ScienceShot: Andromeda Chock Full of Black Holes

X-ray observations have more than tripled the number of known black holes in the nearby Andromeda galaxy. Lying just 2.5 million light-years from Earth, Andromeda (main image) is a giant spiral, the largest of the approximately six dozen galaxies populating the so-called Local Group; the Milky Way ranks number two in size and boasts about 50 known black holes. In the 20 June issue of The Astrophysical Journal, astronomers will report finding 26 likely black holes in the Andromeda galaxy. Although the black holes themselves emit no radiation, material spiraling into them from orbiting stars gets heated to such extreme temperatures that it emits x-rays (inset). With this discovery, Andromeda now has 35 known black holes, each weighing several times more than the sun, plus a far larger one at its center, giving our giant neighbor more known black holes than any other galaxy outside our own.

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