Space

The Andromeda galaxy's dusty pink arms contrast with older stars that glow blue in a striking new image from the Spitzer Space Telescope.

Using about 3000 separate exposures, Pauline Barmby of the Harvard-Smithsonian Center for Astrophysics in Cambridge, US, and her team found that the spiral galaxy has about a trillion stars.

This measurement is the first census of Andromeda's stars in the infrared part of the spectrum and agrees with previous estimates of the stars' combined mass.

"The Spitzer data trace with startling clarity the star-forming material all the way into the inner part of the galaxy," says George Helou, deputy director of NASA's Spitzer Science Center at the California Institute of Technology.

Gas and dust

"The challenge is to understand what shapes the distribution of this gas and dust, and what modulates the star formation at different locations."

Andromeda, also called M31, has also been the target for the Gemini North Telescope on Mauna Kea, Hawaii, US. It took the deepest and highest-resolution pictures yet of the galaxy's central bulge and inner disc in near-infrared light.

Using adaptive optics that compensate for distortions caused by turbulence in the atmosphere, Knut Olsen, an astronomer with the US National Optical Astronomy Observatory, found that most stars in the galaxy's bulge and inner disc are old.

This suggests that the disc has existed in its current form for at least 6 billion years.

Relative calm

Galaxies are thought to form by mergers of material over time, but a galactic disc needs a relatively stable environment to thrive. "The discs of galaxies can't form in these violent collisions," Olsen told New Scientist.

Given the age of the observed stars, Olsen's data suggest that this violent phase of Andromeda's evolution lasted until 6 billion years ago. This concurs with computer simulations that indicate that the disc could have formed at this time.

Another group, led by Tim Davidge of the Herzberg Institute of Astrophysics, Canada, took Gemini data and found a young, violent star near the centre of the Andromeda galaxy.

The discovery of an "asymptotic giant branch" star came as a surprise because scientists thought that the gravitational field of the nearby supermassive black hole would destroy the gas and dust clouds needed to nurture young stars.

This type of star is short-lived and is also found in the Milky Way. "Now we see that the centres of M31 and the Milky Way may be more similar than once thought," Davidge says. The two galaxies are expected to collide in roughly 5 billion years.

The research was presented at the American Astronomical Society Meeting in Alberta, Canada, on Monday.

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