Life and Death of Stars

H-R Diagram – a graph of luminosity (absolute magnitude, M) versus temperature (stellar type).

Gas and dust – nebula.  Collapses.  Why?

4.568 billion years ago – our solar system is born!

Protostar heated by gravitational collapse.  Leftover material forms planetary system.

Too little mass - <0.1 solar masses – failed star / brown dwarf

The larger the birth mass, the shorter the time to get to the Main Sequence (MS) – tens of millions of years (less than a solar mass) to tens of thousands of years (10+ solar masses).

Nuclear fusion powers MS stars.

Low-mass stars:  H to He

High-mass stars must be hotter to offset their larger gravity.

Higher temperature means larger luminosity and shorter lifetime.

Our sun:

G2 star

Absolute magnitude:  M = -4.83

Apparent magnitude:  m = -26.72

Compare to Sirius (m = -1.43, M = 1.47)

We’ll spend about 10 billion years on the MS, whereas a 10 solar mass star might only spend 10 million years on the MS.

Low mass evolution

H starts to run out, pressure in core begins to drop – gravity “wins”

Outer layer cools and expands, engulfing all inner planets.  Sorry.  Red giant phase.

Outer layers eventually “flake away” and expand more – planetary nebula, which are super pretty.

Eventually, a small hot core is left – white dwarf

For more massive stars:

H used up rapidly – expand outward

Red supergiant (Betelgeuse)

There is not enough pressure to counter the immense gravity:  star explodes – supernova!

What is left in core is a neutron star (mostly neutrons), incredibly small relative to their original size – imagine a many-solar-mass star shrunk to the size of Baltimore!

What about the most massive stars?  They may eventually become a black hole.