Monday, March 17, 2008

How fast is it?

Police officer models a radar gun
Image Credit: Speed Measurement Laboratories Inc.

This weekend I worked on trying to measure how fast a certain star was moving. It didn't go very well. It's an interesting star (a white dwarf with a very strong magnetic field that is pulling gas off of a companion star), and I found the star by accident in some data I took a year ago. Because it was a serendipitous discovery (meaning I was looking for something else), I didn't get quite the right measurements, and I'm trying to kludge something together.

You will often hear about astronomers measuring how fast an object is moving. We detect planets around other stars by measuring changes in the star's speed caused by the planet's gravity. We know that the Universe is expanding because all distant galaxies seem to be moving away from us, at faster and faster speeds the further we look. How do we measure speeds of stars and galaxies?

Astronomers use a technique called the Doppler Effect. The one sentence summary of the Doppler Effect in astronomy is that an object's movement slightly changes the wavelength (or "color") of light coming from that object.

A police radar gun uses the Doppler Effect. The radar gun sends out radio waves with a wavelength of about 1/3 of an inch (or a little less than 1 cm). The radio waves bounce off your car, and the car's motion causes the wavelength to change a little bit. The radar gun measures the change, and calculates the speed of the car. And, as long as the police officer has remembered to calibrate the radar gun (by pointing it at something that isn't moving; many radar guns do this automatically now), the measurement is pretty accurate.

The only difference with measuring the speed of astronomical objects, like stars and galaxies, is that we don't send out a light beam to bounce off of the star. That would take decades to millions of years to get a measurement! And, it's not needed, since stars and galaxies put out their own light. Another small difference is that astronomers tend to use optical wavelengths of light, which are only the size of a bacterium, and it's much harder to measure the differences in that wavelength than the radar wavelengths of a third of an inch.

But, like the police officer, we have to make sure to get proper calibrations. Astronomers use several different calibrations. We have lamps that emit light at only very certain wavelengths, and we know those lamps are not moving. We can use Earth's own atmosphere, which glows faintly at very specific wavelengths, as a double-check. And, to triple-check the measured speeds, we look at stars with known and well-measured speeds. Astronomers looking for planets sometimes even use a fourth level of calibration, by making the starlight to pass through a container of iodine gas (which absorbs light at many very specific and unchanging wavelengths).

My problem: I wasn't out to measure the speeds of stars when I took my data. So, while I did use the lamps I talked about above. At the certain colors of light I was looking at, the atmosphere doesn't glow. I didn't look at a star of a known speed (I didn't think I'd be needing that information!). And, since I wasn't looking for planets, the container of iodine stayed safely stored.

So, I have some very accurate measurements of changes in the star's speed during a single night, because I can inter-compare all the data I took. But I don't know the absolute speed. (It'd be like a police officer measuring that you slowed down by 15 miles per hour once you saw the cop car, but the officer not knowing if you were ever actually over the speed limit).

Yes, I could go back to the telescope and get a new and proper look at this star, but I need to try and make do with the information I have first. I have a few more tricks to try, too. We'll see what happens.

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