When I was a grad student, right before the launch of Hubble Space Telescope, I was fascinated by planetary nebulae, the weird, beautiful halos of gas surrounding dying stars. I still am! But back then, a big mystery was the transition stage between the period when a star expands into a red giant and blows this gas off, and the time the core of the star contracts, heats up, and lights up that gas by flooding it with intense ultraviolet light.
This stage, called the protoplanetary nebula stage, only lasts a few thousand years – pretty short on the galactic timescale. That makes PPNe rare and difficult to observe… but times have changed. Hubble has been up a while, and has spotted several of these uncommon beasts, like Minkowski 92:
Isn’t that lovely? The double-lobed shape is a pretty good indicator that the star in the center isn’t alone; it either has a companion star or was possibly circled by planets. When the star expanded into a red giant its rotation slowed, but if there were a nearby star or some planets it would’ve swallowed them up. While they slowly boiled away, they actually sped up the star’s spin like a whisk mixing up batter in a bowl. This created a disk around the star’s equator, and the gas blown off by the star flowed more freely up and down along the star’s poles, creating those lobes.
The gas is being lit up by reflecting the star’s light. Mostly, at least: if you look in the lobe on the left, there are faint filaments that are probably gas ionized by shock waves traveling through the material. Those are glowing on their own, but they’ll soon be outshone. In a few hundred or maybe a thousand years, the last gasps of the star will puff out the final bits of gas, exposing the bright hot core. This dense ball, called a white dwarf, will blast out UV photons and light the gas up like a neon sign.
Until that time, we can add Minkowski 92 to our very short list of these rare, delicate, and beautiful objects.
Credit: ESA/Hubble & NASA