Childhood's End

The Main Sequence

The Sun: the nearest Main Sequence star to the Earth. This is an X-ray image from the Yohkoh satelite.

As the star collapses the temperature and pressure at the core increases. After about 10 million years or so the core gets hot and dense enough for fusion reactions to begin. These reactions convert hydrogen into helium and liberate energy in the process. This energy in turn heats up the star and halts the collapse. This phase of stellar evolution is called the Main Sequence and the star remains relatively stable for a long time (a star like the sun has a Main Sequence phase lasting 10 billion years.) The star has left its childhood behind and settles into a long middle age.

The disk that was formed early in the star's life and began to dissipate in the Weak-lined T Tauri star phases may have formed into planets by now. Perhaps as many as one half of all pre-Main Sequence stars have circumstellar disks and this may mean that half of all Main Sequence stars possess planets.

The Main Sequence star maintains some of the properties of T Tauri stars, although in a weaker form. Main Sequence stars have X-ray emission at a level about 100-1000 times weaker than T Tauri stars. While Main Sequence stars do not have strong outflows, they do possess a stellar wind of charged particles streaming outward from the corona, similar to the Sun's own solar wind. We have been able to learn much about such processes by observing our Sun, which -- by astronomical scales -- is very near-by.

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