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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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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Why do young stars contain so much lithium? That might seem an odd question, but it’s a question that has nagged astronomers for quite some time.

The story of lithium in the universe begins with the big bang. In the early moments of the big bang, the first elements were created through a process known as nucleosynthesis. Most of the matter created through nucleosynthesis was hydrogen and helium (numbers 1 and 2 on the periodic table), but the big bang also produced a bit of lithium (the number 3 element). According to our model of the big bang, for every 10 billion hydrogen atoms produced, only one lithium atom would form. This might seem like an extraordinarily small amount, but it’s actually more lithium than we actually observe in the universe. It would seem our prediction didn’t quite match reality.

But we also know that lithium can be consumed in the heart of a star. Stars produce light and heat through a complex process of nuclear reactions. As a result, lighter elements such as hydrogen can be fused into heavier elements like carbon, oxygen and iron. But some elements are easier to fuse than others, and it turns out that lithium readily fuses into other elements, a process known as lithium burning. As a result, stars can reduce the amount of lithium in the universe. Perhaps that would explain why there is less lithium than our initial model predicted. It’s a good idea, but there’s just one problem. If lithium burning were the solution, then we would expect older stars to have more lithium and younger stars less, since over time there would be less lithium available. What we actually observe is that older stars actually have less lithium than younger stars. What started with one mystery then became two.

But back in the 1970s it was proposed that the higher levels of lithium in younger stars might be due to nova explosions of older stars. Unlike a supernova, where a star is completely destroyed (with the exception of a remnant neutron star or black hole) a nova is due to a runaway nuclear reaction on the surface of a white dwarf. This nuclear reaction could produce lithium, and thus create the abundance of lithium in younger stars. New observations have actually confirmed this effect.

The results come from a nova known as V1369 Cen, which was a bright nova that appeared in the constellation Centaurus in 2013. Because of its brightness, the team was able to observe its spectrum in detail, and they found clear evidence of lithium in the nova. This lithium was cast out into interstellar space, making it available for later stars that happen to form. The measured amount of lithium was small, but combined with the estimated billions of novae that have occurred throughout the history of our galaxy, it is enough to account for the observed rise in lithium levels.

So it seems that the lithium enigma isn’t such an enigma after all.

Paper: Luca Izzo et al. Early optical spectra of nova V1369 Cen show presence of Lithium. ApJ 808 L14 (2015)

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Giants lurk the heart of the Westerlund 1 cluster. It’s an open cluster that contains some of the most massive stars in our galaxy. It contains yellow hypergiants, red supergiants, Wolf Rayet stars and supergiant stars. Given its size and density, Westerlund 1 will likely evolve into a globular cluster.

Because these different types of massive stars have particular lifetimes, we can actually get a pretty good handle on the age of the Westerlund 1 cluster. Red supergiants, for example, generally don’t form until the star is about 4 million years old. On the other hand, Wolf Rayet stars tend to die off after about 5 million years. So the cluster should be around 4 – 5 million years old.

Since Westerlund 1 is only about 12,000 light years away, the cluster provides an excellent opportunity to study the dynamics of large stars. Since large stars create strong radiation fields, and eventually explode to release the gas and dust of heavier elements, they play a central role in the evolution of galaxies.

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There are five small planets orbiting a star known as Kepler-444. These planets are all smaller than Earth, and they are all very close to their parent star, with “years” lasting less than 10 days. None of this is really a big deal given the vast number of exoplanets we’ve discovered, but what is unusual is the age of the star, which is estimated to be about 11 billion years old.

The traditional view of planet formation is that rocky Earth-like planets wouldn’t form around early stars. The reason for this is that elements other than hydrogen and helium are formed within stars, so only after some of the first stars died and exploded would things like iron, carbon and silicon be available for rocky worlds. But we now know that model is a bit too simplistic.

Observed transits of the five planets. Credit: Campante, et al.

