|Discovering Black Holes
in Centers of Galaxies Center of the Galaxy
A galaxy is collection of billions and billions of stars which all orbit around the galaxy's center or nucleus. The center is usually has the most concentration of stars. Many things are unknown about galaxies such why they form and what mysteries await in the center.
You cannot see the black hole, but you can see what’s happening around it. With a binary system, you watch how the black hole interacts with a star. With a supermassive black hole, you watch how stars interact with the black hole.
Quasars (Quasi-Stellar Objects or QSOs) are the brightest objects in the Universe. Like other Active Galaxy Nuclei (AGN), Quasars are extremely compact in size and release enormous amounts of energy.
In 1960, Allan Sandage used the Mount Palomar Telescope to look for a star at the location of a strong radio source. The star's spectrum exhibited a series of bright lines that nobody could identify. Within a couple of years, several more similar 'stars' were discovered, each associated with a strong source of radio waves.
These new objects were at first thought to be an usual type of galaxy. However, these new radio sources turned out to be very compact, and their energy output exceeded that which could be accounted for by the stars they appeared to contain. Optical or visual observations confirmed the objects as star-like, rather than fuzzy, as they would appear to be if they were galaxies.
Since quasars have continuous spectra with emission lines crossing them, they were initially thought to be simply a type of star. But the spectral lines did not match up with any known elements.
The puzzle was solved in 1963, when astronomer Maarten Schmidt compared the spectrum of quasar 3C273 with the Balmer series of hydrogen spectrum. It was found that the two matched up when the spectra was shifted along relative to one another.
The hydrogen lines had been shifted by 15.8% towards the red end of the spectrum. This means that the frequency of the light had decreased and the 'star' was moving away from us. In fact, 3C273 was moving away at a speed of 45 000 km/s (15% the speed of light).
How do these objects exist? Many astronomers currently believe that a supermassive black hole with the mass of billions of suns act as the central engine in a young galaxy. In a way, it seems ironic that quasars, the brightest known objects in the Universe, are believed to be powered by the darkest known objects in the Universe.
In the center of our own galaxy, the Milky Way, rests a starving monster. Only 25,000 light years away, Sagittarius A-star (SgrA*) is a black hole with 2.6 million suns. SgrA* has been the subject of many debates as to whether SgrA* is actually a black hole. Recent evidence may have laid these disputes to rest.
In 1995, Andrea Ghez of UCLA began tracking over 200 stars circling the Milky Way’s galactic center using the Keck I Telescope in Hawaii. Ghez noticed that 20 stars around the black hole were being influenced by a very powerful gravitational force. These stars were moving at around 3 million miles per hour, over 10 times the speed that normal stars travel. Ghez also witnessed the disappearance of a star as it was either pulled into or moved behind the black hole.
Using a technique where computers compare thousands of pictures called "infrared speckle interferometry," Ghez was able to clear away many distortions that previously prevented better observations of the galaxy’s center.
As you can see, new findings in the field of black holes are always being made. In fact, a new class of black holes have been discovered very recently in the spring of 1999.
Continue to Middleweight Black Holes.
Copyright © 1999 ThinkQuest Team EH - 25715.