Category Archives: astronomy research

Getting to Know the First Stars

They Lived Fast, Died Young, but Weren’t Lonely

I’ve long been fascinated with the earliest objects in the universe. And, we live in a time of astronomical discovery and research when scientists are getting closer to “seeing” the earliest objects in the cosmos and understanding how they formed. No they aren’t actually “seeing” the first stars in high-resolution. But, they can use what they do know about the early universe — including the abundances of the elements hydrogen and helium — to come up with very good computer simulations of what was happening back then. Those simulations are causing them to rethink the idea that the first stars were massive, lonely giants.

Let’s start at the beginning… the REAL beginning.

The standard story of the universe starts with the Big Bang — an event that heralded the creation and expansion of the universe.  A few million years after this event, the universe was not something that we could detect with our eyes (if we’d been there at the time).  It was a smooth, uniform mass of material expanding outward.  There were a few fluctuations (variations) in the newborn universe’s temperature and density (that is, the amount of material it had and how it was distributed). If you were there, you would sense only a dark existence — a time called the “Cosmic Dark Ages”, where the “stuff” of stars and galaxies was still a sort of “amorphous” blob. The only radiation that existed was shifted by the expansion of this new universe into wavelengths was what we can detect from OUR point in time as infrared radiation.

That changed when the first stars coalesced out of this “stuff” and began to shine. They gave off ultraviolet and visible light, lighting up the universe. They ushered in a time called the “Epoch of Reionization”.  To put it simply, the infant universe got “lit up” by the first stars.

So, what were those first stars like?  For a long time, astronomers have theorized that they were supermassive, hot, and lonely — meaning they were not clustered together close together in space. However, that picture is changing slightly.

According to some very high-end computer simulations created by astronomers at the University of Texas at Austin and the Center of Astronomy at Heidelberg University, the Max-Planck Institute for Astrophysics in Garching (Germany), those early stars may actually have formed with stellar companions in their protostellar disks.

What’s a protostellar disk?  It’s the nebulous cloud of gas that is set in motion as a giant swirling disk. Eventually the central region in the disk gets hot and dense enough that the nuclear reactions that power stars switch on. The gravitational pull of the stuff in the center wants to suck in more material, but the heat of the stuff in the center is trying to escape. This is true of stars being born today and it was true of the births of the first stars.

A supercomputer simulation of the birth of a primordial star. A spiral pattern forms inside the disk surrounding the star, leading to enhancements in density. One of these density perturbations is large enough to trigger the formation of a secondary protostar. Distances are measured in astronomical units (AU), which is the distance between Earth and our Sun. Credit: Clark, Glover, Smith, Greif, Klessen, Bromm (Univ.of Heidelberg, UT Austin); Texas Advanced Computing Center

However, in some senses, primordial star formation was a very different process. To be sure — there was still that push-and-pull action between the gravitational attraction pulling the star-forming gas together, and thermal energy trying to push it apart. But, for the early stars, things were a bit different during the formation process.

As gravity squeezed the material, the gas heated up. For gravity to win, the gas needed to “lose” the extra heat produced during the collapse. This was more difficult for gas in the early universe than in galaxies like our Milky Way today. This is because when the universe was first formed, its gas did not contain elements such as carbon or oxygen, which cool the gas and make it easier to collapse.  Stars that are born “today” in clouds of gas and dust are rich in the “cooling” elements.

This “lack of cooling elements” was one reason give why astronomers thought thought that primordial stars were solitary massive objects. However, the calculations by the teams in the U.S. and Germany demonstrate that this simple picture needs considerable revision due to the physics of the disks that build up around primordial stars as they form.

We know that the disk around the young Sun that fragmented to build up the planets in our solar system. As it turns out, the accretion disks that formed around the first stars were also found to be highly susceptible to fragmentation.  But, instead of forming planets — since the heavy materials to form worlds didn’t exist — they formed additional stars.

So, instead of forming in isolation as massive single stars, some of the first stars seemed to have formed as members of multiple stellar systems, with separations as small as the distance between Earth and the Sun.  At the end of the birth process for these early stars, it’s far more likely that a massive double star would emerge.  The pair would produce high-energy photons. As they aged, they would produce some of  the first heavy chemical elements — like carbon and nitrogen.

