The Evolution of Low-mass Stars

After birth, newborn stars are very large, so they are very bright. Gravity causes them to contract, and they become fainter because of their smaller sizes. After contracting for millions of years, stars eventually become hot enough to fuse hydrogen, which halts their contraction. They now maintain a stable size and luminosity until they run out of hydrogen fuel. This is the main sequence. Stellar Birth and Reaching the Main Sequence newborn stars contracting Main sequence (hydrogen fusion here) contracting brown dwarfs contracting

All stars spend most of their lives on the main sequence, but the length of time on the main sequence (i.e., the time spent fusing hydrogen) depends a lot on a star’s mass. The Main Sequence Lifetimes of Stars A star with a higher mass has a core with:  higher gravity  higher pressure  higher temperature  fuses hydrogen faster Although more massive stars have more hydrogen fuel, they consume that fuel at a much higher rate than less massive stars. As a result, more massive stars exhaust their fuel much faster and have shorter main sequence lifetimes. The lifetimes for the most massive stars are only a few million years while the least massive stars fuse hydrogen for trillions of years. A star with the mass of the Sun fuses hydrogen for 10 billion years.

The Main Sequence Lifetimes of Stars Sun few million years >1 trillion years 10 billion years

The Main Sequence Lifetimes of Stars

After a star exhausts its hydrogen fuel, its fate is determined by its mass at birth. Low-mass stars (<8 M  ) end their lives as white dwarfs High-mass stars (>8 M  ) undergo supernova explosions, leaving behind neutron stars or black holes Main sequence (hydrogen fusion here) Evolution Beyond the Main Sequence

Central Temperature 100,000 K H The protostar collapses and gets smaller, causing the pressure and temperature to increase in the center. Gravity pulls the star inward Gas pressure resists gravity

Central Temperature 1,000,000 K H The protostar collapses and gets smaller, causing the pressure and temperature to increase in the center. Gravity pulls the star inward Gas pressure resists gravity

Central Temperature 10,000,000 K H The star’s center eventually becomes hot enough to ignite hydrogen fusion, which stops its collapse. The star is now on the Main Sequence.

Central Temperature 10,000,000 K He H Lots of hydrogen out here, but it’s not hot enough to fuse Eventually, fusion converts all of the hydrogen in the core to helium.

Central Temperature 30,000,000 K He H Without fusion to hold it up, the core of the star contracts because of gravity.

Central Temperature 50,000,000 K He H As it contracts, the core grows hotter, and a shell of hydrogen surrounding the core becomes hot enough to fuse.

Central Temperature 50,000,000 K He H The radiation from the shell of fusing hydrogen is so intense that it pushes on the outer layers of the star, causing the star to expand to a huge diameter. The star is now a red giant.

Red Giants Because the surface of the star has expanded so far from the core, it becomes cooler, and hence redder. This is why red giants are red. Although cooler objects produce less light (for a given size), the larger diameter and surface area more than make up for this, and the star’s luminosity increases a great deal. red giants Main sequence (hydrogen fusion here) expanding

Central Temperature 100,000,000 K He fusion Expanded view of core; outer layers of star not shown The contracting core eventually becomes hot enough to begin fusing helium. This new energy source halts the core’s collapse. H fusion

Central Temperature 100,000,000 K Eventually, the He in the core is converted to C and O. The core resumes its contraction once again. H fusion C and O He fusion

Central Temperature 200,000,000 K H fusion C and O He fusion

Central Temperature 300,000,000 K H fusion C and O He fusion If the core’s mass is <1.4 M , the contraction is halted when the core becomes so dense that the atoms can’t be packed more tightly. This resistance to further compression is called electron degeneracy.

C and O The radiation from the core pushes the outer layers of the red giant into space, forming a planetary nebula. After this nebula dissipates, only the core of the star remains. This is called a white dwarf. It is not hot enough to fuse C and O, so it will cool and fade very slowly forever. planetary nebula white dwarf

A star with an initial mass of <8 M  will produce a core that has a mass of <1.4 M . In other words, stars with masses <8 M  end their lives as white dwarfs. So the Sun will one day become a white dwarf. initial mass of entire star = 8 M  eventual C/O core = 1.4 M 

Evolution of Low-mass Stars after the Main Sequence planetary nebulae white dwarfs red giants

Planetary Nebulae Helix Nebula Ring Nebula When astronomers first looked at planetary nebulae through telescopes, the colors reminded them of planets like Mars, which is how they were given their name. We now know they they are unrelated to planets, but the term is still used.

Planetary Nebulae Eskimo NebulaNGC 6751

Planetary Nebulae Hourglass NebulaM2-9

White Dwarfs White dwarfs have diameters that are similar to that of the Earth, but they can have as much mass as the Sun, so they are very dense. White dwarfs have masses <1.4 M  because if the mass was higher, gravity would be strong enough to overcome electron degeneracy, and it would collapse and become an even denser and more compact object, either called a neutron star or a black hole.

White Dwarfs White dwarfs are not hot enough to fuse the carbon and oxygen within them, so they become steadily cooler and dimmer for the rest of eternity. Eventually, they will cool enough to crystallize, and will resemble a diamond the size of Earth!