The spiders that eat stars, and the scientists who catch them
Spiders can eat their partner. Some of the most extreme stars can do that too.
The professor is often on the lookout for spiders and is pleased when he finds a black widow or a red-backed one. But Manuel Linares Alegret is not a biologist, he is a physicist. He finds the spiders far out in space.
“These are stars that could vaporize their neighbors,” says Linares, who works at NTNU’s Department of Physics.
These binary stars are called “spider pulsars”.
The stars got this name because they could eat their partner, just like some spiders do.
The comparably tiny spider pulsar blasts its much bigger neighboring star with so much energy that the side facing it gets twice as hot as the Sun’s surface, slowly burning the star away. Credit: NASA’s Goddard Space Flight Center
Gathers all spiders in one place
Linares leads a group of scientists who study and search for such spider pulsars. In an article in The Astrophysical Journal, they present more than 100 of them in a database. This database is called SpiderCat, which is openly available to anyone interested.
Several physicists have contributed to the catalogue, most notably senior researcher Karri Koljonen.
SpiderCat
- SpiderCat is a catalogue of all known spider pulsars in the Milky Way, except for those in globular clusters orbiting the Galaxy. The catalogue includes the main types of redbacks and black widows, but also candidates and unusual systems such as transitional, huntsman, and tidarren.
- SpiderCat is a tool for research on pulsar development, particle acceleration and the physics of neutron stars.
- The first version of SpiderCat has over 100 systems. The catalogue brings together key information such as rotation and orbital properties as well as data from radio, optical, X-ray and gamma-ray observations among other parameters.
- Lately, scientists have found many more spider pulsars. This is thanks to NASA's Fermi-LAT and other very advanced telescopes.
“SpiderCat is a large database that shows all known “spider” pulsars in our Galaxy, except for those in the globular clusters that orbit the Galaxy,” Koljonen sums up. He led the work on the article and the publication of SpiderCat.
Physics students and engineer Bogdan Voaidas have also contributed to the software machinery behind the scenes.
“SpiderCat is like a living library of these star systems in our Galaxy. It helps astronomers understand how these binary stars work and change over time,” says former student Iacob Nedreaas, who wrote his master’s thesis on SpiderCat.
But what exactly are these greedy stars?
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What is a pulsar anyway?
First, we need to figure out what a pulsar is, and the short version isn’t too tricky. They are so-called neutron stars, and these are born in a dramatic way.
“A neutron star can form when a massive star explodes,” says Linares.
These stars are tiny, perhaps with a radius of just over 10 kilometers. But at the same time, they are incomprehensibly densely packed. If you could weigh them, one cubic meter would weigh up to a quintillion kilograms. We are talking about a number with 18 zeros.
“A pulsar is a neutron star that spins around up to several hundred times per second,” he says.
You can check why it goes around so quickly in the fact box. But you can try to imagine that our Sun is spinning around like a hand blender in the sky. Quite different, right?
In short, a pulsar is the remains of a giant star that has exploded. It spins around quickly.
Pulsars
- If you have a swivel chair or carousel on the playground, you know that it is much easier to spin around a small child than it is to spin a large adult. It's a bit like that with stars too.
- An ordinary star has a certain spin, or angular momentum, that determines its rotation speed.
- After the star's core has collapsed and it has turned into a neutron star, it retains more or less the same angular momentum. But because it is now tiny, the rotational speed is much, much higher.
- Pulsars can also rotate faster if they accrete mass from a companion star, which adds more rotational momentum.
- Some pulsars can spin several hundred times per second. Their rapidly rotating magnetic fields generate strong electric fields, which in turn produce beams of electromagnetic radiation at the star’s poles. This radiation is emitted into space and can be detected using radio telescopes.
- From our fixed observation point on Earth, this radiation is seen as pulses that arrive each time the poles point toward us.
And a spider pulsar is?
“A spider pulsar is a rapidly rotating pulsar that has a star with a small mass next to it,” Turchetta explains.
This type of pulsar thus has a less densely packed companion. But how does the pulsar eat its friend?
“The pulsar emits intense radiation and particle winds. These gradually wear away the partner. Like a spider eating its mate.”
The researchers divide into several different spider pulsars, named after different spiders in English. The two main types are:
- Redbacks: These pulsars have a companion star that is less massive but much larger than the neutron star.
- Black widows: Here, the second star is very light, and almost gone.
In addition, there are several variants of spiders that researchers cannot neatly fit into the other types, such as the huntsman and tidarren types.
The SpiderCat logo.
So, what about SpiderCat?
The new catalogue SpiderCat describes how fast the neutron stars spin, and how long it takes for the two stars in the system to orbit each other. It also provides an overview of the mass of the companion star.
The researchers have also studied how the stars look in different forms of light, such as radio waves, X-rays, visible light and gamma-rays.
The catalogue is an aid so that researchers can study how these systems work and develop. They can learn more about the physics behind neutron stars, understand extreme particle acceleration, and explore how matter behaves under the most intense conditions in the Universe.
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NTNU’s people found the closest spider
NTNU has several people who are deeply involved in spider pulsar research through the LOVE-NEST group.
PhD candidate Marco Turchetta has led an all-sky search for spider pulsars in our Galaxy.
“Among other things, we have found the closest system that could be a spider pulsar. This system is only 659 parsecs away,” says Turchetta.
Parsecs is a measure of distance, not time. 659 parsecs correspond to approximately 2149 light years, or 20.3 quadrillion kilometers. It is this number: 2.03 × 10¹⁶.
This isn’t exactly in the neighborhood either, but maybe that’s just as well.
References:
Marco Turchetta, Manuel Linares, Karri Koljonen, Jorge Casares, Paulo A. Miles-Páez, Pablo Rodríguez-Gil, Tariq Shahbaz, Jordan A. Simpson. COBIPULSE: A Systematic Search for Compact Binary Millisecond Pulsars. arXiv:2410.18199. https://doi.org/10.48550/arXiv.2410.18199
Karri I. I. Koljonen, Manuel Linares. SpiderCat: A Catalog of Compact Binary Millisecond Pulsars. arXiv:2505.11691. https://doi.org/10.48550/arXiv.2505.11691
