Scientists have noticed some odd behavior from some of the stars near the very center of the Milky Way, our home galaxy. Some of these central stars appear to be in wild orbits, whipped around by an invisible force, acting somewhat like bees swarming around a hive. One particularly interesting star, known simply as S2, zooms around at speeds of over 3000 miles per second, completing a full galactic orbit in only 15.2 years. Considering most of the stars in Milky Way take millions of years to complete an orbit around the galaxy, that’s incredibly fast for a star.
A recently completed 16-year study of 28 of these so called “hypervelocity stars,” conducted by an international team of astronomers and published in the Astrophysical Journal, has given scientists a better idea of what is causing the stars’ odd orbits. The observational data point to an enormous gravitational pull, emanating from a huge concentration of mass in the small central galactic region known as Sagittarius A* (pronounced Sagittarius A star). The data from the study lend strong evidence to the existing hypothesis that the invisible force at Sagittarius A* is most likely caused by a gigantic black hole. A black hole is a lot of matter squeezed into a very tiny space, producing a gravitational field so strong that nothing falling into one can ever escape, not even light.
Observations of other galaxies have shown that galaxies with a supermassive black hole at their center are not uncommon. In fact, one may exist in almost every galaxy. Take for example the galaxy M87, located 55 million light years away. Streaming from the center of this large galaxy is a jet of high-speed electrons 6500 light years in length. Astronomers believe this is the signature of a black hole billions of times more massive than our sun. Many other galaxies also show evidence of large central back holes.
The black hole at the center of our galaxy is estimated to have a mass of about four million times the mass of our sun. That’s one monstrous black hole, easily capable of eating whole stars. But we don’t have to worry about this monster swallowing us. The distance between our solar system and the center of the galaxy is a comfortable 26,000 light years (153 million billion miles).
Brown, W. R., “Hypervelocity Stars and the Galactic Center.” Galactic Center Newsletter.
Gillessen, S., et al. “Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center.” The Astrophysical Journal 692.2 (2009): 1075-1109.
Nova. “Monster of the Milky Way.” By Julia Cort and Jonathan Gruppe. PBS Television Network. WGBH, Boston, MA. 31 Oct. 2006.
Sol Company. “S2 and Central Black Hole.” http://www.solstation.com/x-objects/s2.htm