Astronomers at UC Santa Cruz have discovered a surprisingly massive halo of bright, old stars around our galactic neighbor Andromeda, pushing the outer limits of the galaxy to 1.5 million light years, and teaching them how spiral galaxies behave.
The researchers, who are presenting their findings at the American Astronomical Society meeting in Seattle this week, used a series of giant telescopes in observatories to see beyond the Milky Way into our neighbor's backyard. The stars they found are called red giants, often 10 times bigger than the sun, and the brightest objects they could see. The furthest red giants were approximately 562,000 light years from Andromeda's center. The furthest stars in the Milky Way are about 50,000 light years away.
"If you could see the whole halo in the sky," said Karoline Gilbert, an astronomer on the UC Santa Cruz team, "it would stretch as far as 50 full moons side by side."
She calculated previous estimates of Andromeda end-to-end equaled about 15 moons.
Spiral galaxies have three typical features: a flat disk, a central bulge of tightly packed older stars, and an extended halo with far fewer stars. By studying the Andromeda halo, the astronomers can learn how spiral galaxies like it and the Milky Way originally formed billions of years ago.
Galaxies often merge, said Gilbert, and in the process, get torn apart. Small galaxies get eaten by larger ones, for example. When the dust settles, she said, there are streams of stars and other cosmic debris left behind. Closer analysis of the distant stars in the streams can reveal their age.
Younger stars may be remnants of a galaxy devoured, but older stars could have formed with the birth of the galaxy. The red giants are not the only things out there, said Gilbert.
"There are definitely all types of stars out there," she said, "but with telescopes on the ground, they are the only ones we can reasonably see."
With the discovery of the halo, said Raja Guhathakurta, the UC Santa Cruz team leader, Andromeda science has taken on a whole new aspect.
"We now believe that previous groups have been mistakenly identifying the outer parts of the Andromeda bulge as its halo," he said.
The previous findings led researchers to believe Andromeda and the Milky Way were composed differently.
In finally getting a glimpse of the Andromeda halo, said Gilbert, they have learned the Milky Way and Andromeda are actually quite alike.
Michael Rich, an astronomer at UCLA who was part of the study, said the apparent age of some of the stars in the halo makes him think Andromeda is not just made of stars born with it. Other younger stars have probably fallen in, he said.
The next step, according to Gilbert and Rich is learning more about the stars themselves and the halo they form. But ground telescopes only give you pieces of the sky, Rich said. He hopes the Hubble telescope, if working properly, can help them see the whole halo, at much greater detail.
"It's like the proverbial elephant," he said. "A blind man feels the tail, and says it must be like a snake, the tusk and it's made of metal or stone, the sides, and it's a wall. We want the whole elephant."
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