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earthman2110
2004-Mar-14, 03:26 PM
From my very limited understanding of black holes and how they work, I thought that they dont pull any object that they didnt pull before they became a black hole. in that case, how can this black hole be "sucking up" this star, if the star wasnt close enough to be affected by the gravitational field of the star-now-blackhole before?

http://antwrp.gsfc.nasa.gov/apod/ap040224.html

Kaptain K
2004-Mar-14, 03:43 PM
Note that from the diagram, the star is moving. When it gets to close, tides rip it apart.

Espritch
2004-Mar-14, 04:25 PM
You are correct that gravitational effect of black holes is identical to that of a normal star at any distance beyond the diameter of the original star that formed the black hole.

As Kaptain K noted, in this case, the other star just got a little too close. Even if the black hole wasn't a black hole (i.e. still an uncollapsed star), an approach that close would have probably produced interesting results. What happens when two stars collide?

P.S. I should also note that the link you gave is referring to a star getting too close to the massive black hole at the center of a galaxy. Most galaxies have been found to have a massive black hole at their center comprising a mass equal to hundreds of thousands of stars. These black holes are much more massive than those produced by the collapse of a single star. I'm not sure if anyone knows what produces these black holes. There are also large black holes at the centers of star clusters. There appears to be a relationship between cluster or galaxy size and the size of the central black hole, but don't ask me why.

Brady Yoon
2004-Mar-14, 06:17 PM
Stars are always moving relative to each other in space. This is called proper motion.

JohnOwens
2004-Mar-14, 08:29 PM
From my very limited understanding of black holes and how they work, I thought that they dont pull any object that they didnt pull before they became a black hole. in that case, how can this black hole be "sucking up" this star, if the star wasnt close enough to be affected by the gravitational field of the star-now-blackhole before?

http://antwrp.gsfc.nasa.gov/apod/ap040224.html

A most basic answer to the first part of the question: Yes, but, even before a star becomes a black hole, it's pulling on everything else in the universe (well, allowing for the probable light-speed limit of gravitational propagation). So of course it was pulling on that star all along. The only point at which the pull becomes any different for the former star vs. the black hole is when you're at a point closer to the black hole than the surface of the star was.
On the other hand, black holes at the centers of galaxies certainly aren't formed as simply as the collapse of a single star, but I don't think it's clear yet whether they started with a single star and then got bigger as they sucked in other stars, or if they form in some other manner.

AGN Fuel
2004-Mar-14, 11:31 PM
From my very limited understanding of black holes and how they work, I thought that they dont pull any object that they didnt pull before they became a black hole. in that case, how can this black hole be "sucking up" this star, if the star wasnt close enough to be affected by the gravitational field of the star-now-blackhole before?

http://antwrp.gsfc.nasa.gov/apod/ap040224.html

Your understanding is correct - a black hole has the same gravitational attraction as a star of equal mass. However, the APOD is a representation of a supermassive black hole at the heart of a galaxy - in such locations the stellar density is far greater and motions more chaotic, leading to a greater chance that a star might wander 'too close'. In such an event, the differential in forces between the near & far side of the star imposed by the SMBH will 'disrupt' (read: shred) the star.

A slightly different situation can occur if a black hole is part of a binary system with second main sequence star. The two stars may happily orbit their common centre of gravity for billions of years. However, when the main sequence star swells to become a red giant, the surface of the star 'gets closer' to the BH and material may be drawn off to form an accretion disc before disappearing down the maw of the hole. :o

Kaptain K
2004-Mar-15, 12:23 AM
A slightly different situation can occur if a black hole is part of a binary system with second main sequence star. The two stars may happily orbit their common centre of gravity for billions of years. However, when the main sequence star swells to become a red giant, the surface of the star 'gets closer' to the BH and material may be drawn off to form an accretion disc before disappearing down the maw of the hole.
Just to clear up a possible ambiguity. The whole star will not disappear down the black hole. The central core will be left as a white dwarf.