Category Archives: AAS meeting stories

The First Stars and Ancient Gas Clouds

Tracing Chemical Elements in the First Stars

first stars and gas cloud
A simulation of the first stars in the universe, showing how a nearby gas cloud might have become enriched with heavy elements. Courtesy Swinburne University.

I continue to be fascinated by the existence of the first stars in the universe. What were they like? How long did they live? How can we spot evidence of them? These are all questions astronomers who study the universe continue to ask and try to answer.

At this meeting, we heard from astronomers who are searching out evidence of those first stellar objects. They’re not exactly easy to spot. You can’t really just stare hard across more than 13 billion light-years and see these things. They’re too dim, too far away, and hidden behind clouds of gas and dust. It seems like that would be the kiss of death for learning anything about those stars, but instead, astronomers found a way to turn that “bug” into a feature by performing spectroscopy on the cloud (studying the characteristics of star light after it passed through the cloud).

Neil Crighton and Professor Michael Murphy from Swinburne University of Technology in Melbourne, Australia, and Associate Professor John O’Meara from Saint Michael’s College in Colchester, Vermont, U.S., studied the light from distant stars after it filtered through a distant cloud of gas that lies in the line of sight between us and the stars. It turns out the cloud may bear the signature of those very first stars and tell astronomers something about them and the lives they led.

What Light Filtered through a Cloud Reveals

The distant gas cloud is billions of light-years away from us, and appears as it looked 1.8 billion years after the Big Bang. That’s about the time the first stars were forming.  The cloud has an extremely small percentage of heavy elements. These are carbon, oxygen and iron. It turns out that’s less than a thousandth the fraction of the same elements observed in the Sun, which was born a few generations AFTER the first stars, in a universe that was significantly richer in heavy elements.

So, what’s the deal about the lack of heavy elements in a distant cloud in the early universe? Crighton explained it. “Heavy elements weren’t manufactured during the Big Bang, they were made later by stars. The first stars were made from completely pristine gas, and astronomers think they formed quite differently from stars today,” he said. We can thank those stars for their role in the enrichment of the universe by heavier elements.

The first stars — which astronomers call “Population III stars” — were born as quite massive stars out of the original hydrogen supplies in the infant universe. Then, they did what stars do — fuse hydrogen in their cores to form helium. Then, they went on and fused heavier and heavier elements — the first stars to do so.

Then, those stars began to die in supernova explosions, which spread their heavy elements into surrounding pristine clouds of gas, infusing them with an unmistakeable chemical record of the first stars and the “star stuff”  their deaths hurled into space. Astronomers read that record like detectives study a fingerprint at a crime scene.

The way they do it is to study the light that passes through the clouds. Different elements absorb different wavelengths of light, and we see those as “dropouts” in the light. Those dropouts tell you what elements are in the cloud, and since they’re heavy elements, they had to have come from the nearby stars that lived, died, and spread their carcass materials through space.

“Previous gas clouds found by astronomers show a higher enrichment level of heavy elements, so they were probably polluted by more recent generations of stars, obscuring any signature from the first stars,” Dr. Crighton said. That makes it the first cloud to show the tiny heavy element fraction you would expect to see in a cloud enriched by the first stars.

What’s Next?

AS I mentioned in a previous article, the best science requires more than one example of something to study. That would be true with these distant clouds and the story they tell of the first stars. So, astronomers want to find more of these systems, where they can measure the ratios of several different kinds of elements. By finding new clouds with traces of the more heavy elements, astronomers can trace the existence of more early stars and track how they began to enrich the infant universe with elements that got used later to make the next generations of stars, form planets, and seed Earth (and hopefully other worlds) with life.

Looking for Eta Carinae Twins in Distant Galaxies

If There’s One, There are Probably Others

eta carinae from HST
A huge, billowing pair of gas and dust clouds are captured in this stunning NASA Hubble Space Telescope image of the supermassive star Eta Carinae. Courtesy NASA/ESA/STScI

I didn’t get to write about this yesterday, what with all the other exciting astronomy results being thrown at us this week.  But, there’s a big, massive, bright, dying star out there that’s going to blow up.  It’s called Eta Carinae, visible from Earth’s surface from the Southern Hemisphere. It’s so fascinating, astronomers want to know if there are any others like it out there.  So, they looked through data from Spitzer Space Telescope and Hubble Space Telescope to see if any are hidden away in other galaxies.

