12 Stellar Evolution

1. Leaving the Main Sequence

2. Evolution of a Sun-like Star

Solar Composition Change

Hydrogen (yellow) and helium (orange) abundances are shown

At stage (b) only a few percent of the star’s total mass has been converted from hydrogen into helium. This change speeds up as the nuclear burning rate increases with time.

Hydrogen- Shell Burning

As a star’s core converts more and more of its hydrogen into helium, the hydrogen in the shell surrounding the non-burning helium “ash”  burns ever more violently.

By the time shown in below figure,

Stage8: The Sub-giant Branch

Stage 9: The Red-giant Branch

Stage 10: Helium Fusion

Helium Flash 

When He begins to burn, physics has to be changed from "classical" to "quantum":

Horizontal Branch

A large increase in luminosity occurs as a star ascends the red-giant branch, ending in the helium flash. The star then settles down into another equilibrium state at stage 10, on the horizontal branch. 

Stage 11: Asymptotic Giant Branch

3. Death of a Low-Mass Star

This graphic shows the entire evolution of a Sun-like star. Such stars never become hot enough for fusion past carbon to take place.

Stage 12: Planetary Nebulae

The end result

(a) Abell 39, some 2100 pc away, a spherical shell of gas about 1.5 pc across.

(b) The brightened appearance around the edge of Abell 39 is caused by the thinness of the shell of glowing gas around the central core. Very little gas exists along the line of sight between the observer and the central star (path A), so that part of the shell is invisible. Near the edge of the shell, however, more gas exists along the line of sight (paths B and C), so the observer sees a glowing ring.

(c) Ring Nebula, at 1500 pc away and 0.5 pc across, is too small and dim to be seen with the naked eye. 

a) The Eskimo Nebula clearly shows several “bubbles” (or shells) of material being blown into space (distance ~1500 pc).

(b) The Cat’s Eye Nebula possibly produced by a pair of binary stars (unresolved at center) that have both shed envelopes (about 1000 pc away and 0.1 pc across).

(c) M2-9 shows surprising twin lobes (or jets) of glowing gas emanating from a central, dying star and racing out at speeds of about 300 km/s (some 600 pc away and 0.5 pc end-to-end).

Stage 13: White Dwarfs

4. Evolution of Stars More Massive than the Sun

5. Stellar Evolution in Star Clusters

(a) Initially, stars on the upper main sequence (MS) are already burning steadily while the lower MS is still forming.

(b) At 107 years, O-type stars have already left the MS (as indicated by the arrows), and a few red giants (RGs) are visible. 

(c) By 108 years, stars of spectral type B have evolved off the MS. More RGs are visible, and the lower MS is almost fully formed.

(d) At 109 years, the MS is cut off at about spectral type A. The subgiant and RG branches are just becoming evident, and the formation of the MS is complete. A few white dwarfs may be present.

(e) At 1010 years, only stars less massive than the Sun still remain on the MS. The cluster’s subgiant, red-giant, horizontal, and asymptotic-giant branches are all visible. Many white dwarfs have now formed.

Young Cluster H–R Diagram

(a) The Hyades cluster is found 46 pc away.

(b) The H–R diagram for this cluster is cut off at about spectral type A, implying an age of about 600 million years. A few massive stars have already become white dwarfs.

Old Cluster H–R Diagram

(a) The southern globular cluster 47 Tucanae.

(b) Fitting its main-sequence turnoff and its giant and horizontal branches to theoretical models gives 47 Tucanae an age of between 12 and 14 billion years, making it one of the oldest-known objects in the Milky Way Galaxy.

The inset shows many blue stragglers (massive stars lying on the MS above the turnoff point, resulting perhaps from the merging of binary-star systems). The thickness of the lower main sequence is due almost entirely to observational limitations.

6. Evolution of Binary-Star Systems

Stellar Roche Lobes

(a) In a detached binary, each star lies within its respective Roche lobe. 

(b) In a semidetached binary, one of the stars fills its Roche lobe and transfers matter onto the other star, which still lies within its own Roche lobe.

(c) In a contact, or common-envelope, binary, both stars have overflowed their Roche lobes, and a single star with two distinct nuclear-burning cores results.

The evolution of the binary star Algol. 

(a) Initially, Algol was probably a detached binary made up of two MS stars: a relatively massive blue giant and a less massive companion similar to the Sun.

(b) As the more massive component (star 1) left the MS, it expanded to fill, and eventually overflow, its Roche lobe, transferring large amounts of matter onto its smaller companion (star 2).

(c) Today, star 2 is the more massive of the two, but it is on the MS. Star 1 is still in the subgiant phase and fills its Roche lobe, causing a steady stream of matter to pour onto its companion.