The farther backwards you can look, the farther forwards you are likely to see. -- Winston Churchill
Sometimes, we point our most powerful telescopes at the sky, peering as deeply as we possibly can, hoping to shed some light on what the Universe was like oh-so-long ago, as close to the big bang as we can. The Hubble Space Telescope can get us distant galaxies as they were just a few billion years after the big bang.
But Hubble still has never seen one of the elusive, Holy Grails of astronomy: a metal-free star.
You see, immediately after the big bang, the Universe was filled with protons and neutrons, which finally fuse together (when the Universe cools enough) to create hydrogen, helium, and lithium nuclei. A few hundred thousand years later, the Universe cools enough to turn these nuclei into stable, neutral atoms. But that's it: beyond those three elements, there's nothing heavier. The Universe can't make them, not until the first stars form.
And someday, that's what we'd love to find: a metal-free star. These first stars, without any traces of heavier elements, are responsible for exploding and enriching (or polluting, depending on your perspective) the surrounding space with elements much heavier than lithium. Well, we just determined that a Gamma-Ray Burst earlier this year shattered the distance record:
At a redshift of eight, it's the most distant object ever discovered. This was light emitted around 13 billion years ago, when the Universe was less than one billion years old. And yet, looking at the spectrum of this one, it's still full of heavy elements!
Why?
When you form stars, anywhere, you make many, many little, low-mass stars, like red dwarfs. But you make a few very high-mass stars, called O-type or B-type stars. These stars are huge. Compared to a G-type star like our Sun, there's simply no contest.
Huge! Well, there's a big problem with being huge. What is it? Let's ask Bladerunner:
The light that burns twice as bright burns for half as long - and you have burned so very, very brightly, Roy. Look at you: you're the Prodigal Son; you're quite a prize! --Dr. Tyrell
Bladerunner got the scaling wrong: the star that burns with twice as much mass lives only one-eighth as long! So if a star like our Sun lives for 10 billion years, a star 10 times as massive lives for only 10 million years, and one 100 times as massive lives for just 10,000 years!
So that's why this gamma-ray burst we've found, despite being only an estimated 630 million years into the birth of the Universe, is still chock-full of these heavier elements.
In fact, instead of a redshift of eight, we'd have to get all the way out to a redshift of around forty before we expect to start seeing a metal-free star. And the continued observation of this Gamma-Ray Burst confirms that, despite occurring 95% of the Universe's lifetime ago, the Universe was very, very similar then to the way it is now. The same stars, the same stuff, the same explosions as the ones we see now. There's never been a more distant, more comforting observation than this, that tells us pretty much exactly what we expected.
Gamma-Ray Bursts come from stars that die in very certain ways, and this one, from 13 billion years ago, is just like the ones that happened recently -- and close -- to us. By finding very few differences, one amazing piece of the picture comes into view: the Universe looked a lot like it does now a very short time after the big bang. That we can see things when the Universe was only 5% of its current age is like me looking back on my life and remembering everything that happened when I was 18 months old. Only, I wasn't able to do all the things I can do now back then. But the Universe can, and did, and now we've seen the first pieces of evidence for that! So thank you to that massive star that died all those billions of years ago. It's shown us that the Universe grows up very, very quickly!
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It isn't immediately apparent from the article, but - is the existence of a star that is composed of only hydrogen, helium, and lithium feasible? Wouldn't they start fusing, say, beryllium nuclei, if the correct conditions were present almost immediately upon starting the fusion process? What length of time is it from when hydrogen is first fused into helium to formation of heavier atoms?
Katharine: No, the fusion of helium to heavier elements does not happen right away. Stars start out by fusing hydrogen into helium, because that requires lower temperatures and achieves higher energy output than fusing heavier nuclei. (Fusing helium requires a three-body collision: there are no stable nuclei with mass 5 or 8, so you have to skip directly to carbon-12.) Once fusion starts, the energy it releases stops the collapse of the core. Only when the hydrogen in the core is exhausted do you start fusing heavier elements, because the core collapses until you get temperatures high enough to fuse helium; meanwhile, hydrogen fusion continues further out. (In stars with masses comparable to or less than the Sun's, this step never happens, and the star turns into a white dwarf.) For sufficiently massive stars, this process moves up the periodic table until the star forms an iron core. Iron-56 has the highest binding energy per nucleon of any nucleus, so no further energy is available from fusion, and the subsequent collapse produces a supernova. It is only when you get the supernova that these heavier elements are released (and some of the energy of the explosion goes into forming elements heavier than iron).
Okay, that I understand, particularly the bit about unstable nuclei. I know it takes a certain amount of time to start forming heavier metals such as iron-56, and the time before formation of metals varies with the size of the star.
I suppose what I'm trying to ask is - what conditions would be necessary for the technology we have to find a star of sufficient size containing only hydrogen, helium, and lithium?
