for the most up to date guidance, please visit government and NHS websites. You can find a full list of links on our coronavirus information page.

We’d also recommend speaking to your cancer team if you have any questions or worries about coronavirus.

1 May – Health and Social Care Committee launch inquiry into delivering NHS services in England

The inquiry will cover issues such as balancing coronavirus and ‘ordinary’ health and care demand as well as addressing the potential backlog of tests and treatments that have been delayed because of the coronavirus pandemic. The committee held an oral evidence session on 1 May, where cancer care was one of the areas of focus. We’ve submitted our initial response to the inquiry to ensure that cancer services are prioritised.

Nobody knows the full impact yet of COVID-19 on people affected by cancer. But with over 350,000 people hearing the words “you have cancer” annually, it’s clear that continuing cancer diagnosis and treatment must be a government priority. pic.twitter.com/QGdCYExVhQ

— Cancer Research UK (@CR_UK) May 1, 2020

For coronavirus and cancer updates from March and April, please visit our previous blog post.

Katie 

If you have questions about cancer, you can talk to our nurses Monday to Friday, 9-5pm, on freephone 0808 800 4040.

from Cancer Research UK – Science blog https://ift.tt/2VYt27t

• 29 April – NHS England announces second phase of NHS response to COVID-19
• 27 April – NHS campaign urges people to get help if they need it
• 21 April – Urgent cancer referrals fall across the UK
• 17 April – Cancer care needs mass COVID-19 testing, says charity.
• 21 March – Shielding measures introduced to protect people at high risk of COVID-19
• For coronavirus and cancer updates from March and April, please see our previous blog post.

With news about the coronavirus pandemic developing daily, we want to make sure everyone affected by cancer gets the information they need during this time.

We’ll be monitoring the latest government and NHS health updates from across the UK and updating this blog post regularly as new guidance emerges. But for the most up to date guidance, please visit government and NHS websites. You can find a full list of links on our coronavirus information page.

We’d also recommend speaking to your cancer team if you have any questions or worries about coronavirus.

1 May – Health and Social Care Committee launch inquiry into delivering NHS services in England

The inquiry will cover issues such as balancing coronavirus and ‘ordinary’ health and care demand as well as addressing the potential backlog of tests and treatments that have been delayed because of the coronavirus pandemic. The committee held an oral evidence session on 1 May, where cancer care was one of the areas of focus. We’ve submitted our initial response to the inquiry to ensure that cancer services are prioritised.

Nobody knows the full impact yet of COVID-19 on people affected by cancer. But with over 350,000 people hearing the words “you have cancer” annually, it’s clear that continuing cancer diagnosis and treatment must be a government priority. pic.twitter.com/QGdCYExVhQ

— Cancer Research UK (@CR_UK) May 1, 2020

For coronavirus and cancer updates from March and April, please visit our previous blog post.

Katie 

If you have questions about cancer, you can talk to our nurses Monday to Friday, 9-5pm, on freephone 0808 800 4040.

from Cancer Research UK – Science blog https://ift.tt/2VYt27t

Milky Way could be catapulting stars into its outer halo

This isn’t a real galaxy. Instead, it’s part of a super-powerful computer simulation of our own galaxy, the Milky Way. In this simulated image, the “arm” you see extending out from the center spans more than 200,000 light-years. That’s wider than the Milky Way itself! The structure shows the prominent plumes of young blue stars, born in gas that was blown outward by supernova explosions. Read more. Image via Sijie Yu/ UCI.

Though mighty, the Milky Way and galaxies of similar mass are not without scars chronicling turbulent histories.

So say scientists at the University of California, Irvine, who used the “hyper-realistic, cosmologically self-consistent” computer simulations generated via the FIRE-2 collaboration (FIRE stands for Feedback in Realistic Environments) to model our Milky Way galaxy’s rotation over time. In this way, they’ve learned that our galaxy may sometimes launch newly forming stars into the space around itself – that is, into the halo of our galaxy – via outflows triggered by supernova explosions.

UC Irvine physicist Sijie Yu is lead author of this new study, which was published in March 2020 in the peer-reviewed Monthly Notices of the Royal Astronomical Society. Yu said the findings were made possible partly by the availability of a powerful new set of computing tools. She said in a statement:

The FIRE-2 simulations allow us to generate movies that make it seem as though you’re observing a real galaxy.

