Sunday, March 14, 2010

Finding a Stellar Grandpapa

On March 3rd, Anna Frebel of the Harvard-Smithsonian Center for Astrophysics, issued a press release that her team had discovered a really old star. Really old. Dawn-of-time old. While not earth shattering to those of us living in the twitter-timezone, Ms. Frebel's team completed a missing piece of a cosmological puzzle that's been plaguing astronomers and cosmologists since before I was in grad school: where the hell are the old stars? It's been a bit of an embarrassment, really, but before we go there I just want to explain the backstory, silly bookkeeping, and why this discovery is so important.

For the past several decades, astronomers and cosmologist have based the operating assumption of the physical universe on a few principles:
  • The Universe started from a singularity in a massive explosion, commonly referred to as "the Big Bang." For many years there was a competing theory called "steady state," which said that the Universe always was and always will be, but the mountain of evidence to the contrary has basically shouted that viewpoint down. Steady State was poetic, and vaguely religious, but wasn't consistent with observed facts.
  • After the Big Bang, the Universe went through a rapid evolutionary process during its first 3 minutes of existence, in which the literal framework of the Universe was established: all of its physical dimensionality, physics constructs, the flow of time, energy distribution... oh yeah, it was quite a party time.
OK, over simplified, but that's the basic gist of it - everything flows from here. After those first few minutes, everything else began to shake out, including primitive stars. In the beginning of the Universe, you see, there weren't a hell of a lot of building materials. Well, really just hydrogen and a little helium. So, stars that arose from that first boom, were composed almost entirely of hydrogen and helium (with a smattering of the early metals: lithium and beryllium, but such trace amounts that it only counts when dealing with really off-the-beaten-path cosmology issues).

After they lived a few billion years (call it 13B) and burned themselves out, they exploded and sprayed crap all over the bran', spankin' new Universe. They were the frat boys of the universe, beer bottles everywhere the morning after the party. And, by "beer bottles" I mean "heavier elements." Helium, nitrogen... but more importantly: the beginning of the metals...well, "metaloids," actually. Check the periodic table, you'll see them there on the right.

OK, now the fun starts - when these bad boys pop, the crap they spill out is pretty much everything else you see around you: oxygen, iron, heavy metals, the gold in your teeth.... all the rest of the elements. Essentially, the now.

The weird bookkeeping comes into play when you consider how cosmologists categorized these three groups of stars: essentially, in the order of observation. The sun, and all the stars you see when you look up on a cold night are called "Population I" stars. See? They were seen first...

...the second group? The bad boys above that exploded and filled the skies with all the current stuff? They are "Population II," cuz they were found next. Get it?

And the last group....well, yeah, "Population III" stars. When were they observed? They weren't ever observed. Not directly anyway. They are long gone corpses, dried up cinders of their former selves.

(As an aside: I call the bookkeeping "weird" because it never made intuitive sense to me. The first stars should be Pop I's, in my opinion, but that's just me.)

OK, back to Miss Anna at the CfA, and the missing link she just found. If you were following my stream-of-consciousness explanation above, you get the drift that modern stars arose from the ashes of older stars. Similarly, the formation of galaxies (collections of stars bound together by common gravity) such as our own Milky Way underwent their own evolution. Outside the Milky Way galaxy, and other big spiral galactic formations which contain 10's of millions of stars,  are these weenie little malformed galaxies called Dwarf Galaxies. (I know, not Politically Correct, but "Little People Galaxies" didn't quite flow off the tongue.)

Dwarf Galaxies have a few 10's of 1000's of stars at best. You see, in the Population II era, there just weren't a lot of stars yet, so not too many buddies to gang together with... so, the Pop II's did the best they could... hung out together, went to the movies, and watched why all the big galaxies made fun of them. Eventually, some of the Dwarfs hung around to the modern age, because when their Pop II contents exploded, they made some new Pop III friends to hang with. A lot of these bigger, heavier stars were ejected, and they banded together to form the larger spirals.

It wasn't an even distribution, of course, because life isn't like that. Some Dwarf's had Pop III stars in them, and some Spirals had Pop II's. But, as time passed, the distribution of old versus new stars began to change - heavily weighted to the newer Pop IIIs. If you look in our own galaxy, near the center usually, you'll find the Pop II's sitting in their stellar old age retirement communities, taking Viagra and trying to be interested in the television.

And in the remaining Dwarf Galaxies? The ones older than our Milky Way? They should be chock-a-block full of Pop II's, right? Right? Yeah...uh...oops. There's the embarrassment. None. Nada.

Enter Anna and her team - 290,000 light years away, in the Dwarf Galaxy of Sculptor - which, I know, sounds like a Farscape villian - lies Lores: the first metal-poor (Pop II) star found in a Dwarf Galaxy.

Whew. The difficulty in finding a Dwarf Galaxy Pop II makes sense - a Dwarf has fewer stars, remember, so therefore a higher probability that most, if not all, of the Pop II's would have been swapped out for Pop IIIs.... still, it made everyone nervous that no one ever found one before. It called into question the theory of stellar evolution.

But, fortunately, Ms Frebel found Lores: Old, decrepit, and pinching the nurses asses at the nursing home. The Universe is as it should be.

Respect your elders, kids.

Side note: I don't want to leave you thinking that this story is as simple as I describe. Observing Pop II and Pop I stars becomes confusing and interesting the further you look from earth. Taking the speed of light into account means that the farther you look from us, the farther back in time you can see - and the harder it becomes to register and understand the light (diffusion, red-shifting, and other interesting artifacts come into play.). Pop II and even Pop I stars have been observed in abundance by using this lens back into the past. The issues with finding Pop II's in Dwarf galaxies arise when looking at Dwarf galaxies near us, so they are in the same relative "time frame" as we are here on earth.

Also, I'm hoping that the press release from Frebel's team misquoted her. She almost certainly did not say that Lores was as old as the Universe, since that would make it a Pop I star, but she probably said it was as old as the Milky Way galaxy, which would make more sense.

There, I think that does it...

...well, unless you start talking about, next time.


Chip Correra said...

Ah, finally I have a plausible explanation of last week's NCAA bracket upsets. Time to revisit the super-collider.

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