Quasars Looks Can Be Deceiving

After seeing the spectacular optical images of emission nebulae (see Chapter 18, "Stellar Careers,") and galaxies such as Andromeda (see Chapter 21, "The Milky Way: Much More Than a Candy Bar,") a quasar can make a disappointing first impression. While the optical counterparts of the brightest of them can be seen with an amateur telescope on a dark night—3C 273 is 12th magnitude—they are undistinguished. In fact, they look so much like stars that astronomers at first thought they were simply peculiar stars (quasi-stellar objects).

But keep in mind how incredibly distant these objects are. The closest quasars are some 700 million light-years away! Though their apparent brightness might be small, their luminosities (the amount of energy that they put out each second) are astounding.

Star Words

Quasar is short for "quasi-stellar radio source." The first quasars were detected at radio frequencies, though most quasars do not emit large amounts of radio energy. Quasars are bright, distant, tiny objects, which produce the luminosity of 100 to 1,000 galaxies within a region the size of a solar system.

Star Words

Quasar is short for "quasi-stellar radio source." The first quasars were detected at radio frequencies, though most quasars do not emit large amounts of radio energy. Quasars are bright, distant, tiny objects, which produce the luminosity of 100 to 1,000 galaxies within a region the size of a solar system.

Quasars appear optically faint for two reasons: (1) much of their energy is emitted in the nonvisible part of the spectrum, and (2) quasars are very distant objects.

The great distance to quasars truly became apparent in the 1960s. The first quasars were discovered at radio frequencies, and optical searches at these locations showed objects that looked like stars. But the spectra of these "stars" told a different story. In the early 1960s, the astronomer Maarten Schmidt made a stunning proposal. The four bright spectral lines that distinguish hydrogen from the rest of the elements in the universe were seen to be shifting to longer wavelengths—much longer wavelengths. These redshifted lines, along with Hubble's Law, indicated that quasars were very distant—billions of light-years away, in fact.

This image from the Very Large Array shows the detailed structure of Cygnus A, located in the constellation Cygnus, the swan. One of the brightest radio sources in the sky, Cygnus A is one of a large number of radio galaxies that has been discovered, with "jets" that arise from the region around a central black hole.

(Image from NRAO)

Quasar 3C 273 (the 3C stands for the Third Cambridge Catalogue) has its spectral lines redshifted in velocity by 16 percent of the speed of light! As we know, light from a source is redshifted when the object is moving away from us. In the context of Hubble's Law, such dramatic redshifts mean that quasars are receding at tremendous speeds and are very far away. The quasar 3C 273 travels at some 30,000 miles per second (48,000 km/s) and is some 2 billion light-years (640 million parsecs) distant from us.

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