Astronomers, it seems, like eggs. Drifing somewhere out in the skies, you can find the Egg Nebula, the Rotten Egg Nebula*, and now there’s a new addition to this ovoid contingent — the Fried Egg Nebula. You have to admit, it does indeed look rather like a fried egg. This egg, however, has a rather special yolk. At the centre of this nebula lurks a yellow hypergiant star.
Yellow hypergiants are interesting things. They’re the rarest of the rare, with only a small handful (7, as I understand it) being known of. They’re so rare because they’re part of a very short lived phase in the life of a massive star. Stars like to be red or blue. The only time they ever spend being yellow is while they’re changing colour, rapidly, from red to blue. Yellow hypergiants like this one are thought** to have evolved recently from red supergiants, on their way to becoming luminous blue variables, before eventually becoming violent Wolf-Rayet stars.
The Fried Egg (more formally known as IRAS 17163-3907) is interesting because, as it happens, it’s quite nearby. The image above (taken in infrared) is one of the best images to date of a yellow hypergiant. The two circles you see that give it it’s egg-like appearance are due to pulsations within the star, which caused it to suddenly shed huge amounts of stellar material. A vast, billowing cloud of starstuff several times as massive as our Sun lies adrift around this star, having been shaken off like water from a dog’s back. Some of that material has since condensed into dust, shining brightly in infrared light. Massive stars all share in common the fact that they have trouble holding on to their outer layers. This image gives incontestable proof of that.
Yellow hypergiants hold a somewhat special place in my heart. Simply, I find them fascinating, as I’ve ranted about in the past, along with hypergiants in general. For one thing, there’s the fact that the star itself is massive enough that if it were to replace the Sun in our solar system, it would reach Jupiter’s orbit. For another, with all of that stray starstuff being puffed off into space, I’ll bet there are some interesting things in that star’s environment. It rather makes me want to take a closer look and see what I might find…
*Amusingly, the Rotten Egg Nebula is so-named for the huge amounts of sulfur found in it.
**I say thought to evolve. Stars live on such massive timescales, that all we can ever witness is a snapshot. For a scientist, this is a little frustrating. We can observe only this single moment in time, and never actually perform experiments to test hypotheses.