Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2002 October 18 
Posted on 10/18/2002 5:02:21 AM PDT by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2002 October 18
Explanation: At the center of our Milky Way Galaxy lies a black hole with over 2 million times the mass of the Sun. Once a controversial claim, this astounding conclusion is now virtually inescapable and based on observations of stars orbiting very near the galactic center. Using one of the Paranal Observatory's very large telescopes and the sophisticated infrared camera NACO, astronomers patiently followed the orbit of a particular star, designated S2, as it came within about 17 light-hours of the center of the Milky Way (17 light-hours is only about 3 times the radius of Pluto's orbit). Their results convincingly show that S2 is moving under the influence of the enormous gravity of an unseen object which must be extremely compact -- a supermassive black hole. This deep NACO near-infrared image shows the crowded inner 2 light-years of the Milky Way with the exact position of the galactic center indicated by arrows. NACO's ability to track stars so close to the galactic center can accurately measure the black hole's mass and perhaps even provide an unprecedented test of Einstein's theory of gravity as astronomers watch a star orbit a supermassive black hole.
Thanks for the daily post and ping!
Photo 23b/02 shows an infrared NACO image of a ~ 2 x 2 arcsec2 area, centred on the position of the compact radio source "SgrA*" at the centre of the Milky Way Galaxy; it is marked by a small cross. The image was obtained in the Ks-band at wavelength 2.1 µm in May 2002 and the angular resolution (image sharpness) is about 0.060 arcsec. At about the same time, the star designated "S2" came within 0.015 arcsec of the radio source. At the distance of the Milky Way Center, 1 arcsec on the sky corresponds to 46 light-days [5]; the bar is 20 light-days long (0.44 arcsec). In PR Photo 23c/02, "SgrA*" and S2 are identified in the left panel. The right panel displays the orbit of S2 as observed between 1992 and 2002, relative to SgrA* (marked with a circle). The positions of S2 at the different epochs are indicated by crosses with the dates (expressed in fractions of the year) shown at each point. The size of the crosses indicates the measurement errors. The solid curve is the best-fitting elliptical orbit - one of the foci is at the position of SgrA*. The 2002 data points come from NACO observations done during the early commissioning, fine adjustment, and Science Verification phases for this instrument; these data were promptly made public through the ESO Archive, cf. the NACO data webpage.
--But, do you know it will take 5000 years to get there from here?
Oh, that is a long time, we better start today.
The jury is still out, but since most large galaxies show signs of harboring one or more central black holes, I'd say they develop at the galactic core very soon after galactic formation.
How? Theory: a very large star - 100 solar masses or more - develops at the hub and quickly runs out of fusion fuel. Maybe it has one or more companions. Anyway, said massive star explodes, creating a supernova. All the material from the star's outer layers is blown outward, along with the outer layers of any companion stars. At the same time, the core implodes due to the lack of radiation pressure. It's so massive in itself that it cannot stop collapsing. It shrinks past degenerate material, past neutronium. It winks out of known existence, leaving only a "singularity", a point of infinite gravity. This warps spacetime. How big the sphere of influence depends on its mass.
As the crowds of stars, dust and gas get too close to the black hole, they get whirled around the accretion disk then get sucked in like water going down a drain. Thus the black hole grows.
If the infalling material overloads the accretion disk, some of the x ray-hot plasma will be thrown out along the spin axis as a jet thousands of light-years long. See the 10-17-02 APOD or look up Seyfert galaxies and quasars.
A heckuva lot longer than that. IIRC, the center of the Milky Way is about 40,000 light years from Earth. About 250,000,000 years away at the speed of a Voyager type probe.
Freedom, Wealth, and Peace,
Francis W. Porretto
Visit the Palace Of Reason: http://palaceofreason.com
Freedom, Wealth, and Peace,
Francis W. Porretto
Visit the Palace Of Reason: http://palaceofreason.com
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