# Blackhole & Galaxy size

• chris2112
In summary, there is no exact relationship between the size of a galaxy and the density of its black hole. While the mass of the black hole is about 1/700 of the mass of the bulge in big galaxies with bulges, there is no confirmed correlation between galaxy and central black hole mass. However, the density of black holes is consistent across all sizes, with stellar mass black holes having very high density and supermassive black holes having densities similar to ordinary matter. The idea of an infinitely dense singularity at the center of a black hole may not be accurate and is expected to be resolved with a working theory of quantum gravity.

#### chris2112

Is there an exact relationship between the size of a galaxy and the density of its black hole?

For big galaxies with bulges, the mass (not density) of the black hole is about 1/700 of the mass of the bulge. No one knows why but some kind of joint evolution is the most popular theory.

No firm correlation between galaxy and central black hole mass has been established to date. The central black hole masses of Andromeda and our galaxy are quite modest compared to the monster black holes in some similarly massive galaxies. Black hole density, however, is an easier question. All black holes are equally dense.

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"All black holes are equally dense."
This is not true unless you are thinking of the infinitely dense singularity.
The radius of a black hole is proportional to its mass.
The volume is proportional to its radius cubed.
Therefore its density is inversely proportional to the mass squared.
It is very high for stellar mass black holes, but comparable to ordinary densities for supermassive black holes.

Yes, I was thinking about that thing at the center. I do not believe it is infinitely dense, but, so close as not to matter. Infinities suggest the model has broken. When we have a working theory of quantum gravity, the infinite density thing should go away.

## 1. How do scientists measure the size of a black hole?

Scientists measure the size of a black hole by observing its effects on the surrounding matter and light. By studying the orbits of stars and gas around the black hole, they can calculate its mass and use this to determine its size.

## 2. What is the relationship between a black hole's size and its gravitational pull?

The size of a black hole is directly proportional to its gravitational pull. This means that the larger the black hole, the stronger its gravitational force.

## 3. How big can a black hole get?

There is no known limit to the size of a black hole. However, the largest black holes known to exist are supermassive black holes, which can have a mass equivalent to billions of suns. These are found at the centers of galaxies.

## 4. Is there a correlation between the size of a galaxy and the size of its black hole?

Yes, there is a correlation between the size of a galaxy and the size of its central black hole. The larger the galaxy, the larger the central black hole is likely to be.

## 5. Can the size of a galaxy affect the formation and growth of black holes?

Yes, the size and structure of a galaxy can play a role in the formation and growth of black holes. For example, spiral galaxies tend to have larger and more active black holes compared to elliptical galaxies. This is because spiral galaxies have a greater supply of gas and dust, which can fuel the growth of black holes.