Blog Post 9, Free Form: Sagittarius A*

In the last few weeks we have dissected the Milky Way galaxy and analyzed it's composition and structure.  Central to the Milky Way's construction is it's reigning Supermassive Black Hole, Sagittarius A* (pronounced "Sagittarius A-Star").

https://upload.wikimedia.org/wikipedia/commons/7/7e/Sagittarius_A%2A.jpg

Firstly, black holes are the remnants of exploding stars that actually impose on themselves, leaving a singularity in spacetime.  This draws on some intensive math form General Relativity, but basically, it goes like this.  Matter and energy "warp" the fabric of spacetime around themselves.  To use a classic example, imagine a stretchable rubber sheet suspended by it's edges.  Now imagine putting a bowling ball on the sheet.  The "spacetime" will warp around the bowling ball creating a gravity well.  Now imagine placing an extremely dense and heavy BB on the sheet, causing it to stretch downward to almost a point.  That is a good analogy for a black hole.  In reality, the mass of the exploding star implodes on itself, and riches a density that sufficiently warps spacetime to cause a singularity, or a curvature of spacetime too great for light to escape.  That's a normal black hole.

Supermassive Black holes are even more fun.  So, the average "normal" black hole weighs in at about  5-50 solar masses.  For reference, that's about \(10^6\) to \(10^7\) Earths.  Supermassive black holes are a whole other story.  They weigh in at about 100,000 to 1,000,000,000 solar masses.  So, that's about \(3\times 10^{10}\) to \(3\times 10^{14}\) Earths.  These are far to large to be caused by a single star's collapse.  So, how do they grow?  They grow just like anything else: by eating.  Black holes by definition draw everything near them into them.  This let's them accumulate more and more matter through a process called accretion.  Nearby stars, gas clouds, and other black holes are simply consumed by the sheer size of the supermassive black hole and once accreted, simply add to its power.

So, this leads us to Sgr A*.  Our central black hole is the primary force that holds the galaxy together.  By this I mean, it holds the core together, which holds the matter outside of it, which holds the matter outside of it... etc.  Current estimates for Sgr A*'s mass are between 1-5 million solar masses, so it's a nice sized supermassive black hole.  It is likely that in the Milkomeda collision, Sgr A* and Andromeda's central black hole will combine to form a new black hole that will begin to eat it's surrounding post-collision matter and may even ignite into a quasar.

References:
https://en.wikipedia.org/wiki/Andromeda–Milky_Way_collision
https://en.wikipedia.org/wiki/Supermassive_black_hole
https://en.wikipedia.org/wiki/Sagittarius_A*
http://arxiv.org/abs/astro-ph/0210426