The life of a star is determined by its mass. Large stars live short lives that end in supernova explosions, while smaller stars live longer, ending their lives as white dwarfs. Knowing the mass of a star helps us understand not only the life of a star, but the evolution of galaxies. But determining the mass of a star can be difficult. 

The best way to weigh a star is to measure how strongly it pulls on another star. If two stars are a binary pair, the speed at which they orbit each other is governed by the gravitational pull between them. By measuring their orbits over time, we can determine the mass of each star. But many stars are solitary. The nearest star to them can be light years away, and its gravitational pull on these stars is too small to measure. So we need another way to determine its mass.

One alternative is to look at the temperature of a star. Larger mass stars burn hotter than smaller ones, so the higher a star’s temperature, the greater its mass. But this has a few downsides. For one, this relation between stellar temperature and mass is only true for main sequence stars. For another, stars get slightly hotter as they age. An old star with the Sun’s mass has a higher temperature than a young solar mass star.

A new way to measure a star’s mass looks at its surface gravity. A ball dropped near the surface of the Earth will fall at a rate of about 9.8 m/s². This is Earth’s surface gravity. Far away from the Earth gravity is weaker. The Moon, for example, “falls” around the Earth at  only about 2.7 mm/s². The surface gravity of a planet or star depends upon its mass and its diameter. By determining the distance to a star, we can use its apparent size to determine its diameter. Determining surface gravity is a bit more tricky.

If you bounce a ball against the ground, it takes a certain amount of time to rise to its maximum height and fall back to the ground. That time depends in part on surface gravity. If you were to bounce a ball in the same way on Mars, the time between bounces would be longer, because Mars has a smaller surface gravity. We can’t bounce balls on a star, but there are surface fluctuations that rise and fall. The surface of a star often churns a bit like boiling water, creating rising and falling pockets known as granules. The rate at which these granules rise and fall is determined by the star’s surface gravity. So by measuring the rate at which a star flickers in tiny ways, we can determine a star’s mass.

A recent paper looked at the observational limits of data from GAIA (currently gathering data) and TESS (planned to launch in March). They found that GAIA could determine a star’s mass give or take 25%, and TESS should be able to determine stellar mass to within 10%. Since these satellites will observe millions of stars, this could become a powerful tool in the study of stars.

Paper: Keivan G. Stassun, et al. Empirical, Accurate Masses and Radii of Single Stars with TESS and Gaia.  arXiv:1710.01460 (2017)

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Out of the vast sea of stars in the night sky, a few are special. Not because of their size, or color, or age, but because they have a name. When we name a star, we make it a part of our collective cultural heritage. Their names inspire epic stories, or remind us of our history.

Of course many stars have multiple names. The brightest star in the night sky is most commonly known as Sirus, from the Latin. But Geoffrey Chaucer referred to it as Alhabor. In Arabic it is Mirzam Al-Jawza, and in many cultures it is known as the dog-star or wolf-star. All human cultures have a history of astronomy, so lots of stars have multiple names. So how do we deal with this in astronomy? Traditionally we have relied upon the names from Western astronomy. That meant mostly Latin names for the bright stars and Arabic names for dimmer ones, since Arabic astronomers of the Middle Ages were so meticulous in their observations, and later Europeans relied upon their catalogs. Another way is to use the order of brightness within a particular constellation. So Rigel, in the constellation Orion, is Beta Orionis, since it is the second brightest star in Orion, after Betelgeuse. Some stars are most commonly known by this, such as the brightest star in the constellation Centaurus, Alpha Centauri. In Chinese astronomy it is known as Nán Mén Èr, or the second star of the southern gate.

The problem with this naming scheme is that the official constellations mainly derive from European tradition,  so they ignore the long history of astronomy in other parts of the world. We could just stop using names and instead use catalog numbers. But HD 172167 doesn’t appeal to us in the same way that Vega does. Names connect us both to the stars and our history, so why not use the names we have. The challenge is to use names that honor both the history of astronomy and the diverse cultures that have contributed to our common understanding of the stars. This is the goal of the International Astronomical Union (IAU) Division C Working Group on Star Names (WGSN). Over the past several years they have built a list of officially recognized names for stars. Recently they have added 86 new names to the list, bringing the total to 313.

