It's sometimes hard to fathom that only a hundred years ago, there was still a spirited argument going on in the astronomical community over whether the Milky Way was the only galaxy in the universe -- and that the other "nebulae" might be merely small-ish features lying in the outskirts.
The center of the "Milky Way is all there is" faction was the famous astronomer Harlow Shapley, who was their spokesperson in the 1920 "Great Debate" with Heber Doust Curtis, who believed the "nebulae" (or at least some of them) were very distant galaxies more or less like our own. Neither man came away from the Debate convinced of the other's reasoning, but the whole affair was conclusively settled a few years later when Edwin Hubble discovered Cepheid variables in the Andromeda Galaxy. Cepheid variables are a curious type of star that experience a regular periodicity in brightness, and the brilliant astronomer Henrietta Swan Leavitt had showed that their periods of variability were related in a straightforward way to their intrinsic brightness. Because of this, they can be used as standard candles -- just as you can estimate how far away a motorcycle is at night if you compare how bright the headlight appears to be with how bright you know it actually is.
So the discovery of Cepheids in Andromeda gave Hubble a way of figuring out how far away it is from us, and it turns out not to be a small cloud in the fringes of our own galaxy, but an "island universe" of its own that's actually slightly larger than the Milky Way, and 2.5 million light years away. And of course, Hubble and others went on to discover red shift and the expanding universe, and better telescopes showed that there are billions of galaxies out there, some of them so far away that the light they emit has been traveling for most of the age of the known universe in order to get here.
A cool postscript is that Shapley, confronted with this evidence, admitted defeat, and went on to make major contributions to galactic astronomy. It's what I love about science; it self-corrects, and the best scientists look on these reversals as opportunities rather than embarrassments. (Although Shapley did allow himself a moment of rueful laughter at his own error, calling Hubble's paper on Cepheids in Andromeda as "the letter that destroyed my universe."
In any case, we now live in a cosmos so much vaster and richer and stranger than the one they knew a century ago -- one can only imagine what more we'll know a hundred years from now.
These musings come up because of a wonderful piece of research out of the University of Chicago, where a group of undergraduate astronomy students were assigned by their teacher, Alex Ji of the Sloan Digital Sky Survey, to analyze recent SDSS data for anything anomalous, and they found something astonishing; an ancient star that not only appears to date from the very early universe, but has migrated here from the Large Magellanic Cloud, a star cluster than neighbors the Milky Way.
Astronomers can make a shrewd guess about how old a star is based upon its metallicity -- what proportion of its makeup is anything other than hydrogen and helium. (Astronomers confusingly call all other elements metals, which must annoy the hell out of the chemists.) Since all of the hydrogen, and a good fraction of the helium, were formed during the Big Bang -- and virtually all of the other elements have been created since then through nuclear fusion and energetic events like supernovae and neutron star collisions -- the quantity of metals in a star tells you how many cycles of birth and death occurred prior to the star's formation. The Sun, for example, is fairly metal-rich, and is probably a third- or fourth-generation star; its contents were enriched from the activity of previous generations of stars. (It's still not as high in metals as the bizarre Przybylski's Star, which is so anomalously high in rare heavy elements that there's a credible case to be made that it was seeded by technological aliens.)
The newly-discovered star, however is the opposite; it's called SDSS-J0715-7334, and from its spectrum it appear to be almost entirely hydrogen and helium. Even relatively lightweight elements like carbon are so rare in it that they're well-nigh undetectable. In fact, its total metallicity is 0.005% of the Sun's -- making it by far the most metal-poor star ever detected.
Me, I wonder how this star lived as long as it has. Most of the universe's first-generation stars have long since exhausted their fuel and either collapsed into white dwarfs or else flared out as supernovae. How has this one persisted all this time?
Weirder still, the star didn't start out in the Milky Way. Using its current motion, the students calculated that it originated from outside of our galaxy, and backtracked its path to the Large Magellanic Cloud.
Okay, I'm super impressed. These are a bunch of undergraduates, for cryin' out loud. As an undergraduate, I was mostly focused on eating pizza and hanging out with my friends and earning grades that were at least high enough not to get me kicked out of the university. These young people?
They're getting their names in the author line on papers in Nature Astronomy.
"These students have discovered more than just the most pristine star." said Juna Kollmeier, the Director of SDSS. "They have discovered their inalienable right to physics. Surveys like SDSS and Gaia make that possible for students of all ages everywhere on Earth and this example shows that there is still plenty of room for discovery."
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