They show us that as the galaxy center is rotating, a bubble driven by supernova[s] is developing, with stars forming at its edge. It looks as though the stars are being kicked out from the center.

Here’s an example of one of the movies from the FIRE-2 simulations:

Visualization of one of the outflow events discovered in one of the FIRE-2 simulations. Left is the mock starlight movie, consisting of mock Hubble Space Telescope-type images (blue shows sites of young star formation, red/brown shows where dust has obscured the starlight). The right one shows the gas distribution. These gas images are a mock 3-color composite showing the cold neutral gas.

James Bullock of UCI is a study co-author. He commented:

These highly accurate numerical simulations have shown us that it’s likely the Milky Way has been launching stars in circumgalactic space in outflows triggered by supernova explosions. It’s fascinating, because when multiple big stars die, the resulting energy can expel gas from the galaxy, which in turn cools, causing new stars to be born.

The statement explained:

Astronomers have long assumed that galaxies are assembled over lengthy periods of time as smaller star groupings come in and are dismembered by the larger body, a process that ejects some stars into distant orbits. But the UCI team is proposing ‘supernova feedback’ as a different source for as many as 40% of these outer-halo stars.

A typical spiral galaxy, like our Milky Way, has a faint, extended stellar halo. The new FIRE-2 study proposes that outflows from more central regions of the galaxy – triggered by supernova explosions – account for as many of 40% of the outer-halo stars. Image via COSMOS.

Bullock said he did not expect to see such an arrangement because stars are such “tight, incredibly dense balls” that, he said, are generally not subject to being moved relative to the background of space:

Instead, what we’re witnessing is gas being pushed around, and that gas subsequently cools and makes stars on its way out.

The researchers said that while their conclusions have been drawn from simulations of galaxies forming, growing and evolving to the present day, there is actually a fair amount of observational evidence that stars are forming in outflows from galactic centers to their halos. Yu said:

In plots that compare data from the European Space Agency’s Gaia mission – which provides a 3-D velocity chart of stars in the Milky Way – with other maps that show stellar density and metallicity, we can see structures similar to those produced by outflow stars in our simulations.

Bullock added that mature, heavier, metal-rich stars like our sun rotate around the center of the galaxy at a predictable speed and trajectory. But the low-metallicity stars, which have been subjected to fewer generations of fusion than our sun, can be seen rotating in the opposite direction.

He said that over the lifespan of a galaxy, the number of stars produced in supernova bubble outflows is small, around 2%. But things change when the galaxy is undergoing starburst events, that is, events where the galaxy begins undergoing furious rates of star formation. Yu added:

There are some current projects looking at galaxies that are considered to be very ‘starbursting’ right now. Some of the stars in these observations also look suspiciously like they’re getting ejected from the center.

This mock Hubble Space Telescope image shows how star formation happens at the edges of a supernova bubble. The portion highlighted in pink shows the stellar birth region. Blue shaded areas show young stars; red/brown shows where dust has obscured the starlight. The simulation shows clearly where stellar outflow shells are being generated. Image via Sijie Yu/ UCI.

Bottom line: Scientists used computer simulations from the FIRE-2 collaboration to learn that our Milky Way galaxy may sometimes launch newly forming stars into the space around itself – that is, into the halo of our galaxy – via outflows triggered by supernova explosions.

Source: Stars made in outflows may populate the stellar halo of the Milky Way

Via University of California, Irvine

from EarthSky https://ift.tt/2VXWJp9

This isn’t a real galaxy. Instead, it’s part of a super-powerful computer simulation of our own galaxy, the Milky Way. In this simulated image, the “arm” you see extending out from the center spans more than 200,000 light-years. That’s wider than the Milky Way itself! The structure shows the prominent plumes of young blue stars, born in gas that was blown outward by supernova explosions. Read more. Image via Sijie Yu/ UCI.

Though mighty, the Milky Way and galaxies of similar mass are not without scars chronicling turbulent histories.

