The green ghost of a distant dead star

Contributed by
Nov 22, 2011
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160,000 light years away sits the Large Magellanic Cloud, an irregular dwarf galaxy that orbits our own Milky Way galaxy. It's a fascinating object, actually, filled with stars, gas, dust, and all the usual trinkets a galaxy has.

It also has an assortment of globular clusters -- roughly spherical collections of a few hundred thousand stars bound by their own gravity orbiting the cluster center like bees buzzing around a hive. NGC 1846 is one such globular cluster, and it looks like most of the others, if a bit sparse and loosely distributed. But it has something that does make it rather special. You can see it if you peruse this lovely Hubble Space Telescope picture that was just released:

[Click to embiggen, or get a much larger version.]

Isn't that pretty? As much as I like it, the most interesting thing in it, though is actually rather difficult to see here. Look at the center of the cluster, then let your eye go straight down, nearly to the bottom of the frame. See the green spark there? It's the only green thing in the entire picture. It's not a star -- there are no green stars -- but it used to be... Here it is, zoomed up. See now? That's definitely not a star!

So what's the deal here? When a star runs out of fuel in its core, it can no longer stably fuse hydrogen into helium. When that happens, the core heats up, and the outer layers of the star respond by expanding (that's what a gas does when it's heated!). The star turns into a red giant, and starts to blow off a huge amount of gas into space. Eventually, it sheds all of its outer envelope, exposing the hot, dense core to space. That remaining bit, called a white dwarf, floods the expanding gas with ultraviolet light, making it glow fiercely. It's literally like a neon sign!

This kind of object is called a planetary nebula, because when they were first discovered they looked like planets: green disks. But in reality they are trillions of kilometers across! They tend to look green because of the presence of extremely tenuous oxygen. It's spread so thinly that a cubic kilometer of the gas would have the same number of atoms in it that a cubic centimeter would of Earth's air at sea level. When hit with ultraviolet light, the atoms of oxygen in this gas emit light at various wavelengths, but the strongest emission is in the green. Planetary nebulae can look positively verdant through a telescope.

Only a handful of planetary nebulae have been found in globular clusters; literally only four others are known to my knowledge. PNe, as they're nicknamed, don't last long, just a few thousand years; after that time the gas dissipates so much it's too thin to glow. Globulars are also very old, generally billions of years old. That's important because it takes a massive star to make a planetary nebula. The leftover core of the star has to be hot enough to emit UV light, and it takes a biggish one to do that. It's not even clear if the Sun is massive enough to eventually become a PN when it dies, in six billion years. Stars more massive than the Sun tend to die after a few billion years, which is younger than the age of the typical globular cluster. So, by now, all the massive stars in a globular cluster are long since dead, so there is none left to become a planetary nebula.

Objects like this are rare, and that makes them interesting. How can they exist at all? In this case, it's possible we're seeing a coincidence: the PN happens to be in a point in space that is on the line between us and the globular. That's not too weird; after all, if you look carefully at the picture you'll see distant galaxies right through the cluster! Coincidental alignments are common in astronomy. However, careful measurements seem to indicate the nebula is in fact in the cluster itself.

As I was preparing this picture for the blog though, I noticed something: you might think the bright star near the center of the nebula is the white dwarf, the light source for the gas. But I don't think so. It's off-center, for one thing. That also happens sometimes, but look more carefully. Just above that star, overlapping it, is a very faint blue smudge. Can you see it? I suspect that is the actual central star of the PN. A hot white dwarf should appear very blue in the picture, so my guess is that's the guy.

And I'll add one final note: we know of many such planetary nebula in the Large Magellanic Cloud. When I was working on Hubble, I found one by accident! Called M94-20, it was previously known, but never before seen in such detail. I was able to glean some information from it, like its size, what it was composed of, and some characteristics of its central star... which is all pretty remarkable, given that it's 1.6 quintillion kilometers away!

Image credit: NASA and The Hubble Heritage Team (STScI/AURA), with P. Goudfrooij (STScI)

Related posts:

- Happy 20th anniversary, Hubble!
- Another nearly perfect circle in space!
- Why are there no green stars?
- Followup: Green objects in space