HIGH MASS STELLAR EVOLUTION

In higher mass stars, the fusion process continues in shells around the core and also in the core. Each shell has a lower temperature than the shell below it and is fusing a different set of elements. The core is the hottest. The stage of a star's evolution is defined by what elements are fusing in the core. As the evolution progresses, the time the star fuses a certain element in its core is shorter and shorter. The star eventually explodes when the core is composed of iron.

Supernova Explosion

Once the core of a massive star is composed of iron, the process of nuclear fusion stops in the core. It turns out that fusing atomic nuclei with iron nuclei requires energy input (whereas all other fusion releases energy). When the core reaches a mass of 1.4 suns (the Chandrashekar Limit), the core collapses and implodes. Without energy generation in the core of the star, the pressure drops and the star collapses. The collapse is very fast, about 0.1 seconds! At some point in the collapse the star becomes so compacted and violent that it explodes in a massive outward shock wave. This explosion is called a supernova. The energy released in the first second of a supernova is equal to all the energy emitted by all the stars in the universe during that same second of time.

Stellar Remnant

The aftermath of a supernova explosion is usually a big glowing and expanding nebula. Left over where the core of ther star was, may be either a neutron star, or a black hole.

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HR Diagram and Telling The Ages of Star Clusters

First, it is assumed that all stars in the cluster form at the same time (roughly). Then, we utilize the following knowledge.

• "O" stars live the shortest, thus, an old cluster will have no "O" stars and a young cluster will have "O" stars.
• "B" stars live longer than "O" stars, but shorter than "A" stars. So a cluster with no "O" and no "B" stars is older than a cluster with no "O" stars but with "B" stars.
• If you follow the above logic, it follows down the spectral sequence.

So, one looks to see which are the hottest, massive, most luminous main sequence stars on the HR diagram. This point is called the Main Sequence Turnoff.

Since we know how long each type of star live on the main sequence, we can estimate the age of the cluster to be about the age of the life times of the most massive stars remaining on the main sequence. Very old star clusters also have horizontal branch stars and red giants.