The Star That Exploded At the Dawn of Time

sciencehabit writes To probe the dawn of time, astronomers usually peer far away; but now they've made a notable discovery close to home. An ancient star a mere thousand light-years from Earth bears chemical elements that may have been forged by the death of a star that was both extremely massive and one of the first to arise after the big bang. If confirmed, the finding means that some of the universe's first stars were so massive they died in exceptionally violent explosions that altered the growth of early galaxies.

Comments

Why can't hydrogen cool?

[ShanghaiBill]

From TFA:

The big bang produced only hydrogen, helium, and a little lithium, and gas clouds containing only these elements can't cool.

Can someone please explain this? What would prevent a cloud of primordial elements from cooling?

Re:Why can't hydrogen cool?

[tysonedwards]

It's deceptive. At this point in time, the universe was quite small relatively speaking. As such, the density of those materials was still sufficiently high that the materials were in a persistent plasma state even though they weren't a "star", however fusion was still taking place converting Hydrogen to Helium. At this point, we're talking about all matter in the Universe occupying something not much larger than the Milky Way after all, maybe a little larger considering that we're talking about very, very fast expansion of the universe and the difference of a second amounts to a light year or so of growth. As the Universe continued to expand outwards and the material became less dense, it allowed for the material to actually split apart from one giant clump of hydrogen, helium and lithium and begin to actually get some empty space. As that empty space formed, then this plasmatic cloud could coalesce into the first stars. As long as there was these plasma clouds and not "empty space", then yes, there was "nothing" yet in which the material could *cool* into.

Re:Why can't hydrogen cool?

[ShanghaiBill]

I appreciate your explanation, but honestly, what you say makes no sense. To be blunt, I don't think you know what you are talking about. You say that the Universe was the size of the Milky Way, and expanding by a light year per second. Since the Milky Way is only 120,000 light years across, if the Universe was really expanding that quickly, it would be bigger than the Milky Way in ONE DAY. You also say that hydrogen fusion was occurring, but according to this graph, fusion stopped three minutes after the big bang. There is nothing that you say that would only apply to H-He-Li and would not apply to heavier elements.

I found the following explanation here:

Hydrogen and helium are, by far, the most abundant elements in interstellar clouds. However, these elements are very poor coolants because they cannot be collisionally induced to emit photons at the low gas temperatures characteristic of molecular clouds. Two decades of theoretical studies have consistently predicted that a large fraction of the total cooling is borne by a few other atoms and molecules, notably gaseous water (HO), carbon monoxide (CO), molecular oxygen (O), and atomic carbon (C).

Re:Why can't hydrogen cool?

[mmell]

It can - now. Back then, where would the heat go . . . out of the Universe?

Re:Why can't hydrogen cool?

[stevelinton]

Basically the conditions (temperature, density, amount of ionizing radiation around) thought to apply, the gas would be made up of atoms that tend to simply bounce off one another when they collide. This doesn't change the total energy in random motion of the cloud, ie the temperature.

More complex atoms or molecules can interact in more complicated ways when they collide, so that part of the energy ends up as vibration in a molecule, or extra energy of an electron in an excited state. These vibrating molecules or excited atoms then relax back to their ground state releasing a photon and so actually cooling the cloud.

Re:Why can't hydrogen cool?

[mdsolar]

The universe cools as it expands. Once the background radiation is cool enough then the heat of contraction can dissipate. Initially, growth of structure in the universe happens only in dark matter because the normal matter smooths out destiny fluctuations. But after recombination, the normal matter begins to catch up. http://books.google.com/books?...

Population II stars and globular star clusters

[torsmo]

Why are they present mostly in the galactic halo? Also, stars like SM0313 are supposed to have formed only a 100mn or so years after the Big Bang. Is that enough time for Population III stars to have formed and gone supernova?

I took this for granted

[Anonymous Coward]

Big Bang -> a few ridiculously massive stars -> a few little Bangs -> many massive stars -> many hypernovae -> many many large stars -> many many supernovae -> ... -> ... -> Michael Bay

It's explosions all the way down.

