Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: katieHaylor on 19/09/2017 15:45:39

Title: How does a star become a black hole?
Post by: katieHaylor on 19/09/2017 15:45:39
Matthew says:

Why is it that a star with a lot of mass can be visible, but then explode shedding mass but then convert to a black hole? After losing mass shouldn't the star be less likely to have the mass to create a black hole? What role does density have on the effect of gravity of an object?

What do you think?
Title: Re: How does a star become a black hole?
Post by: Kryptid on 19/09/2017 15:58:59
According to shell theorem, density has no direct impact on the strength of a gravitational field as experienced by a distant observer: replace the Sun with a black hole of equal mass to the Sun and Earth's orbit would not change although the black hole is significantly more dense. It does have an impact on an object's surface gravity, but only because a more dense object has its surface closer to its center than a less dense object of equal mass does. So it's not really the density per se that makes the gravity stronger, it's just that the distances involved are smaller.

However, the fact that a more dense object has a higher surface gravity than a less dense one of equal mass means that the escape velocity on its surface is higher as well. At some critical point, an object can be so dense that its "surface" escape velocity reaches the speed of light. This occurs when the mass of the object is inside of its own Schwarzchild radius. Black holes don't exactly have a surface, but the radius of their "event horizon" (the point of no return) is equal to their Schwarzchild radius.
Title: Re: How does a star become a black hole?
Post by: jeffreyH on 19/09/2017 17:35:49
When an object becomes dense enough it is able to overcome electromagnetic forces. This means that protons would no longer be able to repel each other. Under these conditions it is theoretically possible to have neutrons form spontaneously via electron capture. This will result in a neutron star. One stage on from this is the black hole. What lies beyond the event horizon would have to be open to speculation since it can never be observed outside it.
Title: Re: How does a star become a black hole?
Post by: evan_au on 19/09/2017 21:39:36
It is estimated that a star with mass around 20 times the mass of the Sun will burn through all its Hydrogen fuel much faster than the Sun (in around 20 million years), and explode as a supernova, leaving behind a black hole with a mass around 5 times the mass of the Sun.

Once all the Hydrogen has been burnt, this star will enter a Red Giant phase, burning heavier and heavier elements and producing iron in its core. In its Red Giant phase, this star would easily swallow Earth and Mars, and it emits a lot of light.

After producing iron, nuclear fusion can produce no more heat to sustain the star against its own enormous gravity, and it will collapse to a neutron star; the inertia of the infalling matter will further compress it until the escape velocity in the core equals or exceeds the speed of light.

Because nothing material can exceed the speed of light, nothing will come out of this central black hole, not even light.

On the other hand, the outer parts of the star's atmosphere that were falling inwards are hit by a massive blast of neutrinos from the (temporary) neutron star, and these are blasted into space, to seed new planetary systems with heavy elements. This supernova can outshine a whole galaxy for a short time.

The remaining black hole absorbs far more radiation and matter than the tiny amount of Hawking radiation that it emits, so it is effectively black and cold (with an effective temperature measured in nanokelvins).

See: https://en.wikipedia.org/wiki/Black_hole#Gravitational_collapse