Ionized Emission NebulaeThe other bright sources of radiation in the Milky Way are the ionized emission nebulae. In these objects, a very hot star has produced ultraviolet radiation that has ionized and heated the interstellar gas around it. This hot, charged gas is an excellent emitter of radio waves produced by thermal processes. These sources of radio emissionÑone of the brightest is the Great Nebula in OrionÑare more prominent at the higher radio frequencies because the spectrum of synchrotron radiation is most intense at low frequencies, even though thermal emission has about the same intensity over a broad frequency range. StarsSeveral dozen normal stars have been detected as faint radio sources in our galaxy. The detectable radiation appears to come from hot shells or coronae, like that of the Sun, surrounding these stars. Interstellar Atoms and MoleculesA very important feature of the galactic radio emission is the strong radiation on specific wavelengths, or so-called spectral lines, that are emitted by atoms and molecules in the interstellar gas. The most prominent of these is the radiation of neutral atomic hydrogen, the most abundant element in the universe, at the wavelength of 21 cm. The atoms in this case and others radiate at a specific frequency but may arrive at Earth at a different frequency due to the Doppler effect associated with the motion of the atoms toward or away from the Earth.
By studying this Doppler shift in the frequencies of the observed spectral lines, the astronomer can deduce the temperatures and motions of the gas clouds that the radiation comes from. In this way the velocities and arrangements of gas clouds in our galaxy and others have been determined.In addition to atomic hydrogen, spectral lines have been discovered, surprisingly, from more than 40 different molecules. Those found so far are primarily organic molecules. The largest contain as many as 13 atoms and have molecular weights of more than 100. Carbon monoxide is found to be very abundant in space, as is formaldehyde. The molecules that are most effective in producing molecules important to biology in laboratory experiments are also abundant in space. This discovery suggests that interstellar chemistry perhaps played a role in the development of life on Earth and elsewhere, or else that the process was mimicked here.