Some of the list simply makes the most popular names official. Sirius, Betelgeuse, and Vega keep their names. Others change slightly, such as Alpha Centauri becoming Rigel Kentaurus (the foot of the Centaur). But other names are drawn from across the globe. The fifth brightest star in the constellation of the Southern Cross (Epsilon Crucis) now has the name Ginan. It derives from the astronomy of aboriginal Australia, which is perhaps the oldest culture of astronomy on Earth. A star in the Hyades cluster commonly called Theta-2 is now recognized as Chamukuy, from the Mayan. Zeta Piscium now has the Hindu name Revati. All of these names have been used in some circles of astronomy, but the IAU designation now recognizes them as the preferred name.

These names are just the beginning. The stars will continue to call to us. As we further understand them, learn of planets that orbit them, and perhaps even travel among them someday we will continue to give them names. They will continue to inspire stories of our history, wonder, past and future.

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The constellation of Orion is popular among winter stargazers in the northern hemisphere. It is easily identifiable by three bright stars that form the belt of Orion. Slightly below the belt is a fuzzy region known as the Orion Nebula. It is about 1,300 light years away, which is relatively close in astronomical terms, and it is an active stellar nursery. It is thought that all stars, including our Sun, were produced in stellar nurseries, where vast quantities of gas and dust come together to produce hundreds of stars at a time before galactic rotation and intense stellar winds rip the nebula apart. But new observations of the Orion Nebula find that stellar nurseries could produce stars in multiple waves rather than a single burst. 

Using data OmegaCAM, which is a wide angle optical camera at Paranal Observatory,  a team looked at more than 200 pre-main sequence stars. That is, stars that are only a few million years old and are still in the process of settling down into a stable (main sequence) star like our Sun.  They measured the apparent brightness of these stars in both visible and infrared wavelengths, and used this data to compare brightness and temperature. Very young stars heat up as they age, so this comparison allows astronomers to estimate their age. They found these stars didn’t cluster around a similar age, but were instead grouped into three clusters, each a few hundred thousand years apart. This would imply that star formation within the Orion nebula occurred in multiple waves rather than all together. If true, this would be a surprising result, and could force astronomers to reassess the details of stellar formation models.

The distribution of stars within the Orion nebula for each age group. Credit: G. Beccari, et al.

But determining the ages of very young stars is difficult, and some research has shown that the luminosity-age relation isn’t always reliable. Stars do heat up over time, but this can be greatly affected by things such as the rate at which the star accretes material. The apparent clustering of stars could be due to stars heating up at different rates, rather than being different ages. So the team looked at another method to estimate the age of stars, specifically the rate at which stars rotate. Early on, a young star can rotate rapidly, but they slow down as they reach the main sequence. If the clustering effect is due to age, one would expect star in the youngest group to rotate more quickly than stars in the oldest group. This is exactly what the team found, further confirming that the Orion nebula has had multiple waves of star production.

The team also looked at the distribution of these stars within the Orion nebula, and found them to be located within similar regions. This means that the stars did not form within distinct regions of the nebula. It seems clear that the Orion nebula has gone through active and quiet periods of stellar formation over the past few million years.

Paper: G. Beccari, et al. Tale of Three Cities: OmegaCAM uncovers three discrete episodes of star formation in the Orion Nebula Cluster. Astronomy and Astrophysics DOI: 10.1051/0004-6361/201730432 (2017)

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Ten light years away there is a star that could tell us about the origins of our solar system. Known as Epsilon Eridani, it is a bit smaller and cooler than our Sun, but similar in composition. It is also only about 500 million years old, giving us a view of what our own solar system may have been like in its youth. New work now finds the system is similar to our own. 

We’ve known for a while that Epsilon Eridani has a disk of debris surrounding it. It is in keeping with the idea that planetary systems form from such disks during a star’s youth. We understand the basic process of planetary formation pretty well, but where the details get fuzzy is how and when planets form. Do they form further from the star and migrate inward over time? Do large planets form first and dictate where other planets might form? Computer models can only take us so far. To make matters worse, we now know that our solar system is a bit unusual, so we can’t rely on it as a typical model. But Epsilon Eridani could help.