One of the ways we determine the age of a star is by its metallicity. That is, the amount of “metals” (anything but hydrogen and helium) a star contains. That’s because the less metal a star has, the older it is likely to be. Our Sun is only about 5 billion years old, for example, and has a relatively high metallicity. We’ve seen some correlation between the metallicity of a star and the type of planets that might form, specifically that higher metallicity stars are more likely to have large Jupiter-like planets. Kepler-444, by contrast, is a metal-poor star with a low metallicity. It isn’t the type of star we’d expect to have a planetary system, and yet it clearly does. Given the size of these planets, they are likely to be rocky worlds as well.

The low metallicity of Kepler-444 would imply it is an older star, but astroseismology (the stellar version of helioseismology) gives an age of about 11.2 billion years, give or take a billion. That means it formed when the universe was only about 2 – 3 billion years old. So it seems that planetary systems could form early on, even when metals were fairly rare.

It would seem then that planets have been around almost as long as there have been stars.

Paper: Campante T. L. et al. An ancient extrasolar system with five sub-Earth-size planets. ApJ 799 170. (2015)

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Earlier this year I wrote about a diffuse band of gamma rays coming from regions above and below the Milky Way. The regions spanned about 25,000 light years above and below the galactic plane, and are thought to have formed from an active period of our galaxy’s supermassive black hole about 2 million years ago. While we could determine the size of these regions from their x-ray and gamma ray emissions, it has been difficult to determine their motion. But yesterday at the American Astronomical Society Meeting, new results from the Hubble telescope are measuring the motion of these regions using an interesting trick.

The team looked at ultraviolet light from a distant quasar. The light from this particular quasar passes through the gamma ray bubble before reaching us, and so interacts with the material there. By looking at the spectrum of this light, you can see dark lines where the intervening material absorbs light at particular frequencies. By comparing these absorption lines with ones from materials on Earth we can determine what the material is made of. But you can also determine the speed at which the material is moving due to its Doppler shift.

Normally when we talk about the Doppler effect, it is a shift in the wavelength of light toward the red or blue due to the motion of the light source. But in this case, the material in the region relative to the quasar, and the light it “sees” is redshifted or blueshifted due to its motion. This means if the material is moving toward us the absorption lines are blueshifted, and if they are moving away from us they’re redshifted. Since motion is relative, we can calculate the speed of the material by the shift of its absorption lines.

What the team found was that the material is streaming away from galactic center at about 800,000 m/s. That’s fast, but not quite active black hole fast. So it seems like these lobes of material may be the result of a burst of star production in the central region of the Milky Way. To know for sure we’ll need a more detailed analysis of material motion in this region. Fortunately there are about two dozen quasars that happen to have a line of sight through the region.

So more backlighting will likely illuminate this mystery.

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Meteorites provide one of the best sources of evidence for the history of our solar system. We have a wide range of them, with origins at different epochs of the solar system, from which we can analyze chemical composition and even the geology of other planets. The oldest meteorites are a stony type known as primitives. These look somewhat like rusty sandstone, and were formed in the earliest period of the solar system. They are called primitive because they were never reheated or combined with other material after their initial formation about 4.5 billion years ago. So they provide clues about the formation of solar system.

Credit: Roger R. Fu, et al.

Recently a paper in Science looked at the magnetic properties of a primitive meteorite known as Semarkona, and discovered a surprising fact about the early solar system. The authors studied small iron particles within small olivine chunks (chondrules) within the meteorite. These chondrules formed when regions of the early solar system were heated to a temperature hot enough to melt iron. The region then cooled and the iron droplets were flash frozen. This meant that any magnetic alignment due to an external magnetic field was locked in to the iron grains.

What the authors found was that within the chondrules the iron grains were aligned along a similar direction. This meant that they formed together within a magnetic field. By measuring the magnetic strength of these iron grains, they found the external magnetic field was likely around 50 microtesla, which is about the same strength as Earth’s magnetic field today.

This is much higher than modern magnetic field strengths in the interplanetary regions of the solar system, and it actually supports our model of the early solar system. One of the mysteries of protoplanetary formation is just how they could form out of the gas and dust surrounding a young star. Computer simulations relying simply on gas dynamics find that protoplanets take a long time to form. But the interactions of ionized gas in a moderate magnetic field leads to early protoplanet formation. It’s generally been thought that protoplanets form early, and thus our solar system should have had a stronger magnetic field in its past. This new work shows that in fact there was a stronger magnetic field.

