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
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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.
The Strange case of Epsilon Aurigae July 12, 2009Posted by jcconwell in Astronomy, Epsilon Aurigae, IYA 2009, Observatory, stars.
Tags: EIU, Epsilon Aurigae, International Year of Astronomy, IYA 2009, Observatory, stars
When I was a freshman in high school and first developed my interest in Astronomy, two of the more fascinating sources of knowledge I had were the books, “The Universe” by Issac Asimov and the “Guinness book of World Records”.
I still remember running across, in Guinness, the record “the largest star” ….which refers to the diameter of the star, not the mass of the star. Back then the record holder, according to Guinness, was Epsilon Aurigae B, the second member of the binary system (hence the B). The brighter member of the system, Epsilon Aurigae A , is a FO supergiant star visible to the naked eye as a 3.0 magnitude star. Given the temperature from its spectra, and at a distance of about 700 parsecs or 2300 light years, that means its about 100 times the diameter of the Sun and about 50,000 time more luminous.
You can find the star in the East before dawn, just to the right and slightly above the bright star Capella.
The real interesting object , is not the FO star, but its companion. The system is what astronomers call an eclipsing binary. The system first caught the eye of astronomers when it was noticed that it was a variable star. A star that varied in brightness. In this case, it change between 3.0, and dims to 3.8 magnitude and back again to 3.0, over a cycle of 27.1 years. Now some star are what are called intrinsic variables, meaning the stars pulsate and actually change in brightness, not so here.
The companion of the FO star happens have its orbit alligned to our eye so it passes in front of the primary star, blocking some of the light … hence eclipsing binary. Now eclipsing binary stars not uncommon, but in this case, the eclipse last for over 2 years! Meaning, whatever the companion is, it’s VERY big.
Notice I’ve stopped calling the companion a star, since it’s also very dark. Much darker than any star it’s size has a right to be. So dark, that astronomers don’t know for sure what it is. The best theory is it’s a large disk of gas and dust surrounding a hidden star that orbits Epsilon Aurigae. If you look at the light curve, above, you’ll notice it brightens in mid-eclipse. Some speculate there might be a double star rotating in the center of the disk that clears out a hole for the light of the main star to shine through.
Much of this is speculations, since 27 years ago astronomers weren’t able to get a good spectra of the object. So one of the projects, at the EIU observatory, are students trying to get spectra before and going into the eclipse. We hope the edges of the disk will be thin enough that we can see a change in the spectra as light starts to dim. You don’t need a big telescope since at 3rd magnitude the object is quite bright. So wish us luck, and if we see something we’ll let you know.
Listen to the PODCAST about Epsilon Aurigae at 365 days of astronomy
For More information on how you can contribute go to web site: citizensky.org