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One big spiral arm

Check out Arp 184 (aka NGC 1961), an odd-looking but lovely spiral galaxy recently imaged by Hubble:

Usually weird-looking galaxies are the result of recent (or ongoing) mergers, but there’s no evidence for that in this case. There’s no obvious companion galaxy near by and the galaxy doesn’t have double cores, which you’d expect to see if it had recently merged. If its last merger was a long time ago, it should look more like a normal spiral galaxy. Why it looks like this, with its one large spiral arm, is a bit of a mystery.

I particularly like this older photo of the galaxy from the Mount Lemmon Observatory, which shows a bit more clearly how the spiral arm wraps all the way around the galaxy:

The single spiral arm might be a little misleading, because (at least, to me) it gives the impression that Arp 184 is a relatively small galaxy. My first thought was that maybe it’s a weird shape because there’s just not enough material there to form the usual spirals that we’d expect to see, but actually, it’s a fairly large galaxy.

Our galaxy, the Milky Way, is about 90,000 light years across, and our nearest large neighbour, Andromeda, has a diameter of about 150,000 light years. Arp 184 is a whopping 240,000 light years across., the size of the Milky Way and Andromeda combined.

A galaxy that size pretty much must have undergone a bunch of mergers and other interactions with other galaxies in its past. That merger history would probably explain how the galaxy ended up looking like it did, but until someone works out the details all we can do is admire the galaxy’s odd appearance.

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Solar System

• Juno is producing results: using a microwave radiometer it has taken measurements of cyclones beneath Jupiter’s clouds and it has used infrared observations to find molten magma under Io’s surface. [NASA, Earth.com, Space.com]

• Japan’s Resilience lander has arrived in orbit around the Moon, ready to attempt a landing next month. The lander is carrying a handful of scientific experiments and a rover, named Tenacious. [Space.com, Phys.org]

• Vesta was thought to be a protoplanet because measurements suggested it had a separate core, mantle, and crust. New analysis suggests it’s actually a chunk of a different planet, blown off in a collision in the early solar system. [Space.com, EarthSky]

Galaxy

• A new analysis of data from Gaia reveals a “family” of 1,000 young stars in the process of dispersing unusually quickly across the Galaxy. [EarthSky, ScienceDaily, Universe Today]

• A new record for the coldest exoplanet: a super-Jupiter which is a chilly -87 °C. It was detected because it is orbiting a white dwarf, which are less luminous than main sequence stars, making it easier to make the spectroscopic measurements to measure the temperature. [Universe Today, Space.com]

• Flares on magnetars can create heavy elements through the “r-process” – a type of nuclear fusion that relies on having a lot of free neutrons which can be captured very quickly. Magnetars are a type of neutron star, which as you’d guess have plenty of neutrons to spare. [Earth.com, Sky & Telescope, Space.com]

• Brown dwarfs, which are less massive than stars and more massive than planets, have weather in their atmospheres and it has been measured for a nearby brown dwarf using JWST. [Universe Today]

Universe

• Galaxy or star cluster? A tiny collection of stars discovered last year, Ursa Major III/Unions 1 (UMa3/U1), could be either. Previous research suggested it is a galaxy dominated by dark matter, and a new paper suggests it’s a dark-matter-free star cluster. [Universe Today]

• Quasi-periodic eruptions might be due to objects smashing through the accretion disk of a supermassive black hole. New X-ray measurements of the brightest known QPE suggest each collision sends roughly the mass of Jupiter bursting out of the disk at 15% of the speed of light. [Phys.org, ScienceDaily]

• New polarisation measurements from IXPE show that supermassive black hole jets produce X-rays by scattering of light by electrons (Compton scattering), solving the mystery of whether it is electrons or protons doing the scattering. [NASA]

Finally

A poetic thought on the re-entry of Kosmos 482:

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].