WHAT’S THAT STAR MADE OF?

Hi folks, Sean Lally here.

So here’s a question for you to ponder – how on Earth (literally) can we tell what a star is made of? We clearly can’t travel to a star – nearly all are way too far away to be reachable in our lifetimes with normal space travel, and more importantly – they’re really hot! (More on this later.)

Quick problem for you to consider: Traveling at a “typical” spaceship speed, say 25,000 miles per hour (that was an average speed for rockets heading to the Moon), how long would it take you to reach the nearest star, our very own Sun (about 93,000,000 miles away)?

Hmmmm….

Well, if keep things simple and just use the equation (v = d / t), what do you come up with?

How about traveling to the next closest star system, Alpha Centauri (4.37 light-years away)? How far would it take you to get there? It will be helpful to note that a light-year, the distance that light would travel in one year, is 5,878,630,000,000 miles.

Think we could make it there quickly?

So, all that aside – it’s not too practical to take a space-drive out to a star, even the nearest one, grab a cup of star-stuff and bring it back to Earth for analysis. So then, how do know what it’s made of?

For this exercise, you’ll need access to a spectrum tube power supplies and tubes containing several gases. You’ll also need spectroscopes or diffraction gratings.

Do you remember our earlier exercise about prisms? Well, prisms are wonderful are breaking up light into its constitutive parts, but it’s not the only game in town. Another tool that breaks light up is called a diffraction grating – that’s the central part of the modern spectroscope, though prisms were used for quite a long time (and still work quite well). Diffraction gratings are cheap and simple to produce in large quantities, making them somewhat preferable to prisms. And they perform the same task, break up light according to wavelength, though the process is different for diffraction gratings.

Let’s put aside the details of how this happens. A diffraction grating is made of many tiny slits through which light can travel. However, if the slits are similar in size to the wavelengths, light can break up and “interfere” with itself – crests of light waves adding to other crests, troughs adding to other troughs, and some crests and troughs canceling each other out. In short, light hits a diffraction grating, and the various waves add or cancel: a pattern emerges and the spread of the pattern is related to the wavelength of the light. (For more details of the process, see consult the references below.) Of course, the wavelengths have to be there in the first place to actually emerge from the grating. But like sunlight through a prism, the colors are spread out in a pattern related to their wavelengths.

Of course, the material casting off light may have many different wavelengths – you can only see them after they have passed through a grating or prism. But once they do, you can see a pattern that you could assign to that particular light-emitting stuff.

Let’s see how this works in practice.

Ask your teacher to help you set up the spectrum tube power supply – be careful, as these can generate 2000 volts or more!

Have a look at the spectra of several gases. I recommend Hydrogen, Helium, Oxygen, Carbon Dioxide among others. To do this, you will need to position yourself around a meter from the spectrum tube, and look through the diffraction grating (or spectroscope, if you are using one of those). What do you notice? Are there differences? Describe?

What in the world is going on here? At high temperatures, things glow and each element has its own unique “emission spectrum,” not unlike a fingerprint. So in principle, if you had a library of emission spectral images, you could tell what the particular gas is made of. It is, of course, more complicated with compounds (chemical collection of multiple elements), but the principle is the same. If you had a large enough set of fingerprints, you could conceivably compare your pattern to the set and then maybe tell what the stuff actually is. Behold the beauty and power of spectroscopy!

Additional resources

http://www.teachersource.com/

Good source for diffraction gratings, spectroscopes and other many other materials for teaching color.

http://chemistry.bd.psu.edu/jircitano/periodic4.html
http://www.800mainstreet.com/spect/emission-flame-exp.html
http://ioannis.virtualcomposer2000.com/spectroscope/amici.html#colorphotos

These include libraries of spectral images.


http://www.starspectroscope.com/
A small diffraction grating optic that you can attach to a telescope eyepiece to examine some stellar spectra of your own.

ALL TEXT AND IMAGES COPYRIGHT SEAN LALLY 2009 (except where noted)