H-R Diagrams

Spectral Types

Saha equation predicts which species are prominent at each temperature

Some species have prominent absorption lines

O stars

M ~ 20 Msun

L ~ 20000Lsun T = Tsun/1000 ~ 10 mission years

Very brief lifetime

M stars

M ~ 0.2 Msun

L ~ 0.01 Lsun T = 20 Tsun ~ 300 billion years

Last a LONG time

When H is exhausted, He begins burning

Clusters in the same area of diagram are all the same age

Stellar Evolution

Stars spend from 300 billion to 1 million years, then they start burning helium

When He burning commences, the size of the star increases. When this happens to our sun, the size will extend to the Mars orbit. However, the core shrinks

He burning RT ~ constant

Core would be approximately the size of the Earth

10^-5 times less dense than air

The star is then held up by radiation, and it begins moving rapidly on HR diagram

Loses Mass

What is to hold matter back?

Much of the mass of the star boils off and returns to interstellar space

After He is burned, the star shortly collapses to become a white dwarf.

We know this to be true because we can look at globular clusters formed at once from the same material – only the mass differs.