Binary Neutron Stars

Way back when we spent a number of posts surveying the stars, we covered binary systems. These are star systems that contain multiple stars. Imagine if our sun had a companion, and two stars rose and set in our sky over the cycle of day and night.

It might surprise you that the majority of stars in the universe are actually in binary systems. Our solar system seems to be an outlier in that regard. Most stars have a companion or two or six…

…and so do some neutron stars.

Remember that neutron stars are the collapsed remnants of massive stars that have gone supernova. If most stars are part of binary systems, then naturally, some of these stars will evolve into neutron stars and still be part of their birth system.

Not all neutron stars are still part of their birth system. As I covered in my last post, many neutron stars rocket through space at incredible velocities, leaving their birth system behind.

Those that stay, though, provide astronomers with fascinating insight into the nature of neutron stars.

First of all…how do astronomers discover binary neutron stars?

To answer this question, let’s review what we know about how astronomers discover ordinary binary systems.

As I covered in my post on spectra, a star’s spectrum is probably the single most powerful tool astronomers have to study it. The main idea is that, as you see above, elements such as hydrogen—when excited—emit light at very specific wavelengths.

What do I mean by that? Well, think of every tiny variation in the color of a rainbow as a different wavelength. You can see a better representation of it on this diagram of the electromagnetic spectrum, the spectrum of all light in the universe…

Visible light spans a very small range of wavelengths compared to the entire EM spectrum, but still, each variation in color is a different wavelength.

So as I was saying, different elements emit light at different wavelengths. By studying which wavelengths get blocked (absorbed) and which get emitted, we can learn a lot about a star—and about anything in the universe, including the gas and dust that permeates the space between the stars. Depending on the type of spectrum, we can even learn if a gas is hot or cold, ionized or neutral.

Anyway. What’s with those spectra above, anyway? How come it looks like the spectral lines—the dark bands where specific wavelengths have been absorbed by, in this case, hydrogen, and have failed to reach our instruments—sort of separate, merge, and separate again?

That’s because of a super-cool mechanic called the Doppler shift.

The Doppler shift can be separated into two different shifts in spectral lines: redshift and blueshift. Redshift occurs when an object moves away from the observer, and any radiation it emits gets stretched into a longer (redder) wavelength. Blueshift occurs when an object moves toward the observer, and any radiation it emits gets squeezed into a shorter (bluer) wavelength.

You can see how this might be useful for detecting binary systems. As the two stars orbit one another, they each periodically move toward and away from an observer on Earth. The result: their spectral lines get redshifted and blueshifted.

Neutron stars are very small—around 10 km in diameter—so they are very faint. That would make them very difficult to study, and binary neutron stars even more difficult to find, if not for their powerful magnetic fields that produce beams of radiation at the magnetic poles.

If a neutron star happens to be oriented so that, as it spins at around a hundred times a second, its beams sweep directly over Earth, humans see very brief pulses of light. That visual phenomenon is called a pulsar.

Pulsars have very regular periods (cycles of pulses), almost as exact as an atomic clock. So when, in 1974, a pulsar was detected whose period grew longer and then shorter over a regular course of 7.75 hours, it was pretty clear that the pulsar was getting Doppler shifted.

The first binary neutron star had been discovered.

It turned out that this pulsar, dubbed PSR B1913+16 (I know, so imaginative), was not only part of a binary system—its companion was also a neutron star, but a “silent” one (that is, not a pulsar as seen from Earth). The two were separated by a distance roughly equal to our own sun’s radius.

But that’s not the coolest thing about this neutron star pair. As they orbit one another, they are emitting something called gravitational radiation.

Yeah. Gravity waves.

Gravitational waves are literal waves in space-time. If you imagine the fabric of space-time as like the surface of a trampoline, you can imagine that as an object moves, it sends ripples through the trampoline not unlike ocean waves. And just as light—a form of energy—exists as waves, gravitational waves are a form of energy.