What gave rise to that stronger magnetic field is still an unanswered question. But now we have one more piece in the puzzle of our solar system’s origin.

Paper: Roger R. Fu, et al. Solar nebula magnetic fields recorded in the Semarkona meteorite. Science, 13 November 2014

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Recently in Astrophysical Journal Letters a paper was published on the oldest “solar twin” yet discovered. The star, HIP 102152, is about 250 light years away from us, and about 4 billion years older than our Sun.

When you think of a twin, you probably think of two people who are siblings. For stars, that might be two stars that originated from the same stellar nursery. For this reason, the term solar twin is a bit misleading. In astronomy, a solar twin is a star that appears physically similar to the Sun, regardless of whether they share a common solar origin. In this case, since HIP 102152 is 4 billion years older than the Sun, they couldn’t possibly have formed from the same stellar nursery. Perhaps a better term would be solar doppelganger.

HIP 102152 is physically similar to the Sun in two ways. The first is that its mass is almost identical (about 97% the mass of the Sun). The second is that its metal abundances are similar to those of our Sun. When astronomers speak of “metal,” they mean element other than hydrogen and helium. The reason stems from the fact that most of the universe is made of these first two elements, and all the other elements are much, much less common.

Through careful observations of the line spectra of the star, the team made precise determinations of the abundances of twenty of the more common elements. They found that with a few exceptions the abundances were within about 5% of the Sun’s values. This means HIP 102152 is chemically very similar to the Sun.

With a similar mass and chemical makeup, one would expect HIP 102152 to evolve in a similar way to the Sun. But this star is on the edge of entering its red giant stage, so we know its about 4 billion years older than our Sun. As such it gives us a glimpse of our Sun’s possible future, in much the same way that a 30-something person could look at a 60-something person of similar appearance. This is why solar twins are useful. They allow us test our understanding of the evolution of our Sun.

One aspect this particular paper focused on was the level of lithium in the star. Lithium (the third lightest element) existed in trace amounts in the early universe, so you would generally expect that stars would have a similar level. But some stars, such as our Sun, have much less lithium than expected.

We know that lithium is consumed in the cores of stars as other, heavier elements are formed (a process known as lithium burning). HIP 102152 has significantly less lithium present in its atmosphere than even our Sun. This, combined with observations of other solar twins seems to indicate a correlation between the amount of atmospheric lithium in a sun-like star and its age.

If this correlation holds up, then it would provide a useful tool in determining the age of stars. Main sequence stars can be difficult to age because they are very stable. It is similar to trying to determine the age of someone between 30 and 50. By observing atmospheric lithium, we can better gauge where a sun-like star is in its life cycle.

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In the early moments of the universe, hydrogen and helium were formed through a process known as baryogenesis. Trace amounts of other elements such as lithium were also produced, but none of the heavier elements. This means that the first generation of stars were composed of hydrogen and helium, and it is only through fusion in their cores that the heavier elements we see today were created. The carbon, oxygen and iron in our bodies was produced through that process, which is why it is often said that we are star stuff.

Because heavier elements are only produced in stellar interiors, one way we can determine the age of a star is by the amount of these elements in its atmosphere, what is known as its metallicity. Younger stars will tend to have a higher metallicity, since they tend to form from the dust of earlier stars. Because of this, there has been a great deal of effort to discover very low metallicity stars. We have found some stars with quite low metallicity, indicating that they are second generation stars.

There has also been a search for first generation stars, but this has not been successful. Many of these stars would likely have been quite large, and therefore would burn through its hydrogen rather quickly, dying in a tremendous supernova explosion. One of the unanswered questions about these first generation stars is just how large they could get. The larger a star is, the shorter its lifetime. The general consensus is that first generation stars could have been as large as 200 – 300 solar masses, but proving that is a challenge.

