The Milky Way is nothing else but a mass of innumerable stars planted together in clusters.
The key to proving that there's a black hole is showing that there's a tremendous amount of mass in a very small volume. And you can do that with the motions of stars. The way the star moves around the center of the galaxy is very much like the way the planets orbit the sun.
One of the big mysteries about the black hole at the center of the galaxy is, 'Why don't we see emission from matter falling onto the black hole, or, rather, the black hole eating up its surroundings?'
There's a large cluster of stars that are orbiting the center of our galaxy. And by measuring the motion of stars, and in particular, their orbits, we can figure out whether or not there's a central black hole.
All of the stars rotate, have orbits around the center of the galaxy, and most of them go around the center of the galaxy in nearly circular orbits. They vary a little bit from circular, but they're predominantly circular.
I'm interested in finding whether or not there is a really massive, what we like to call 'super massive' black hole at the center of our galaxy. And the reason this is interesting is that it gives us an opportunity to prove whether or not these exotic objects really exist.
If you want to see a black hole tonight, tonight just look in the direction of Sagittarius, the constellation. That's the center of the Milky Way Galaxy and there's a raging black hole at the very center of that constellation that holds the galaxy together.
If we're thinking about old civilizations, those that formed a long time ago and there were stars and planets around long before Earth even existed, then these are going to be towards the center of the galaxy. That is the place to look if you think there are ancient civilizations that have made beacons or some other way of attracting our attention.
Scientists have discovered a black hole that is 12 billion times the size of our sun. It's full of Hillary Clinton emails.
Out Milky Way is the dwelling; the nebulae are the city.
Our galaxy's pretty ordinary, garden-variety. So if we believe our galaxy has a super massive black hole, that tells us that most, if not all, galaxies host such a black hole at their centers.
The great spirals... apparently lie outside our stellar system.
Data suggest that central black holes might play an important role in adjusting how many stars form in the galaxies they inhabit. For one thing, the energy produced when matter falls into the black hole may heat up the surrounding gas at the center of the galaxy, thus preventing cooling and halting star formation.
As an astronomer, I get to ignore the details of the things that we don't understand. There's a lot of work that we can do on scales that we do understand, and there is actually a finite size that I can associate with a super massive black hole.
The question that I started off with was, I thought, very simple. It was just 'Is there a massive black hole at the center of the Milky Way?' But one of the things I love about science is that you always end up with new questions.
The central region of the Milky Way, known as the bulge, is stuffed with literally tens of billions of stars. And most of these are old - considerably older than our Sun or its neighbors - because this part of the galaxy formed first. Consequently, bulge stars are generally deficient in heavy elements.
