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The Strange case of Epsilon Aurigae July 12, 2009

Posted by jcconwell in Astronomy, Epsilon Aurigae, IYA 2009, Observatory, stars.
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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.

Epsilon Aurigae in mid July before dawn

Epsilon Aurigae in mid July before dawn

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.

Light Curve from the last eclipse

Light Curve from the last eclipse

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

TONIGHT: “Stars that go Bump in the Night” April 22, 2009

Posted by jcconwell in Astronomy, IYA 2009, stars.
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Tonight at 7:00 PM in Phipps lecture hall, our last IYA speaker for the semester Dr. Robert Mathieu.  Professor Mathieu is Chair of the Astronomy Department at University of Wisconsin, Madison, and has done extensive work in the field of star clusters.

Dr. Robert MathieuHis talk,  “Stars that go Bump in the Night” will explore the strange world in the center of star clusters.

One Week from Today! April 15, 2009

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Our third International Year of Astronomy Talk

starbump_page_01Wednesday, April 22

7:00PM Phipps Lecture Hall

Extreme Universe: Hottest White Dwarf! December 25, 2008

Posted by jcconwell in Astronomy, Extreme Universe, stars.
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Between finals and jury duty the December blog has been a bit neglected. So let’s close out the year with some of the more fun, extreme objects of the year.

Astronomers have found a white dwarf star with a surface temperature of 359,500 degrees Fahrenheit (200,000 Celsius). It’s so hot that “its photosphere exhibits emission lines in the ultraviolet spectrum, a phenomenon that has never been seen before,”

M.S. Sliwinski and L. I. Slivinska of Lunarismaar

Credit: M.S. Sliwinski and L. I. Slivinska of Lunarismaar

Stars from one to eight times the mass of the sun, end their life as an Earth-sized white dwarfs after the exhaustion of their nuclear fuel. During the change from a normal nuclear-burning star to the white dwarf stage, a star becomes very hot.

The white dwarf, named KPD 0005+5106, lives in the globular cluster M4, 7,200 light years away is among the hottest stars ever known.

Discovered in 1985, KPD 0005+5106 attracted  attention because it’s spectrum  suggested that this white dwarf is very hot. It belongs to a class of rare white dwarfs whose atmospheres are dominated by helium. Studies revealed  emission lines from calcium, and detailed stellar  modeling confirmed their origin in the star’s photosphere. The analysis proves that the temperature must be 200,000 Kelvin, for these emission lines to be present.

The measured calcium abundance (1-10 times the solar value) in combination with the helium-rich nature of its atmosphere represents a chemical surface composition that is not predicted by stellar evolution models.

Citation: Discovery of photospheric CaX emission lines in the far-UV spectrum of the hottest known white dwarf (KPD 0005+5106), by K. Werner, T. Rauch, and J. W. Kruk. Astronomy & Astrophysics Letters, 2008, volume 492-3, pp. L43.

Twinkle, Twinkle little star, how I wonder what shape you are. August 4, 2008

Posted by jcconwell in Astronomy, stars.
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One of the most exciting talks at the American Astronomical Society meeting last month was about an old friend, stars. The problem that stellar astronomers have is that stars are so very far away, compared to their size, that they appear as points in photographs. Close galaxies can be as wide as 1 degree of arc in the sky, twice the size of the moon, but a close star may only a few millionths of a degree, or about 5 millisecond of arc ( 5mas).

Now most stars rotate, our sun completes one rotation in about 25 days, but many heavy and brighter stars are rapid rotators. Since stars are gas, when they rotate their equators bulge out as illustrated by Achernar

illustrtion of Achernar

illustration of Achernar

The problem is how do we know it’s real shape? Until recently we could not image the shape of any star except for the sun. Diameters could be measured, but not imaged, using interferometers. They were first used in 1920 at Mt Wilson observatory by Michelson and Pease to measure the size of Betelgeuse. In the last few years one of the first stars to be imaged by the Hubble space telescope was Betelgeuse, an easy target since it is 45 mas , compared to 5 mas. for bright stars.

To form an image you effectively need several pairs of telescopes, along different oriented paths, and different spacings. This was first done at radio wavelengths, and now it has final been done in the infrared. The shorter the wavelength, the better the detail (or resolution), but the harder it is to keep the phases from different telescopes in sync.

Thanks to Dr. John Monnier and associates at the U. of Michigan, and Georgia State U. we have some of the first pictures of Altair in the infrared, using the CHARA interferometer at Mt Wison.

U of Mich.

Credit: U of Mich.

What is seen here is not only the distorted shape, but the distribution of temperature, deep red is cooler, yellow hotter. Rapid rotators were thought to be cooler along the equator compared to the poles, up to 1000 degrees difference. Now, for the first time, the effects of rotation can be observed. One of the modifications that might be needed in the near future is a modification of the old H-R diagram to include modification of spectral class due to rotation.

In the future, this tool can open up new vistas, such as observing differential rotation, the poles rotating differently than the equator, along with convection, star-spots, and how all of these effects can vary in time.