The system has at least one Jupiter-sized planet. This planet, known as Epsilon Eradani b, or AEgir, has about the same distance as Jupiter in our solar system. It also has an asteroid belt within AEgir’s orbit, just as we have one within Jupiter’s orbit. Far beyond AEgir’s orbit is a comet belt, similar to the Kuiper belt beyond Neptune. It’s hard to determine the details beyond that, since each region of debris within the system emits light at different wavelengths. In particular the long infrared wavelengths often emitted are largely absorbed by our atmosphere, making them impossible to observe from the ground.

This is where SOFIA comes in. SOFIA is a 2.5 meter telescope mounted in a Boeing 747. SOFIA can observe these long infrared wavelengths because it flies high above much of Earth’s atmosphere. New observations from SOFIA found Epsilon Eridani has two asteroid belts. In addition to the one within AEgir’s orbit, there is a narrow asteroid belt between AEgir and the comet belt. Thin belts of debris would tend to spread out over time, so it is likely that this belt is shepherded by one or two additional planets. The spacing of this new belt and the comet belt implies two planets, similar to Uranus and Neptune. Over time, the gravitational tug from these planets and AEgir would cause material from the outer belt to migrate toward the inner one. The inner asteroid belt is likely stable over time.

These new planets still need to be confirmed, and even the existence of AEgir can be debated. But the formation of gaps within the debris disk of Epsilon Eridani validates models for the formation of our solar system.

Paper: Kate Y. L. Su, et al. The Inner 25 au Debris Distribution in the epsilon Eri System. The Astronomical Journal, Volume 153, Number 5 (2017)

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Our Sun is adrift among the stars. As our home star moves through the galaxy, so to do the other stars. This means that the apparent positions of the stars change over time. Because of the great distances of stars this shift is minuscule and difficult to measure. For years we have only been able to measure the motion of a few close stars. But that’s beginning to change. 

From 1989 to 1993, the Hipparcos spacecraft made high precision measurements of more than 100,000 stars, cataloging their positions and distances, as well as a measure of their proper motion across the sky. This data was compiled in the Hipparcos Catalog in 1997. A less precise catalog of more than 2 million stars, known as the Tycho catalog was also published. While the accuracy of the Hipparcos data for some stellar clusters has been debated, it has proved to be quite accurate for most stars.

Then in 2013 the Gaia spacecraft was launched, with the goal of measuring the position and motion of more than a billion stars. In 2014 the Gaia team published its initial data, including measurements of more than 2 million Hipparcos stars. This gave us the opportunity to see just how far these stars had moved over the course of 25 years.

Fortunately the data from both Hipparcos and Gaia are freely available. So the United States Naval Observatory (USNO) analyzed the data to calculate both the location and motion of these 2 million stars, giving the most accurate proper motions thus far. They then went one step further, and compared the positions of these 2 millions stars with the positions of million that had been measured by the USNO in 1998 and 2004, and were able to determine the proper motions of millions more stars. A new catalog containing this data will be released soon.

We’ve long known the stars moved over time, but we are now able to determine this motion accurately for millions of stars. This will help us understand not only the dynamics and evolution of our Milky Way galaxy, it could also provide clues to things like dark matter.

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Although our Sun is the only star in our solar system, that isn’t the case for every planetary system. It’s estimated that about than half of stars could be binaries, though the exact percentage is still hotly debated. What we do know is that binary stars are common. This has raised the question about how planets might form in binary systems. 

The usual view of planetary formation is that planets form within disks of material around a young star. While such protoplanetary disks are common around single stars, there has been debate about whether they are common around binary stars. While we have observed debris disks around young binary stars it hasn’t been clear whether such disks would be stable long enough for planets to form. After all, the gravitational interactions between two orbiting stars might make the surrounding region hostile to stable orbits.

So far all the exoplanets we’ve found around binary stars have been large, Jupiter-like planets. They would have formed in the outer icy regions of the system. But what about rocky, Earth-like worlds? Could they have formed closer to the stars where orbits might not be so stable? A newly observed binary system suggests that they could.

The system is called SDSS 1557. It consists of a white dwarf about the mass of our Sun, orbited by a large brown dwarf about 60 times more massive than Jupiter. Recently astronomers have observed a rocky asteroid belt surrounding the system.

Diagram of SDSS 1557 showing the debris ring around the two central stars. Credit: J. Farihi, et al.