Unless things have changed greatly in the last thirty years since I took any astronomy courses(and they have), I believe that the larger stars, say F and above, utilize the carbon cycle while the smaller stars, G and below, use the proton-proton cycle. I don't recall how much carbon is necessary percentage-wise of the stellar composition for this to work, but surely these very early stars would not be able to use this reaction. How would this affect their evolution? Would they have longer lifetimes, for example?
Besides the aforementioned redshift of 40.
Forgive my short knowledge about stars; I'm a biology student, not a physics student.
This gets to me more personally than my having been an Astronomy professor. I am one of the rare human beings who has had it repeatedly confirmed that I vividly remember some events that happened when I was 18 months old. Most people cannot, and one popular theory is that one's neural operating system changes around that time. Which gets my to something I've said in peer reviewed Quantum Cosmology publications. It is NOT valid to assume that random distributions in the cosmic past, the present, and the cosmic future are the same at all. The key word (common in Mathematical Economics, but not yet in Cosmology) is Heteroscedasticity. In statistics, a sequence of random variables is heteroscedastic, or heteroskedastic, if the random variables have different variances. The term means "differing variance" and comes from the Greek "hetero" ('different') and "skedasis" ('dispersion'). In contrast, a sequence of random variables is called homoscedastic if it has constant variance.
Katharine: also, the hydrogen fusion destroys the lithium. This is important in understanding Brown Dwarfs. And in modeling how much Lithium remains behind from the Big Bang. Am I right, Ethan?
what conditions would be necessary for the technology we have to find a star of sufficient size containing only hydrogen, helium, and lithium?
The window for forming such stars would have been fairly small. As Ethan mentioned in the post, a star with 100 times the mass of the sun would exhaust its hydrogen fuel in about 10,000 years. Fusing heavier nuclei would extend that a bit, but less than a factor of two. As it happens, the largest stars we have ever seen are right around 100 solar masses. Assuming that that is the largest star size that can form, we see that metals should have started to appear about 10,000 years after the start of star formation, which (if memory serves) would have begun several hundred thousand years after the Big Bang. Of course, you can get this sort of (as Marvin the Martian would say) earth-shattering kaboom with a star smaller than 100 solar masses--IIRC 10 solar masses is big enough, giving you a window of about 10 million years for the necessary (but not sufficient; you can also get such stars to form with the debris from an earlier supernova) conditions to have a metal-free star go supernova. I presume Ethan's estimate of a redshift of 40 corresponds to when this window closes.
ScentOfViolets: The lack of a carbon cycle should not have a big effect on stellar lifetimes. The fact that stellar lifetimes scale as M^-3 comes from the fact that the temperature a star must have to maintain equilibrium between gravity and pressure (and thus the amount of power it has to produce) scales as M^4, and of course the amount of fuel available is proportional to M. You get the same amount of energy out of converting four protons to a helium-4 nucleus plus two positrons plus two neutrinos whether you use the direct process or the carbon cycle (if there is a difference, it is because the star's temperature and pressure determine which one has the lower barrier).
Loved the use of the Tyrell quote.
Is it still possible we could find the supernova of such an early star? Is there a possibility that we are just too late for such observations and that the light of those super nova's already past us, dissipated beyond recognition or are outside the observable universe?
So, does that mean that the early universe was heavy on really big stars, and only very little hydrogen was getting "safely" stored away in slow burning small stars? How much hydrogen does a heavy star burn through before it explodes?
Here's my question:
What would an observer at that gamma ray burster see looking back along the same line of site towards us? Would they see what this region of the universe looked like 13 billion years ago? What would they observe?
Second question. What would that same observer see from their position observing along the same line of site in the same direction we observe them?
An essay like this by Ethan is quite excellent. In bringing together the idea of a relation between 40 redshift stars and metal-free stars; you have given an inquisitive layman such as myself information that can only be gotten from an inquisitive professional who takes the time to translate obscure data, observations and theory into intelligible information. Much thanks.
Of course I am still a skeptic. But today let me use the words of Roger Penrose another skeptic, "Nevertheless we are still stuck with the big bang and that also seems untidy. But here there appears to be no way out." Elsewhere and more recently Penrose sober and heretical continues, "What reason is there to believe that such an inflationary picture of the universe is likely to be close to the truth? Despite its evident popularity, I wish to give my own reasons for casting considerable doubt upon the entire idea!" Personally, I agree and I suggest that the phrase "apparent big bang", "apparent age of the universe" and other such "apparent's" be used.
The fact that there are no metal-free stars is one more observation that our "visible universe" whether viewed in stars in our galaxy or from stars in 13 billion year old galaxies is essentially the same. Let me make a prediction. Astronomers will never find a metal-free star; or, they will find them in young stars in young galaxies as well as the most distant galaxies.
