Hubble: Zoom Into Andromeda Galaxy's Double Nucleus [720p]

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This zoom dives deep into the nucleus of the neighboring spiral galaxy M31, also known as the Andromeda galaxy. The new Hubble Space Telescope image centers on the 100-million-solar-mass black hole at the core of the galaxy and the young blue stars surrounding the black hole. This is the sharpest visible-light image ever made of the nucleus of an external galaxy. Astronomers are trying to understand how apparently young stars were formed so deep inside the black hole's gravitational grip and how they survive in an extreme environment.

The event horizon, the closest region around the black hole where light can still escape, is too small to be seen, but it lies near the middle of a compact cluster of blue stars at the center of the image. The compact cluster of blue stars is surrounded by the larger "double nucleus" of M31, discovered with the Hubble Space Telescope in 1992. The double nucleus is actually an elliptical ring of old reddish stars in orbit around the black hole but more distant than the blue stars. When the stars are at the farthest point in their orbit they move slower and give the illusion of a second nucleus.

The blue stars surrounding the black hole are no more than 200 million years old, and therefore must have formed near the black hole in an abrupt burst of star formation. Massive blue stars are so short-lived that they would not have enough time to migrate to the black hole if they were formed elsewhere.

Astronomers are trying to understand how apparently young stars were formed so deep inside the black hole's gravitational grip and how they survive in an extreme environment.

The fact that young stars are also closely bound to the central black hole in our Milky Way galaxy suggests this may be a common phenomenon in spiral galaxies.

credit: NASA, ESA, and G. Bacon (STScI)