Craig: This is a really good question. In most circumstances, there are not enough pulsars to literally collide head-on. However, if two pulsars are orbiting each other in a binary pulsar system then they will slowly inspiral due to emission of gravitational waves. Eventually (billions of years) the pulsars will coalesce. It is thought that inspiral-based neutron star coalescence is one source of gamma-ray bursts (GRBs), which are the most powerful explosions in the universe. The explosion will be an immense conversion of nuclear-density matter (i.e. a giant gamma-ray explosion) and conversion into a into a black hole.

Craig: Great question. For the most part, pulsars only have the energy left over from the supernova that created them. They are hot, so they can radiate thermal X-ray emission, and they are often rotating quickly, so they can somehow radiate their rotational energy. (in this case, it would be somewhat like a dipole radiator). Once this energy is exhausted, it is believed that pulsars will turn off and become a cold and dark neutron star. There are strong suspicions that our galaxy is a “graveyard” of old neutron stars floating around.

That being said, there are exceptions. If a neutron star is orbiting a binary companion, that companion can donate matter (gas), which will come crashing down on the star. The star can glow so hot from this interaction, that it will glow in X-rays (millions of degrees). Also, eventually enough hydrogen gas can collect on the surface of a neutron star that a short flash of fusion can occur (about 10 seconds), which converts the hydrogen to helium. This can go on for the life of the binary system.

Craig: Yes, this is known as accretion-based collapse of a neutron star into a black hole. Neutron stars are supported by the quantum pressure of degenerate neutrons. That is a mouthful! Degeneracy pressure is a powerful thing, but it is not limitless. If you pile enough matter onto a neutron star, the matter can overcome the degeneracy pressure and cause the star to collapse into a black hole. The source of this matter is almost always a companion star. In other words, two stars that formed together as a binary pair, and one went supernova to a neutron star, and the other stuck around to donate mass.

Craig: Black hole vs. neutron star mass. Generally speaking in an astronomical context, a neutron star is between about 1 and 2 solar masses, whereas a black hole can be larger. In a cosmological context, it is possible there could be microscopic black holes, but these are formed in the early universe, not by stellar processes.

More common? This is an open question. There are probably many more neutron stars in our galaxy because the stellar evolution processes that produce them are more common. However, many neutron stars in our galaxy could be hidden if they are cold and/or are not near a source of matter that could “light up.”

Does a pulsar emit light? Optical light is possible. The Crab pulsar is well known to be an optical pulsar. Many pulsars are radio, X-ray or gamma-ray emitters.

Does a planet still orbit around a pulsar? Yes, there have been claims about planets around pulsars. These are discovered because the planet disturbs the motion of the pulsar, and this can be detected by detailed timing measurements.

How is a pulsar formed? It’s too difficult to answer here in short form. Most pulsars are formed when a massive star evolves to supernova; the outer layers of the star is ejected in the supernova explosion, and the hot nuclear core of the star remains. If it is rapidly spinning, it will likely be a pulsar.

Craig: It’s difficult to answer such a question in such a short format. Massive stars, about 3-10 times our sun’s mass, evolve much more quickly. The cores process nuclear material rapidly until they exhaust their nuclear fuel. In this case, it means that fusion occurs between hydrogen and hydrogen, helium and helium, nitrogen, oxygen, etc., until iron is produced. Fusion of nuclei heavier than iron require energy input rather than producing energy output, so that is the end of the stable fusion sequence. Eventually enough mass of iron is produce in the center of the star, with too little nuclear fusion, that radiation pressure cannot support the core any longer, and collapse occurs. This is a supernova explosion. The central core of the star collapses, and the outer shells of the star is blown out, which forms the supernova remnant. The core becomes a neutron star, and if it is rapidly spinning, a pulsar. This is the approximate stellar evolutionary process; of course there are many details along the way that I skipped!

Craig: There are such things as “star quakes” (which are better known in a pulsar context as pulsar glitches). This would be a sudden de-coupling between the rigid/brittle crust and the more fluid interior. Some pulsars are known to be more glitchy than others.