Ethan, can you tell me. Have astronomers been able to identify young galaxies or proto galaxies and examine the protogalaxy's stars to determine if the youngest stars of a protogalaxy are of the same metal composition as the stars of a mature galaxy. Ideally, such a protogalaxy would be isolated from other galaxies by a great distances. I would predict that the early stars of a proto galaxy have less metal than the stars of a mature galaxy. This by the way would suggest such galaxies are formed of "new matter" (not "dark matter"); "new matter" being recently converted by some process from (I've speculated elsewhere). Based on your statement that, "So if a star like our Sun lives for 10 billion years, a star 10 times as massive lives for only 10 million years, and one 100 times as massive lives for just 10,000 years!"; I think I'd want to look at a protogalaxy that is less than 10 million years old. This is a serious question; are astronomers able to observe such protogalaxy and their stars in our near cosmic neighborhood (i.e. not more than 500 million light years)? A layman like myself can speculate; but my skeptical speculation is constrained by the data, observations and the bedrock theory of professional physicists and astronomers.
Epic Win for using the Tyrell quote.
Thomas@14,
I find it interesting that you want;
"such a protogalaxy would be isolated from other galaxies by a great distances."
and
"This is a serious question; are astronomers able to observe such protogalaxy and their stars in our near cosmic neighborhood (i.e. not more than 500 million light years)?"
I must admit to being skeptical of the basis for your skepticism. For instance, how can something be isolated if it is only 500 million light years away?
In answer to Thomas@14.
Wikipedia list the 114 nearest galaxies to the Milky Way. They vary from a distance of .025 million light years away from Earth to a distance of 11.5 million light years away.
Wiki also gives the dimensions of the Milky Way as .1 million light years in diameter and .001 million light years thick.
Thus if within 11 million light years there are 114 galaxies; I quickly estimate that within 500 million light years of Earth there are arounds 50,000,000 galaxies. By isolated I simply mean that they are like the black dots on a soccer ball, their material is essentially not interacting (no I'm not disputing clusters). Now amongst those 50,000,000 galaxies; I assume that there are also a large number of baby galaxies or protogalaxies forming (at least a 1000). And I also assume that these protogalxies are mostly forming in the white space of the soccer ball; not in the black space (i.e. not in the galaxies of our universe), (i.e. new galaxies form where there aren't currently galaxies).
Further, I do not assume that protogalaxies are composed of recycled material from stars and older galaxies. Rather in a string theory sense, I suggest the possibility that our visible universe is but a subuniverse of an 11-dimensional space time. And that our visible universe is part of a cosmic cycle of exchange of matter and energy between the other subuniverse which compose our 11-dimesional space time. And by the way if you believe in black holes; then it is not honest for you to dis-believe in higher spatial dimensions beyond the 3 of our visible universe (i.e. if matter inside a black hole leaves our 3-dimensional visible universe forever; then it must be in some kind of extra dimension). So protogalaxies could be formed from either recycled star matter or from newly minted matter entering our subuniverse from other subuniverses in a quantum gravity cosmic cycle pg 174.
Oh by the way let me quote Einstein, "As a result of this discussion, a most interesting question arises for astronomers and physicists, and that is whether the universe in which we live is infinite, or whether it is finite in the manner of the spherical universe... On the contrary, the results of calculation indicate that if matter be distributed uniformly, the universe would necessarily be spherical (or elliptical)." Here Einstein is talking about spherical Riemann space (i.e. a 3-sphere) not flat-euclidean space. Einstein's interesting question has been avoided by everybody since he posed it.
In order to proceed, it is necessary to be able to visualize 4, 5, all the way to 10 dimensional space plus 1-dimension of time; or it is necessary to be able to do the calculation in those higher dimensions. I suggest starting page 72 that it is easier to learn to visualize higher dimensions. Now even my mother can do it. And furthermore, without being able to visualize higher dimensions; the calculations are likely to be wrong (e.g. if we can't visualize the difference between an apple and an orange; then the answer of adding apples and oranges will probably be nonsense.)
Wow, 13 billion years ago in our reference time-frame. And no time at all for those shifty photons. I wonder if the universe will only be a hundred years older in another hundred years. Why can't it be a trillion years old? or a billion trillion?
I believe the universe is infinate, i dont believe in a big bang There was no begining. The further we look away the older it seems but we are not looking at no begining just another region of space, of our iniverse. Objects can be trillions and billions of light years away..if we were there it would be no diffrent than looking our way and saying wow that region of space is far away. We need to stop assuming we are in some kind of little bubble universe, we are not special there is proably others looking our way from trillions of light years away and wondering if we exist. Wait we wouldnt exit because to them we havent been produced yet or better yet they cant see us because our light has not reached them yet. 13 billion years is pathetic, did time not exist from lets say 100billion years ago? yes it did we just cant see it with our technology ...will we ever see trillions to the trillion light years away? no we wont. Or maybe we will but can light travel that far??