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Mysterious Stars Surround Andromeda's Black Hole
UltimaGuy writes to tell us that Yahoo is running a story about a recent discovery that shows the source of strange blue light coming from the center of the Andromeda galaxy. The light is actually a cluster of stars circling the galaxy's central black hole with immense orbital velocity. From the article: "Such frenetic activity was thought to prevent star formation. Stars form when a knot of gas and dust collapses under its own gravity."
Actually its neutron accretion that produces elements heavier than iron, not fusion. Iron disintegrates at temperatures lower than what it will fuse at.
No, because hydrogen is a single proton, not a single neutron. I forget the exact mechanics of neutron accretion, as its not my field of direct study, but it occurs in two forms, the r-process (rapid) and the s-process(slow) (we physicists are not known for being creative with names). The r-process occurs in supernovae when heavy nuclei are bombarded by many neutrons, ad rather than splitting the target nuclei, the neutrons stick (at the same time the nuclei are radiating particles away, but not as fast as they are gaining them). Once the process stops, the new, super neutron rich nuclei give off beta radiation (changing neturons into protons) until they reach a stable configuration. The s-process occurs in large, but otherwise stable stars. This process however only produces elements as heavy as lead. Anything heavier is produced by the r-process.
That's quite possible. If they formed in a wider orbit around the black hole, for example, they could well have been caught by tidal drag and slowly moved into lower orbits.
The real question is how they can exist at all in such a low orbit (or, more accurately, how they can exist in such a strong gravity gradient). What happens is that if they tidal difference between the two sides of the star (the difference between the black hole's gravity at the closest edge of the star and that on the furthest edge) exceeds the star's escape velocity, matter will be able to leave the star and it just falls apart. The implication (which the Yahoo! article was too low-tech to get right) was that the stars must be very dense. A dense star will have both (a) less distance between that nearest and furthest edge and (b) a steeper gravity well for material to get out of in the first place.
The other interesting bit is the rather close estimate of the black hole's mass. Most of the other estimates of galactic center black hole masses I've seen are based on things orbiting them far more distantly, such as 10-100 light years.
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