3.3: Stellar Evolution

Stellar evolution of high mass stars: the top 0.1%

Our world has risen from the ashes of massive stars. Those massive stars are rare: they make up just 0.1% of all stars. The synthesis of heavy elements occurs during the final 10% of the lifetime of these stars. Except for hydrogen, the atoms in our bodies were all formed by nucleosynthesis in stars. Without those stars, or specifically without the death of those stars, we would not have the material needed to build planets and needed for prebiotic and biological chemistry.

It bears repeating that the the mass of the star is what drives the rate of fusion reactions and therefore the evolution of a star. The definition of "high mass" is generally taken to be a few times the mass of the Sun. The evolutionary path is determined by the mass of the star; stars that are less than a few times the mass of the Sun are by far the most common. These lower mass stars take the high road in Figure \(\PageIndex{1}\) below and end their lives as compact white dwarfs, roughly the size of the Earth. Higher mass stars follow the lower path in the figure below and end their lives as exotic neutron stars or black holes.

Figure \(\PageIndex{1}\): The evolution and final state of a star are largely determined by the initial mass of the star. What will happen to our Sun after it is no longer able to carry out nucleosynthesis?

Stellar evolution of stars: the bottom 99.9%

Roughly 99.9% of the stars are AFGKM spectral type stars. These stars fuse hydrogen in their cores on the main sequence and then evolve into red giants when the hydrogen is depleted. The red giant stars puff off about half of their mass (hydrogen enriched with helium, and trace metals like carbon, nitrogen and oxygen), polluting the interstellar medium like an industrial chimney stack. As the outer envelope of the red giant (confusingly called a planetary nebula because astronomers were originally uncertain about these objects) mix with the interstellar medium, the core of the remnant red giant collapses. If the core mass is less than 1.4 solar masses, it becomes a white dwarf. This is the ultimate fate of our Sun.

White dwarfs

A white dwarf is about the size of the Earth and it is an incredibly dense object - one teaspoon of white dwarf material weighs several tons. Nuclear fusion is no longer taking place in the white dwarf (except for a short period of hydrogen fusion on the surface). Now, electron degeneracy supports the white dwarf against further gravitational collapse. This supporting pressure arises from the quantum mechanical nature of electrons. Once the lowest spin energy state is occupied by an electron the Pauli exclusion principle tells us that another electron cannot have that same spin energy state; the other electrons are forced into higher and higher (faster moving) energy states. As long as the mass of the remnant white dwarf is less than about 1.4 times the mass of the Sun, electron degeneracy can support it against further collapse.

Stellar evolution of stars: the top 0.1%

As fragments of the cold molecular cloud begin to contract and form stars, hundreds to thousands of stars are born, however, only about 0.1% of the newborn stars will have enough mass to become O or B type stars. These massive stars contract quickly and carry out hydrogen fusion at a furious pace for 1 or 2 million years. At that point they have burned through their endowment of hydrogen. The core collapses until helium fusion begins and the outer shell expands, forming a red super giant star. As described above, there are several cycles of fuel depletion, contraction, and re-ignition as the core of the red super giant develops an onion layer structure, with stratified shell burning of different elements. Once the core contains iron, the star collapses again, but iron fusion does not produce energy and cannot support the star against gravitational collapse. The core of the star now hits a fork in the road:

• if the core mass is between 1.4 and 3 solar masses, then the star becomes a neutron star
• if the core is greater than 3 solar masses, then the star becomes a black hole.

Betelgeuse is a red super giant star that is 12 - 20 times the mass of the Sun with a radius that is almost 900 times the radius of the Sun. Betelgeuse would sweep out almost to Jupiter if it were the center of our solar system. The star can be seen in the Orion constellation - at the shoulder of the famed hunter.

This star has been in the news lately, because astronomers noticed that this red super giant started dimming in October 2019. Is this pre-super-nova behavior that might be typical of other stars? Or normal variable star behavior? We have a ring-side seat to watch the evolution of this star, but it is impossible to know if we will see this happen next week, or over the next several thousand years. Because Betelgeuse is 640 light years away, it is possible that the star has already gone super nova and we just haven't gotten the memo yet.

