I'm such a sucker for emission nebulae, the sites of intense star formation. Part of that is because I spent years researching other types of gaseous clouds, but also because they're just so darn pretty, like this shot of NGC 371:
[Click to ennebulanate, or get the 2000 x 2000 pixel version).]
NGC 371 is in the Small Magellanic Cloud, a companion galaxy to our Milky Way. That puts it at a distance of about 200,000 light years, or 2 quintillion (2,000,000,000,000,000,000) kilometers. In this kinda-sorta false color image from the ESO's Very Large Telescope, blue shows ionized helium at a wavelength of 468.6 nanometers (which is roughly blue to the eye), green is unionized helium (587.6 nm, green to the eye), and red is warm hydrogen (656 nm, again, red). It's not really true color because the filters used to make this image are what astronomers call very narrow, letting through only a very thin slice of a given color. What we call red to the eye is actually a wide range of wavelengths, covering 650 to 700 nanometers. But the red filter used in this image only lets through a teeny sliver of red light, where hydrogen tends to emit. Any red outside that wavelength doesn't get through, and the same is true for the colors of the other filters.
This lets astronomers analyze specific elements in the gas, and that's very useful. For example, trivially, we know this nebula has lots of hydrogen and helium in it! That's no surprise to modern astronomers, but it's still pretty cool that we can taste the elements of a cloud of gas so far away.
When I was younger, these types of objects were sometimes called Strömgren spheres, after the astronomer who figured them out. Basically, a cloud of gas surrounding a hot star or stars can be "excited" by the light emitted by the stars. The electrons in the atoms of the gas get raised to a higher energy or are stripped from the atoms entirely. When the electrons recombine (or fall to lower energy states), they emit light at characteristic colors, as I described above. Since the light emitted from the stars gets weaker with distance, the ionized/excited region of the gas takes on a spherical shape. There may be lots of gas outside this sphere, but the starlight is too weak to light it up.
That's what's happening with NGC 371. Usually when you see this circular shape (and assume it's really a sphere) it's either because something is expanding like the debris from an exploding star or a planetary nebula, or it's a Strömgren sphere. In the former case, you usually get a bright edge as material piles up, like snow in a snowplow. But in NGC 371 the edge is fuzzier, a strong indication we're seeing the edge of the excited region. Astronomers call this the ionization edge (as opposed to the mass edge; again it's only the limit where the gas is no longer excited), and the ionization source is the cluster of stars you can easily see embedded in the gas.
I don't think the term "Strömgren sphere" is used much anymore, which is a pity; it has a wonderful mad scientist feel to it. But I suppose what we call it isn't that important. After all, it would still retain that dear perfection which it owes without that title*.
Image credit: ESO/Manu Mejias
* Yes, an astronomer who has read Shakespeare. All the Universe's a stage.