In many ways, the evolution of our galaxy is not totally clear. Were all the hundreds of billions of stars born in the Milky Way? Or were they stolen from smaller galaxies?
Until recently, there was a discrepancy between the stars that can be found in our galactic disk and those that we find in the dwarf galaxies that orbit within our galactic halo. If the Milky Way was some kind of ‘galactic cannibal,’ devouring these small local galaxies (containing only a few billion stars), surely there must be some stars in the existing dwarf galaxies that resemble the ones stolen from ancient dwarf galaxies now residing in the Milky Way?
Now it looks like we have our answer; astronomers hunting for ancient stars have found a primordial massive star (designated “S1020549″) hiding inside the dwarf galaxy Sculptor, some 290,000 light years away.
This is a huge achievement as primordial stars in other galaxies are very difficult to spot as they are generally very dim and far away.
“This was harder than finding a needle in a haystack. We needed to find a needle in a stack of needles,” said a Caltech astronomer Evan Kirby. “We sorted through hundreds of candidates to find our target.”
One galactic evolution model is known as the “bottom up” model that theorizes that galaxies grow by sucking in smaller galaxies. For this model to be correct there must be evidence of dwarf galaxy stars inside bigger galaxies (like the Milky Way).
A good place to start is to search for “metal poor” stars. To astronomers, the term “metal poor” means that a particular star contains few of the heavier elements that might be found in other younger stars like our sun. A metal poor star is composed mainly of hydrogen, helium and small quantities of lithium (with only trace amounts of other elements).
The lack of heavier elements means that metal poor stars are typically very old, as old as the oldest stars in the Universe (born only a few hundred million years after the Big Bang). The early Universe was also metal poor as only stars have the ability to create any element heavier than lithium (through nuclear reactions and supernovae), so metal poor stars are in fact the first stars to be born, some are still alive today, at over 13 billion years old.
“The Milky Way seemed to have stars that were much more primitive than any of the stars in any of the dwarf galaxies,” said co-author Josh Simon of the Observatories of the Carnegie Institution. “If dwarf galaxies were the original components of the Milky Way, then it’s hard to understand why they wouldn’t have similar stars.”
After careful spectroscopic measurements by Carnegie’s Magellan-Clay telescope in Las Campanas, Chile, it turns out that S1020549 has a metal abundance 6,000 times lower than the sun and five times lower than any other star found so far in any dwarf galaxy.
Our galaxy contains an abundance of metal poor stars, so this new finding appears to agree with the “galactic cannibal” theory that many dwarf galaxies died in the making of the Milky Way.
“If you watched a time-lapse movie of our galaxy, you would see a swarm of dwarf galaxies buzzing around it like bees around a beehive,” said Anna Frebel of the Harvard-Smithsonian Center for Astrophysics, lead author of the Nature paper reporting these findings. “Over time, those galaxies smashed together and mingled their stars to make one large galaxy — the Milky Way.”
Image: Artist impression of a primordial star (David A. Aguilar, CfA)