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Rockin’ Stars

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The 21-centimeter Band

No, it’s not the name of a punk-rock band, although I wouldn’t be surprised if some enterprising and musically inclined astrophysics grad students didn’t form a band in their “spare time” and name themselves that. There’s a great tradition of geeky names for scientist-led bands, such as the Eigenfunctions, the Algo-Rhythms, and one of my favorites, the Titan Equatorial Band, an impromptu group that featured such folks as one of my former colleagues Kelly Beatty (Sky & Telescope Magazine), Cassini Mission scientist Carolyn Porco,the late great science journalist Jonathan Eberhart, and many others. They gathered and played during Voyager spacecraft flybys.

But, that’s not the kind of band I’m talking about in this entry. The 21-centimeter Band is a wavelength of light that is more attuned to a single note: the radio frequency (1420 MHz) emitted by changes in atoms of neutral hydrogen. It’s right smack in the radio portion of the electromagnetic spectrum, and so radio astronomers have been using it for years to probe a variety of conditions in the universe.

Yesterday, I went over to Haystack Observatory to hear a talk about using 21-cm band emissions to study clouds of material being emitted from Asymptotic Giant Branch stars. These may sound like weird, far-out, geeky stars. And they are. But they’re also part of the final act in the lives of stars that are less than eight times the mass of the Sun… including the Sun. As they slip into old age, these stars cool down, they expand, they get brighter, and through all this, they spend their nuclear fuel (which is running low) faster and faster. As they cool, their atmospheres get just chilly enough that dust grains can “freeze out” and create a dusty shell around the star. Think of this phase as a last burst of lively activity before settling into very old age (not unlike the antics of some elderly rockers doing successive world tours (not that there’s anything wrong with that)).

Well, some of the larger AGB stars also start to pulsate, and these heavings send a stellar wind blowing away from the star, shoving the dusty shell out away from the star, along with a cloud of neutral hydrogen. Now, we can study the dust by looking for its signature in the infrared (where astronomers commonly detect warm (but not too hot) glowing things. And, voila, we can study the progression of the mass loss (that is, how quickly the stellar wind is shoving mass away from the star) by examining the 21-centimeter emissions from the neutral hydrogen in the shell.

Four scenes from an animation showing Mira and its 13 light-year-long tail.
Courtesy Galex Mission.

It’s still a work in progress, but we did see some fine examples of 21-centimeter emissions from the stellar tail trailing out along the line of travel of the star Mira A which looks like it’s got a comet tail. That tail is glowing in ultraviolet light, but 21-centimeter band studies show more detail in the neutral hydrogen that is also being carried along. If the work (which is still in progress) plays out as the astronomers expect, they should be able to figure out a pretty accurate timetable for when this material started streaming off the star (and hence, how old the tail is), and give us some new insights into the rockin’ activity in these geriatric stars.

astrophysics, chemistry, spectroscopy, stars

Doing Astronomy Through Chemistry

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It’s Elemental, Dear Watson

In the last entry, I referred to a star that astronomers studied to understand its chemical makeup in an effort to figure out where it came from. That raised a question about how astronomers figure out the chemical makeup of a star.

They use a technique called spectroscopy. That’s really a $25.00 word that means “breaking the light up into its wavelengths” and then comparing the data to the spectral fingerprints of known chemical elements. This is something that chemistry folks (who study the elements in the universe) do all the time, and a technique that let astronomers look at the radiation emitted from an object in space in new ways. It’s fair to say that when astronomers began using spectroscopy to study stars and galaxies, the science of astrophysics took a huge leap forward.

Astronomers use specialized instruments called spectrographs, which were first used by chemistry researchers to study the spectral fingerprints of elements in the lab. (Read more about them here). Astronomers employ spectrographs to break up the light from stars, galaxies, planets, nebulae, etc. into its component wavelengths. The data from these instruments is then plotted, which lets the researchers analyze the chemical signatures in the light and compare them to the signatures of known elements.

The “prism” view of a spectrum of a star with hydrogen in its atmosphere might look something like the images below. The top image shows what it looks like when hydrogen absorbs light as it is emitted from an object. This means that hydrogen exists in or near the object. The bottom image shows what it looks like if hydrogen is emitting radiation (while it is heated). Each chemical element has a unique absorption fingerprint.

Hydrogen absorption spectrum, courtesy www.solarobserving.com.

Each element has a typical “absorption” pattern that shows up in the spectrum of a star where the element exists. An object in space can also have an emission spectrum, which tells us that some element is being heated and glowing brightly. There’s a rather nice tutorial about spectra here if you’re interested in learning more about them.

So, the short answer to the query about how the astronomers figured out the chemical makeup of the star HE 0437-5439 is, they studied the light it radiates and compared what they found to the known chemical signatures of elements, particularly metals. They then compared THAT information to spectral studies of regions in the LMC. From that, they can draw a pretty good assumption that the star came from that region.

One other thing about spectra: you can also tell an object’s velocity through space and the direction it’s traveling, all using spectra. There’s a gold mine of information locked away in the light and other wavelengths of radiation being emitted from objects in space. It’s an amazing treasury that astronomers tap into every time they study an object through a spectroscope.)