First Spectra of Epsilon Aurigae July 30, 2009Posted by jcconwell in Astronomy, Epsilon Aurigae, IYA 2009, Observatory, stars.
Tags: EIU, Epsilon Aurigae, International Year of Astronomy, IYA 2009, spectra
I was up last night from 2:30 am to 3:30 am looking at clouds. Fun if you in meteorology, but not astronomy. I was trying to get my second good spectra of Epsilon Aurigae, a mysterious eclipsing binary (see earlier post) . Most of the people looking at this object are doing photometry, measuring the brightness of the star either visually or with a camera (usually a CCD digital camera). Since I have a larger telescope (16″) on a nice permanent equatorial mount, and since the star is bright at 3rd magnitude, I decided to take spectra. Most information about an astronomical object, chemical composition, doppler shifts, temperature, magnetic fields, come from looking at spectra.
Now you may not know that the reason the “arms race” for bigger and bigger scopes began in the early 1900’s to take spectra. You need telescopes that are big “light buckets”, because the light that the telescope would normally put into one point to make a nice image on a camera has to be spread out. The light is diluted by a prism or diffraction grating into a long strip of light to make a spectrum. If it’s a color camera it would look like smear from a rainbow. Since what use to land on a few pixels of my camera is now landing on several hundred the image is MUCH dimmer. So to take a good spectra you either have to take a much longer exposure, stick to much brighter objects, or get a bigger telescope. Brightness or exposures increase by a factor of 100, or for you astronomy experts about 5 magnitudes in brightness.
Now instruments are stupid (as are theoretical physicists trying to be observational astronomers at 3:00 am in the MORNING), they don’t know how the position of the light in the camera is related to wavelength. So when I take the spectra of a star, I also take a spectra of a Mercury lamp with known spectra lines for calibration. I take both spectra, making sure I don’t change anything with the camera or telescope (like focus). That way I can tell my computer that this pixel means this wavelength (color). As Shown below:
Now you may notice the star’s spectrum has dark lines because it’s an absorption spectra, while the mercury spectrum is a bright line or emission spectra. Once the computer knows what the wavelengths are we can look at a plot of a star’s spectrum, a lot easier to read that the picture. There are other steps, like subtracting out spectral lines from the Earth’s atmosphere, but I thought you’d like to see a preliminary result.
With any luck, clear weather, we’ll be able to take some more spectra in the next few days to see any changes in the spectra as the eclipse stars. That way we hope to learn about the object causing the eclipse.