in the constellation Centaurus
130 million light-years
8 million to 10 million times the mass of the Sun
About half the size of Mercury's orbit around the Sun
The bright core of the galaxy NGC 3783 has been studied by several orbiting telescopes, each of which sees a different type of energy. Hubble Space Telescope provides images in visible light and ultraviolet light, while the Far Ultraviolet Spectroscopic Explorer detects higher-energy ultraviolet rays, and the Chandra and XMM-Newton Observatories collect and analyze the super-high-energy X-ray radiation from the galaxy.
It is fortunate that such a wide variety of instruments are available to astronomers, because the core of NGC 3783 is a complex and turbulent place, and it would be difficult to get a clear picture by studying just one part of the total spectrum.
With careful analysis, a coherent picture of this galaxy core is taking shape, resolving the apparent discrepancy among the different instruments. The X-rays come from gas clouds in the hottest part of the core, within a few light-years of the central black hole, and show the highest velocities -- some of the gas clouds are moving away from the center at a million miles per hour. Because things are moving around so quickly at the center, the X-ray signals get brighter and dimmer pretty quickly, too, flickering by 50 percent from day to day. Farther from the center, the gas clouds are cooler, and emit most of their energy in the ultraviolet. The fluctuations in the ultraviolet light are slower as well, taking weeks or months to change appreciably. Finally, the visible-light spectra change most slowly, on timescales of years.
Since astronomers cannot pinpoint the exact source of the x-ray and ultraviolet energy, they must use mathematical models of this energy to estimate the black hole's mass. Because of this additional uncertainty, the black hole mass estimate is not as secure as those made with more-direct techniques.
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This document was last modified: January 20, 2011.