Unique Neutron Stars

Neutron stars—the compact remains of massive stars that have gone supernova—are some of the most extreme objects in the universe, narrowly beaten by black holes (and, as we’ll talk about in future posts, active galaxies and such).

Dense balls of pure neutron material with diameters barely larger than Los Angeles, neutron stars have strong magnetic fields that produce beams of radiation at the magnetic poles. Their speedy rotation makes these beams sweep across the sky like a lighthouse.

When one of their beams crosses directly over Earth, human astronomers observe rapid pulses of light called pulsars.

These objects are whacky, to say the least. And there’s more…

In my last few posts, we covered how neutron stars, as they blast energy away via their radiation beams, gradually slow down their rotation. In other words, over time, neutron stars gradually rotate slower.

It stands to reason, then, that younger neutron stars should pulsate faster, and older neutron stars should pulsate slower…right?

Actually, some of the fastest pulsars are very old. But why?

Think of it this way…

Okay, I know what you’re going to ask me. What does a water wheel have to do with a neutron star?

Well, remember how a number of neutron stars are found in binary systems—star systems with more than one star? And many of those binary systems engage in mass transfer—the transfer of mass from one star to the other.

Because neutron stars have ridiculously high gravity, they’re going to be the ones “stealing” the mass, so to speak. And when mass transfer occurs, the mass can’t just fall directly onto its recipient star. It has to fall first into an accretion disk that whirls around the star in the same direction that it’s rotating.

In other words, mass transfer is like pouring water onto a wheel. A very old neutron star would have had plenty of time to get spun up to a fantastic rate by mass transfer from a companion.

The next question becomes…do these weirdly fast-spinning neutron stars actually orbit companion stars?

The answer is…sort of.

But what does that mean?

Many speedy neutron stars—dubbed millisecond pulsars due to their rotation periods of around a millisecond—are indeed in binary systems. One in particular, XTE J1751-305 (I know, creative), is gaining mass from a companion star that’s currently only 0.014 M.

People…that’s not much more than a hundredth the size of our own sun. It’s only 15 times the mass of Jupiter. That’s ridiculously small for a star.

Clearly, the star has lost mass to its companion neutron star…and that explains why the neutron star has been spun up so much.

Here’s the problem, though. There are also a few millisecond pulsars that are not part of binary systems. So…how could they have gotten spun up?

Meet millisecond pulsar PSR B1957+20, more creatively named the Black Widow.

This pulsar isn’t actually one of the mysterious speedy neutron stars that lack companions. It’s orbited by a brown dwarf.

Wait…what’s a brown dwarf, again?

It’s essentially a star-like object that doesn’t quite make the cut. That is, it formed like a star, as the central object in the dense core of a giant molecular cloud. But it wasn’t massive enough to ignite hydrogen fusion, and thus is very similar to Jupiter—and not much bigger.

The question is, was the brown dwarf orbiting the Black Widow once a main-sequence star, and has it been gobbled up by its companion neutron star?

There is no evidence of current mass transfer. But…remember from my last post that neutron stars emit lots of powerful radiation and high-energy particles? Well, stellar spectra show that these blasts of radiation and and pulsar winds are evaporating the brown dwarf.

Astronomers can’t observe cause and effect in order. Stuff in space happens too slowly for us to do that in our lifetimes. We have to piece together the story of stellar life cycles from observations of star clusters, for example.

A binary system like the Black Widow gives us a “missing link,” so to speak, in the chain of causality. It’s like finding a fossil that has features of both marine and terrestrial life—the missing link that shows when life on Earth evolved to crawl on land.

What does this tell us? All of the other lonely, lonely millisecond pulsars may have once also been part of a binary system and eventually evaporated their companion, just like the Black Widow.

There’s something weirder than millisecond pulsars out there, though. How about pulsar planets?

…yeah. I’m not kidding you. There are planets orbiting some neutron stars.

Not only that, some have been discovered that orbit their neutron star more closely than Venus orbits our sun.

At first glance, this doesn’t make any sense at all. Neutron stars are the remains of a massive star’s core, after a supernova—a very violent explosion—has blasted apart the star’s interior and atmosphere. And even before the whole violent explosion bit, the star had expanded to seriously massive proportions.

And on top of that, the majority of a massive star’s mass goes into creating its supernova remnant, the expanding cloud of gases that results from a supernova explosion. In comparison, a neutron star’s mass is significantly reduced, and consequently, so is its gravity. So many of the planets in the system should have been allowed to escape their orbits.

My point? It would take a miracle for planets orbiting this star that closely to survive and still be there when the core is a neutron star.

So…how the heck do neutron stars have planets?

Here’s one possible explanation…

Anyone think at first that this was some crazy artist’s rendering of Jupiter?

Yeah, I know it’s a bit of a stretch. But look how similar this object looks to our largest Jovian planet! And guess what—it’s a brown dwarf.

We said earlier that brown dwarfs are objects that formed like stars but more closely resemble planets, right? So…what if some of these pulsar planets are actually the devoured remains of main-sequence companion stars?

It’s even possible that a neutron star could acquire multiple planets in this way. For one thing, many stars are part of trinary, quaternary, or even larger systems—that is, systems with three, four, or more stars orbiting one another—and a neutron star could potentially gobble up all its companions.

In the same vein, I imagine that the tidal forces close to a neutron star could strip apart a single companion and let it re-accrete into multiple smaller objects which resemble planets more than stars.

The Spitzer Space Telescope offers one more possibility. Observations in the infrared have revealed a ring of gas and dust around another neutron star. Astronomers aren’t actually sure how it was formed, but if the star’s supernova explosion was somehow responsible, then maybe planets can form in such rings just as they do by accretion in the protoplanetary disk around a newborn star.

So, now we’ve covered just about all the basics of neutron stars. Next up is a topic I’m sure you’re all looking forward to—black holes!

Questions? Or just want to talk?

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