Image reveals galaxy's violent past. (Starry View).

Often referred to as twins, the Milky Way and the nearby galaxy Andromeda have a similar spiral shape, size, and mass. What's more, Andromeda appears as unperturbed as the Milky Way. But a new image unveiled this week belies that serene portrait. The most detailed visible-light picture ever taken of the heaven's reveals that Andromeda was walloped by an intruder galaxy much more recently than the Milky Way was.

The image, the result of 3.5 days of observations with the recently installed Advanced Camera for Surveys on the Hubble Space Telescope, is several times as sensitive as the Hubble Deep Field images (SN: 3/1/03, p. 139). The new picture for the first time resolves faint, sunlike stars in a galaxy other than the Milky Way. The picture shows 300,000 individual stars in a portion of Andromeda's halo, a spherical cloud of stars that extends from the galaxy's center. The brightness and colors of these stars indicate that they range in age from 6 billion to 13 billion years. In contrast, the stars in the Milky Way's halo are almost all between 11 and 13 billion years of age.

The relative youth of many of Andromeda's halo stars suggests that an intruder crashed into the galaxy some 8 billion years ago, hurling young stars from Andromeda's central disk into the halo. Also, the collision may have led to a new wave of star birth in the halo. The subsequent breakdown of the intruder galaxy may have released young stars into the halo.

Alternatively, says Tom Brown of the Space Telescope Science Institute in Baltimore, many small galaxies rather than a single massive one may have pummeled Andromeda.

"Every astronomy textbook says that halos are old, but we had only looked at our own galaxy," notes Brown. "Now, it's hard to know what's typical." He reported the findings at the institutes annual May symposium.

Before the Hubble observations, astronomers could image only the very brightest stars in Andromeda and therefore couldn't determine the age of most of the stars in the galaxy's halo. Other galaxies are still too distant for such determinations.

"This is pretty 'wow' data, a real step forward," comments Mario L. Mateo of the University of Michigan in Ann Arbor.

Brown notes that it's now up to theorists to explain why Andromeda looks to be as settled as the Milky Way even though our own galaxy was last hit by a massive intruder at least 2 billion years before Andromeda was.

The Hubble image covers only a small chunk of Andromeda's halo, Brown cautions.

Furthermore, he says, spectroscopic studies of the brightest stars in Andromeda's halo with a ground-based telescope could reveal their composition and suggest whether the stars were born during a giant wave of star birth generated by a collision with a single, massive galaxy or by a series of gentler encounters.