Today’s issue number, 1054, doesn’t seem special in any way, yet is a number any astronomer will recognize immediately.

It’s the year the light from the Crab Nebula supernova explosion reached Earth.

The Crab is one of the most (and I do mean the most) iconic astronomical objects in the sky (heaven knows I’ve written about it enough times). It’s faint enough that you need a pretty good pair of binoculars to see it at all, but some structure becomes apparent with even a small telescope. And of course, with something like Hubble you get this sort of magnificence:

I mean, it looks like the expanding debris from a cosmic explosion. Because it is. A massive star ran out of fuel, and its core collapsed. This created a colossal explosion of energy that blasted away the star’s upper layers at soul-freezingly high speed… and mind you, we’re talking about 4.5 times the mass of the sun worth of gas, so an incredible amount. After nearly a thousand years of expansion the result is the Crab Nebula.

The collapsed core of the star became a super-dense neutron star, one of if not the best studied examples of its kind. The fierce radiation of the neutron star — technically a pulsar — also heats and excites the gas around it.

The star was so bright when it exploded that it was seen during the daytime from Earth despite being about 6,500 light-years away. It was recorded by Chinese astronomers, which is how we know that not only was it seen in 1054, but the date it was first noticed translates to July 4 now. So it’s almost the anniversary of that as well.

Happy birthday, Crab! You’ve been a huge help to astronomers; we’ve studied you to learn about supernovae, pulsars, the interstellar medium (gas between stars), stellar nucleosynthesis, X-ray emission from neutron stars, and so much more. We’ve even watched you grow over the years.

And for the jokes. Whenever I happen to see the time is 10:54, I say. “It’s Crab Nebula time!” to the point where my wife doesn’t even roll her eyes anymore when I do.

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I love brown dwarfs: objects more massive than planets, but lacking enough mass to ignite nuclear fusion in their cores to become true stars. Some people call them failed stars, but I reject that; they are their own beasts, and besides, it’s just as correct to call them extremely successful planets.

Anyway, they have masses between about 13 and 77 times Jupiter’s, and the first were discovered in the 1990s (I studied them for a while). Now we know of thousands, and JWST is finding more all the time.

They are commonly found as binaries, a brown dwarf (or BD) orbiting a normal star or another BD. In general they’re not terribly close together, with typical orbital periods of more than a year.

However, a team of astronomers went searching for BDs with far shorter orbital periods. Why? Because that means the pair would be very close together, close enough that they could be interacting with each other, with the more massive of the two physically drawing matter off the other. This is called an accreting binary, and we know of zillions of regular stars that do this, as well as ones orbiting white dwarfs, neutron stars, and black holes. None with a brown dwarf has ever been found.

Until now! They found one, called ZTF J1239+8347 (let’s call it J1239 for short). It’s about 1,100 light-years away, and exceedingly dim, which is why it’s evaded previous discovery [link to journal paper]. They found it in the Zwicky Transient Facility (an observatory that scans the skies looking for objects that change in brightness or move across the sky) survey data, and used Gaia observations to further constrain its physical nature. The brown dwarf binary system shows a clear change in brightness over a 57-minute period. Assuming that’s the orbital period, I ran the numbers and get a separation of less than 200,000 kilometers between them: roughly half the distance from Earth to the moon! They’re close.

That’s so close that the gravity of the slightly more massive brown dwarf can literally pull material off the other. In most situations like this that matter forms an accretion disk around the receiving object, but they’re so close together that instead the donor material slams directly into the receiving brown dwarf! That creates a hot spot in the atmosphere, and they find it’s at about 8,600°C or 15,500°F. Hot! Hotter than the sun’s surface, in fact.

They can’t measure the masses of the brown dwarfs directly, but the observations indicate they have between 60 – 80 Jupiter masses — right around the value needed to ignite fusion.

But the receiving BD is gaining mass…which implies that over time it may gain enough to become a true star. How cool is that?

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