A runaway visits a nebula

This is a nebula called Sh2-46, which is located about 6,000 light years away in the constellation Serpens, seen in a new photo from the European Southern Observatory:

Sh2-46 is an emission nebula. The red colour is caused by hydrogen gas becoming ionised (losing electrons) because of the radiation from that bright star near the centre. That star is called HD 165319, it’s a large, hot, O type star, and it’s really only passing through.

HD 165319 is what’s called a runaway star. That’s pretty much just what it sounds like: a star that is travelling extremely fast and has left the place it was born. HD 165319 seems to have started out in an open cluster in the nearby Eagle Nebula and appears to be punching right through Sh2-46.

How did an otherwise normal star end up careening through the Galaxy, colliding with nebulae? There are two ways for stars to become runaways. In this case, the star was likely thrown out of the cluster it was born in by an unlucky gravitational interaction with another star. Stars in clusters are relatively close together and that means they interact with each other through gravity fairly often. In the right circumstances that can accelerate one of the stars out of the cluster, sending it flying out into the Galaxy. It’s similar to the gravitational slingshot manoeuvre that allows us to send probes across the solar system, and the technical name for this is dynamical ejection.

The other way for stars to become runaways is through binary supernovae. Sometimes, when a star in a binary system undergoes a supernova, the explosion can disrupt the binary system and send both the remaining star and the new black hole or neutron star (created in the supernova) flying apart at high speed.

There are hundreds of runaway stars known, and a good number of them were discovered recently thanks to the Gaia mission. Studying these stars tells us a lot about the lives of stars in clusters and about supernova explosions.

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Solar System

• If you were deliberately trying to find ways to make the US less prestigious, absolutely wrecking NASA would probably be near the top of the list. The Trump administration’s budget cuts will be a disaster. [Time]

• Venus’ crust is thinner than expected, given the planet’s lack of plate tectonics. It looks as though the crust can only get so thick before the lowest parts break off and melt into the mantle. [NASA]

• Voyager 1’s primary thrusters are working again after being broken for 20 years! The craft has been relying solely on its backup thrusters to keep the spacecraft’s antenna pointing at Earth, but they were at risk of clogging from over-use. [CNN, Space.com]

• JWST has captured Jupiter’s aurorae in stunning detail, complete with video. [NASA, ESA]

Galaxy

• For the first time, asteroseismology has been used to measure the age and radius of a star which is cooler than the Sun. It’s over 10 billion years old (for comparison the Sun is 4.6 billion years old) and the radius is smaller than expected. [Phys.org, EarthSky]

• Using radio observations, astronomers have mapped the accretion disk around a newly forming massive star for the first time. Previously it wasn’t known if a stable accretion disk could exist during the formation of such massive stars, because of the large mass infall rate required. [Phys.org, Universe Today]

• JWST has detected water ice in a newly forming star system, the first time water has been detected in crystalline ice form outside of the Solar System. [Webb Telescope, NASA]

Universe

• Everything decays if you wait long enough, but it turns out we don’t need to wait nearly as long as we thought! (But it’s still an incomprehensibly long time: 10⁷⁸ years). [Phys.org, Universe Today, Space.com]

• For the first time, a tidal disruption event (where a supermassive black hole tears apart a star) has been detected outside the centre of its host galaxy. This suggests that galaxy has two supermassive black holes, with the second likely being left over from a merger with another galaxy. [UC Berkeley, Ars Technica]

• A new study raises doubts about the established model explaining long gamma ray bursts. The existing theory is that long GRBs happen when the jet of a hypernova is pointed right at us. New observations from the Swift gamma ray observatory suggest this only explains about half of known long GRBs. [Universe Today]

Finally

This week I’ve been wowed by this photo of the Milky Way taken by astronaut Don Pettit on the ISS. It was taken with a homemade tracker to counteract the orbital motion. It kind of looks like an aurora, but that's actually clouds just before dawn. The purple colour comes from nitrogen scattering sunlight.

Check out the New York Times article for more of his photos.

What is Three Alpha? Other than being the name of the newsletter you’re reading now, the name “three alpha” comes from the triple-alpha process, a nuclear chain reaction in stars which turns helium into carbon. Read more here.

Who writes this? My name is Dr. Adam McMaster. I’m an astronomer in the UK, where I mainly work on finding black holes. You can find me on BlueSky, @adammc.space.

Let me know what you think! You can send comments and feedback by hitting reply or by emailing [email protected].