RADIO SOURCES WITHIN THE GALAXY When the sky is scanned with a radio telescope, a very bright band of radio emission is found
that coincides with the Milky Way. The brightness of this band relative to the other cosmic sources of radio emission is much greater than the relative brightness of the Milky Way as compared to the ordinary stars as seen with the unaided eye. This
radiation from the disk of our galaxy is, at most radio frequencies, synchrotron radiation from cosmic-ray electrons spiraling in the magnetic fields of our galaxy (see cosmic rays). In addition to the bright radiation of the Milky Way, the sky is filled with distinct sources of radio emission. Some of these come from solar system objects, while others arise in more distant parts of the galaxy or outside of the galaxy. None of the bright radio sources is associated with any of the bright, well-known optical stars; instead, these sources of radio emission are associated with objects that in all cases are rather faint optically but radiate large amounts of radio energy due to synchrotron radiation or very high
temperature. Aside from the solar system, the most prominent sources of radio emission within the galaxy are supernova remnants, pulsars, most ionized emission nebulae, and interstellar atoms and molecules (see interstellar matter). The Solar System The Sun is an interesting and complex source of radio waves. It radiates a steady level of thermal radio emission because of its high temperature. When observed at long radio wavelengths, however, the radio emission observed is very intense and is typical of a hot object whose temperature is 1,000,000 K rather than the 5,800 K temperature of the solar surface. This high-temperature radiation comes from the solar corona, which becomes the visible "surface" of the Sun when observed at long radio wavelengths. In addition to this steady emission, the Sun exhibits at least six other kinds of complicated, time-variable radio emissions. All of these seem to be associated with sunspot activity and the solar flares that accompany this activity. One type of radio emission varies slowly as the Sun rotates. Another is noise storms, which last hours to days. Also occurring are at least four different types of solar radio burstsÑlarge increases in radio emission from a small region of the Sun, usually near a sunspot. One type of burst may last only a few seconds, while another type may persist for minutes or hours. Often the radio emission making up a burst occurs only at a very limited range of frequencies, a range that changes as time goes on, usually to lower frequencies. This occurrence provides evidence that the burst is caused by material ejected from the Sun that moves up in the solar atmosphere to less dense regions where the typical frequencies for radio emission are lower. Radio observations of Venus have shown that the temperature of its surface is much higher than once imagined, about 480¡ C (900¡ F), a result since confirmed by spacecraft. This high temperature is maintained by a greenhouse effect caused primarily by the dense carbon dioxide atmosphere and the clouds of Venus. Radio observations of Mercury have shown that temperature on its surface rises as high as 425¡ C (800¡ F). Observations of the Moon have shown that the mean temperature of the lunar surface is only slightly less than that of the Earth, but that the temperature of the surface plunges to about -100¡ C (-150¡ F) at night. Similarly, the temperature on the surface of Mars, which barely reaches the freezing point of water during the day, approaches -100¡ C (-150¡ F) at night. The biggest surprises in solar-system research have been provided by Jupiter, which exhibits three different types of radio emission. First, radio emissions are caused by thermal emission from the body of the planet. Higher and higher temperatures are observed as observations are made at longer and longer wavelengths for two reasons: because of the greenhouse effectand because the radio telescope looks deeper into the Jovian atmosphere at the longer wavelengths, a consequence of the fact that the atmosphere is made partially opaque by ammonia gas, with the amount of obscuration less at long wavelengths. Indeed, temperatures little different from terrestrial temperatures are found deep in the Jovian atmosphere. A second type of radio emission, called decimeter emission, is synchrotron emission from a vast system of radiation belts that are held within the magnetic field of the planet. These belts are like the Van Allen radiation belts of the Earth but contain many more energetic particles whose origins are not understood. The third type of radio emission, called decameter emission, is a sporadic, very intense radio emission that is observed only at low frequencies of about 20 MHz. It consists of groups of short bursts of radio emission usually lasting only fractions of a second, with a group of bursts lasting sometimes tens of minutes.