That’s why we say that these two neutron stars are emitting gravitational radiation. The stars are orbiting one another so closely and so rapidly that they are emitting orbital energy as gravitational radiation and slowly spiraling in toward one another.

Astrophysics has made great strides since the discovery of PSR B1913+16. We’ve actually detected gravitational waves directly for the first time. But back then, this binary neutron star system was heralded as confirmation of Einstein’s theory of general relativity.

Another incredible discovery was made in 2004, in the form of a double pulsar. These two neutron stars are also part of a binary system, and instead of one being a pulsar and the other being “silent,” both have radiation beams that sweep directly over Earth.

These two pulsars are also emitting gravitational radiation, and their separation is decreasing by 7 mm each year. Within the next 85 million years, the two neutron stars will likely merge.

What happens when two neutron stars merge? Well, because they’re supported by the material strength of degenerate neutrons, we know they have a mass limit of about 3 M (solar masses). So if two neutron stars merge to form a mass greater than that, they must collapse into a black hole.

Until then, these two pulsars present an incredible opportunity for astronomers. They’re nearly edge-on to Earth, meaning the plane of their orbits are nearly flat when viewed from Earth. That means that every orbit, each pulsar gets eclipsed by its companion.

When this occurs, studying the spectrum of each allows us to study two stars, not just one. We’re able to study the size and structure of their magnetic fields.

Another case study of a neutron star system is Hercules X-1, the first X-ray source found in the constellation Hercules. It contains a neutron star orbiting a 2 M main-sequence star.

The main-sequence star in this system is much like our sun, only twice its mass and around the same age. Normally, main-sequence stars don’t engage in mass transfer. But this one orbits its neutron star companion so closely that it does lose mass to its companion.

It’s not a gentle process at all. Mass transfer doesn’t tend to be, but neutron stars are very extreme objects with powerful gravitational fields. An apple landing on the surface of a neutron star would produce an explosion as if it were a 1-megaton nuclear warhead. Mass transfer isn’t usually that violent.

In the case of Hercules X-1, matter from the main-sequence star falling onto the neutron star flows into an accretion disk and emits a powerful X-ray glow.

Because the system is edge-on to Earth, the neutron star gets eclipsed (hidden behind its companion) every orbit, and the X-rays shut off completely, only to turn back on again when the neutron star peeks back out from behind its companion.

There’s yet another phenomenon that can happen in binary neutron star systems: X-ray bursters.

X-ray bursters are neutron stars that accumulate a layer of degenerate helium until it suddenly fuses violently to produce, as the name goes, a “burst” of X-rays.

How does this happen?

It’s not fully understood, but the general consensus is illustrated by a particular system very imaginatively named 4U 1820-30. This system contains a white dwarf and a neutron star which orbit their center of mass every 11 minutes.

Yeah. Crazy, right? Only 11 minutes. Imagine if Earth had 11-minute orbits.

Anyway, these two compact objects are crazy close together, only about a third of the distance between the Earth and the moon. That kind of distance in a binary system is hard to explain.

Theorists have suggested that a neutron star rocketed through space, as is the norm for these objects, and collided with a giant star. It would have actually gone into orbit around the giant star’s core…inside the star’s envelope.

Thanks to its impressive gravitational field, the neutron star would have slowly eaten away at the giant star until its core, rather than going supernova and collapsing into a companion neutron star, would have collapsed into a white dwarf.

Because this system is so extraordinarily close together, the white dwarf would be forced to transfer mass to its companion despite its tiny size. This matter would flow into an accretion disk onto the neutron star, which would eventually fall onto the surface of the object itself and then release an X-ray burst.

Anyhow, that’s it for binary neutron stars. They’re some pretty crazy objects with extreme conditions, and we’ve barely scratched the surface. Next up, we’ll cover some particularly unique neutron stars…and then we’ll dive into black holes.

Questions? Or just want to talk?

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