I spent last week attending the “Formation and Evolution of Black Holes” conference at the Aspen Center for Physics, organized by Andrea Ghez, Vicky Kalogera, Fred Rasio, and Steinn Sigurdsson (who blogs over at the Dynamics of Cats). It was a great mix of observers and theorists, and we covered the full range, from stellar-mass black holes in our galaxy to supermassive black holes on the far side of the Universe. I was particularly interested in two topics: gravitational-wave recoil and black hole binary inspiral (I’ll blog about both soon enough). And I made another pilgrimage to the Highlands bowl, this time with 15″ of virgin powder.

The Aspen Center runs a public lecture series in conjunction with each conference. So last Wednesday Andrea Ghez gave a lecture on the black hole at the center of our galaxy. It’s our closest big black hole, roughly 25,000 light years (2×10¹⁷ kilometers) away, and four million times the mass of our Sun. Andrea has been leading a team studying the motion of stars orbiting around this black hole. These orbits are one of the best ways (short of the detection of gravitational waves from black hole mergers) of confirming that black holes exist. The orbits tell us the mass of the central object. And the innermost passage of the closest orbit gives us an upper limit on the size of the central object. Combining these numbers gives us a lower limit to the density of the “dark object” at the center of our galaxy. At this point, a black hole is the only viable model for what we see. There is no way to make sense of the orbits using a cluster of (dark) stars at the center, or a massive gas cloud, or anything else we can think of. Gravity tells us that any normal stuff we put there (including “conventional” dark matter) will evaporate or collapse to a black hole. We are not yet probing the horizon of the black hole (in some sense, its surface), but we are getting closer and closer with each passing year.

But, more importantly, Andrea is responsible for one of the coolest movies in all of science:

This shows the orbits of stars around our galactic center. This isn’t an artist’s conception. This isn’t some abstraction of other data. This is a real movie of stars circling the black hole over the last 15 years. In particular, watch S-02. It loops around the black hole, and closes its orbit; we have watched it over one full S-02 “year”. It is an incredible feat of observational astronomy to make these movies. It requires adaptive optics on the largest telescopes in the world (the Keck telescopes on Mauna Kea). We used to think of the heavens as eternal and unchanging. Now we watch movies of stars orbiting black holes.