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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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 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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Two million years ago, the early humans known as homo habilis lived in Africa. Nearly 450 light years away a new star was just beginning to form in the constellation Taurus. It’s now known as CI Tau, a T Tauri star about 80% the mass of our Sun. A T Tauri star looks like a red or orange star. Their surface temperature is a bit cooler than the Sun’s, but they are brighter than a typical main-sequence star of the same type. The reason for this is that they have not finished collapsing, so they are larger than a similar mass star. 

A map of the circumstellar disk around CI Tau. Credit: Stephane Guilloteau/University of Bordeaux

As T Tauri stars continue to collapse under their own weight, the surrounding gas and dust begins to flatten into a disk of material out of which planets can eventually form. We have several examples of planets forming within a circumstellar disk, but planet formation can be difficult to observe in very young stars because the surrounding gas and dust can obscure them from view. For CI Tau, astronomers studied the emission spectra of the star looking for small shifts in their wavelengths over time. These Doppler shifts are due to the relative motion of the star. Any planet orbiting the star will cause the star to wobble slightly. They found evidence of a planet about 11 times more massive than Jupiter, orbiting the star once every nine days.

It’s a bit surprising that such a large planet has formed around such a young star. It demonstrates that planets can form quite quickly, and the formation of these early planets could affect the formation and location of smaller planets as the star system matures.

Paper: Christopher M. Johns-Krull, et al. A Candidate Young Massive Planet in Orbit around the Classical T Tauri Star CI Tau.  arXiv:1605.07917 [astro-ph.EP] (2016)

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The most massive star known is R136a1. It is a Wolf-Rayet star with a mass about 265 times larger than the Sun. Such a star is more massive than the assumed upper limit for a traditional star, and it is not entirely clear how such large stars can form. The most popular view has been that such supermassive stars form when two large stars collide, but new observations cast doubt on that model. 

The largest known star is part of a cluster of stars known as R136, which is in the Large Magellanic Cloud. The cluster is only about 1.5 million years old, and has about 70 bright blue (O-type) stars. A recent ultraviolet survey of the cluster found 9 stars with masses greater than 100 solar masses. Given the cluster’s age, it’s extremely unlikely that 9 pairs of large stars would have merged within that time. The most massive stars in this cluster also have a bright emission line known as  He II λ1640, and since the largest stars in the cluster are also the brightest, this emission line is prominent when the spectrum of the cluster stars are averaged together. This emission line also appears in another young star cluster in the galaxy known as NGC 3125. This would support the idea that the R136 cluster isn’t some unusual fluke.

So young star clusters might form supermassive stars directly rather than through mergers. If that’s the case, we’ll have to develop new models to account for them.

Paper: Paul A. Crowther, et al. The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of He II λ1640 in young star clusters. MNRAS 458 (1): 624-659 (2016). doi: 10.1093/mnras/stw273

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HL Tau is a young star, only about a million years old. Despite its young age, the star is already busy at making a family. 

The star is probably most famous for an early image from the Atacama Large Millimeter/submillimeter Array (ALMA), which showed a disk of gas and dust surrounding the star. What was most striking about the image was the clear gaps in the dusty disk, which seemed to be due to young protoplanets as they began the process of formation. ALMA’s strength is the ability to observe light at millimeter wavelengths, which is the type of light emitted by cold gas and dust. Unfortunately the resolution of ALMA isn’t high enough to observe individual protoplanets, which left some doubt as to whether the gaps were caused by some other process.

Combined ALMA/VLA image of HL Tau.
Credit: Carrasco-Gonzalez, et al.; Bill Saxton, NRAO/AUI/NSF.

But new evidence from the Very Large Array (VLA) confirms the existence of at least one protoplanet. The VLA observes at longer radio wavelengths, but the VLA antennas can be spread across 36 kilometers vs ALMA’s maximum spread of 16 kilometers. This larger spread means that the VLA can make higher resolution images (though only at radio wavelengths). Recent observations of the central region of HL Tau clearly shows a clump in the inner ring of material, indicating a protoplanet in the early stages of formation.

These results show the power of combining observations from different facilities and at different wavelengths. They also demonstrate how astronomy can still surprise us. With an age of only a million years HR Tau is still in the earliest stage of its life, and yet it is already on the path toward becoming a solar system.

Paper: Carlos Carrasco-Gonzalez, et al. The VLA view of the HL Tau Disk – Disk Mass, Grain Evolution, and Early Planet Formation. arXiv:1603.03731 [astro-ph.SR] (2016)

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Most of the star systems we’ve discovered have been small systems with planets close to a red dwarf star. This is largely due to the fact that large planets close to a star are easier to detect than smaller ones farther away. Our own solar system is fairly large by comparison, with the most distant planetary bodies being about 300 AU (or perhaps 700 AU) from our Sun. It raises an interesting question about just how large a planetary system could be. If a recent discovery pans out, the answer could be much larger than we expected. 

The size of a solar system is largely driven by gravity. Since gravity follows an inverse square relation, the closer you are to a massive body the stronger its gravitational pull will be. With increasing distance the gravity of a star never quite goes away, but it becomes so small that other gravitational tugs can overpower it. As a result, the most distant objects of a solar system would be susceptible to the gravity of stars that pass close by (on a cosmic scale). Over the past 20 million years about 40 stars have made a “close” approach to our solar system, and these can disturb objects in the Oort cloud.

It would seem reasonable that beyond a thousand AU or so the odds of having a large planetary body would be rare. On cosmic time scales a stellar close encounter would likely make the orbit such distant planets unstable. But recently astronomers have found a system with a radius of 4,000 to 6,000 AU. The team found an L-class brown dwarf at that distance from a red dwarf star.

Now the fact that two stars happen to be close doesn’t automatically mean they are orbiting each other. It’s possible that they just happen to be making a close flyby at the moment. At several thousand AU, it’s also not possible to determine their motion precisely enough to confirm that they are orbiting. However there are a few clues that point toward being a common system. For one, their proper motion is basically the same. This means they are moving through the galaxy in the same direction, which would be unusual for a random close encounter. Another is that they are both young bodies, and seem to be the same age, which again would be unlikely for a random encounter. It’s possible that the two bodies formed within the same stellar nursery and simply happened to be expelled in the same general direction, but again that would be unusual. The alternative is that they really are a binary system. Since the brown dwarf is likely in the range of planetary mass, this would then be the largest known solar system.

So it’s possible that solar systems can be very large indeed. Of course this raises questions about how such systems could form, and whether they would be stable over millions or billions of years, but that’s a mystery for another day.

Paper: N.R. Deacon, et al. A nearby young M dwarf with a wide, possibly planetary-mass companion. Mon. Not. R. Astron. Soc. 000, 1–18 (2013)

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