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.

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.