# What's in the center of a Galaxy?

Ok, so it's supposed that there is a black hole in the center of any galaxy, that makes my head explode because:

1. There is light in the center of the galaxy, but it is supposed that black holes, also attract light.
2. For a black hole to exist there should be a star explosion, so what size does the star needed to be to create such a galaxy.
3. Why does the black hole doesn't eat us, I mean what objects are pulling us away from the extreme gravity of the black hole.

I'm not god, I didn't study Astronomy, I'm just a kid fascinated by the Universe. (This question should be dumb for anyone who studied Astronomy)

But if you think my question is dumb, (which isn't) or if you have any recommendation for further asking, let me know!

• Welcome to Astronomy @Antonio! – andy256 Jun 3 '15 at 3:50
• Related, though specific to the Milky Way. – HDE 226868 Jun 3 '15 at 12:26

## 4 Answers

At the center of our galaxy is a powerful radio source named Sagittarius A*, which is believed to be a super massive black hole (SMBH). This blackhole would contain far more mass than your run-of-the-mill supernova remnant. Our galaxy is believed to contain a SMBH containing the mass of likely a bit above 4 million times the mass (Gillessen) (2) (Ghez) of our Sun. For reference, I don't think we've ever discovered a star more massive than 600 times that of our Sun.

It's also important to understand that while many people look at blackholes as mystical or all-consuming, they actually have to follow the same rules as everyone else in the stellar neighborhood. The stars that make up our galaxy don't fall into the black hole for the same reason our planet doesn't fall into the Sun. Our star orbits black hole, our star system's velocity in equilibrium with the attractive force of the galaxy's center of gravity. This should hopefully resolve point 3.

For point 1, we should make clear that the 'black' part of the black hole is only true once you cross the event horizon. This is the case because at this point the escape velocity to escape the gravity of the black hole because greater than the speed of light. The light that isn't within the event horizon and is moving away from it is free to escape. So we can see light around it. But why is there so much light? Well, as it happens there are a lot of rather young and large stars in this area. It is not completely understand why this is the case. Lots of stars, lots of light! There are other factors that can contribute to this too, such as there simply being lots of stars between us and the center, not just in the center itself. The accretion disk of a black hole can also be exceptionally bright. Hopefully that clears up part 1.

Now for part 2. As far as I know, we don't really have any way of determining exactly where our SMBH came from originally. Black holes aren't necessarily formed just from a supernova event, there are a handful of other ways they can be created in nature. What is apparent, however, is that SMBHs contain far too much mass to be from a single star. It has probably consumed plenty of other black holes to grow to what it is now.

One interesting and notable difference between the comparison of a star system and a galaxy is the distribution of mass. While our Sun is believed to contain 99.8% of the mass of our solar system, the SMBH at the center of the Milky Way is not nearly as massive as the total mass of the Milky Way. The ratio can vary a lot, and there are some galaxies which are believed to host no SMBH at all.

Gillessen, Stefan et al. (23 February 2009). "Monitoring stellar orbits around the Massive Black Hole in the Galactic Center". The Astrophysical Journal 692 (2): 1075–1109.

Ghez, A. M. et al. (December 2008). "Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits". Astrophysical Journal 689 (2): 1044–1062.

• @RobJeffries Edited that portion out. You're correct, I mispoke (miswrote?). However, the last I had read on the subject opinions were a bit mixed as to whether a SMBH was necessary for the formation of large galaxies, or whether a SMBH was the result of the formation of a large galaxy. – Mitch Goshorn Jun 3 '15 at 7:00
• Wouldn't surprise me much if Sagittarius A* didn't occasionally wander outside the center of mass of the galaxy. Given the size of Milky Way, and propagation speed of forces, things will be a bit sloppy. – Wayfaring Stranger May 5 '18 at 15:39

If you understand the facts, your head won't explode!

1. You are right, light cannot escape from even a black hole, so why is there so much light in the center? Well, simply because there is a lot of light in the center of the galaxy, because there is a high concentration of stars. These stars are very far away from us, so the light from each together create a "halo" effect in the center, which makes it bright.