Now a new paper in Science has found evidence to support this idea. The team looked at a low metallicity star known as SDSS J0018-0939. It is a dim orange star that seems to be a second-generation star. They looked at the abundances of 18 different elements within the star’s spectrum, and found that elements with an even atomic number (such as titanium and nickel) are more abundant than elements with odd atomic numbers (such as scandium and cobalt). This suggests that its progenitor star had some dying mechanism to produce more even elements than odd.

But this is something you would expect to see in the death of supermassive stars. When very large first-generation stars had fused most of their hydrogen into helium in their cores, they would then collapse quickly under their own weight. This would trigger a very large supernova (hypernova) known as a pair-instability supernova. In such a supernova, the gamma rays produced by nuclear collision are so energetic that they decay into electrons and positrons. These interact less strongly with helium, allowing more helium nuclei to fuse to produce heavier elements.  Since helium has an atomic number of 2, more even elements would be produced.

Simply finding higher even/odd element ratios in a single star isn’t enough to prove that first generation stars were quite large, but it does show that the idea is feasible. We’ll have to look at more second-generation stars to see if the idea truly holds up.

Paper: W. Aoki, et al. A chemical signature of first-generation very massive stars. Science, 345 (6199): 912-915 (2014).

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The seven sisters, also known as the Pleiades, is an open cluster of stars, meaning it is a loosely bound star cluster.  It is distinctive in the sky because its brightest stars are brilliant blue stars.  At least six stars in the cluster are easily seen with the naked eye. With a pair of binoculars or a small telescope, you can see hundreds of stars in the cluster, which makes it an excellent object for amateur viewing. 

The Pleiades is a fairly young cluster, which actually makes it a challenge to date accurately. One way to determine the ages of star clusters is to look at the color and brightness of its stars with what is known as a Hertzsprung-Russell diagram. But this can overestimate the ages of young bright stars. Another way is to look at the amount of lithium in the spectra of small cool stars in the cluster.  Since lithium is consumed in such stars over time, the amount of lithium remaining gives an indication of stellar age.  These two methods combined give the age to be about 120 million years.

Computer simulations of stellar motion within the cluster indicated that in its youth it was likely a stellar nursery similar to the Orion Nebula. This means the Orion Nebula and the Pleiades can be seen as two snapshots of stellar formation. Since the Pleiades is only loosely gravitationally bound, it will eventually be ripped apart by the differential gravity of the Milky Way.  Over the next few hundred million years, the stars of the cluster will smear out, and the stars formed in the cluster will make their own way through the galaxy.  The same thing happened to the Sun’s stellar nursery billions of years ago.

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Recently I wrote about the average color of the universe, as determined by a survey of more than 230,000 galaxies. While knowing the overall color of these galaxies is a fun little factoid, it isn’t particularly useful from a scientific standpoint. However the color was determined by the average spectrum of the galaxies, which is quite scientifically useful. This “cosmic rainbow” tells us about the history of star formation in the universe.

You can see the average spectrum in the figure below. One of the things you’ll notice is that isn’t simply a continuous spectrum. There are wavelengths that are particularly bright or dark. These are emission (bright) or absorption (dark) lines that are particularly common for the galaxies. Several of the lines are labeled by the element that causes the line. By looking at the relative brightness of these lines, we can determine the relative abundances and temperatures of typical stars. This is because young stars have hotter atmospheres than older stars, so the emission and absorption lines of a star changes over time.

Credit: Karl Glazebrook & Ivan Baldry

Given this average, you can fit it to models of historical star formation. If most stars formed earlier in the universe, then the line spectra would resemble older stars, since most of the present stars would be older. If instead stars formed at a fairly constant rate, then you would see much less bias toward older stars.

What we find is that stars haven’t been produced at a continuous rate within the universe. Instead, there was a peak of star production between 6 and 10 billion years ago, and that the rate of production has been declining ever since. Most of the stars we observe are more than 5 billion years old. New stars are still being formed, but the level of star production is nothing like what it was. The universe has shifted into middle age.

Of course this just further supports that the universe began with the big bang. A peak of star production is exactly what you’d expect in a universe that begins with raw hydrogen and helium, with each generation of stars releasing some material back into the wild via supernovae and the like, but locking part of the material into red dwarfs, neutron stars and the like.

Hints of the big bang in a cosmic rainbow.

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