I’ve read the same glossy description…that standard text repeated in almost every reference that glosses over the details, but I remain unclear on the specifics, such as: When a main sequence star goes “red giant” and swells to enormous proportions, does its mass increase, too? Or, is the mass constant (compared to just moments before going “red giant”)? If the former, where does the additional mass come from?
Of course the mass does not increase. As you asked yourself, where would the additional mass come from?
As it swells, its average density goes down. The mass stays the same.
As the density goes down so does the temperature, which is why it cools down to red.
OK, then on a related note, I was once under the impression a dying star becomes a super-massive back hole for which I asked (in a previous thread) where does the additional mass come from? The SDopers informed me the mass remains the same. I thought the gravity of a black hole somehow became exponentially huge; yet, this seems to be false…perhaps a false image spawned by Hollywood. (I mean, if mass is not increasing, and mass equates to a measure of the gravity, then gravity is not increasing, either).
So, if all the above is correct, how is it that light can leave a star - even a red giant - but cannot escape a black hole when that star collapses in on itself? Something here seems a bit paradoxical, don’t you think? Please feel free to clarify if you see where my logic has gone astray.
It’s all about density.
A big star is well, big. When it dies, and it’s fusion power can no longer force the atoms apart, it collapses, and if it’s big enough, it can become a black hole. The mass is the same, but the density goes up immensely (the diameter of the star becomes much smaller). Note that this ignores the whole supernova stage…
Mass alone does not make an object a black hole, rather cramming mass together closely enough so that escape velocity exceeds the speed of light. The relevant radius is 2G/c[sup]2[/sup] times the mass, where G is the gravitational constant and c the speed of light.
Regular stellar-remnant black hole: 5 to 10 times the mass of the Sun.
Supermassive black hole = big. Sits in the center of a galaxy. Small average density; mass is millions to billions of suns. You are not getting that mass from a random supernova, and not exponentially; it would have to be swallowed over time.
Finally, why is a (small or big) star not a black hole? Because you would have to squeeze the whole thing down to a few kilometers’ radius, but the star generates pressure that keeps it from collapsing.
Most of the red giant is low density gas;
“In a red giant a huge, cool, low-density hydrogen envelope (with a density of about 0.1 kilograms/m3) encloses a small, hot, high-density helium core (with a density of about 1,000 tons/m3)”
So the envelope, which with our sun would reach out past the Earth’s orbit, is thin with a density of about 3.5 ounces per cubic meter. The core remains very dense and that is where the fusion continues until the star exhausts itself.
While much of the mass will be blown off into space, forming a nebula, the remaining core is what collapses into a black hole (given sufficient mass, which would be more that of our sun, which…)
“The final naked core, a white dwarf, will have a temperature of over 100,000 K, and contain an estimated 54.05% of the Sun’s present day mass. The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to a hypothetical black dwarf.”
A Black Hole of a given mass has no more gravitational influence at planetary distances or greater than any other type of star with the same mass. The effects get weird when you get closer than the radius of the normal star. Some of the mass of the star would then be above you, and therefore pulling you in the opposite direction. At the center of the star all the mass is “above” so you would be pulled upward evenly and experience no gravitational acceleration.
In a neutron star the mass is below you for much longer, and in a Black Hole it is always below you.
From a distance, a spherical cow acts as a point mass, the closer you get to the point, the stronger the gravity. If the sun were to collapse into a black hole right now (it can’t, but if it did) the planets would continue orbiting normally as if nothing had ever happened. If the Earth did so, did the same, there would be some effect (being an imperfectly spherical cow before but not as a black hole, orbits would smooth out, and maybe (can anyone clear this up?) the moon would stop receding from tidal drag) but noting would get “sucked in.”
The fact that real objects are fluffy and not literal points only matters when you are reasonably close to the object. But if you take out all of that pesky empty space from the matter and collapse it into a black hole, it begins to actually be a pointish mass (we think.) Dig thousands of miles down into the fluffy Earth and gravity decreases because part of what is above you will cancel out part of what is below you. Travel those same thousands of miles from the former surface towards a black hole Earth, and gravity never stops increasing.
It’s currently unknown exactly how supermassive black holes form (this is one of the known unknowns we hope to learn from observations of gravitational waves). You could, in principle, make one by starting with a stellar-mass black hole and feeding it a lot, but it’s believed that this would be too slow, and our Universe too young, for this to have actually happened. Alternately, they could form from mergers of smaller black holes. How exactly this would happen is also unknown: It could involve a single large hole that steadily eats smaller holes, or it could be holes of comparable size merging to form larger holes, and then those larger holes merging with each other, and so on.
Or they could be inherent to galaxy formation, which strikes me as obvious. They were always there. Galaxies and super-massive black holes are two sides of the same process.
They probably are inherent to galaxy formation, in that you need something as big as a galaxy to get that much mass together in one place. And we’ve never seen a galaxy without a supermassive black hole, or vice-versa (though I wouldn’t bet that there aren’t any out there somewhere). But that still doesn’t address how the hole itself grows as the galaxy coalesces. They weren’t “always there”, because the galaxies themselves weren’t always there. Though it is perhaps fair to ask if one formed before the other, and facilitated the formation of the other, or if they formed at the same time: That’s another question we’re not entirely sure about.
Not that I have the slightest understanding of this subject, but don’t forget dark matter. It makes up the vast majority of the mass of a galaxy. How the total mass of a galaxy compares to the mass of a super-massive black hole I have no idea. But While the black hole will be a significant portion of the visible stars, it probably is small compared to the mass of the entire galaxy.
Generally dark matter makes up around ninety percent of the mass of a galaxy. The mass of the supermassive black hole compared to the galaxy varies, for the Milky Way it’s something like one ten thousandth of the stellar mass or one millionth of the total mass. That’s pretty typical, and the biggest supermassive black holes are estimated to be no more than one part in a hundred of the galaxy’s mass, nowhere near the dominant mass.
And it’s simple to calculate μ.
That’s udderly terrible Tom.
I thought that constant pertained to cats.
Dark matter is probably indirectly important in black hole formation, in that it’s important for galaxy formation. But it’s probably only a very small portion of the mass which went into the hole. Any dark matter which reaches the horizon, a black hole will eat just fine, but dark matter won’t form
(or be affected by) the accretion disks which make it much easier for matter to end up falling in.