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Galactic archeologists create the first map of Milky Way’s stellar halo

| Monday, November 23, 2015

If you’ve ever looked up at the stars and wondered how they got there, a group of galactic archeologists on campus might have some answers for you. 

Research assistant professor of astrophysics Daniela Carollo said her group looks at “ancient stars.”

“First of all, what we do in the galactic archeology group is we observe, study and analyze ancient stars in the Milky Way,” Carollo said. “By ancient stars, I mean stars that formed while the galaxy was being assembled, and even before.”

Carollo said these ancient stars are the “fossils” of our galaxy — hence the name galactic archeology.

Research assistant professor Vinicius Placco, another member of the galactic archeology group, said the group has two goals: to identify the chemical composition of ancient stars and to understand how they move in the galaxy.

“We couple the chemistry with the movement of the star, and then we try to come up with explanations of how the stars move the way they do and why,” Placco said.

In the group’s most recent paper, the researches created an age map that dated 4,700 stars in the Milky Way’s stellar halo, Carollo said. 

“The Milky Way is a complex system — there is this disc where most of the stars are concentrated, and there is this very extended stellar halo that surrounds all the other structures. … This halo is where the fossils reside,” she said. 

She said the fossils can be 12 or 13 billion years old.

“So the universe started 13.8 billion years ago, with the big bang,” she said. “There was a point of high density and high temperature which started to expand very fast, and time and space began in that particular moment.”

Carollo said the first stars were formed two to three hundred million years after the big bang, but they were massive and quickly exploded in supernova. The elements and gasses released by these supernova formed the next generation of stars, and those with low mass became our stellar fossils. Smaller stars burn their hydrogen fuel slower, which is why we still see them in our galaxy 13 billion years after they were born.

However, she said, not all fossil stars are the same age — they can differ by two to three billion years. Examining the age of fossil stars and where they are located allows researchers to make guesses on when and how the ancient stars assembled into the galaxy, and ultimately why the Milky Way looks the way it does.

Placco described the method researchers use to estimate the age of stars.

“We have to use a very specific type of star, that’s called blue horizontal branch star — it just means that the star is burning helium in the core,” he said. “And with those particular stars we can turn colors into age estimates. So for this particular map, we selected stars from the SLOAN Digital Sky Survey. We were just measuring fluxes with different filters, so we can just get those magnitudes and colors.”

Once they had those colors, the researchers correlated them with an age and created the first map of the Milky Way halo system.

According to Placco, the researchers found a sphere of very ancient halo stars in the center of the galaxy and increasingly younger stars towards the outer edges.

“People have been doing simulations of how the galaxy formed and how it evolved, and why does it look like the way it does today, for many many years,” Placco said. “In these simulations the old stars are [predicted to be] concentrated towards the center of the galaxy. Our work demonstrates this important property for the first time.”

Carollo said the younger groups of stars on the edges are also significant — they tell us when certain groups of stars merged into the galaxy at later times.

“For example, if the galaxy started to assemble one billion years after the big bang, then this younger [group of stars] merged into the galaxy something like five years to ten billion years after the big bang,” she said.

But for Carollo, the most significant part of the paper is the discovery that stars near the galactic center are very ancient.

“It’s the first time we have demonstrated that the center of the galaxy is very old,” she said.

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