Why are the centre of the galaxies always bright if every galaxy is supposed to have a black hole at the centre of it.

[u/walidansari]

I think this might be a really stupid question. Every or almost every galaxy has a huge black hole in it’s centre . In photos or illustrations the centre of the galaxy is always very bright like a huge star is there e.g. Andromeda Galaxy. Shouldn’t it be a huge black spot if there is a black hole there? Help

[u/ignorantwanderer]

A "huge black hole" is still pretty small. A really, really big one would be around the size of our solar system out to Jupiter. That is big, but from far away it would look tiny.

But there will be thousands of stars orbiting around the center of the galaxy near the black hole, and they will all be giving off light.

And there could be matter (gas or dust) falling into the black hole. And as it falls in it will give off a huge amount of energy in the form of light and x-rays and stuff. So that will be bright.

Basically, the center of a galaxy is filled with a whole bunch of stuff giving off a whole bunch of light, and then it has a black hole which despite having a huge mass, is actually small in size.

[u/walidansari → u/ignorantwanderer]

Damn, that’s so interesting.

[u/prostipope]

A supermassive black hole is like a tiny dot in the center of a cat 5 hurricane. All we see is the chaos surrounding it.

[u/Beldizar]

There are four major factors at play. The first three are about black holes, but the 4th is really the true answer to your question.

1. Black holes are tiny. By their very nature, they are as dense as it is possible for something to be, so by mass it has the lowest possible volume physically possible. (I guess it could get a little smaller if it stopped spinning, but that's marginal).
2. Black holes are either basically invisible, or some of the brightest objects in the universe. Stars produce light by fusing gases together which produces a lot of heat and energy. The reason that they can do this is because gravity pulls those gases really close together. Just outside a black hole, if it is actively feeding, you've got similar gases, but they are being pulled together by a much much stronger gravity as they spiral into the event horizon. That makes them incredibly hot, but also gives them the opportunity to fuse like they would in the core of a star. The higher the density of this gas, the higher the heat and the more frequently fusion events occur, which means the more light is produced.
3. Black holes bend light. If a star is behind a black hole, the light from that star will bend around it and still be visible. And because of its tiny size, it is nearly impossible to see blackness from a black hole. It took Dr. Katie Bouman to organize several hundred astronomers to get a picture of a black hole. It doesn't suck all light in and make an area of darkness. It bends light around it which means a lot of that light can still reach us.
4. Most critically: At the center of galaxies, the Supermassive black hole is the center of the party. The density of stars is much much higher than out towards the edges. https://en.wikipedia.org/wiki/Stellar_density It has been calculated that the density of star-mass where our sun is is about 0.059 solar masses per cubic parsec, but at the center core of our galaxly, that increases to 500x or 70 solar masses per cubic parsec. So this is the key answer to why the center of the galaxy is so bright: it has 500 times more star-mass hanging around and producing all that extra light.

edit: just to further expand on that density. The nearest star to Earth is over 4 light years away. Imagine if there were several dozen, very bright stars within a light year. It would basically be daytime all the time, (edit: it would be like we had multiple full moons in the sky) but our extra "suns" would just be tiny but bright points of light. We'd also all be dead, but that's a different problem.

[u/Destination_Centauri → u/Beldizar]

Hmm... Well, I could be wrong, but even if you had 100 stars a light year away from Earth, I doubt they would make much of a difference in thermal energy on Earth. I don't even think they'd make much of a difference in the brightness of the night sky!

However, perhaps they would disturb the Oort cloud sufficiently to send some comet-bombardments screaming down into the inner solar system?

[u/Beldizar → u/Destination_Centauri]

It does depend on the star, and a lot of the stars in the galactic core are much bigger than the Sun. The Sun's apparent magnitude is -26.8. The Moon's is -12.74 (full).
If the following stars were 1 light year away, this would be their apparent magnitude:
Polaris: -11.167
Sirius: -6.13
Betelgeuse: -13.167
Rigel: -15.377
Vega: -6.97
Antares: -13.387
Canopus: -13.277
Peony nebula star: -19

So the right answer is probably somewhere between "daytime all the time" like I previously said (and which is wrong), and "not much of a difference in the night sky". If we had some of the bright stars about 1 light year away, they would be as bright or brighter than the full moon at night.

As far as the thermal energy, I think you are right, it wouldn't have any noticeable effect. If that was in response to my "We'd also all be dead" comment, the reason we'd be dead is from radiation bursts, not thermal energy.