Jul 4, 2013

Sagittarius A*: The black hole at the center of the Milky Way galaxy

An infrared portrait of the core of the Milky Way galaxy revealing bright star clusters and hundreds of thousands of massive stars amidst dust and swirls of hot ionized gas. The galaxy’s black hole lurks within the luminous Central cluster at right. (National Geographic)

It's hard to be modest when you live in the Milky Way. Our galaxy is far larger, brighter, and more massive than most other galaxies. From end to end, the Milky Way’s starry disk, observable with the naked eye and through optical telescopes, spans 120,000 light-years. Encircling it is another disk, composed mostly of hydrogen gas, detectable by radio telescopes. And engulfing all that our telescopes can see is an enormous halo of dark matter that they can’t. While it emits no light, this dark matter far outweighs the Milky Way’s hundreds of billions of stars, giving the galaxy, a total mass of one to two trillion times that of the sun. Indeed, our galaxy is so huge that dozens of lesser galaxies scamper about it, like moons orbiting a giant planet…

Because we live within the Milky Way, we actually know less about its overall appearance than we do about distant galaxies – just as absent a mirror you know more about your friend’s face than your own. Nevertheless, in the past decade astronomers have made numerous new discoveries about our galaxy, beginning with revelations about the huge black hole at its heart.

Every star in the Milky Way revolves around  this black hole, named Sagittarius A* (abbreviated “Sgr A*” and pronounced “Sagittarius A-star”). The sun, 27,000 light-years away, completes a revolution once every 230 million years. Within just a light-year of the black hole swarm more than 100,000 other stars caught far more firmly in its grip. Some take only a few years to complete their orbit. These paths reveal that Sgr A” is four million times the mass of the sun, somewhat more massive than had been thought a decade ago.

Every now and then, the black hole swallows a bit of gas, a wayward planet, or even an entire star. Friction and gravity heat the victim to such high temperatures that it lets out a scream of x-rays. These light lip nearby gas clouds, preserving a record of the black hole’s past feasts. For example in 2004 scientists reported an v-ray echo in gas clouds some 350 light-years from the black hole. Since x-rays travel at the speed of light, the echo indicates that an object fell into the black hole around 350 years ago. The x-ray intensity suggested that it had the mass of a small planet. Another object took the plunge as recently as the 1940s. (National Geographic)