Variability begins a* soon as a star forms by the gravitational contraction of an interstellar gas and dust cloud. Within the contracting cloud, a core of dense gas forms, heats up and begins to produce energy. This transition from a dark cloud to a self-luminous star would certainly be visible were it not for the veil of infalling material which still surrounds the star. The star may first appear when its light is able to penetrate the veil. Two pos¬sible examples of this process have been observed. In 1937, the star FU Orionis brightened by six magnitudes and it has remained bright since. This star has all the properties that would be expected of a very young star. It is located in a region thick with gas and dust. It is a giant F-type star, still much larger than a main-sequence star. It is rich in the element lithium, which is a strong indicator of youth. FU Orionis is also a strong source of infrared radiation, which presumably arises in the cloud of warm gas and dust which still surrounds the star. A second star, V1057 Cygni, brightened by six magnitudes in 1969, and has remained bright since; this star has properties similar to those of FU Orionis.

Elsewhere in Orion and along the Milky Way there are thou-sands of other very young stars. We can recognize them in many ways: by their association with gas and dust clouds, by their membership in short-lived stellar associations, or in young galactic clusters. NGC 2264 is one such cluster. Its colour-magnitude diagram (equivalent to an HR diagram) is shown in figure 2.11. There are many hot, bright 0- and B-type stars on the main sequence, from which we can deduce that the cluster is only a few million years old. The fainter, less massive stars in the cluster have not yet had time to contract fully to the main sequence, hence they lie above and to the right of it. From such bona fide pre-main-sequence stars, we can learn some of the unique properties of this particular type of star

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