Neutron stars are the end-of-life result for stars many times more massive than our Sun, but not massive enough to collapse into a black hole. These stars have shed much of their mass and size in their supernovas and are left with tightly packed cores of neutrons, and to a lesser extent, protons and electrons. A recent discovery of a 3.2 millisecond neutron star reveals a highly unusual and intriguing system that includes a binary pair with a third companion orbiting the two.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode, we're going to talk about an interesting pulsar. And as we usually do, let's refresh our memories of what a pulsar is, beginning with neutron stars.
Stars that are about 12 to 25 times the mass of our sun will end up as a neutron star.
They will have a normal hydrogen fusion stage like our sun, and when that fuel runs out, they will fuse helium, then carbon, then oxygen, and on and on until they build up an iron core. That core is crazy compressed as a few billion times the density of our sun and a few billion degrees Fahrenheit. It's so dense that eventually the core will attempt iron fusion, but iron fusion consumes energy rather than making it, and that ends support for the star.
The core, which is 50% more massive than our sun, on average, collapses, and within a fraction of a second, the elements break down into their fundamental electron, proton, and neutron particles. The negatively charged electrons and positively charged protons try to repel each other, but the core's gravity and momentum are too strong, and they're forced to merge into electrically charge-less neutrons.
But neutrons have nuclear charge, which is strong enough for them to repel each other and provide support for the core.
The inward momentum is abruptly halted as the core springs into support, and the resulting rebound causes a supernova, which from the outside looks like an exploding star. What's left behind is a neutron star — a Springfield-sized piece of star more massive than our sun, which is so compressed that inside it can only contain neutrons, or possibly even sub-neutron particles, called quarks. This material is squeezed so tightly that we don't really know what it would be like. Scientists think it should form a superconducting superfluid, which is sort of like a liquid without any friction or resistance to flowing charge.
But particle accelerators on Earth can only create this environment for a tiny fraction of a second and in tiny quantities, so not large enough or for long enough to see how the particles would flow.
While the outside of a neutron star can be crazy hot, perhaps even 10 million degrees Fahrenheit, unlike normal stars, it forms a solid crust, a crystalline lattice of neutron-rich iron, 10 billion times stronger than steel. The inner part of the crust is called a nuclear pasta, where protons and neutrons under immense pressure form elongated structures. Really interesting for studying nuclear particles and systems under extremely extreme conditions.
Neutron stars have incredibly strong magnetic fields.
If that magnetic field is not aligned with spin, then it beams out radio waves along the poles, sweeping around with the star's spin like a radio lighthouse. If those beams point toward us, we see the neutron star as a pulsar. But we have to couple that with spins of neutron stars.
As the core collapses, it spins up to about one revolution per second, and so we see pulsar beams about once per second. As pulsars age, they also slow down.
The interesting pulsar we're talking about this week has the lovely name of PSR J0435+3233, but let's just call it J.
It was recently published in the Astrophysical Journal Letters by Zonglin Yang of the Chinese Academy of Sciences and Collaborators. Several things make this pulsar extra interesting, and one of those is its spin period — just 3.2 milliseconds. Yes, it spins more than 300 times per second. Much faster than a regular pulsar, and most likely this is because of its companion.
J has a white dwarf companion in just an 8-day orbit.
Recall that white dwarf stars are the end states of stars like our sun, and J's white dwarf friend is even a special white dwarf called an extremely low mass, or ELM, white dwarf. So early in J's companion's life, after surviving the explosion of J itself, when it became a red giant star, J began eating it. This caused J to spin faster, and the white dwarf to have a very low mass.
But to make it even more interesting, J has a second companion, a sun-like star with a wide orbit around the inner pair with a period of 73.5 years, and the second companion's orbit is tipped almost completely perpendicular to the first pair.
In Dr. Yang's paper, they calculate that the system only had a 35% chance of remaining together after J's explosion, and so this system is quite unusual. J is roughly Springfield-sized, but more massive than our Sun. Its white dwarf companion is only 30% the mass of our sun, and about twice the size of the Earth, and the second companion is a sun-like star, but older, just after it's run out of hydrogen in its core. Soon, it will become a red giant.
So this system will be very useful for studies of stellar dynamics and evolution.