Eta Carinae is actually two stars in a 5.5-year orbit. The larger, brighter one (the primary) is a luminous blue variable that started out its life with at least 150 times the mass of the Sun. Over time, it has lost the equivalent of about 30 Suns. That’s the monster star which will blow sometime soon. By that, I mean, it will explode as a class of supernova so huge and powerful and bright that astronomers call it a “hypernova”.

Once it does blow up, essentially “any time now”, Eta Carinae will appear incredibly bright in our sky. It would be much brighter than Venus, but  it’s not likely we’d suffer any ill effects from it, due to distance.

Eta Carinae lies some 7,500 light-years away and it suffered an eruption in the 1840s that caught everyone’s attention. At that time, it blew off about ten Sun’s worth of material off at that time. What we see now as a sort of “dumbbell” shape is that ejecta traveling out to space. Add up the massive star, the expanding material, and the known evolution of this thing, and you’ve got a one-of-a-kind object that continues to fascinate astronomers.

Are There More of them Out There?

In astronomy and science in general, we don’t like to see just “one” of anything. While our Sun is special to us, there are more stars like it. The more sunlike stars we study, the more we understand them (and our star) as a “type” and where they fit into the pantheon of all possible stars. It’s the same with Eta Carinae. It’s fascinating, but if there’s one, there should be others. And, they can tell us more about the type of stars Eta Carinaes are.

It turns out there are others out there.  A study using archival data from Spitzer and Hubble found five similar objects in other galaxies. And that’s good.

Why Look for more Members of the Eta Carinae Family?

As one of the nearest laboratories for studying high-mass stars, Eta Carinae has been a unique, important astronomical touchstone since 1840s activity. To understand why that eruption occurred and how it relates to the evolution of massive stars, astronomers needed additional examples. Catching rare stars during the short-lived aftermath of a major outburst approaches needle-in-a-haystack levels of difficulty, and nothing matching Eta Carinae had been found prior to the discovery of others in the Hubble and Spitzer data.

Finding Eta Carinae Twins

Dust forms in gas ejected by a massive star. This dust dims the star’s ultraviolet and visible light, but it absorbs and re-radiates this energy as heat at longer, mid-infrared wavelengths. Those “signals” show up in Spitzer data, and could be matched with objects seen in Hubble observations. But, first astronomers had to find candidate objects.

eta carinae candidates in other galaxies
Researchers found likely Eta twins in four galaxies by comparing the infrared and optical brightness of each candidate source. Infrared images from NASA’s Spitzer Space Telescope revealed the presence of warm dust surrounding the stars. Comparing this information with the brightness of each source at optical and near-infrared wavelengths as measured by Hubble, the team was able to identify candidate Eta Carinae–like objects. Top: 3.6-micron images of candidate Eta twins from Spitzer’s IRAC instrument. Bottom: 800-nanometer images of the same sources from various Hubble instruments.
Courtesy NASA, ESA, and R. Khan (GSFC and ORAU)

An initial survey of seven galaxies from 2012 to 2014 didn’t turn up any Eta twins, which meant they must be rare, indeed. It did, however, identify a class of less-massive and less-luminous stars of scientific interest, demonstrating the search was sensitive enough to find objects like Eta Carinae, had they been present.

In a follow-on survey in 2015, astronomers found two candidate Eta twins in the galaxy M83, located 15 million light-years away. There were also likely Eta Carinaes lurking in NGC 6946, M101, and M51, located between 18 million and 26 million light-years away. These five objects mimic the optical and infrared properties of our galaxy’s Eta Carinae, indicating that each very likely contains a high-mass star buried in five to ten solar masses of gas and dust.

While astronomers study those new candidates more fully, our own Eta Carinae continues to tick along, approaching its hypernova stage. It’s going to be magnificent when it goes.

One question I get from people is, “Has it already blown up?”

That’s entirely possible, and we just haven’t seen it yet. After all, it IS 7,500 light-years away, so if it has already died, there’s a bright surprise waiting us in the future.  You can read more about this finding at the Hubble Space Telescope Web site.

Stay tuned!