This is important for two reasons. First, since the main star is a white dwarf, this is an old system. Our Sun will eventually become a white dwarf, but only after another 5 billion years when it reaches the end of its life. So it is possible that this asteroid belt has been stable for quite some time. Plenty of time for planets to form. Secondly, since the astroid belt is rocky and high in metals, it could form planets that are much more similar to Earth in size and composition.

None of this means that the system actually has rocky planets. In our own asteroid belt no planets formed because of the gravitational pull of Jupiter. A similar type of gravitational dance might have occurred near SDSS 1557. But what this discovery does show is that we can’t rule out the possibility that Earth-like planets might exist with two Suns.

Paper: J. Farihi, et al. A Circumbinary Debris Disk in a Polluted White Dwarf System. arXiv:1612.05259 [astro-ph.EP] (2017)

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When you think of a family tree, you probably think of human ancestry, and how we can trace our ancestors back to different geographical regions. All living things have a common family tree, which can be seen in our genetic code. While stars aren’t living things, they have a similar family tree, and we’re starting to gather enough data to piece it together. 

We’ve long known that stars can be categorized into generations depending on their origin. The very first generation of stars formed from the hydrogen and helium formed in the big bang. When the largest of those stars died in supernovae, the remnant gas and dust collapsed to form a new generation of stars. The largest second-generation stars later exploded, a third generation of stars formed, and so on. Since heavier elements such as carbon and iron forms within stars, later generations of stars tend to have a higher metallicity. Our Sun, for example, has a relatively high metallicity, and is therefore a third or fourth generation star.

Diagram showing how our Sun is chemically related to other stars. Credit: Paula Jofré, et al.

As our observations of stars has increased, there have been efforts to find stellar siblings of our Sun.  Stars don’t form on their own, but rather form with other stars in large nebulae known as stellar nurseries. Stars formed in the same stellar nursery would have similar ages and similar chemical compositions. The more similar the composition and age, the more likely the stars are to be related.

With sky survey satellites such as Gaia, we are finally gathering this kind of information on millions of stars, so in principle we should be able to study the connections between stars and groups of stars. A new paper in MNRAS shows how this can be done. By piecing together a stellar family tree, we can better understand the dynamics of stellar evolution within our galaxy.

Paper: Paula Jofré et al. Cosmic phylogeny: reconstructing the chemical history of the solar neighbourhood with an evolutionary tree. MNRAS 467 (1): 1140-1153. (2017)

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If you ask someone to name the closest star, Alpha Centauri is perhaps the most common answer. Some will recognize it as a trick question and correctly answer “the Sun,” but the closest star to our Sun is a faint star known as Proxima Centauri. One of the long-standing questions is whether Proxima is actually part of the Alpha Centauri system, or whether it just happens to be in the same general vicinity. We now have a pretty solid answer. 

The Alpha Centauri system compared to the Sun. Credit: Wikipedia

We’ve known since the late 1600s that Alpha Centauri is a binary pair. The two stars (A & B) are each similar in mass and luminosity to our Sun, and orbit each other every 80 years. Proxima Centauri is much smaller and dimmer, and wasn’t discovered until 1915.  By the 1950s we were able to confirm that Proxima wasn’t simply in the same direction as the others (known as an optical double star) but was in the same general region. It also had roughly the same proper motion, meaning that it’s slow drift across the sky is in the same general direction and speed as Alpha Centauri A & B. This meant that odds were good that the three stars are part of a trinary system. But proving this was a different matter.

Although Proxima is in the general region of the other two, it is still about a quarter of a light year away. That’s pretty close for stellar distances, but it’s quite far for an orbiting star. It’s quite possible that Proxima is simply making a close flyby of Alpha Centauri, but doesn’t plan on staying. The key point is whether Proxima has enough speed to escape the gravity of Alpha Centauri, or whether it is gravitationally bound to them. To determine that you need two measurements: radial velocity and proper motion. Radial velocity can be found by looking at the spectrum of a star. As a star moves toward or away from us, its light is redshifted or blueshifted due to the Doppler effect, and we can see this shift in the spectral lines. Getting a precise measure of Proxima’s proper motion is much more difficult because it’s so faint. But recently measurements from the HARPS detector at La Silla Observatory have finally nailed down a precise measure of Proxima’s motion. It’s speed and direction is well under the limit for being gravitationally bound, which means Proxima and Alpha A & B are indeed a trinary system. It’s orbital period is about 550,000 years.

