Seyfert galaxies
Another type of spiral galaxy is notable for having a very bright nucleus and very faint spiral arms. Such as galaxies were first studied in the 1940s by the American astronomer Carl Seyfert, and are now termed Seyfert galaxies. Because their centres shine so brightly, they can sometimes be mistaken for stars.
Seyfert galaxies are one type of active galaxy, one that has an unusually powerful energy source at its centre. Other galaxies are very strong emmiters of radio waves, and are termed radio galaxies.
The study of radio galaxies started when astronomers began "turning into" the heavens with radio telescopes. One of the first radio sources found was located in the southern constellation Centaurus and named Centaurus A. In 1949 Australian astronomers identified this source with the bright galaxy NGC 5128.
Twin lobes
In light photgraphs Centaurus A looks like an ordinary elliptical galaxy, as do most radio galaxies. But as radio wavelengths they can outchine an ordinary galaxy by up to a million times. Strangely, the radio waves do not come from the centre of the galaxy. They come from twin lobes, regions of space on each side of the galaxy. In the case of Centaurus A, the lobes extend over 2,500,000 light-years. Yet the galaxy is only 30,000 light-years across!
Images of the radio galaxy M87 indicate how the radio
waves are emitted. They show a jet, or stream of matter shooting out of
the nucleus. The jet is a high-speed beam of electrons. As they pass through
the magnetic field surrounding the galaxy, they are forced into a spiral
path around the magnetic lines of force. This constant change of direction
triggers off the radiation we detect as radio waves.
| The Sun orbits arond the center of the Galaxy at a speed of about 900,000 km/h. It is now travelling on its 23rd orbit. |
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