Segue 1 is a tiny, nearby galaxy that circles the Milky Way. It only has a few hundred stars, and researchers now reveal that they have low abundances of elements heavier than helium. Having fewer metals than any known galaxy suggests that Segue 1 stopped evolving nearly 13 billion years ago, making the dwarf galaxy one of the oldest ever formed.
Nuclear fusion of basic elements like hydrogen and helium atoms take place inside the cores of stars. And when these stars explode, atoms that are even heavier are created. All of these go into making new stars, and so each succeeding generation has more of the heavier elements like metals.
Using the Magellan Telescope at Las Campanas Observatory in Chile and the High Resolution Echelle Spectrometer Keck Observatory in Hawaii, MIT’s Anna Frebel and colleagues collected elemental composition data from six of Segue 1’s brightest stars.
They found that the stars are made almost entirely of hydrogen and helium, with trace amounts of iron. “Segue 1 is so ridiculously metal-poor that we suspect at least a couple of the stars are direct descendants of the first stars ever to blow up in the universe,” study coauthor Evan Kirby of the University of California, Irvine, tells Scientific American.
“Segue 1 is the least chemically evolved galaxy known,” the researchers conclude, making it a fossil left over from the early days of the universe.
Additionally, the team found that the stars in Segue 1 were formed very quickly, and all at once. When massive stars explode, they make magnesium and calcium; when smaller stars explode, they form iron. Stars in Segue 1 contain products of high-mass stars, and few from low-mass stars -- which makes sense because the very massive ones die younger. And it looks like the dwarf galaxy experienced just one burst of star formation, and then stopped.
“Segue 1 is the only example that we know of now that was never enriched by these low-mass stars, meaning it formed stars really quickly, in the blink of an eye,” Kirby tells SciAm. “If it had formed stars long enough those low-mass stars would have to contribute.”
It’s possible that the dwarf galaxy stopped making stars because of reionization. When the universe started to cool, the isolated protons and electrons started to form atoms in the gas. Primordial stars were born and sent out radiation that energized the gas surrounding it, reionzing the gas and killing star formation in the galaxies that existed around 13.2 billion years ago.
The work appears in The Astrophysical Journal this week.
[Via Scientific American]
Image: A. Fujii via NASA