# Why the center of our galaxy doesn't absorb us?

Depending on the theories, the center of our galaxy is a super massive black hole, this is easy to accept as a truth, but what I couldn't simply devour is how the solar system is orbiting around it while not getting absorbed to the inside ? It's simple to understand how earth orbits the sun, but the black hole is something more energetic and at most pulls everything to it's center. By looking to this image for example:

If we follow the bright lines it looks like everything is really going to the true center like a vortex. If you have any simple ways to enlighten me or any references to read I will be thankful, because sometimes I don't know what topic should I search for to find answers without posting questions like this one.

And also, would our galaxy run out of stars since the black hole devours them fast while they take too long to reproduce ?

• Possible duplicate: physics.stackexchange.com/q/73705 – Kyle Kanos Oct 18 '13 at 18:32
• Hint: look up just how fast our solar system is whizzing around the galactic center. Compare it to how fast the planet goes around the sun for good measure. – David H Oct 18 '13 at 18:34
• Not to mention it looks like you put the sun in the wrong galaxy :-) Even if it were a representation of the Milky Way, we are further out than your nice-looking picture has us. So that means we have much more time to worry about being absorbed. :)) – Howard Pautz Oct 18 '13 at 18:45
• For the same reason that the Earth doesn't go whizzing into the sun. – Pranav Hosangadi Oct 18 '13 at 18:53
• Spiral arms are density waves, not streams of matter being sucked into the center. See for instance How do spiral arms form? and How can a spiral galaxy exist? – user10851 Oct 18 '13 at 20:52

The spiral arms don't mean that the mass is getting sucked to the center. They're just wave-like density patterns.

The bodies in orbit around the center of the galaxy are in stable orbit; just like the Earth around the Sun and the Moon around the Earth. What happens is that gravity accounts for the centripetal force (in the orbiting frame, gravity is balanced by the centrifugal force), so there is no net radial acceleration "left over" to suck the body in.

The only reason things would fall into the center is if they were headed there. This can happen if two stars pass by each other and are slingshotted in opposite directions, one of which gets sent to the center of the galaxy.

• There are other ways to draw gas mass into the center, such as gas. Non-axisymmetric density patterns in galactic gas can lead to torques that draw gas into the center, and this may be part of the way that massive black holes grow. But that doesn't seem to be happening to any significant extent in our galaxy now. – kleingordon Nov 27 '13 at 5:16
• Also, I'd assume there is some level of radiative losses of energy, though I doubt they are even observable. – Jerry Schirmer Apr 1 '15 at 15:33

To the original poster: You appear to be operating under the "hollywood" misconception that a black hole somehow "sucks harder" than the same amount of mass in a non-black-hole form. However, this false "black holes produce an enormous sucking" misconception is one of the many, many concepts of physics that "hollywood" gets totally wrong; a black hole of a given mass produces exactly as strong a gravity field as an object made of "normal matter" having the same mass. If, for example, the Sun were somehow instantaneously replaced with a 1 solar-mass black hole, the orbits of all the planets in the solar system would remain unchanged in the slightest, and the only way anyone would know anything had happened would be that "the Sun suddenly went dark."

Nor despite superficial appearances is the galaxy a "bathtub vortex" draining down the central supermassive black hole; as other posters have noted, the spiral arms are not "streams of matter," but rather concentrations of bright, hot, short-lived stars that form in the wake of "density waves" propagating through the gas and dust of the galactic disk.

Thus, as other posters have noted,stars do not "fall into" the central supermassive BH for the same reason that the the planets do not "fall into" the Sun: They are in stable orbits around the galaxy.

Moreover, please also note that the mass of the central black hole in a spiral galaxy is minuscule compared to the mass of the galaxy itself --- only a small fraction of a percent --- albeit that mass does appear to be correlated with the mass of the host galaxy, see e.g. http://iopscience.iop.org/0004-637X/737/2/50/. (Why the mass of every supermassive BH appears to be about the same small fraction of a percent of the total mass of its host galaxy is still an open question.)

• gdp, your post here has been flagged by another user who thinks the tone is too critical of the questioner. As a group the moderators seem to feel it is below the threshold for us to take peremptory action, but you might consider re-reading the post with a view to how it will sound to future visitors. Perhaps an edit is in order? – dmckee Mar 25 '14 at 13:05
• This post is critical of the mindless misconceptions promoted by Hollywood, and IMHO Hollywood can't be criticised strongly enough for promoting misleading concepts of physics. When they make no effort whatsoever to present the truth, they fully deserve all the brickbats they get. – Ed999 May 15 at 23:59
• Gravity is primarily a local effect, because if a star's gravity is measured at a distance of 1 astronomical unit (1 au), then measured again at a distance of 8 au, the field strength at 8 au (8-times the distance) will have fallen to only 1 percent of the strength it had at 1 au. Thus an object in orbit at a distance of (say) 16 au will find all the mass 'pulling' it in a single direction; and if the mass is not a star but a gas cloud (with a diameter of say 4 au, but identical mass) the effect is the same. But if the orbit is at 2 au, the mass will not all be lined up in a single direction. – Ed999 Jun 3 at 0:31

