We humans like to put things into neat, little boxes. Naming, labeling, categorizing, classifying: it's normal, but can easily confuse us when the process goes awry. What if something is on the edge of one category versus another; teetering on the cusp of one state of being before plunging into another? That challenges our desire for static status, and can make things more confusing.
With that, may I present the lovely and bizarre object IRAS 13208-6020, as seen by the Hubble Space Telescope?
[Click to ennebulenate.]
Isn't that weird? When I first saw it I thought it was a planetary nebula; the (poorly-named) structure that forms around a dying star. As stars age, their cores shrink and heat up. Through complicated physics, they wind up blowing off a dense stream of particles like a solar wind. There can be several stages to this process, but it ends with the star totally ejecting its outer layers, exposing its tiny, über-dense but incredibly hot core, surrounded by eerily-shaped clouds of gas glowing due to the ultraviolet light from the exposed core.
But I was wrong: it's not a planetary nebula, but a proto-planetary nebula. That is, the star is blowing that wind, enveloping itself in clouds of gas, but the core is not yet hot enough to make the gas itself glow on its own like a fluorescent light bulb. Instead, the gas is merely reflecting the light from the star. I know that doesn't sound like much of a difference, but it really is. The physics of how and why this object gives off light is very different, for one. This image here is in two colors; the kind of light we see (visible light) which is tinted blue in the picture, and infrared light (falsely colored red in the picture). Usually, planetary nebula are faint in the IR, but this one is quite bright in that light; the gas itself doesn't emit much IR, but the star sure does, and the gas is reflecting it.
Normal planetaries tend to glow green due to the presence of atomic oxygen; it emits very strongly in that part of the spectrum. But again, the star here doesn't have enough oomph to get that process going, so the gas just sits there. It looks like a planetary nebula to me, but only kinda sorta. The shape is right, but the essence of it is not.
The shape is really weird, too. We see lots of planetaries like this, but still. A star all alone in space would normally emit gas in a rough sphere around itself, but in this case something is shaping the wind. Most likely it's some sort of companion: a star orbiting very close in, perhaps. Or, it's possible that the star we see here had planets, and as it died it expanded, engulfing those planets. As they orbited inside the star, frying the whole time, they would help accelerate the star's spin. That too could produce the wonderful bipolar shape of the gas around the star.
We're not precisely sure how objects like this evolve; how they get their start, how they get their structure, and how they eventually turn into a full-fledged planetary nebula. Stars don't stay in this stage for long, so finding one is a rare opportunity to learn more about them and get answers to those questions. Sometime in near future -- a few thousand years, perhaps -- as the star gets hotter, it will eventually reach that special moment when it starts to blast out ultraviolet light. At that point it will have the power needed to make the gas glow, and IRAS 13208-602 will become a real planetary nebula, joining hundreds of others we know about.
Until then, it sits in its own box, or perhaps on the lip of two. Only time will be able to tip it from one to the other.
Image credit: ESA/Hubble & NASA