After my shark article, I decided to write a parody with the above title. Then, wandering through an airport, I saw the latest issue of Scientific American and found that someone has already beaten me to it:
The Benevolence of Black Holes
The matter-eating beast at the center of the Milky Way may actually account for Earth’s existence and habitability…
The connection between black holes and life is complex, but our galaxy’s central black hole seems to have made numerous contributions to our ability to exist at this place and time.
Take that, you haters, with your irrational racist prejudice against darker celestial objects.
The article itself is annoyingly written–lots of the meaningless but simple-sounding phrases that scientists put in their writing because they think it makes lay readers more comfortable–but still worth reading. (I like any article about Sgr A*, or about sharks for that matter.) The author, Prof. Scharf, suggests that a galaxy’s central black hole can play a crucial role in regulating the temperature of the gas in a galaxy, and hence the star formation rate, and thus indirectly influence the habitability of a galaxy. Basically, if you want to form stars out of gas, the gas has to overcome its pressure support to collapse under its own gravity. Higher temperature means higher pressure, so the way to keep stars from forming is to make the gas hot. There are lots of things that can heat galactic gas: shocks from supernovae and galaxy collisions being two of the best known influences. The outflows from active black hole regions could be another. (Of the gas sucked toward the black hole, some of it ends up being ejected at high speeds for reasons we really don’t understand but suspect have something to do with magnetic fields. Some models predict that this feedback from the central black hole on the surrounding galaxy can influence the galaxy in various ways, but the influence of the central black hole is not well understood or definitively established.) Too much star formation, and you get lots of supernovae, which could sterilize any inhabited planets within a dozen parsecs or so. Too few, and you don’t get enough supernovae seeding the galaxy with the heavy elements needed for life as we know it. Sharf believes that our supermassive black hole, Sgr A*, is just the right size for a habitable galaxy.
It’s a cute argument but we shouldn’t make too much of it. There are a lot of known influences on star formation rate, some of which are probably more important, so we shouldn’t give all the credit to this one. Also, galactic supernovae being as rare as they are, I’m pretty sure that the population of OB stars in the Milky Way could be significantly larger than it is before the average inhabited planet would stand a strong chance of being sterilized on any given passage through a spiral arm. I suspect the “galactic habitability zone” is in fact quite large, at least for spiral galaxies at the current cosmological epoch.