34. A CHOICE OF ASTRONOMIES

The introduction showed the possibilities we have to get information about the stars and galaxies by observing objects at wavelengths other than those of visible light. Here follows another example:
Four views of the Andromeda Galaxy M31, at different wavelengths give an impression of he power of "multispectral astronomy". By visible light (4) the galaxy appears as a spiral, disk-like aggregation of stars with as bright central bulge.
Infra-red rays (5) come from the galaxy's dust clouds and cool stars, and the rings colour-coded yellow may show where new stars are forming. In radio emissions at 21 centimeters (6) the central bulge disappears. Instead what we see is the galaxy's outlying stock of hydrogen gas, which approaches us (blue) or recedes (red) as the whole galaxy rotates in a clockwise fashion. The fourth view is a close-up of the central region of the galaxy as seen by X - rays (7). Two of the scattered sources registered by the European EXOSAT satellite have flared up since the same region was observed by the American EINSTEIN satellite a few years arlier.