The observation of this orbit, reported by Rainer Schödel of the Max Planck Institute (Germany) and by his colleagues, is a triumph of Earth-based astronomy. Although S2 is much larger and brighter than the Sun, its visible light is obscured by dust and does not reach us. However, infra-red (IR) light emitted by S2 can penetrate, and a sophisticated IR camera was used, attached to an 8-meter (diameter) telescope of the Southern European Observatory in Chile. The giant mirror telescope overcame the image-blurring twinkling from the atmosphere by using "adaptive optics" with rapidly adjusting mirrors, and attained the incredibly fine resolution about 1/100 of a second of arc (note the scale on the graph above!).
Monster on a Restricted Diet
Far from being a voracious devourer of stars and of interstellar gas, "our own" black hole is rather benign. Gas falling into it causes X-rays to be emitted, but the emission is weak, apart from occasional "flares" thought to come from the capture of comet-sized chunks of matter. A report in "Science" (30 May 2003, page 1356) calls it "The Milky Way's Dark, Starving Pit" and also suggests an explanation. The galactic center is now inside an expanding bubble of gas, apparently created some 10-50,000 years ago when a supernova exploded nearby. The suggestion is that the front of the bubble sweeps away interstellar gas and keeps down the gas density surrounding the black hole.
So is this black hole what holds our galaxy together? Probably not. If it did, then the motion of stars around it would slow down with increasing distance, in accordance with Kepler's third law. The star S2 obeys Kepler's laws, and other stars near the center do so, too. However stars distant from the center do not slow down as much as expected, suggesting their motion is determined, not just by the attraction of the concentrated central mass, but also by some unseen "dark mass" spread out through the galaxy.
This very significant observation is further discussed here.
The article announcing the discovery (with scientific details): A Star in a 15.2 year orbit around the supermassive black hole at the center of the Milky Way, by R. Schödel et al (22 co-authors), Nature, vol. 419, p. 694-6, 17 October 2002.
A short report in the same issue, describing that work for general readers: Into the heart of darkness by Karl Gebhardt, Nature, vol 419, p. 675-6, 17 October 2002.
Postscript 7.31.2003: Note "The Black Hole at the center of Our Galaxy" by Fulvio Melia, published this year by Princeton University--201 pp, $29.95, reviewed in "Science", 18 July 2003, p. 314. A more detailed discussion of the subject... and it is no more than a coincidence that the titles of the book and of this section are the same!
And by the way...
The Greek legend about a divine mother's milk strewn across the sky is also the source of the word "galaxy," since "gala" in Greek means milk. The ancient Jewish name for the milky way was "river of fire" (nahar di nur).
Questions from Users:
Around what does the Sun revolve? (asked before this section was written).
What holds galaxies together?
"Proper Motion" of Stars
*** Spiral arms of our galaxy
*** Earth crossing Galactic Equator?
*** Is our galaxy held by a Black Hole?
*** Solar system motion through the galaxy
*** Are galactic spiral arms opening or closing?
*** Crossing our galaxy's equator