More Evidence On A Milky-Way Black Hole

admiral2001 writes: "On FoxNews here is an article describing the most decisive evidence to date of the existence of a black hole at the center of the Milky Way. They witnessed and measured a flare of x-ray activity that allowed them to determine the size of the object. Given the mass, the only explanation is a black hole."

Black holes are cool

[Dancin_Santa]

Even if we couldn't observe the emitted X-rays, wouldn't the observation that the galaxy isn't flying apart from the centrifugal force caused by its apparent rotation show that we are indeed held in place by a large gravitational force?

I'm of course not saying that finding external evidence like this isn't cool. Am I completely off in my statement above?

Dancin Santa

Re:Black holes are cool

[krlynch]

Even if we couldn't observe the emitted X-rays, wouldn't the observation that the galaxy isn't flying apart from the centrifugal force caused by its apparent rotation show that we are indeed held in place by a large gravitational force?

The fact that the galaxy is not flying apart is proof that there is a large gravitational force holding it together, but it is NOT proof that there is a large, massive object at the center holding it together. In fact, by the study of galactic rotation curves (plots of the velocity of the luminous objects in the galaxy against their distance from the center of the galaxy) we know that the majority of the stuff holding galaxies together is NOT clustered at the center; it is more diffuse and spread across the entire galactic sphere. (This is the so called "dark matter" because it doesn't interact with light.) The existence of a large black hole at the center of the galaxy is not, therefore, a foregone conclusion. Strong evidence in favor of such a black hole is very interesting, if not entirely unexpected.

Re:Black holes are cool

[Anonymous Coward]

It may very well be that black holes are common in spiral galaxies. It seems to me that spiral galaxies are either very ripe for creation of supermassive black holes, or some spiral galaxies form as accretion disks to a supermassive black hole. But not all galaxies are spirals, and those ones are held together, too. Whether black holes are common in the center of spiral galaxies or not, they probably have a negligible responsibility in holding the galaxy together.

Re:Black holes are cool

[groebke]

>> Even if we couldn't observe the emitted X-rays, wouldn't the observation that the galaxy isn't flying apart from the centrifugal force caused by its apparent rotation show that we are indeed held in place by a large gravitational force?

Firstly, there is no "centrifugal force." There are the forces of inertia, angular momentum, etc. You are looking for angular momentum, (inertia in a arc or circle). Secondly, if the mass of the object at the center of the Milky Way galaxy was large enough to hold the galaxy in place, it would distort the overall shape of the galaxy, removing its spiral characteristic. Thanks, Gerald Roebke.

Re:Black holes are cool

[Yazeran]

Second, why would the black hole need to be so strong that it distorts the shape of the galaxy?

The reason for the beformentioned effect is that if a massive object at the center of the galaxy was responsible for the gravitational pull in the stars orbiting it (e.g. the rest of the galaxy) then the stars orbiting close to the center would have to have a orbital path-speed much higher than the stars further out. This combined with the much shorter orbit would result in the inner stars 'overtaking' the outer ones, thus destroying any radial structure in the galaxy. We know such exsist (e.g. the spiral arms) and thus no single big object can account for the gravitational pull.

If you look at the formulas for celestrial motion (Keplers laws) then this would become obvious. They can be derived from the general gravitational formula: F = G * m1 * m2 / r^2. Where G is the gravitational constant F is the force between the objects (with mass m1 and m2) and r is the distance between them.

Yours Yazeran

Plan: to go to Mars one day with a hammer.

Growth Rate

[FortKnox]

Isn't it a fact that black holes grow at a steady rate? At what rate will it affect the orbit of our solar system?

Re:Growth Rate

[krlynch]

Isn't it a fact that black holes grow at a steady rate?

No, it isn't. Outside of a black hole, it looks gravitationally just like a star. And just like stars do not grow at a steady rate, black holes need not grow at a steady rate. In both cases, rates of growth depend on the local environment surrounding the object; once they have "eaten" everything in their neighborhood, they will not grow anymore.

Re:Growth Rate

[FortKnox]

I thought the phenomenon about black holes is that it growth rate isn't affected by what it "eats". For example, a black hole eats Jupiter, but doesn't grow the size of Jupiter. It just grows at its normal rate. Giving it the phenomenon that we don't know where jupiter went to.

I need to go fish out my physics book...

Re:Growth Rate

[pubudu]

I thought the phenomenon about black holes is that it growth rate isn't affected by what it "eats". For example, a black hole eats Jupiter, but doesn't grow the size of Jupiter. It just grows at its normal rate.

Black holes do not grow if left to themselves; in fact, they shrink (at least according to Mr. Hawking) -- the smaller they are, the faster they shrink. Of course, most observable black holes would radiate less energy than the cosmic background radiation, so this is all rather academic, but the point remains the same: if black holes aren't absorbing energy, they're losing it.

(yes, I know I said black holes aren't black; this is precisely Mr. Hawking's point).

Re:Growth Rate

[CheshireCatCO]

Even if the black holes 'eats' all of the stars in its area, it won't affect the Sun's orbit about the galactic center. All the that Sun sees at this distance is how much mass is as close to the galactic center as it is or closer. What form that that matter is in turns out not to matter. Black holes don't get really wacky, graviationally speaking, until you get close to them. This is exactly why they are so hard to identify with the existing data.