Going Out With a Bang (Life of Stars, Part IV)

Dylan Lee, Stellar Astronomy

In the last post, we explored the later life and death of low-mass stars. They consumed all the hydrogen in their core, before the resulting helium core compressed and reigniting fusion in a shell around the core, which caused the star to expand into a red giant. The compression of the helium core also ignited fusion of the helium itself, which created a carbon-oxygen core that was too massive to be fused by these low-mass stars. When the star was unable to continue fusion, its life ended tragically and rather quietly with its collapse into a white dwarf: a tiny, dense ball of degenerate matter.

The Fate of Massive Stars

For stars that are more massive, however, death will not occur after the formation of the carbon-oxygen core. Compression in the core of these stars can raise temperatures to fuse elements up to sulfur, which is almost three times as heavy as carbon. However, even the most massive stars meet an inevitable limit. The fusion of sulfur produces iron, which is almost five times as heavy as carbon. Even if compression of the core produces enough heat to fuse iron, iron fusion actually requires energy, instead of producing energy like the fusion of all the elements up to sulfur (recall that the energy produced by fusion was what released heat and light and counteracted gravity). Therefore, nuclear fusion in the star stops. Gravity once again takes over (that's becoming a theme now, isn't it?) and forces the star to contract, forming a temporary white dwarf.

In these formerly large stars, though, the mass of the white dwarf is so great that electron degeneracy pressure cannot counteract gravity. The particular mass for this to happen is called the Chandrasekhar limit, which is about 1.4 times the mass of the Sun. Keep in mind that stars lose much of their mass in the red giant stage, so this requires the star to actually begin with a mass of about 10 times that of the Sun. In this scenario, the white dwarf continues to compress until the degenerate electrons in the core are forced to collide with the protons in the core. When an electron and a proton collide, they combine to create a neutron, which is over 1,800 times as massive as an electron (most of this mass comes from the proton). Besides producing a neutron, this phenomenon produces a neutrino, a particle with almost no mass and a huge amount of energy. (If you are familiar with the form of radiation called electron capture, you could think of this whole collision process as a "forced" version of electron capture.) We will discuss the significance of the neutrons and the neutrinos separately.

Neutrons, like electrons, are fermions and thus must obey the Pauli exclusion principle (see the last post for details on what this means). Therefore, the neutrons will also exert a degeneracy pressure under the extremely high compression of the star. When the collisions of protons and electrons have turned essentially the entire core into neutrons, the neutron degeneracy pressure is strong enough to counteract gravity, and the core becomes the remnant of the star. Demonstrating their creativity (or lack thereof) once again, astronomers call this remnant a neutron star.

An X-Ray image of the Cassiopeia Supernova, formed by the collapse of a star.

Meanwhile, the neutrinos cause chaos. Neutrinos generally don't interact with matter because they are so small. However, the star is now so dense that the neutrinos produced by collisions in the core actually can interact with matter. The neutrinos fly out of the core with extremely high energy, and then transfer this energy to the matter outside of the core. This energy transfer is enough to cause a huge explosion of the star's outer layers into space, known as a supernova. These violent explosions are usually the brightest and most beautiful phenomena in the universe. The ejection of elements from supernova actually spread the elements throughout the universe, where they can form anything - including other stars, planets, and even the life on Earth. That means everything that you can see or touch - including yourself - is made out of the remnants of stars.

Stellar Lighthouses

A neutron star's size compared to Manhattan in New York. Keep in mind that over 1.4 times the mass of the Sun is squeezed into that tiny star.

Neutron stars have several properties that make them some of the most interesting stars in the universe. First of all, they are the densest of all objects in the universe (aside from black holes, which have infinite density). The extreme compression that formed the neutron star squeezed all of the mass in the star's core into a ball that is only 10 to 20 kilometers in radius. That means that a neutron star is about the size of a small city, or borough, like Brooklyn. The fact that all this mass is compressed in such a small volume means its density is huge (if a grain of sand had the density of a neutron star, its mass would be about that of a cruise ship).

An important consequence of all the mass being compressed into such a tiny space is a huge increase in the rate that the neutron star spins. Many neutron stars make hundreds of revolutions per second. The reason for this is the conservation of angular momentum, which is a physics concept you can learn more about with this video here. As a brief analogy, think of a figure skaters pulling in their arms to spin more quickly in the air.

Figure skaters spin faster by pulling in their arms. This demonstrates the conservation of angular momentum, which also causes the rapid spin of neutron stars.

This is exactly what happens in stars. When stars are born, the random motions of molecules cause it to have a tiny bit of angular momentum in a certain direction, which causes it to spin on its axis. Since there is essentially no friction in space, stars keep spinning throughout their entire lives. Then, when the star collapses into a tiny neutron star, all of the star's mass becomes very concentrated near the center, close to the axis of rotation (like the rightmost figure skater above). This forces the neutron star to spin extremely fast, achieving the hundreds of revolutions per second mentioned earlier.

The neutron star's magnetic field causes radiation to be emitted along the magnetic axis, which differs from the axis of rotation. This makes the neutron star similar to a lighthouse.

Neutron stars also have huge magnetic fields (the origin of these fields is actually debated amongst physicists), which exert huge forces on particles with an electric charge. These particles are forced away from the neutron star in beams of radiation that point in the north/south direction of the magnetic field. Since the axis of rotation is usually different from the direction of the magnetic field, it turns the neutron star into a kind of lighthouse, with beams of radiation as the "light" emitted from the star. In certain cases, these beams will emit flashes in the direction of the Earth, and then astronomers on Earth can detect them. Similar to how a lighthouse looks from sea, the beams of these neutron stars look like regular pulses that alert astronomers to the presence of neutron stars. For that reason, we call these neutron stars pulsars.

These pulsars were actually used in the discovery of neutron stars, and they almost serve as a tiny, persistent reminder of the once-living stars that created all the elements of the universe.

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Fraknoi, Andrew, et al. Astronomy. Houston, Texas, OpenStax, Rice University, 2017.

NASA. "Neutron Stars." Objects of Interest, 2017. https://imagine.gsfc.nasa.gov/science/objects/neutron_stars1.html.

Redd, Nola Taylor. "Neutron Stars: Definitions and Facts." Space, 24 February, 2018, https://www.space.com/22180-neutron-stars.html.

All photos linked to sources.

Cover photo is the Crab Nebula, photographed by the Hubble Space Telescope: https://en.wikipedia.org/wiki/Crab_Nebula#/media/File:Crab_Nebula.jpg