Evolution, Page 3

Evolution, Page 3: [Previous Page] [Next Page] [Class Notes] Textbook pages 235-246

Young Stars and Stellar Clusters

The Formation of a young star

Recall, the main-sequence is a time when stars are fusing hydrogen into helium in their cores. Before this starts, the star is called pre-main-sequence, and it gets it energy from gravitational contraction. The stars contract to be smaller and hotter with time, a short time for high mass stars, a very long time for low mass stars.

At any one time, a star has a Temperature and a Luminosity. If we plot the location on the HR Diagram with time, we are showing the evolutionary track as the star contracts down to the main sequence (as shown to the left). Proto-stars enter the diagram at the far right, along the birth line, contracting and moving mostly left until stopping at the zero-age main sequence to begin H fusion.

Low mass pre-main-sequence stars are called T Tauri Stars. These stars show great activity, x-ray flares, jets of material, and remnant accretion disks (conserving angular momentum) all consistent with them being very young.

Young Stellar Clusters
Stars don't form in isolation but in large gravitationally bound groups called clusters. If there are many high mass stars forming (spectral types O and B), these very hot stars produce a strong radiation field of Ultraviolet photons which ionize the gas and light up dust in the molecular cloud they just formed out of. These lit up nebular emission regions are called HII Regions because the Hydrogen is ionized from neutral HI --> ionized HII.

Regions of the sky might show many groups of young OB clusters, making up an OB association. As time passes, the UV radiation from the OB stars destroys the gas and dust, leaving just the stars as an open cluster.

These open clusters (see left) typically do not live as gravitationally bound groups for very long and the stars scatter independently through out the galaxy with time.