An ionized rose would smell as sweet

By Phil Plait | March 30, 2011 10:20 am

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.

Related posts:

A WISE view of a small neighbor
A vast cloudy brain looms in a nearby galaxy
The Orion VISTA
The terrible beauty of chaotic starbirth

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (18)

  1. Phil, you always come up with the coolest pictures!

  2. Joerg

    Strömgren sphere is still a very much used term, but not for objects, for which it was never intended, but for the region that can be ionized by a UV source. A quick abstract search on ADS gives a lot of recent papers:

  3. HP

    I hear Scott Walker’s going after unionized helium next.

  4. Jenna

    What’s in a name?

  5. Dys

    I read ‘unionized’ as an American spelling of the thing you do when you form a union.
    Does unionised Helium get better health benefits?
    Sorry >.<

    And yes, astronomy does get the best pictures.

  6. DrFlimmer

    Strömgren sphere, or also HII region, if we speak of the hydrogen part of that cloud.
    And, yes, HII means ionized hydrogen (one electron stripped off the proton), HI would be the neutral atom. Nomenclature is strange in astronomy, and sadly, it will never be changed!

  7. Old Muley

    NGC 371 has a strong tradition of protecting the rights of elements. I’m sure helium as well as other elements have benefited from being unionized and seen their standard of living improved thanks to collective bargaining. Hopefully NGC 371 doesn’t have a governor like Scott Walker!

  8. Brian

    Dys: That’s actually an old engineering student joke: How can you tell a mathematics student from a chemistry student? Answer: write the word “unionized” on a chalkboard and ask them to pronounce it.


    Actually, “unionized” should be non-ionized or, if you’re a limey, non-ionised. 😉

  10. Yotam

    Thanks for (another) interesting post. One thing, if you tell us what a quintillion is (and I admit I don’t remember), can you use exponents? It’s quite annoying trying to count all those zeros. 2 X 10^18 is much neater, IMHO.

  11. Messier Tidy Upper

    Part of that is because I spent years researching other types of gaseous clouds,

    Er , BA .. Are there any other types? Liquid and solid clouds maybe? 😉

  12. Messier Tidy Upper

    @ ^ About two minutes after posting that I thought of dust clouds (solid particles) and clouds of insects. (Ditto)

    But by then I had already shut the computer down and left the house. D’oh! :-(

    Sorry BA you were right to specify. (Blushes)

  13. Don’t be embarrassed, MTU. Phil’s referring to the bio class he had every day right after pork, beans, and cabbage in the school cafeteria.

    However, I will add fog to your list. Not quite gas, not quite liquid.

  14. Nigel Depledge

    The barber of civility (13) said:

    However, I will add fog to your list. Not quite gas, not quite liquid.

    Yeah, fog is an aerosol. Fine liquid droplets suspended in a gas.

  15. Nigel Depledge

    @ Yotam (10) –
    It is actually quite systematic.

    In the American counting system, you count the multiples of three powers of ten after thousands (each multiple of three powers of ten being a factor of 1000 larger than the preceding one).

    So, one multiple of three powers of ten after 1000 gives you a million. Two gives a billion. Three gives a trillion. And so on for as long as your Latin (or is it Greek?) holds out.

    In the older (and obsolescent), British, system, you count the mutliples of six powers of ten, without having to allow extra for your starting point. One multiple of six powers of ten is a million. Two is a billion (but is 10^12, not 10^9). Three is a trillion (but is 10^18, not 10^12). And so on. But this system is almost never used any more, because it is easily confused with the American system, and the American system makes it easier to get sensational-sounding large numbers. (Hence the American system is more appealing to mass media).

  16. Matt B.

    “Unionized” is totally going in my book about spelling reform, right next to “resume”.


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