Friday, September 3, 2010

Around Sagittarius A*

Our Sun is one of the hundreds of billions of stars in the Milky Way Galaxy and it is located approximately 26000 light years from the center of the galaxy. One light year is the distance light travels in a year and its value is 9.46 trillion kilometers. A supermassive black hole named Sagittarius A* sits right in the center of the Milky Way Galaxy and this colossal black hole is estimated to have a mass of four million Suns.

Located within the close vicinity of Sagittarius A* is a very intriguing group of stars called the “S stars”. These stars are the closest known stars to the center of the Milky Way Galaxy and they orbit around Sagittarius A* at very high orbital velocities. Each of the “S stars” are several times more massive than our Sun and being much more massive than our Sun, these stars undergo a much more rapid rate of nuclear fusion which means that they have a lifespan of just several million years. In comparison, our Sun has a lifespan of over 10 billion years.

The “S stars” are intriguing because the environment around such a supermassive black hole is hostile to the formation of stars and the “S stars” would have to form somewhere much further out before migrating to their current extraordinarily close proximity to Sagittarius A*. However, the timescales involved in such a migration is longer than the short ages of the “S stars” and hence, these “S stars” constitute a “paradox of youth”.

S2 is the designation given to one of the “S stars” and what distinguishes S2 from the other stars is that S2 is by far the closest star in orbit around the supermassive black hole - Sagittarius A*. S2 orbits Sagittarius A* in a highly elliptical orbit and in such an extreme gravitational environment near a supermassive black hole, S2 takes just 15.5 years to complete one orbit around Sagittarius A* even though S2 is located at an average distance of about 140 billion kilometers from Sagittarius A*. In comparison, Pluto orbits the Sun once every 248 years at an average orbital distance of 6 billion kilometers from the Sun.

At its closest approach, S2 comes within just 17 light-hours from Sagittarius A* and this is roughly three times the distance of Pluto from the Sun. The highly elliptical orbit of S2 also brings it out as far as 10 light-days from Sagittarius A*. During closest approach, S2 zips around Sagittarius A* at a incredible velocity of over 5000 kilometers per second (about 2 percent the speed of light). The remarkable orbit of S2 around Sagittarius A* makes it uniquely valuable for testing various general relativistic and even extra-dimensional effects.