So now we can say the closest stellar system to the Sun includes both Alpha and Proxima Centauri.

Paper: P. Kervella, et al. Proxima’s orbit around Alpha Centauri.  arXiv:1611.03495 (2016).

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A popular dream of science fiction has been the creation of a vast database containing all known information about the galaxy, an Encyclopedia Galactica. It would list the characteristics of stars in the Milky Way, their planets, and even the history of human alien civilizations. While still a dream in most ways, the foundation of such a cosmic Library of Alexandria is being laid with the first release of data from the Gaia spacecraft. 

The initial data release contains information on more than a billion stars. It gives the position and brightness of 1,140,622,719 stars in our galaxy and nearby dwarf galaxies, with distance and motion data for about 2 million stars. If a cosmic encyclopedia is created, it will begin with the data from Gaia. What makes the data so powerful is its combination of size and precision. Other spacecraft such as Hipparcos have mapped about 100,000 stars, but the Gaia mission will precisely determine the position and motion of more than a billion stars. That’s still only about 1% of the total number of stars in our galaxy, but it’s large enough that we can study aspect of the Milky Way like never before.

For example, since stellar motion is determined by gravitational interactions, the data will allow us to determine the overall mass and distribution of mass within the Milky Way. Comparing the mass distribution to the distribution of stars, we can better determine the distribution of dark matter in our galaxy. The data will also allow us to determine the motions of stars within clusters, which will not only help us understand the origin and evolution of star clusters, it will help us learn more about our galactic history.

How Gaia’s accuracy and mission varies with distance. Credit: ESA; Background: Lund Observatory

Measuring the positions and motions of all these stars requires precise observations of the stars’ brightness and spectra over time. This data can also be used to discover exoplanets. It’s expected that Gaia will discover any Jupiter-mass planet orbiting stars within 150 light years of Earth. It could detect nearly 50,000 planets over the course of its mission. The data not only determines the location of stars, but also their size and type.  The initial data contained 3,194 variable stars, which not only help us understand stellar evolution, they are also used to determine galactic distances. Overall the precision of the data is so high it will allow us to test theories such as general relativity in new and subtle ways, possibly leading to the discovery of new physics.

Then there are the secondary discoveries that might occur. While Gaia’s mission is to measure stars, in practice it measures the brightness and location of any object over an apparent magnitude of 20. This includes countless asteroids, Kuiper Belt objects, and any large outer solar system planet within 200 parsecs. It will also observe occultations of stars by solar system bodies, such as the recent occultation of a faint star by Pluto, which helped us study Pluto’s atmosphere.

Perhaps what makes Gaia most exciting is that the data is released publicly. Since anyone can access the data, it can be used in ways the Gaia team hasn’t anticipated. It truly is a great encyclopedia available to all.

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A nova occurs when a star brightens by several magnitudes over a very short time. Like supernovae, they’ve been recorded throughout history. We now know novas are caused by a dance between two stars, where a white dwarf orbits close enough to a companion star that it captures material from its companion until it reaches a critical limit and it’s outer layer explodes. Studying the details of this phenomena is difficult, because a nova is usually too faint to be noticed until it brightens. But thanks to large sky surveys, that’s starting to change. 

Recently a team observed a nova in their data, and knew they had captured that region of sky before. So they went back through their data and were able to document the binary system before, during and after the nova occurred. They found the two stars orbit each other once every five hours, putting them at a distance roughly equal to the diameter of our Sun. Before the explosion, the white dwarf was capturing material at an irregular rate, causing its brightness to “sputter” slightly. After the nova the white dwarf captured material at a more regular rate. This would support the hibernation model, where the white dwarf captures material early on, then the rate of capture dies off. It should be stressed however, that the aftermath of the nova is still young, so we’ll need to collect more data to be sure.

In addition to helping us understand novae, observations like these could also help us understand supernovae. Type Ia supernovae in particular are caused by a similar dance between a white dwarf and companion star, but instead of just the outer layers exploding, the entire white dwarf is ripped apart by a cataclysmic explosion.