2. The type of black hole you are thinking of is a stellar black hole, caused by a supernova which eventually leads to a neutron star collapsing. Not all supernovae; become black holes, look up the Chandrasekhar Limit if you want more information. So it is believed that galaxies contain supermassive black holes at the center and the mass of the supermassive black hole is equivalent to the entire galaxy!

3. This black hole won't eat us. You have the typical misconception of black holes. Black holes don't have super sucking power; in fact, they do not "suck" at all. It is only if you get close to the event horizon of a black hole that you cannot escape it's gravitational pull. If the Sun became a black hole we wouldn't get "sucked" into it because it would have the same mass as itself before, and we would orbit it normally. Gravity and inertia is what keeps us in orbit and the stars in our galaxy in orbit, so as long as we stay in orbit, we will be okay. By the way, this supermassive black hole at the center of our galaxy is about 30,000 light years away, so even if we somehow get out of our normal orbit and head towards the center, we won't get "sucked" in for a very long time.

There are no dumb questions, just dumb answers.

• Worth pointing out that we can't "see" the Galactic centre. For that you need a radio or infrared telescope. Your answer to part 3 is not helpful and does not address the central misconception. We are much closer to the Sun, but we don't fall into that either. – Rob Jeffries Jun 3 '15 at 6:51
• I wouldn't consider a SMBH to be equivalent to an entire galaxy (at least not to the galaxy they're in). The mass of the milky way has been measured to be around 0.8-1.5x10^12. A pretty big difference from Sgr A*'s 4.1x10^6. The smallest ratio I know of is M60-UCD1, which hosts a SMBH of about 10% of the galaxy's total mass. – Mitch Goshorn Jun 3 '15 at 7:56
• @RobJeffries you are correct, I have updated my response to part 3 of the question and welcome any further comments. – NuWin Jun 3 '15 at 8:01
• Adding on to what @RobJeffries said: the reason we can't see the center is the large amounts of gas and dust blocking our view. – HDE 226868 Jun 3 '15 at 12:12
• @HDE226868 dust scatters, but how does gas help block the light at visual wavelengths? Or does "see" refer to a wider range of wavelengths? – uhoh Aug 20 '17 at 15:13

I was about to put this in a comment, but it did not let me because it is too long. So, I will just put it here.

There is a certain radius from the center of the black hole, where light inside this radius cannot come out, i.e., event horizon.

The light that we see is from materials those are spiralling inwards into the black hole, and they are outside the event horizon. These materials experience frictions, therefore they are heated up, and lose their energy into radiation.

Black hole can be formed by various mechanisms, according to theories, with or without star explosions (a.k.a. supernovae, SNe). For star-forming black hole, the size, which is typically referred to the zero-age main sequence (a.k.a. ZAMS) mass, is still under debate. But for sure, the star has to be "massive" $> 8$ solar masses in ZAMS. Note that these massive stars might form neutron stars or black holes after the SNe; also the stars might not leave any compact object if the explosions are pair-instability SNe.

For a galaxy's black hole, how it is formed is still unknow. One theory is from the accumulation of stuff after SNe, like you said. But, we don't know exactly at this moment, therefore to answer about the "size," I don't think it can be answered.

A black hole is just another massive object possessing huge gravitational potential, like our Sun. Therefore, there are orbits those are stable and are not attracted into the center of the potential.

• Sgr A* is fairly quiet, with no accretion disk to speak of, so there's not much light coming from its immediate vicinity. But I'm sure you know that and were just talking about the EM radiation from active black holes. – PM 2Ring May 4 '18 at 21:18

In the middle of the Milky Way is a supermassive black hole and a strong radio source in Sagittarius A*. It is 26 thousand light years away from us. Latest estimates estimate the black hole to be 4.31±0.38 million solar masses. It's Schwarzschild radius is about 12Gm(0.08AU).