Ch. 9 – The Lives of Stars from Birth through Middle Age

Second part

The evolution of stars on the main sequence

Stars with Masses between 0.08 and 0.5 times the mass of the Sun

have low core temperatures,

live a long time,

convect helium from the core, so it mixes uniformly,

and will end up composed entirely of helium.

A G-Type Star is similar to our Sun.

The evolution is shown during an imaginary trek through space.

At the end of the red giant stage,

the core is small, the envelope huge, and the outcome depends on the

total mass of the star.

Evolution of stars with more than 0.4 solar masses

Solar Composition Change

During stage 7 hydrogen burning

causes a build-up of helium in the star’s core.

We will follow the evolution of a star like the Sun, with one solar mass.

Hydrogen Shell Burning occurs around an “ash” core, which is mostly helium, and the temperature is T = 10 million K

Helium Shell Burning on the Horizontal Branch

The hydrogen shell burning causes higher

pressure on the envelope, which causes the star to expand into a Red Giant.

The star follows the yellow curve on the

H–R diagram.

Stage 8 is the “subgiant branch” and the radius is about 3 times the

solar radius.

An example is the star Arcturus, M = 1.5 Msolar

and R = 23 Rsolar, the luminosity is about

100 times solar.

Stage 10 follows the Helium Flash,

which is like a huge nuclear explosion of

helium “flashing” or burning quickly into carbon at 108 K

Fusion of 3 He-4 nuclei produces a

C-12 nucleus plus other products

Then there is the Horizontal Branch

Reascending the Giant Branch

occurs in a way similar to the originalmove up to a giant.

Burning in the H and He shells is even

faster than before, so the star expands even more on this

“asymptotic branch”

Quick introduction: Supernova !

Visible supernovae are uncommon and of great interest to astronomers.

They occur when a massive star has burned up most of its “fuel” and suddenly “collapses”. A shock wave is formed which blows off the outer layers of the star.

Supernovae in our own galaxy had not been seen since the 1600’s

until ………………………… 1987

One dramatic result of stellar evolution: a supernova remnant

Heavy Element Fusion- shells like an onion

A Type II Supernova is a “core collapse” and occurs when the core is finally pure iron, which cannot be fused to other elements. The core collapses

to a big ball of neutrons, which causes a shock wave to bounce back outward, which blows off the entire envelope

of the red giant, to form a supernova remnant.

Supernova Remnants

Vela supernova remnant

Other examples:

Cassiopeia A (link) (link) N63A (link)

Crab nebula

M1 – the Crab Nebula

is from a supernova seen in year A.D. 1054

The remnant is 1800 pc away and the diameter is currently 2 pc.

Astronomers have been waiting for hundreds of years for a bright, nearby supernova.

Finally, one night in 1987…

We learn the story of the observation in the movie

“Death of a Star” (from the Nova series on PBS)

Supernova 1987A seen near nebula 30 Doradus

Supernova Light Curves fall into two types