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I did some googling around and couldn't get answer to a simple question of black hole mass compared to the star from which it was created. It is well known that giant stars burn very bright and relatively very quickly. They end up their lives as red giants and eventually collapse into a black hole.

Suppose that the star is mid-life and has some mass M. Over the time it burns its fuel, turns into a red giant and after supernova, turns into a black hole. This black hole has some mass B. Why B >> M?

According to some simple physics laws, mass could not be created nor destroyed.

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    $\begingroup$ What made you think B > M in the first place? $\endgroup$
    – Russell Borogove
    Nov 13, 2015 at 17:58
  • $\begingroup$ @RussellBorogove The fact that when any black hole meets a star, it devours it. That was my misconception that the heaviest "eats" the lightest. $\endgroup$
    – Vladislavs Dovgalecs
    Nov 13, 2015 at 18:00
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    $\begingroup$ Aha. Density is the dominant factor here, rather than mass. Note that in a star vs black hole collision, stellar matter approaching the black hole gets ferociously compressed, which leads to a lot of the matter getting blown clear of the collision -- you'll get a slightly fatter black hole and a lot of radiation emission and loose gas. youtube.com/watch?v=thkKrW23vNU $\endgroup$
    – Russell Borogove
    Nov 13, 2015 at 18:11
  • $\begingroup$ Mass, no. Weight, yes, pressure, yes, density yes. The weight of a dead star grows exponentially as it's size shrinks, even though it loses mass in it's final stages. Mass really can't be created, the mass is just what's left over, but as it contracts the weight grows very fast, and that's why a stellar core can collapse in as little as 1 second once it gets going. $\endgroup$
    – userLTK
    Nov 13, 2015 at 18:41
  • $\begingroup$ @xeon: it's energy that can be neither created nor destroyed. Mass is destroyed in something as simple as a fire. A very (very!) small percentage of the matter is converted into light. Newton knew about this, see Opticks query 30. $\endgroup$
    – John Duffield
    Nov 15, 2015 at 11:51

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The mass of a black hole is always much less than the star from which it formed.

A very large star such as Eta Carinae (which is expected to form a black-hole some day) is about 100 times that of the sun, Having already lost as much as 50 solar masses, blown off the star by powerful outbursts in the past.

The future of the star is not fully understood. It is possible that it will collapse in a supernova, in which most of the rest of the star will be blasted into space, forming a supernova remnant, and the core of the star will collapse. The black hole formed may have a mass of more than 3 times the sun, but much less than the mass of the original star.

So while the black hole is not heavier, it is much denser. A 3 solar mass black hole would have a diameter of about 20km.

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  • $\begingroup$ Interesting. Then the only way (?) this brand new black hole can gain mass (well beyond giant star masses, including the mass of it's creator star) is by devouring other stars, right? What if this young black hole "meets" as super giant which is much more heavy? The star is more massive and should attract it. $\endgroup$
    – Vladislavs Dovgalecs
    Nov 13, 2015 at 17:52
  • $\begingroup$ Yes, if a black hole encounters a star, it will collect much of the star's mass (though some of the mass will be thrown off by high-energy interactions in the black hole's accretion disk). Black holes at the centers of galaxies such as our own have masses millions of times that of our sun, having absorbed many stars in their lifetime. en.wikipedia.org/wiki/Supermassive_black_hole $\endgroup$
    – Russell Borogove
    Nov 13, 2015 at 17:57
  • $\begingroup$ Well Eta Carinae itself is a pair of massive stars. If the first forms a black hole, that black hole may well end up in orbit around the second. If the black hole is close enough, it can start to pull matter off the other star into a disk. As this matter orbits the black hole, collisions between the matter in the disk will cause it to heat up (extremely) and release X rays. This is what is happening around the black hole Cygnus X1 $\endgroup$
    – James K
    Nov 13, 2015 at 18:01

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