The super massive black hole is creating a force acting on the material in the galaxy, but that material still has angular momentum which needs to be conserved. In a similar way, the earth is in orbit around the sun and it is bound in its orbit by the gravitational potential from the sun's mass. If you were to naively calculate the force on the earth as $F= G m_\text{earth}M_\text{sun}/r^2$ you would conclude that the force would pull the earth in, however you would have neglected the angular momentum of the earth, for it to fall in it would need to lose its angular momentum (L) . Since $L=m_\text{earth} v\times d$, where d is the distance to the sun, this means the earth would need to decrease its orbital velocity to decrease L.

Just remember that a Black Hole doesn't have infinite gravity - it just has however much mass created it in the first place. Yes, anything that gets within the event horizon is trapped forever, but that event horizon will actually be smaller than the size of the equivalent amount of mass composed of ordinary matter. This is also why "microscopic black holes" created at, say, CERN, could never suck the planet in - they're simply too small. If we could create a planet-sucking black hole in a lab, we'd already have to know how to violate Thermodynamics and create mass.

• Conventional theory (the Standard Model) implies that there is literally not enough mass in the world to create a black hole, of any radius whatsoever, since the gravitational density is insufficient unless the available mass exceeds 8 solar masses! Plainly, one measly planet does not contain 8 solar masses. So CERN can fiddle about until doomsday, but will never be able to bring together enough mass to build itself a black hole. – Ed999 May 15 at 23:48

I'm not going to present a complex mathematical argument, nor investigate the abstruse corners of the Standard Model of physics that would require a University-level qualification to comprehend.

I merely wish to make a straightforward logical point, that no one ever seems to consider.

The Galaxy has existed for thousands of millions of years (the Earth itself is at least 4.5 thousand million years old). The Universe is about 13.5 thousand million years old. The Sun has been orbiting the centre of the Galaxy for a very long time, therefore. And the Galaxy is even older than the Sun. But both the Sun and the Galaxy still exist.

If the Galaxy was in some manner unstable (this question imagines that all the stars in it are swirling down an imaginary cosmic plughole, like a bathtub of water with the plug pulled out), why would the Galaxy still exist after such a vast length of time?

I suggest that the reason why the Galaxy is still here, after all these thousands of millions of years, is a proof that the Galaxy is stable. Enough time has elapsed for the theorised disaster to occur, and the Sun to be destroyed by falling into the central black hole, but this has not in practice happened.

I suggest this proves that the Sun is in a stable orbit about the galactic centre of mass, such that the radius of the Sun's orbit is not significantly changed in all these eons, and that the length of time the galaxy has existed proves that it exists in a state of equilibrium, in which its stars are in stable orbits.

Black holes pull matter in thus creating a pulling of matter or rotation (gravity) force that matter rotates or spirals around thus creating our shape form and rotational direction of our galaxy. If our sun doesn't die first then yes one day we will be pulled to the center of our galaxy. Every galactic year our solar system speeds up and grows closer to the center. The reason earth doesn't collide with our sun is were all traveling at a galactic velocity constant! Which is in proportion to our galactic center ( black hole) pulling or swallowing matter, gas, and Solar systems down, a gravitional galactic constant.

• Not sure if this is meant to be commentary on one of the other answers, or an actual answer to the question. Either way, it's pretty vague and I'm pretty sure parts of it are just wrong. Would you consider making an edit to clarify: what your first sentence is responding to (this seems a bit confrontational as well), and what mechanism you're suggesting is causing the decay of the solar orbit? Also, the Earth doesn't collide with the Sun because it is orbiting it, which has at bet very little to do with our orbit around the galactic centre. – Kyle Oman Apr 2 '14 at 22:05
• This answer is so wrong. – Brandon Enright Apr 3 '14 at 0:01
• The basic problem with this answer is that it admits that the Sun is orbiting the galactic centre, and correctly points out that this rotation is accelerating the Sun (due to the frame-dragging effect of Einstein's general theory of relativity), but it fails to draw the correct conclusion from this: that the effect of acceleration is to overcome the attraction of gravity, so that the Sun is gradually getting further away from the galactic centre. An object which is accelerating is gaining momentum, so has more energy to resist the (constant) gravitational strength, hence moves outward. – Ed999 Jun 3 at 0:17