Paper: Przemek Mróz, et al. The awakening of a classical nova from hibernation. Nature doi:10.1038/nature19066 (2016)

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Two thousand light years from Earth is a star known as Epsilon Aurigae. It’s a third magnitude star most of the time, but about every 27 years it dims to about half its brightness for nearly two years. The cause of the dimming is a bit of a mystery. 

It’s long been thought that the dimming is the result of Epsilon Aurigae being a binary system. With a companion star in a large orbit, it could pass in front of the primary star, making it appear to dim. The star is indeed a binary star (if not a multiple star) but the details of the dimming mechanism have been difficult to pin down.

Historically there have been two main ideas. The first is that Epsilon Aurigae is yellow supergiant about 15 times the mass of the Sun, with a companion of similar mass obscured somewhat by dust. This idea is supported by the fact that the spectrum of Epsilon Aurigae has many of the signatures common to yellow supergiants. However the companion star has a spectral signature more similar to a B-type main sequence star.

The other idea is that Epsilon Aurigae is much smaller, with a mass of 2 to 4 solar masses. This would make it smaller than the B-type companion with a mass of about 6 solar masses. In order for the companion to be much dimmer than Epsilon Aurigae, it would have to be surrounded by a thick disk of dust, and that disk would have to be aligned edge on when seen from Earth. It would be odd for a main sequence star to have a thick dusty disk, since they are more commonly seen around young stars that are still forming.

When the most recent dimming occurred in 2009 – 20011, both amateur astronomy groups and the Spitzer infrared telescope made observations of the transit. It now seems that both models were at least partly right. The model that now seems to best fit the data assumes Epsilon Aurigae is only about 10 solar masses, but it moving toward the end of its life. This means it is much brighter than a main sequence star of similar mass. The B-type companion is therefore much dimmer by comparison. With smaller masses, the two stars would be close enough that the companion would capture gas and dust pushed away from Epsilon Aurigae, thus explaining the companion’s dusty disk.

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A single star is a wonder. A million stars is a story. 

A star can burn for billions, even trillions of years. With human history spanning mere centuries, how can we possibly understand the lifespan of a star? If we only had the Sun to study, understanding it’s history would be difficult, but we can observe millions of stars, some ancient and some still forming. By looking at these stars as a whole we can piece together the history and evolution of a star. It is similar to taking pictures of a single day on Earth, and using it to piece together the story of how humans are born, live and die.

One of the ways this is done is through a Hertzsprung-Russell (HR) diagram. The brightness of a star is plotted against its color. When we make such a plot, most stars lie along a diagonal line where the bluer the star the brighter it is. Given a large enough sample of stars, we can presume that the ages of stars are randomly distributed. Since most stars lie along this line (known as the main sequence) they must spend most of their lives there. So it’s clear that stars have a long stable period where they burn steadily. Other stars are red, but still quite bright. One would expect red stars to be dimmer than blue stars since they have a lower temperature. In order to be so bright, they must be quite large. These red giants are stars that have swollen up as their cores heat up in a last-ditch effort to continue fusing hydrogen. Some stars are large enough to start fusing helium in this stage. Since helium burns hotter, these stars brighten into blue giants. In the end, however, most stars collapse into white dwarfs when core fusion ends. They become hot but small stars, blue-white in color but quite dim.

While an HR diagram gives us a snapshot of stellar lifetimes, they don’t tell the whole story. Another way to categorize stars is through their spectra. Different elements in a star’s atmosphere absorb particular wavelengths of light. By looking at the pattern of wavelengths absorbed we can determine which elements the star contains. On a basic level can categorize stars by their metallicity. While stars are mainly hydrogen and helium, they contain traces of other elements (which astronomers call metals). The metallicity of a star is by its ratio of iron to helium, known as [Fe/He]. This is expressed on logarithmic scale relative to the ratio of our Sun. So the [Fe/He] of our Sun is zero. Stars with lower metallicity will have negative [Fe/He] values, and ones with higher metallicity have positive values. Since “metals” are formed by fusion in the cores of stars, those stars with higher metallicity must have formed from the remnants of earlier stars. Our Sun is likely a third generation star. One of the things metallicity tells us is that stars toward the center of our galaxy formed earlier than stars in the outer regions. Through millions of stars we not only understand the history of stars but the history of galaxies.

As we continue to gather more data on stars, they continue to tell us a rich collective story.

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