Neutron stars

When the mass of the remnant core is greater than 1.4 times the mass of the Sun, electron degeneracy can no longer support the core against gravitational collapse. As the core collapses, electrons and protons squeeze together to form neutrons with a density similar to an atomic nucleus. The neutrons exert a resisting pressure to gravitational collapse that is similar to electrons. Objects supported by neutron degeneracy are called neutron stars. They are far more compact and far more dense than white dwarfs. The diameter of a neutron star is about the size of San Francisco, measuring roughly 10 kilometers (or about 6 miles); a teaspoon of neutron star material would weigh a billion tons. Neutron degeneracy can support a star against further collapse as long as the total mass of the remnant is less than 2 or 3 times the mass of the Sun.

As the remnant core collapses into a neutron star, it spins up, conserving angular momentum. Neutron stars can rotate hundreds of times per second with a narrow beam of electromagnetic radiation that spins with the star like a lighthouse. If the Earth happens to reside in the path of this beam of light, we see the neutron star as a rapidly blinking source - a pulsar. Of course, most of the time, the synchrotron beam will not be so favorably aligned. Pulsars were theoretically predicted going back to the 1930's. In 1968, Jocelyn Bell observed radio emission pulses that confirmed the existence of neutron stars.

Figure \(\PageIndex{2}\): In 1967, the Irish astronomer, Jocelyn Bell, used radio telescope observations to measure periodic pulses from neutron stars. Why are the dishes of radio telescopes made from meshes rather than mirrors?

Black holes

Electron degeneracy can support a stellar core (or white dwarf) against collapse if the total mass is less than 1.4 M_sun. Neutron degeneracy can support against collapse if the remnant stellar core is between 1.4 and 2-3 M_sun. If the mass of the remnant stellar core is greater than about 3 time the mass of the Sun, there is nothing that can stop the collapse and a black hole is formed. Nothing escapes the black hole - not even light - so it is very difficult to find these stellar ghosts. But there are about a dozen candidate black holes in binary star systems where the second star is still visible. We can measure the orbit of the visible star and deduce the presence of a massive, but invisible star and in some cases, we can see gas being funneled off the visible star and heated up to tens of millions of degrees as it spirals onto an accretion disk around something that cannot be seen. If it walks like a duck and it quacks like a duck....

Type 1a Supernovae

About half of stars like the Sun are members of binary systems - a gravitationally bound pair of stars. The fraction of binary systems is even higher for stars that are more massive than the Sun. This has interesting implications, especially if one star in a gravitationally bound system is more massive than the other. The more massive star in a binary system has a shorter lifetime - it will evolve first. Let's say that the more massive star is originally 2 M_Sun, it evolves through the red giant phase and ends up as a white dwarf with a mass of 1.3 M_Sun (below the 1.4 M_sun Chandrasekhar limit). Now, a billion years later, the other star (with a mass of 1.5 M_sun) in the binary system goes through a red giant phase. As that star expands, it begins to dump some of its outer envelope onto the white dwarf. If the white dwarf crosses over the Chandrasekhar limit, it cannot be supported by electron degeneracy and it begins to collapse into a neutron star. During this process, the outermost shell of the white dwarf is blown off in a Type 1a supernova.

The supernova explosion of a single massive star (called a Type II) supernova can exhibit a wide range of brightnesses. However a Type 1a supernova is a carefully regulated process - mass from the evolving binary companion is funneled onto the white dwarf and when the white dwarf hits the magic limit of about 1.4 M_sun, the Type 1a supernova occurs. It's always the same physical process, so it is always the same brightness. A Type 1a supernova is a very bright standard candle.

The YouTube video below is a TedX talk about stellar evolution by Dame Jocelyn Bell Burnell

What fraction of stars will end up as neutron stars or black holes? How might you estimate the total number of neutron stars or black holes in our galaxy?