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Quasars1 - significant fraction of their light in the...

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Quasars What is strange about the quasars is not their great distance, but, rather, their incredible luminosities. They are hundreds to thousands of times more luminous than ordinary galaxies. Yet, all of this energy is being produced in a small volume of space. Their luminosity varies on time scales of a few months to as short as a few days. Remember from the discussion of pulsars and black holes in the stellar evolution chapter that the light fluctuation time scale from any object gives you an estimate of the maximum possible size of the object. The maximum size = (speed of light) ×(light fluctuation time interval). The quasars that vary their light output over a few months are about the size of our solar system. This is tens of thousands of times smaller than a typical galaxy! The shape of the continuous part of a quasar spectrum is also quite unusual. Stars are luminous in primarily the visible (optical) band of the electromagnetic spectrum. The hottest stars also emit a
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Unformatted text preview: significant fraction of their light in the ultraviolet band and the coolest stars emit a significant fraction of their light in the infrared band. Regardless of the star, though, the spectrum of a star, and, hence, the spectrum of a normal galaxy, rises to a peak at some wavelength determined by the temperature (remember Wien's law ?) and drops off at wavelengths shorter or longer than the peak wavelength. Such a spectrum is called a thermal spectrum because it depends on just the temperature. Quasars have a decidedly non-thermal spectrum: they are luminous in the X-rays, ultraviolet, visible, infrared, and radio bands. They have about the same power at all of the wavelengths down to the microwave wavelengths (shortwave radio wavelengths). The spectrum looks like the synchrotron radiation from charged particles spiralling around magnetic field lines at nearly the speed of light (remember the emission from pulsars ?)....
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