The region of the Galactic center is a pretty interesting place within our Milky Way, and we are constantly learning new things about it as our observational technology improves. Since our Galaxy (like all galaxies) contains lots of mass, the "Galactic center" is the center of mass of the Milky Way. It is the point about which the rest of the Galaxy rotates, once every 220 million years or so. Both the Galactic bulge and the spiral arms are centered on this point, though since the Galaxy is made up of discrete bits of matter, neither is truly "attached" to the center but is merely bound to it by gravity.

In ancient times, before people understood what the Milky Way physically was, what we call the Galactic center was merely a brighter patch of light within the larger band of light arcing across the sky. It wasn't until the invention of the telescope that people could actually resolve the diffuse light of the Milky Way into stars, and people began to consider the Milky Way as a disk of stars. Until the early part of the 20th Century, what we today call the "Milky Way" was considered to be "the Universe". It wasn't until the time of The Great Debate and the work of Vesto Slipher and Edwin P. Hubble that people realized that the Universe was far, far larger than our own Milky Way, and that the "spiral nebulae" were galaxies like our own at great distances. It was then that a more accurate physical understanding of the Galaxy and our place in it began to take shape. We realized that our solar system resides within a galaxy (the Milky Way), that we are approximately 30,000 light years from the Galactic center, and, most importantly, that our Milky Way is just one of millions of other "spiral nebulae" (spiral galaxies) observed in the universe. The universe (or at least our perception of it) grew in size by many orders of magnitude with this realization.

The middle of the 20th century brought some new advances in our understanding of our own Galaxy and the Galactic center itself. For one, the engineer Karl Jansky discovered radio waves coming from the center of the Milky Way, suggesting that astronomical objects other than our Sun could be sources of radio waves. During World War II, the astronomer Walter Baade took advantage of the blacked-out skies of Los Angeles to discover a window through the obscuring dust and gas of the Galactic plane into the core of our Milky Way, discovering a new population of stars within the bulge. He discovered that these stars were different from the stars within the plane: they were older than the stars of the Galactic disk, and probably formed billions of years before our own Sun.

As the Century progressed, new astronomical technologies allowed us to study the center of our Galaxy in even greater detail, particularly at wavelengths of light not visible to our eyes. Low-energy infrared, microwave, and radio observations allow us to peer through the obscuring dust into the very center of our Galaxy. Radio observations were particularly important since they resulted in the detection of a strong radio source -- Sagittarius A* -- in 1974.(1) It is now considered to be coincident with the very center of the Galaxy. Two fairly recent discoveries in radio and infrared astronomy have essentially proved the existence of a black hole at the center of our Galaxy. In the infrared, astronomers have taken time-lapse pictures of stars orbiting about Sgr A*, and by analyzing their orbits, they've shown that the object they orbit must have over a million times the mass of the Sun packed into a very small region.(2) The only stable way to contain that much mass in such a small size is to have it all contained within a black hole. And in a very recent discovery, astronomers used the Very Long Baseline Array -- a radio interferometer nearly the size of the western hemisphere -- to image the shadow of the event horizon of the black hole itself.(3)

Observations at high-energies (X-rays and gamma rays) have allowed us to peer into the very core of our Galaxy as well. It is known now that several bright sources of X-rays lie near the Galactic center, mainly X-ray binary stars that are powered by the accretion of matter from ordinary stars onto white dwarfs, neutron stars, and stellar black holes. Accretion can be a very violent process, and the intense heat and energy created can generate X-rays and gamma rays. There also appears to be diffuse X-ray emission near the center of the Galaxy, which had been assumed to come from supernova remnants of massive stars. Recent work by astronomers using the Chandra satellite suggest that this X-ray light may instead come from the dozens or hundreds of X-ray binaries and other compact stars in the Galactic center region.(4) Finally, gamma rays characteristic of the annihilation of electrons and positrons were discovered coming from the Galactic center in 1997. While this may be considered a signature of the black hole powering Sagittarius A*, the broad spatial distribution suggests the antimatter may also be generated by supernovae or other high energy sources.(5)


Of course, while all of this scientific information is quite exciting and worth knowing, the Galactic center has a very visceral appeal to casual stargazers, too. One of the great tragedies of modern urban civilization is the loss of the night sky, particularly our ability to see the Milky Way at night. On the few lucky occasions I've been able to get far, far away from city lights at night, the sight of the Milky Way was literally breathtaking. The sight of the Galactic center rising over the Andes in July of 1999, high overhead and neatly framed by Sagittarius and Scorpius, was one of the highlights of my life as an astronomer. It is something everyone who loves the night sky should try to do at least once.

The viewing season of the bulge of the Milky Way peaks in the months of June and July, and since the Galactic center is at a declination of -29°, the best viewing locations are in southern hemisphere and low northern latitudes.


Sources:
(1) Balick, B. & Brown, R.L., 1974, "Intense sub-arcsecond structure in the Galactic Center", Astrophysical Journal v194, 265-270
(2) Schödel, R. et al., 2002, "A Star in a 15.2-Year Orbit Around the Supermassive Black Hole at the Centre of the Milky Way", Nature v419, 694-696
(3) Bower, G.C. et al., 2004, "Detection of the Intrinsic Size of Sagittarius A* Through Closure Amplitude Imaging," Science v304, 704-708
(4) Wang, Q.D., Gotthelf, E.V., and Lang, C.C., 2002, "A faint discrete source origin for the highly ionized iron emission from the Galactic Centre region," Nature, v415, 148-150 (see also http://chandra.harvard.edu/photo/2002/gcenter/)
(5) Kinzer, R.L. et al., 2001, "Positron Annihilation Radiation from the Inner Galaxy," Astrophysical Journal, v559, 282-295 (see also http://antwrp.gsfc.nasa.gov/apod/ap970501.html)

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