Neutron Stars

In our last post, we discussed supernovae in all their explosive stellarness, and this week, we’ll discuss some of the “leftovers”, so to speak. Yes, these are the first of our lovely stellar remnants — the neutron star and its relatives the pulsar and the magnetar.

A neutron star (NS) forms from the collapsed core of a star that explodes in a Type II supernova, and much like its name suggests, it is a highly compact ball of neutrons. A typical NS has a few solar masses of matter packed into a sphere 20 km or so in diameter (about the size of a large city), a density roughly equivalent to packing every human being on the planet into the volume of a sugar cube. When the core of the original star collapses, conditions are so extreme that protons and electrons are squished together to make neutrinos and neutrons. The surface of a neutron star is a thin solid crust made mostly of regular nuclei, but as you go towards the core, you find nuclei with more and more neutrons; eventually you would reach the “neutron drip” layer, where neutrons leave their nuclei and move around freely, and then… well, no one really knows what kind of exotic matter lies at the core.

Neutron star structure

Neutron star structure (Credit: Wikipedia)

Much like electron degeneracy pressure keeps a white dwarf from collapsing, neutron degeneracy pressure holds up a neutron star against its own gravity, but if it is above ~3 solar masses (the Tolman-Oppenheimer-Volkoff limit, or TOV limit), even neutron degeneracy pressure can’t hold out against gravity, and the stellar remnant collapses into a black hole instead. Some astrophysicists also theorize that when a stellar remnant has a mass between a neutron star and a black hole, exotic remnants like quark stars may exist. However, the neutron star itself started off as just a theory too — it was first proposed in 1933 by Fritz Zwicky and Walter Baade, only a year after the discovery of the neutron itself!

Now, we skip forward a third of a century to 1967, when Cambridge grad student Jocelyn Bell Burnell, working under adviser Antony Hewish,  discovered a curious radio source that emitted pulses so regularly that it was thought to be a signal from an extraterrestrial civilization, and even named LGM 1 for “little green men”. Astrophysicists discovered several more of these sources, eventually realized that they had to be rotating neutron stars (see here for a detailed explanation), and named them pulsars as a contraction of “pulsating stars”. Two of this year’s DSOs are particularly interesting pulsars — one because it’s among the oldest pulsars ever discovered, the other because it rotates so darn slowly for its relatively young age.

Crab Pulsar

One of the most famous pulsars in the sky – the Crab Pulsar in M1 (Credit: NRAO/AUI and Joeri van Leeuwen (UC Berkeley) / ESO / AURA)

Going back to characteristics of these massive, compact objects.  When the core of a massive star collapses to form a neutron star, it starts to spin very rapidly. The original star was rotating slowly, and now conservation of angular momentum dictates that its angular velocity will increase because its radius — and therefore its moment of inertia — decreases (that is to say, it spins faster because it is now smaller and has less resistance to change in its motion). The classic analogy is a figure skater who starts spinning with her arms extended, and spins faster as she pulls her arms in to her body. The magnetic field also becomes much stronger, because the field lines get closer to each other as the star collapses.

Just like not all rectangles are squares, not all neutron stars are pulsars. We only see a pulsar if the neutron star’s rotational and magnetic axes are misaligned in such a way that the radiation beam produced at the magnetic poles sweeps across earth as the NS rotates. This is called the lighthouse effect, because the pulses we see from a pulsar are analogous to the beam from a lighthouse appearing to blink on and off to an observer far out at sea. Rotation-powered pulsars, such as the Crab Pulsar represented in the image above, spew out synchrotron radiation from high-energy particles above their magnetic poles. Most rotation-powered pulsars are found in radio wavelengths, but a few can also be detected in x-rays or gamma-rays (the Crab can reportedly even be seen in the visible). Accretion-powered pulsars, on the other hand, are typically visible in x-ray wavelengths. These pulsars are part of a binary system, accreting matter from a companion. The pulsar’s magnetic field may direct the accreted matter towards the magnetic poles (in the process heating the matter until it’s hot enough to emit x-rays), where it creates a “hot spot” that emits x-ray pulses as the pulsar rotates.

pulsar diagram

Diagram of a pulsar (Credit: NRAO)

The period of a pulsar slowly increases with age, as it gradually radiates energy into space. However, there are two important ways in which a pulsar can speed up. Glitches are sudden, small decreases in a pulsar’s period that last for a short time before the pulsar resumes its normal slow increase in period. Astrophysicists don’t know for sure what causes glitches, but it is thought that the crust and “mantle” interact in such a way that the crust shifts and the NS shrinks by a tiny amount, increasing its spin speed (again thanks to conservation of angular momentum). The other way in which a pulsar can speed up is accretion from a companion star. Matter spirals in around the NS, contributing angular momentum and spinning it up to a period of a few milliseconds — hence the term “millisecond pulsar” (MSP). They are also called “recycled pulsars” because the infalling matter has restored the pulsar to a faster spin rate.

Vela Pulsar Glitches

Glitches in the Vela Pulsar – individual glitch events are labeled with arrows (Credit: GAE-UCM (High-Energy Physics Group at the Complutense University of Madrid))

Magnetars are pulsars with extremely high magnetic fields — hundreds or thousands of times stronger than the already powerful magnetic fields of a regular NS. It’s thought that they are created with spin speeds much faster than normal pulsars, and this increased spin strengthens the magnetic field, amplifying it to many times more powerful than normal. However, a magnetar also ceases to emit beams of radiation much sooner than a comparable regular pulsar, because its strong magnetic field quickly slow downs its rotation rate. These strange NSs were first theorized in 1992, when Robert C. Duncan and Christopher Thompson attempted to explain how magnetic fields were created around pulsars in the first place. They found that under ideal circumstances, neutron stars could create fields thousands of times stronger than observed in regular pulsars. Magnetars have been used to explain strange cosmic phenomena such as Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs).

Alas, as fascinating as these stellar remnants are, we humans may never be able to study them up close, as one would be quite dead — in multiple ways — before even getting to a neutron star.

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TL;DR — Neutron stars are made of, well, neutrons; they are prevented from collapsing by neutron degeneracy pressure until their mass reaches the TOV limit. Most neutron stars spin very quickly, and if conditions are right, we see them on Earth as pulsars. There are two main kinds — rotation-powered pulsars (high-energy particles in magnetic field) and accretion-powered pulsars (radiation emitted from heated, infalling matter). Pulsar periods can be sped up through glitches or by accretion in “recycled” pulsars. Magnetars are neutron stars with abnormally powerful magnetic fields.

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Sources and links for further reading:

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