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How come black holes have the same mass as the star they were before the supernova if so much of the star's matter has been blasted into space?
By the no-hair theorem, a black hole can only have three fundamental properties: mass, electric charge and angular momentum (spin). It is believed that black holes formed in nature all have spin, but no definite observation of the spin has been recorded. The spin of a stellar black hole is due to the conservation of angular momentum of the star that produced it.
The gravitational collapse of a star is a natural process that can produce a black hole. It is inevitable at the end of the life of a star, when all stellar energy sources are exhausted. If the mass of the collapsing part of the star is below the Tolman–Oppenheimer–Volkoff (TOV) limit for neutron-degenerate matter, the end product is a compact star — either a white dwarf (for masses below the Chandrasekhar limit) or a neutron star or a (hypothetical) quark star. If the collapsing star has a mass exceeding the TOV limit, the crush will continue until zero volume is achieved and a black hole is formed around that point in space.
The maximum mass of a neutron star is not well known. In 1939, it was estimated at 0.7 solar masses, called the TOV limit. In 1996, a different estimate put this upper mass in a range from 1.5 to 3 solar masses.
In the theory of general relativity, a black hole could exist of any mass. The lower the mass, the higher the density of matter has to be in order to form a black hole. (See, for example, the discussion in Schwarzschild radius, the radius of a black hole.) There are no known processes that can produce black holes with mass less than a few times the mass of the Sun. If black holes that small exist, they are most likely primordial black holes. Until 2016, the largest known stellar black hole was 15.65±1.45 solar masses. In September 2015, a black hole of 62±4 solar masses was discovered in gravitational waves as it formed in a merger event of two smaller black holes. As of April 2008, XTE J1650-500 was reported by NASA and others to be the smallest-mass black hole currently known to science, with a mass 3.8 solar masses and a diameter of only 15 miles (24 kilometers). However, this claim was subsequently retracted. The more likely mass is 5–10 solar masses.
There is observational evidence for two other types of black holes, which are much more massive than stellar black holes. They are intermediate-mass black holes (in the centre of globular clusters) and supermassive black holes in the centre of the Milky Way and other galaxies.
originally posted by: Flyingclaydisk
a reply to: chr0naut
Wow...this is a great answer!
I was going to distill things down a little bit, for the sake of simplicity, but your answer is fantastic.
To the OP:
I was going to say...
1. The radius of the event horizon is a function of mass.
2. Time is relative depending on the observers point of view. In other words, the time of "collapse" depends on one's vantage point.
3. The gravitational field before the supernova keeps most of the ejecta within the sphere of influence of the ultimate event. Over time this mass increases.
4. Yes, for a while, but over time the mass of the black hole would increase and steadily draw nearby mass into it.
5. Infinite mass in zero volume is not a possibility. However, remember time factors into this. It's not about volume, but rather mass. Volume has little bearing when contemplating black holes.
Again, more simplistic answers, but Chr0naut's answers are far more thorough.