Luke: You are exactly right. To be useful for navigation (at least in the way we use them for SEXTANT) they need to be predictable, which is really what we mean by stable. The stability of known pulsars varies greatly. I’ll leave it to the pulsar scientists to explain what makes some pulsars so stable and others less so, but a subset of the millisecond pulsars (MSPs) are indeed extremely stable, similar to the stability of the atomic clocks that stabilize the radio beacons from the GPS satellites. The most stable MSPs are predictable to a few microseconds over months or years which translates to a couple parts in 10¹⁴ if we assume, say 1µs in 1yr, which is in the realm of atomic clocks. So far, such stability measurements have been made with radio observations from ground-based telescopes, which are thought to be limited by dispersion effects (frequency dependent propagation speed variations caused by interstellar gas clouds, charged particles, etc). The X-ray signals that NICER detects are essentially immune to dispersion effects, and so NICER, being the most precise X-ray timing instrument ever put in orbit, will establish the stability of MSPs to greater precision than possible before. In fact, this is one of NICER’s science goals.

Zaven: Hi MikeCian, thanks for your question!

Generally speaking, pulsars gradually spin down if they’re in isolation — that is, if they’re not interacting with other stars. That’s because the energy they radiate away (in the form of light and particles) ultimately derives from their spin. So, yes, a slower-spinning pulsar is likely older than a faster-spinning one, but there are important exceptions. Certainly the size and magnetic field strength of the progenitor star has something to do with the spin rate when the pulsar is born (presumably in a supernova explosion), but there are also random events called “glitches” that cause a pulsar to temporarily speed up its spin before resuming the gradual slow-down. More dramatically, a “dead” pulsar can be spun up to super-high rates (thousands of revolutions per second) and brighten again by drawing matter from a companion star, in a process we call “accretion”. The pulsar landscape is very rich, and part of the fun for us is to try to understand how spin rate, magnetic field strength, age, progenitor properties, and many other known and unknown characteristics, relate to each other and give us the collection of neutron stars and pulsars we see today.

Craig: It’s more or less a science fiction question. And in fact, such a pulsar-generator was written about by the science fiction author, futurist and physicist name Robert L. Forward. The book is called “Dragon’s Egg,” and it’s about humans who visit a neutron star and discover that life is living on the surface!

Craig: Thanks for these questions. The magnetic fields are not so strong that they alter objects in our solar system. However, if there is gas near the pulsar, the magnetic fields are strong enough to steer and channel the gas onto “hot spots” on the magnetic poles. While this type of gas can just come from the galactic neighborhood, it more likely come from a star that is orbiting with the pulsar. NICER uses theoretical predictions of hot spots to compare with the data to make constraints on the magnetic fields as well as gravity profile.

Craig: A pulsar is a kind of magnetar. Magnetar is a special kind of pulsar which has the observational signatures of having extremely high magnetic fields. A “normal” pulsar would have a magnetic field in the 10¹² Gauss range (million million Gauss); a magnetar would have a field about 100 times that (10¹⁴ Gauss = 100 million million Gauss). At that level of magnetic field, bizarre effects happen such as the vacuum itself becomes polarized. Because of the strong magnetic field, magnetars tend to emit much of their rotation energy as electromagnetic energy (and related sub-atomic particles), and for this reason most magnetars we observe today are slow rotators. I.e. most of the rotational energy has been emitted away.

Rita: Hi! Glad you asked. Yes, pulsars can be used to detect gravitational waves from particular types of sources, merging supermassive black holes. There are already a few experiments going on: Pulsar Timing Array, NANOGrav, European Pulsar Timing Array.

Craig: Probably not even in billions of years. If the solar system stays around “forever” then yes eventually there will be inspiral, but it will be about 10²³ years. (ref: http://www.tapir.caltech.edu/~chirata/ph236/2011-12/lec15.pdf)

Craig: Pulsars are at the extreme of both gravity and magnetic fields. We are not able to produce such high fields in our terrestrial laboratories, so we observe pulsars with X-ray observatories such as NICER. I would not say our understanding “breaks down,” but rather there are a lot more uncertainties at the extremes, and that is why we seek innovative ways to make measurements to constrain the extremes.

Craig: What happens at the core of a pulsar is an open scientific question. In all likelihood, the core is a superfluid. There are probably exotic states of matter there, such as Kaon Condensation. There are some theories that the properties of the neutron star itself changes as it ages. For example, the magnetic field structure may change.