“One touch of nature makes the whole world kin.” -William Shakespeare
Like most globular clusters, this is a very tight, dense collection of perhaps a few hundred thousand stars, contained in a roughly spherical region just a few light years in size. Unlike most of the globular clusters we know, this one is not located within our own galaxy, but instead is about 160,000 light years away in the outskirts of the Large Magellanic Cloud.
And like all the stars we’re familiar with, this globular cluster contains a great many of the great varieties that star colors are allowed to come in: from hot, blue stars to white ones, to cooler yellow, orange, and even the coolest type of all, red stars. But if you look closely, you’ll see something in this image that ought to puzzle you terribly: a green-looking object.
Now this should bother you, because there is no such thing as a green star! And while astronomical objects are often falsely-colored in order to maximize contrast and visibility, this is not one of those times, and this object really is green.
So what’s going on here? Believe it or not, if we want to understand it, the best way to show you is to take you up in orbit around the Earth.
As you watch the video, you’ll of course notice the Earth’s cities, the clouds, oceans and land masses, and the occasional spectacular aurora. But what I really want you to look at is the atmosphere over the horizon when there’s no aurora present. What you’ll see is a greenish haze (possibly with red mixed in), known as the airglow.
What causes this green glow? Believe it or not, this is caused by oxygen atoms cooling down in the nighttime dark, shielded from the Sun. From the International Space Station, this was the view as the final US Space Shuttle mission — Atlantis — returned to Earth.
How does cooling oxygen do this? Like all atoms and molecules, oxygen has a wide variety of energy levels that its electrons can live in. When Ultraviolet light from the Sun comes in and hits the oxygen atoms, some of it is at just the right wavelength to excite its electrons into one of the highest allowable states, in some cases even partially ionizing it.
But when this Ultraviolet light source goes away, hidden by the intervening Earth during the night, those excited electrons drop through the allowable energy levels, emitting a wide variety of photons at very particular wavelengths.
This physical process is something you’ve heard of before, even if you don’t recognize it by its description: fluorescence. These wavelengths are the same for every single atom or molecule of oxygen in the Universe, as they’re all governed by the same laws of quantum physics.
And what does the emission spectrum of oxygen look like?
For atomic oxygen that absorbs Ultraviolet light, the strongest emission will occur at a wavelength of 558 nanometers: the very strong green line above!
It’s no coincidence at all that this is exactly the dominant wavelength of the green “halos” around dying or recently deceased stars that we sometimes find in planetary nebulae! Have a look at NGC 7009, below.
There are a great many other colors as well, as oxygen is hardly the only element present in planetary nebulae. But when oxygen is abundant, its green glow is unmistakeable, as NGC 6826 shows as well.
Of course, not every planetary nebula has a strong oxygen signature: some don’t, like the Ring Nebula (below, left), but some do, like the Dumbbell Nebula (below, right), with its mix of oxygen and hydrogen.
So when you see that green glow coming from what looks like a very diffuse star, you’ll know what that is. That’s not a star, it’s the oxygen, blown off from the outer layers of a star — much like our Sun — that’s burned up the last of its fuel. The ultraviolet light that permeates the nebula excites the electrons in the oxygen atoms, and when they transition back down, that green glow we see comes from those oxygen atoms fluorescing.
And — from 160,000 light years away — that’s exactly what we’re seeing. Enjoy the brilliant green glow from beyond our galaxy, and know that someday far into the future, our Sun will look an awful lot like that. The green color tells you it isn’t a star, but the halo of oxygen tells you it used to be!