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Andromeda galaxy in infrared (red) and visible (blue) light

NASA recently released the top photo of the Andromeda galaxy (red), taken in infrared light by the Spitzer Space Telescope. Lanes of dust (spawning grounds for new stars) show up as rough rings. Also released was an image in visible light (blue), showing the distribution of old stars.

In a crumby galactic neighborhood, a star is born

A new study of the Andromeda galaxy confirms ideas of how our solar system originated

By Deane Morrison

June 20, 2006

If you've ever stepped outside on a dark night and seen the Andromeda galaxy, you know it as just a faint oval blur. Yet buried in that distant sea of stars lie clues to how our sun, the planets, and even humanity itself originated. Working with infrared images from NASA's orbiting Spitzer Space Telescope, several University researchers were part of a national team that recently confirmed the sun's--and therefore humanity's--origins in cosmic dust. The story begins when a star explodes and spreads dust through a galaxy like pollen in the wind. Eventually, some of the dust coalesces and forms a new star and planets. The next thing you know, we have dinosaurs, woolly mammoths and Ray Charles. It's dust to dust, on a grand scale. In the Spitzer photo recently released by NASA (see red image at right), the galaxy's dusty skeleton stands out in stark relief as cords spiraling in to the galactic center. By showing where the dust is distributed in Andromeda, the photo reveals where new stars similar to the sun are forming. "The image shows that sunlike stars form in areas rich in heavy elements, that is, dust-rich areas that contain the leftovers from the evolution of previous generations of massive stars," says astronomy professor and team member Bob Gehrz. Also on the team were fellow University astronomy professors Roberta Humphreys and Charles "Chick" Woodward and postdoc Elisha Polomski. Pauline Barmby of the Harvard-Smithsonian Center for Astrophysics led the team. Along with this photo, NASA released the blue image, of Andromeda in visible light. It shows the distribution of old stars, which is quite uniform compared to the upper image and its rough, choppy waves of dust that spawn new stars like the sun. Cold dust, of course, produces no heat of its own and thus should not give off any infrared light (the signature of an object that is warmer than its surroundings). These dust clouds do give off heat, however, because they are the nurseries for the formation of young, luminous stars. As they absorb radiation from the stars, the dust clouds warm up and radiate in the infrared, becoming visible to Spitzer. A star is born The dust in Andromeda contains elements like carbon, magnesium, oxygen and iron-all necessary for life as we know it. It also contains heavier elements such as copper and uranium, which are building blocks of planets. Succeeding generations of stars form from dust clouds that represent the "crumbs" of massive stars from previous generations. Images of galaxies like our Milky Way show dark lanes of what appears to be dust spiraling through the planes of the galaxies like cinnamon in a bun.

"These new Spitzer images enable us to see the actual faint heat from the coolest dust," says Gehrz. "The images confirm what we've been suspecting, namely, that the sun and solar system are composed of elements made in other stars that have long since died."

Infrared photos have another use, too: to "weigh" the galaxy. According to sources at NASA's Jet Propulsion Laboratory in Pasadena, Calif., Andromeda shines with the light of about a trillion stars, compared to perhaps 400 billion in the Milky Way. Andromeda's girth is also staggering: about 260,000 light-years, compared to 110,000 light-years for the Milky Way. That is, if Andromeda were six feet tall, the Milky Way would stand at only two-and-a-half feet or so. Getting to know you Our two galaxies both have had smaller "companion" galaxies associated with them. The most famous are the Magellanic Clouds, two blurs of light over skies of the Southern Hemisphere. Their existence was reported by members of Ferdinand de Magellan's crew during their circumnavigation of the world (1519-22). The Milky Way has not devoured the Magellanic Clouds yet, but astrophysicists have recently found remnants of other galaxies that it has torn apart and incorporated.

If you look at the image with this article, you'll see two especially bright spots in the lower photo, which was taken in visible light. Those are companion galaxies to Andromeda. In the upper photo, which shows the distribution of dust, they are invisible because Andromeda has already stripped them of their dust. At some point the stars of both companions will also be sucked into Andromeda. And if you think the rings of dust in Andromeda appear a little warped, you're right. Gravity from its companions has indeed disturbed Andromeda, as the Milky Way may also have been disturbed by its companions.

Andromeda is now about 2.5 million light-years away, making it the closest big galaxy to us, and the gap is closing. "A reasonable scenario has our galaxies merging in about 3 billion years, or about 2 billion years before the death of our solar system," says Gehrz. That spectacle will be a treat for any astronomers who may be around to watch. With a little luck, it'll be as visually enthralling as the famous picture of the Mice galaxies. Besides the Harvard-Smithsonian Center for Astrophysics and the University of Minnesota, the team that produced the Spitzer image included researchers from the Johns Hopkins University and the University of Arizona.