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O5 BO AO FO GO KO MO M5 Spectral type

HR diagram, showing the locations of various types of variable stars.

O5 BO AO FO GO KO MO M5 Spectral type

HR diagram, showing the locations of various types of variable stars.

longer, and are called long period variables. Any individual Mira variable may show fluctuations in its period. The stars change their brightness by about 6 mag, or over a factor of 250 in brightness. For example, the apparent magnitude of Mira ranges from 9 mag to 3 mag. These changes in brightness are accompanied by changes in spectral type. Mira changes from M9 to M5. This means that a temperature change is accompanying the luminosity change.

Cepheid variables are named after the prototype 8 Cephei. Its period is 5.4 days and its apparent magnitude varies from 3.6 to 4.3 mag. In general, Cepheids have periods from 1 to 100 days. We know of more than 1000 in our galaxy. Another familiar Cepheid is Polaris (the North Star), which changes by only 0.1 mag, from 2.5 to 2.6. (For a long time, astronomers did not know that Polaris is a variable, and it was used as a reference for measuring the magnitudes of other stars.) Cepheids have masses of approximately 6 M0, and radii of about 25 R0.

10.2.2 Cepheid mechanism

When we study the spectral lines in Cepheids, we can detect Doppler shifts that vary throughout the light cycle (Fig. 10.5). The Doppler shifts go through a cycle in the same period as the light. This means that the surface of the star is moving. The size of the star changes as the luminosity changes. The spectral type also changes throughout the cycle.

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