Evolution of Low Mass Stars

Protostar-->Main Sequence-->Subgiant-->Red Giant--> Horizontal Branch-->AGB-->Planetary Nebula-->White Dwarf-->Black Dwarf

Press release

The evolution of low mass stars is similar to the evolution of high mass stars. The primary difference is in how far an individual star moves along the nuclear burning chain. This then leads to differences in the ways in which the stars end their lives.

The Sun: The Paradigm for Low Mass Stars

(1)-(2). On the Main Sequence, the Sun converts hydrogen to helium in its core through the proton-proton chain. The Sun first scours out the hydrogen in its center, slowly eating its way outward. Consequently, the Sun slowly gets hotter and brighter (moving to the left and up in the HR diagram).

(2)-(3). After the hydrogen is used up in the core of the Sun, the helium core contracts, and heats the hydrogen rich layer just outside of the core. The hydrogen ignites in the shell around the core and the Sun moves to the right in the HR diagram.

(3)-(4). After the outer layers become convective, the luminosity shoots up and the Sun becomes a Red Giant.

The helium core continues to contract and heat until it reaches the ignition temperature for helium, roughly 100,000,000 K. Once this temperature is reached, 3 helium nuclei will fuse to form a carbon nucleus in what is called the Triple Alpha Process. After a carbon nucleus is formed, the carbon may capture another helium nucleus to form oxygen. At helium ignition (4), the core of the Sun is supported by the hot (normal) gas of helium nuclei produced by the hydrogen burning and by degenerate electrons ===> the pressure due to the electrons does not change very much when the helium ignites ===> the core of the Sun does not expand strongly in response to the ignition of the helium.

The nuclear evolution of the Sun ends at this point and the star is now ready to enter into its final stages of evolution; at this time the star is AGB star characterized by a carbon-oxygen core, surrounded by a helium burning shell and a hydrogen burning shell.

(7)-(8). At this point, the evolution becomes controversial but current wisdom suggests that the shell burning becomes unstable and a series of nuclear burning pulses can occur. This coupled with the fact that as the outer layers of the star cools, the protons and electrons combine to form neutral hydrogen which produces photons and heats the envelope of the star. These effects combine to eject the outer tens of percents of the envelope of the star at a speed of a few tens of kilometers per second in a few million years.

In the left panel, we see a plot showing the shell luminosity of typical thermal pulses in an AGB star. The shell luminosity more than doubles with outburst intervals of a few hundred thousand years. The pulsing leads to the evolution shown in the right panel. This leads to the formation of Planetary Nebulas, e.g., the Cat's Eye Nebula.

Planetary nebula are emission nebulas which consist of a glowing shell of gas (the outer envelope of the star which has been ionized and energized by the hot central star). They were given their name because they resembled planets when viewed through small telescopes. They are short-lived lasting only tens of thousands of years. There are around 3,000 planetary nebulas known in our Galaxy. Planetary Nebulas are important because they return chemically enriched matter from low-mass stars to the Interstellar Medium, material to be used in the next generations of stars.

Why do the Planetary Nebulas to the left appear nearly circular (ring-like)? Are they like smoke rings ejected by the dying stars?

(8)-(9) The hot central core which has just been laid bare becomes a white dwarf star.

What Happens to the Earth as the Sun Evolves?