I’ve decided to switch from sending this every week to sending it every two weeks. This should allow time for me to also write occasional feature-length articles, in addition to these short news summaries.

Striking out on their own

Imagine this. Two stars are orbiting each other. They've been together for their whole lives. They formed from the same cloud of gas and dust, billions of years ago. Normally, they would expect to grow old together. Only these stars are being assailed by a massive, invisible interloper. A black hole, with the mass of a million Suns. Once they're close enough, the black hole's gravity unleashes chaos on the two helpless stars, disrupting their orbits and wrenching them apart. When it’s over, one star is tightly in the grip of the black hole, where it will spend its remaining aeons. The other has been cast out, slingshotted at relativistic speeds out of the very galaxy where it was born.

What I've described is the start of the journey of a hypervelocity star. In the last newsletter, I talked about runaway stars, which have been ejected from their place of birth and forced to wander the Galaxy at high speed. Hypervelocity stars are the speedier cousins to the runaway stars.

Hypervelocity sounds like something from science fiction. These stars are moving faster than anything else in the galaxy. They move at around ten times the normal speed of stars, roughly 1,000 km/s or about 0.3% the speed of light. That is fast. At that speed, in about six months you would catch up to the Voyager 1 space probe, which has been travelling for 47 years. In about 800 million years, you could cross the 2.5 million light year gap between the Milky Way and the Andromeda Galaxy.

So how do these stars end up travelling at such implausible speeds? There are two main ways it can happen. Like runaway stars, they can be ejected from binary systems as a result of their companion undergoing a supernova. This is very rare, though, as the speeds involved are obviously a lot faster. Remember, the star is not really being accelerated in this scenario; it’s already travelling at its final speed, but in orbit around its companion. When the companion is destroyed, it simply carries on at the same speed but in a straight line. So most binary systems will not have a high enough orbital speed to produce hypervelocity stars. This probably does happen occasionally, though, as there are known to be hypervelocity stars which come from the galactic disk. This is far from the galaxy’s supermassive black hole, which is responsible for the other way for hypervelocity stars to be accelerated.

As described at the beginning, the supermassive black hole scenario also involves binary stars which are passing near to the black hole. When the binary system gets close enough to the black hole, it is possible for the black hole to capture one of the two stars into orbit. The other star, now free of its companion’s gravity, is slingshotted away through something called the Hills Mechanism. The way this works is basically this: as the two stars pass close to the black hole, the one closest to the black hole can be captured while the other one is still far enough away to avoid being captured. The captured star orbits the black hole while the other star is accelerated away.

Less than two dozen hypervelocity stars are known. They mostly seem to come from the centre of the galaxy, near the supermassive black hole. The others come from the various globular clusters and dwarf galaxies which orbit our galaxy. At least one originated in the Large Magellanic Cloud. And intriguingly, it has recently been suggested that stars from our largest galactic neighbour, Andromeda, could already be here in the Milky Way.

Hypervelocity stars are a reminder that there is not a scale at which we are isolated from the rest of the Universe. Rocks from space rain down on the Earth all the time, including parts of asteroids and rocks blasted off other planets. We’ve recently learned that interstellar objects regularly pass through the solar system. Moving through the wrong part of the galaxy seems to cause mass extinctions. And even the unimaginable gulf between galaxies is not so great that it can prevent the exchange of whole stars.

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In the News

Solar System

• Reanalysis of 30 year old data from the Magellan mission suggests Venus may have ongoing tectonic activity. Circular features on the surface called coronae seem to sit on top of hot plumes rising up in the mantle. [NASA, Discover, Scientific American]

• The near side of the Moon is hotter than the far side, probably because it contains more radioactive elements which heat it more. (That’s also one of the things that makes the Earth hot inside.) [New Scientist, Science Focus, Earth.com]

• Apparently Jupiter used to be twice as large as it is today (based on the orbits of two of its closest moons). It had the same mass and shrank over time as it cooled. [Space.com, Popular Science, Science Alert]

• Not water: it’s a mystery where streaks on the Martian surface come from, but a new paper rules out water as a cause and narrows down the other options. [Science Daily, Space.com, Universe Today]

Galaxy

• A system that shouldn’t exist: a planet in a retrograde orbit that passes between two stars. New measurements suggest it either started out in orbit around both stars and moved as the stars aged, or it actually formed from material ejected by one star as it reached old age. [New Scientist, Ars Technica, Nature]

• A pulsar has been found orbiting extremely close to a helium star. The two likely went through a “common envelope” phase, when the pulsar was inside the star, throwing out the star’s outer hydrogen layers and leaving behind the helium star that’s there today. [Space.com, Nature, Phys.org]

• A weird, perfectly spherical supernova remnant is raising questions about how it formed. [Live Science, Popular Mechanics]

Universe

• Feeding supermassive black holes produce enough radiation to put an end to star formation in their host galaxies, in a process called “quenching”. For the first time a SMBH has been found quenching star formation in a neighbouring galaxy. [ESO, Reuters, EarthSky]

• Dwarf galaxies orbit larger galaxies, like the Milky Way, and they often line up with each other. New simulations suggest that’s because of the influence of the filaments that helped the larger galaxy grow when the Universe was young. [Space.com]

• The Einstein Probe has detected a fast-evolving X-ray transient (FEXT) which lasted over a month. The origin of FEXTs is unknown, but these observations suggest there was a relativistic jet which could have come from a supernova, a magnetar, or a black hole. [Phys.org]

Finally

Construction of the Extremely Large Telescope, due to be completed in 2029, is progressing nicely:

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].