Of course, there were also massive singleton stars formed in the early universe. These super-supermassive stars weren’t destined to live long — similar to massive stars that exist today. They spent their short lives creating heavier and heavier elements in their cores — and just as with supernovae today, the first massive stars that died as supernovae back in the early days of the universe spread those heavy elements out to the expanding cosmos. Those elements are crucial for the formation of the next generations of stars — and planets — and life. So, think of the first stars as instigators of the ultimate recycling processing in the universe.

The binary nature of the first stars opens up exciting possibilities for detecting them. Astronomers can search them out in  hyper-energetic gamma-ray bursts, or through the strong x-ray radiation they give off as they evolve and die.

The search for and study of the first stars is another major step in understanding just when the Cosmic Dark Ages ended and when actual “First Light” commenced with the births of the earliest stellar objects.

There’s more information about this first stars simulation at the McDonald Obeervatory website.

The Universe Keeps Comin’ At You

Delivering All Kinds of Goodies to Study

Yesterday I talked about how all time is local, in essence, and that the universe’s activity flows along the lines of time.  We get to divide the time into units that humans can understand, but the universe itself doesn’t care about how WE divide it up. It just keeps doing its thing.

One of the things it just keeps on doing is showing us new objects. The universe’s structure is divided into large-scale structure — like galaxy clusters and superclusters.  These stretch across huge expanses of space. In a given supercluster, light from one “end” of the cluster might travel hundreds of millions of light-years to reach the other “end”. They’re huge.

Small-scale structure “lives” inside the large-scale structure. This would be stuff like individual galaxies, globular clusters, stars, and planets.  Humans are at the “micro structure” end of things, comparatively speaking. As we humans keep extending our gaze out beyond our home planet, we keep finding new variations on the “stuff” of the universe. Take planets, for example.  Throughout most of OUR history, we only knew about the planets we could spot from Earth’s surface using our trusty Mark I eyeballs.  So, we had Mercury, Venus, Mars, Jupiter, and Saturn. That lasted until astronomers invented telescopes and aimed them to the skies — thereby allowing us to extend and magnify our vision.

An artist's concept of an exoplanet called WASP-12b. It's the hottest known planet in our galaxy and probably won't last too much longer. Its star is devouring this world. ravitational tidal forces from the star stretch the planet into an egg shape. The planet is so hot that it has puffed up to the point where its outer atmosphere spills onto the star. An accretion bridge streams toward the star and material is smeared into a swirling disk. The planet may be completely devoured by the star in 10 million years. The planet is too far away for the Hubble Space Telescope to photograph, but this interpretation is based in part on analysis of Hubble spectroscopic and photometric data. Credit NASA, ESA, Greg Bacon.

Today we know of eight planets in our own solar system, dozens of dwarf planets, and — astonishingly to folks who haven’t followed all the latest news — HUNDREDS of planets around other stars.

Last year, I reported from the American Astronomical Society about the first planets found by the Kepler satellite. This year, there will no doubt be more discussion of MORE planets found, followed up by discussions of the characteristics of those planets. They range from supersized hot worlds to places only a bit larger than planet Earth. Kepler is specifically scanning the sky looking for the light signatures of planets that are roughly the same size as Earth. In the process, it has found more than 700 planet candidates and confirmed eight of those candidates as actual planets.

And, the search for planets isn’t limited specifically to Kepler. Ground-based surveys are turning up new planets all the time — and in fact, the first exoplanet was discovered in ground-based data.

This continual discovery of new planets, most in the Earth’s neck of the galactic neighborhood bodes well for the existence of worlds elsewhere in the galaxy — and beyond. That we haven’t seen them yet doesn’t matter.  We will.

Beyond the search for other planets astronomers are stretching our collective vision out to study other stars, gathering data that helps them understand how stars work, how they form, how they live, and how they die. In the process, they also learn about how our own star was formed — and the “stuff” of the interstellar medium that is necessary for the formation of stars and planets. This also helps them study galaxies, and the influences that these stellar cities have on the stars and planets and nebulae — and black holes — that populate them. Every discovery uncovers something new that we didn’t know about the universe before. Each finding adds to our collective store of scientific knowledge. What we find is limited only by our ability to see and understand what is discovered. I don’t know about you, but I find this quite exciting. Our universe will keep throwing this stuff at us; it’s our job to step up, take it in, and add to our scientific treasury.