Pair-instability Supernovae

[Rich0]

So, I don't get why pair-instability supernovae happen in the first place, and the Wikipedia article certainly isn't helping.

The argument is that at some point a star gets hot enough that its photons start creating electron/positron pairs, and this causes a collapse. Then that collapse leads to a runaway nuclear reaction.

What I don't get is how the star would ever become supercritical from a nuclear reaction perspective. I get that there might be some kind of transition in pair creation as the whole star gradually increases in temperature and perhaps a large portion of the star crosses some threshold temperature at the same time. I get that this could reduce pressure in the star and allow it to start collapsing.

However, while the transition in pair-creation behavior might happen quickly (this is an event at the quantum level), the collapse of the star involves huge masses of gas falling inwards at macroscopic speeds. I don't see how the migration of a few dozen sun's worth of H/He towards the core is going to happen at a rate anywhere near the rate at which individual H/He atoms are colliding throughout the core. So, if the density started to rise such that you got an increase in nuclear reactions, wouldn't that create additional pressure that stops the collapse?

For there to be an explosion you need to build up potential energy of some kind and then release it all at once. If you light C4 with a match it just burns from the surface which doesn't lead to a dramatic release of energy. If you send a shockwave through it that travels faster than the speed of sound in the medium then the initiation of combustion of the C4 propagates faster than the shock wave produced by the combustion, and as a result the energy of the entire explosive is released seemingly at once. Likewise if you ignite a cloud of pure hydrogen surrounded by normal air it will just burn at the surface hotly, but if you premix hydrogen and air to a stoichiometric mix and light a match, it will detonate, because in that case the ignition will naturally propagate faster than the speed of sound (and is not limited by the diffusion of oxygen/hydrogen to allow mixture).

Nuclear reactors don't explode, because they aren't significantly supercritical - they stay near equilibrium. A nuclear bomb reaches supercriticality because for a very short moment in time the inertial of the collapsing fissile mass allows it to continue to collapse before the energy produced by the initiating chain reaction can blow it apart.

Is that the case for these supernovae? Does it take long enough for the nuclear reactions to start that the mass of the falling gas has enough inertia to allow it to continue to compress even after passing the critical point?

Re:Pair-instability Supernovae

[mdsolar]

This may help: http://articles.adsabs.harvard...

Pair Instability Super Novae

[mdsolar]

The instability that causes the collapse of a stellar core and subsequent explosion comes from turning gamma rays into pairs of electrons and positrons. This turns energy into matter and cuts the pressure that the energy provides. http://en.wikipedia.org/wiki/P... It turns out that these explosions may make observing the early universe easier. One of the most important abundance ratios is the interstellar medium is the ratio of oxygen to carbon. The strength of the carbon monoxide bond is so strong that these two really pair up. Whichever runs out first determines the remaining chemistry to a large degree. Mass losing carbon rich stars produce carbon rich dust, while mass losing oxygen rich stars produce silicate dust for example. But, primordial Pair Instability Super Novae may produce lots of oxygen with little carbon or silicon to combine with. So the very early solid phase of the ISM may be mostly water ice. This happens to increase the far infrared emissivity of this solid phase making early objects brighter in the red-shifted sub-millimeter. Thus very early object may be easy to find in surveys at that wavelength. http://iopscience.iop.org/0004...

Re:"expanding by a light year per second"

[HiThere]

You sure about that? I was under the impression that it was probably STILL expanding faster than light, if measured from edge to edge (which, of course, you can't do, but can only calculate). In fact I was under the impression that it was believed that many areas of the universe may still be rapidly inflating, just not around here.

FWIW, (and in my understanding) there are theoretical reasons to believe that inflation will continue forever. They're just beyond our light cone. Perhaps this "problem" has been generally agreed to be resolved, but if so I haven't heard about it.

OTOH, I am not a cosmologist.