Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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: enter image description here

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 ?

share|cite|improve this question
Possible duplicate: – 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? – Chris White Oct 18 '13 at 20:52
up vote 11 down vote accepted

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.

share|cite|improve this answer
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. (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.)

share|cite|improve this answer
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

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.

share|cite|improve this answer

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.

share|cite|improve this answer

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.

share|cite|improve this answer
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

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.