So say scientists at the University of California, Irvine, who used the “hyper-realistic, cosmologically self-consistent” computer simulations generated via the FIRE-2 collaboration (FIRE stands for Feedback in Realistic Environments) to model our Milky Way galaxy’s rotation over time. In this way, they’ve learned that our galaxy may sometimes launch newly forming stars into the space around itself – that is, into the halo of our galaxy – via outflows triggered by supernova explosions.

UC Irvine physicist Sijie Yu is lead author of this new study, which was published in March 2020 in the peer-reviewed Monthly Notices of the Royal Astronomical Society. Yu said the findings were made possible partly by the availability of a powerful new set of computing tools. She said in a statement:

The FIRE-2 simulations allow us to generate movies that make it seem as though you’re observing a real galaxy.

They show us that as the galaxy center is rotating, a bubble driven by supernova[s] is developing, with stars forming at its edge. It looks as though the stars are being kicked out from the center.

Here’s an example of one of the movies from the FIRE-2 simulations:

Visualization of one of the outflow events discovered in one of the FIRE-2 simulations. Left is the mock starlight movie, consisting of mock Hubble Space Telescope-type images (blue shows sites of young star formation, red/brown shows where dust has obscured the starlight). The right one shows the gas distribution. These gas images are a mock 3-color composite showing the cold neutral gas.

James Bullock of UCI is a study co-author. He commented:

These highly accurate numerical simulations have shown us that it’s likely the Milky Way has been launching stars in circumgalactic space in outflows triggered by supernova explosions. It’s fascinating, because when multiple big stars die, the resulting energy can expel gas from the galaxy, which in turn cools, causing new stars to be born.

The statement explained:

Astronomers have long assumed that galaxies are assembled over lengthy periods of time as smaller star groupings come in and are dismembered by the larger body, a process that ejects some stars into distant orbits. But the UCI team is proposing ‘supernova feedback’ as a different source for as many as 40% of these outer-halo stars.

A typical spiral galaxy, like our Milky Way, has a faint, extended stellar halo. The new FIRE-2 study proposes that outflows from more central regions of the galaxy – triggered by supernova explosions – account for as many of 40% of the outer-halo stars. Image via COSMOS.

Bullock said he did not expect to see such an arrangement because stars are such “tight, incredibly dense balls” that, he said, are generally not subject to being moved relative to the background of space:

Instead, what we’re witnessing is gas being pushed around, and that gas subsequently cools and makes stars on its way out.

The researchers said that while their conclusions have been drawn from simulations of galaxies forming, growing and evolving to the present day, there is actually a fair amount of observational evidence that stars are forming in outflows from galactic centers to their halos. Yu said:

In plots that compare data from the European Space Agency’s Gaia mission – which provides a 3-D velocity chart of stars in the Milky Way – with other maps that show stellar density and metallicity, we can see structures similar to those produced by outflow stars in our simulations.

Bullock added that mature, heavier, metal-rich stars like our sun rotate around the center of the galaxy at a predictable speed and trajectory. But the low-metallicity stars, which have been subjected to fewer generations of fusion than our sun, can be seen rotating in the opposite direction.

He said that over the lifespan of a galaxy, the number of stars produced in supernova bubble outflows is small, around 2%. But things change when the galaxy is undergoing starburst events, that is, events where the galaxy begins undergoing furious rates of star formation. Yu added:

There are some current projects looking at galaxies that are considered to be very ‘starbursting’ right now. Some of the stars in these observations also look suspiciously like they’re getting ejected from the center.

This mock Hubble Space Telescope image shows how star formation happens at the edges of a supernova bubble. The portion highlighted in pink shows the stellar birth region. Blue shaded areas show young stars; red/brown shows where dust has obscured the starlight. The simulation shows clearly where stellar outflow shells are being generated. Image via Sijie Yu/ UCI.

Bottom line: Scientists used computer simulations from the FIRE-2 collaboration to learn that our Milky Way galaxy may sometimes launch newly forming stars into the space around itself – that is, into the halo of our galaxy – via outflows triggered by supernova explosions.

Source: Stars made in outflows may populate the stellar halo of the Milky Way

Via University of California, Irvine

from EarthSky https://ift.tt/2VXWJp9