The Star That’s Secretly a Lawn Sprinkler

In recent years, scientists have used some of the most precise astronomical tools ever developed to detect incredible and extreme objects within our galaxy. ~ One of the latest, reported in 2021, is a small but incredibly dense star that’s spinning on its axis more than twice a minute. It’s the fastest spinning star ever detected.

What’s more, this wildly wheeling star also seems to be spraying stuff out into space, just like a lawn sprinkler! To try and understand why it’s behaving so weirdly, scientists need to look deeper into how this rare and exotic star came to be.

Why is it spinning so fast, and where is all the stuff coming from? It’s worth mentioning that pretty much everything in the universe spins. Our own sun, for example, completes a full rotation about once a month, and the Earth spins once a day… by definition. But the fastest-spinning stars put both of these to shame.

Astronomers use highly sensitive instruments alongside powerful telescopes to detect faint pulses of light from spinning stars. Those pulses are thought to come from bright spots on the stars’ surfaces, and so the speed of the pulses lets us calculate the speed of rotation — a bit like a sweeping lighthouse beam. Until recent times, there seemed to be only one of these record-breaking spinners.

Known as AE Aquarii, it was bright enough to be seen in telescopes from the 1940s, and analysis in 1979 showed that it appeared to be rotating once every 33.08 seconds. That means it’s already spun around more than twice while you’ve been Reading this Article! But in 2021, astronomers finally managed to pinpoint the speed of another star, with the catchy name J0240-1952, or J0240 for short. And it seems to be spinning even faster.

To pick up its faint pulsing signal, they used a high speed camera on the huge Gran Telescopio Canarias, part of an observatory in the Canary Islands. J0240 completes a full rotation every 24.9 seconds. That’s nearly 25% faster than AE Aquarii. J0240 spins more than 93,000 times in the time it takes for our Sun to rotate just once. But both AE Aquarii and J0240 are nothing like our sun. They’re actually white dwarfs.

These small, hot bodies are the cores left over after the death of a typical main sequence star. These are stars like our Sun that fuse hydrogen i n their cores for many millions of years, until their nuclear fuel runs out.

So, instead of being sustained by nuclear fusion, white dwarfs are so dense that the repulsion of electrons in their core is what keeps them going. White dwarfs have at least half the mass of our Sun, but are packed into a sphere about the size of the Earth. That gives them a density some 200 thousand times greater than the Earth, and correspondingly huge gravities and strong magnetic fields. But the fastest spinners in the galaxy aren’t alone. Both AE Aquarii and J0240 are part of binary systems with another, red giant star. ~ And it’s as these two stars interact that things get really interesting.

The intense gravity from the white dwarf pulls material from its partner star, channeling it onto its own surface.

Kind of like a parasite feeding on its host. Remember that the white dwarf has run out of fuel, but it is still incredibly dense. So when fresh hydrogen fuel is pulled onto its surface, the gravity is enough to compress it and start off fusion again.

But because this fuel is in the outer layer, the energy it generates is enough to blow away the remaining unignited gas in a bright flash of light, traditionally known as a nova. That process repeating over and over again gives these kinds of stars a very cool name: cataclysmic variables! There are all kinds of cataclysmic variables out there, and the precise dynamics of accumulation and explosion depends on the white dwarf’s magnetic field. If the field is weak, then the secondary star’s gas forms an accretion disk around the white dwarf’s equator, and material gradually rains down onto the surface.

If the magnetic field is really strong, then the two stars become locked in a dance, where their rotational speed matches orbital speed, a bit like how the moon is tidally locked to the Earth. ~ The material is then channeled in a single stream toward the white dwarf’s poles. But if the magnetic field is somewhere in between, not too strong and not too weak, then the red giant’s material ends up spiralling around and getting tangled up. It occasionally falls to the surface in quick spurts. ~

These spurts do still have some spin from their initial spiral, so as they slap into the white dwarf’s surface, they give it a little extra push around. Repeated over and over again, this has the effect of powering the white dwarf’s spin, just like spinning a carousel faster and faster by pushing it every so often.

With J0240, astronomers have even been able to see the point where it’s being “pushed”, as a spot covering about 2% of the surface that’s unbelievably hot, at about 30,000 Kelvins.

Now, we think this star has been spun up about as fast as it can go without completely flying apart. But the strong magnetic fields here are also having another, unexpected effect on the incoming stellar material. Not all of the gas makes it to the surface of the white dwarf. The star’s incredibly fast spin and powerful magnetic fields are enough to eject some of the channeled gas back out into space.

The result is blobs of gas being sprayed out all around the white dwarf, just like a rotary lawn sprinkler! The phenomenon is called a magnetic propeller, and it’s incredibly rare. AE Aquarii and J0240 are the only two stars that we’ve ever spotted doing this.

The unique alignment of J0240 which allows the red giant to occasionally eclipse the white dwarf from our point of view. That has allowed scientists to literally see the blobs of gas as they’re thrown out into space. Each one is a mini-explosion from the white dwarf’s surface, ejected at about 9.5 million kilometers per hour.

That’s around 1% of the speed of light! The energy for all these ejections comes from the star’s incredibly fast spin, so with each explosion we expect the rotation to slow down by a tiny amount. But right now, the discovery of J0240’s lightning-fast spin and its energetic ejections is still pretty recent. So we’ll have to wait a little longer to see how this strange lawn-sprinkler in the sky will evolve.

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