The Song of Distant Mountains

by Brian Koberlein

This story first appeared at www.briankoberlein.com on January 5, 2024. 

The surface gravity of a neutron star is so incredibly intense that it can cause atoms to collapse into a dense cluster of neutrons. The interiors of neutron stars may be dense enough to allow quarks to escape the bounds of nuclei. So it’s hard to imagine neutron stars as active bodies, with tectonic crusts and perhaps even mountains. But we have evidence to support this idea, and we could learn even more through gravitational waves. 

One of the ways we know neutron stars are active is through pulsars. Pulsars are neutron stars that emit powerful beams of radio light from their magnetic poles. When those poles are aligned toward Earth, we see a regular series of pulses. The pulses are so regular that we can use them as a kind of cosmic clock, measuring everything from the orbital decay of binary systems due to gravitational radiation to the rippling of spacetime caused by the first moments of the Big Bang. 

Because neutron stars radiate energy, their rate of rotation gradually slows down over time. It’s a small effect, but we can observe this slowdown in pulsar data. Sometimes, however, a pulsar will glitch, meaning that its rotation rate jumps up slightly. This can only happen if the shape of the neutron star has suddenly changed. Just as earthquakes can trigger a measurable change in the rotation of the Earth, starquakes change the rotation of neutron stars. So we know there is some kind of tectonic activity on neutron stars, but we aren’t entirely clear what that is. 

One idea is that neutron stars have a rather thin but rigid crust, similar to that of a rocky planet. As a neutron star cools over time, this crust fractures and folds, which leads to quakes, fissures, and perhaps even mountains. While this seems to be a reasonable model, it’s difficult to prove because we can only detect a glitch when something dramatic happens. Imagine trying to study the mountains of Earth when you can only capture earthquake data. But as Fabiarn Gittins shows in his paper, “Gravitational waves from neutron-star mountains,” there could be another way to study the mountains of neutron stars: gravitational waves. 

Gravitational wave astronomy is still a young field, but it has already captured data from neutron stars. When neutron stars merge, they create an energetic burst of gravitational waves, similar to the way merging black holes emit a gravitational chirp. Astronomers have been able to combine gravitational wave observations of merging neutron stars with optical data to study the interiors of neutron stars. This new paper takes the idea one step further. 

If a neutron star has a mountain or surface rise, it is asymmetrical. This means as it rotates the neutron star will emit a continuous stream of gravitational waves. These waves aren’t very intense, but they would hold lots of information about the overall shape of the neutron star. If we can observe these waves over time we could even study how a neutron star precesses due to the dynamic motion of its surface. For neutron stars with intense magnetic fields, known as magnetars, we could even study how the magnetic fields can distort the shape of a neutron star, which is something that may play a role in fast radio bursts. 

If a neutron star has a mountain or surface rise, it is asymmetrical. This means as it rotates the neutron star will emit a continuous stream of gravitational waves. These waves aren’t very intense, but they would hold lots of information about the overall shape of the neutron star. If we can observe these waves over time we could even study how a neutron star precesses due to the dynamic motion of its surface. For neutron stars with intense magnetic fields, known as magnetars, we could even study how the magnetic fields can distort the shape of a neutron star, which is something that may play a role in fast radio bursts. 

Of course, to do all this, we need to be able to detect these faint gravitational waves, and here astronomers are a bit more sanguine. At the moment, the most precise gravitational wave data we have can only place an upper bound on the scale of neutron star mountains. Even then, all we can really say is that they aren’t huge, which we already knew based on glitch data. But as the next generation of gravitational observatories comes online, it could put us in the range of observation. There are still lots of challenges, but they don’t look insurmountable. So in the coming data, gravitational waves could revolutionize our understanding of neutron stars in much the same way as they are revolutionizing our understanding of black holes today. 

We Need to Preserve Our Dark Skies
Now More Than Ever

by Laura Fitzgibbons

Across the globe there are more than 200 designated parks and areas of land that are preserved as dark sky locations. These parks and spaces limit artificial light as much as possible in order to provide amazing stargazing. They are go-to destinations for the public and scientists alike. 

Darkness is in trouble

It may seem like the enjoyment of the stars overhead is as reliable as the night sky itself, but every year more and more light pollution enters the sky making it harder for people to observe the constellations and stars. The more light pollution and resultant skyglow in your area, the less you will be able to see independent astronomical objects.

Light pollution causes ecological harm

Too much light pollution confuses plant and animals species that depend on the onset of darkness to set their body's natural timing. This impacts everything including knowing when to hunt, when the rest, when to create life, and when to seek shelter from predators. Birds, plants, insects, amphibians, mammals, and any living creatures in a confused light and darkness pattern are in immediate threat. The harm is lasting through generations and can even be deadly.  

You can implement responsible lighting practices now

Reducing light pollution is a mindset, and you can start right away with simple habit changes. The easiest way to do this is by simply turning lights off when you're not using them. If you spend time outside, try to use low lights or less light whenever possible. When you are enjoying your space at night and need bright lights, try to remember to close your curtains. When you choose small lamps make sure they have a cover. Flashlights outside should point down. If you want to make a big impact, get involved in your community and encourage them to choose street lights that aim light down without also shining light directly up into the night sky.

How to find the darkest skies near you

One simple trick if you're deciding on a great stargazing location is to check if you can see the Milky Way. It is one of the most beautiful astronomical objects, but a whopping one third of all people in the globe cannot see it. A great resource to find a designated dark sky preserve is Dark Sky International's Dark Sky Place (DSP) finder. If there is not already a great stargazing location anywhere near you, your community will benefit immensely from you creating one yourself.