The last gasp of a dying star

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The last gasp of a dying star

The protoplanetary nebula IRAS 22036+5306, a dying star about 6,500 light years away. Credit: ESA/Hubble & NASA

Some stars die with a bang, exploding as supernovae. Some stars just sorta fizzle out without even a whimper, like red dwarfs, which just slowly run out of fuel and shut down.

There's a lot in between that vast range. Stars like the Sun (and up to about 8 times the Sun's mass) swell hugely into red giants, blow off their outer layers of gas, and then fade away into tiny, hot, white dwarfs. And sometimes, if the star was in a binary system (orbiting another star), that white dwarf can siphon material off the other star; when that happens it can undergo recurring explosions as the material heats up and detonates like a giant nuclear bomb. We call those novae.

And some stars do a little bit from column A and a little bit from column B.

Let me present you with IRAS 22036+5306, a dying star that can't make up its mind what it wants to be.

The protoplanetary nebula IRAS 22036+5306, a dying star about 6,500 light years away. Credit: ESA/Hubble & NASA

Isn't that pretty? It's also a mess.

When a star like the Sun sheds its outer layers and becomes a white dwarf, the dwarf can be hot and bright enough to excite the gas and cause it to glow. We call those objects planetary nebulae, because when they were first discovered they looked like little planetary disks through a telescope.

IRAS 22306+5306 isn't quite there yet; it's a proto-planetary nebula, an intermediate object between a star shedding its layers and one that has finished that period of its demise. This stage in a star's death doesn't last very long — only a few thousand years, if that — so we don't see too many of them. When a star lasts billions of years, this phase is barely the blink of an eye.

The elongated shape of it is odd. When the object was first discovered it was just a bright source in the Infrared Astronomical Satellite (where the IRAS bit in its name comes from; the numbers are its coordinates on the sky) that was identified as a young planetary nebula. But Hubble reveals it's extremely elongated. More than you'd expect. You can also clearly see a very thick ring of material, a torus, around its middle. It's like a napkin ring at a fancy dinner setting… but that torus is about a half trillion kilometers across!

When a star expands into a red giant, it spins very slowly, sometimes taking years to make a single rotation. The gas it expels, therefore, should be a sphere. If you spun the star up really fast, you might get such a torus around it, since the gas would blow outward more along the star’s equator due to centrifugal force. But to do that you'd have to spin the star unreasonably fast! Way faster than it could on its own.

In some planetary nebulae we think this may be due to a giant planet orbiting close in. When the star expands it swallows the planet, which spins the star up. But IRAS 22306+5306 is too elongated even for that.

It gets weirder. Other observations revealed the presence of jets: beams of material blasting away from the center at ridiculously high speeds, like 200 kilometers per second. And the amount of material is huge, equal to 0.03 times the Sun's mass. That may not sound like much, but it's equal to the mass of 10,000 Earths! Imagine what kind of cannon could shoot out an entire planet's worth of material 10,000 times at 700,000 kilometers per hour, and you get a sense of the ridiculous amount of energy expended.

If you look at the speed and length of the jets, you can backtrack to the time they first erupted outward. Amazingly, it happened only about 40 years ago! Whatever huge eruption got these started, it happened in the early 1980s. Holy wow.

Incidentally, this is one of the few nebulae of this sort where the orientation is pretty much what your brain is trying to tell you: The lower left side is the far side, with the gas there heading away from us, while the upper right side is closer, with the gas headed toward us. This was revealed in velocity measurements of the gas. Most of the times when looking at planetary nebulae the near and far side can be ambiguous and can only be understood when velocity maps are made. The shadowing on this one makes it more obvious.

What is probably going on here is that we're seeing a binary star. The dying star was big, 4 or 5 times the mass of the Sun. It initially did blow off material in a spherical shell (evidence of that is seen around the nebula), but these kinds of stars are also prone to eruptive events as they die. It could have blasted out a lot more material rapidly. Some of this gas fell onto the other star, creating a swirling disk of debris around it called an accretion disk. These can be very energetic, and focus beams of material that then get shot away at terrific speeds to form the jets.

Missing media item.

That also explains the torus around the middle; that is material blown off the plane of the orbit of the two stars. It's loaded with thick dust, too (probably another 10,000 Earths' worth of dust with grains bigger than about 1 mm, the size of sand on the beach). That explains another mystery: why this event wasn't seen when it happened in the 1980s. Dust is very good at absorbing visible light, so it hid the flash of energy. It probably got very bright for a short time in the infrared as the dust warmed up, but at that time we didn't have a good enough infrared observatory in space to see it. By the time IRAS launched in 1983 it had already faded.

The energy released in this event was more than a typical nova, but far less than a supernova. In a paper about these sorts of events (authored by my master's degree advisor, Noam Soker; I studied planetary nebulae for that work!), they're called ILOTs, for intermediate-luminosity optical transients — objects of intermediate brightness that only last for a short time. I don't know if that name will stick, but it's descriptive. Incidentally, they think that the actual event that kickstarted those jets probably took less than a year. That's certainly what I would call an eruption!

I hope we get more observations of this odd object over the next few years. It should change noticeably even after a year or two due to the high speed of those jets, which is really cool. But more importantly, it will tell us a lot about what happens in that brief moment between a star starting its death rattle and actually expiring. It's an amazing chance to glimpse an extraordinary moment in cosmic history.

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