First Spectra of Epsilon Aurigae July 30, 2009
Posted by jcconwell in Astronomy, Epsilon Aurigae, IYA 2009, Observatory, stars.Tags: EIU, Epsilon Aurigae, International Year of Astronomy, IYA 2009, spectra
add a comment
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:
Raw First Spectra of Epsilon Aurigae (9/27/09)
Mercury Spectrum used for calibration
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.
Low resolution preliminary spectra of Epsilon Aurigae
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.
Formation of the Elements July 20, 2009
Posted by jmegenhardt in Astronomy, stars, supernova.Tags: Astronomy, stars, supernova
add a comment
James Megenhard has his blog at: http://eastrichlandchemistry.wordpress.com/
Stardust, the Building Block of Everything?
At the birth of our universe the only elements formed in any substantial amount was helium and hydrogen. There were some heavier elements like lithium and beryllium, but these were so minor that they are not even considered. So where did the carbon that all life is made of, the oxygen that all animals need to breathe, or the iron that makes up some of our strongest buildings come from? Hydrogen and helium were formed during the Big Bang, while all of the other elements come from small bangs; the death of stars.
At this time, there are 118 known elements. The simplest element is hydrogen which has only 1 proton and 1 electron.
In order to make hydrogen into another element, protons need to be added; which in turn requires the addition of electrons and neutrons. For example, add a proton, an electron, and two neutrons, and hydrogen has become helium.
Adding a proton, electron, and two neutrons creates lithium. Adding yet another proton, electron, and neutron gives beryllium. By simply adding more protons, electrons, and neutrons, heavier and heavier elements can be formed. It seems reasonable for helium to form, after all, what else is to be done with the neutrons that hydrogen did not use from the Big Bang? The question is, why would more protons, neutron, and electrons come together to make any elements past helium?
The Big Bang roughly states that everything that would one day form the physical universe began as a super hot, super condensed mass. This mass reached a critical point which resulted in the mass exploding. As the material from this mass cooled hydrogen and helium were formed. The hydrogen and helium started pooling together into various gas clouds. These gas clouds were pulled in towards their center, resulting in an increase of mass at center, which created more gravity, which resulted in more hydrogen and helium being pulled in.
|
In the Center of Star More mass in center → More gravity in center ↑ ↓ More gravity in center ← More mass in center
|
The increasing density of that material resulted in more gas atoms colliding. Each time there was a collision, some of that energy was converted to heat. As more gas was pulled in, there were more collisions resulting in heat increasing, until the temperature reaches around 10 million degrees Kelvin at which point nuclear fusion of hydrogen begins; a star is born. To sum it up…
More gravity in center → More density in center → More collisions in center → More heat in center until nuclear fusion is reached.
Nuclear fusion is the process by which two atoms are combined to form a new atom. When two hydrogen atoms fuse they produces helium and energy. Click here to see how hydrogen becomes helium. It is the energy produced by nuclear fusion that runs a star. Since hydrogen is the simplest element with only a proton and electron, the star begins the process of fusion with it, but as hydrogen is used up and temperature increases, the helium produced can undergo nuclear fusion of its own to produce carbon, oxygen, or neon. Carbon can further fuse to form metals like sodium or magnesium, until nuclear fusion produces iron.
Layers of Fusion in a Star
But why did these elements not form during the Big Bang? The answer is repulsion. Hydrogen is a proton and electron, so its formation is easy since opposites attract. Helium, on the other hand, was a little tougherto form since it needs two positive protons in its center; but like charges repel. The only reason the two protons in helium did not repel away from each other was the pressure within the expanding material from the Big Bang was higher than the repulsive force. But this force was not great enough for more than two protons to come together. The only place in our universe where the force is so great that multiple protons cannot repel from each other is in the heart of a star.
The first 26 elements on the periodic table are formed by stars as they produce energy. The remaining elements are formed from a star dying. When a star dies, the gravitational pull upon that star causes the iron center to collapse. As the center collapses, it reaches a point where the energy build-up causes the collapse to stop and reverse just like a rubber ball will collapse so far before it rebounds. In other words, the center implodes and then explodes out. As the center is blown back outwards, it collides with the outer material surrounding the star; which was also being pulled in to the center. Just like in the birth of a star, this increase of collisions results in even more heat and pressure, which means even more nuclear fusion. The net result being that as the star is being blown apart, further nuclear fusion is occurring resulting in elements even heavier then iron. Not only does the stars death form these heavy elements, but it also causes those elements to be blasted out into space, where they can collect and form other astronomical bodies like planets and asteroids.
External resources:
The Strange case of Epsilon Aurigae July 12, 2009
Posted by jcconwell in Astronomy, Epsilon Aurigae, IYA 2009, Observatory, stars.Tags: EIU, Epsilon Aurigae, International Year of Astronomy, IYA 2009, Observatory, stars
2 comments
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
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
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.Tags: EIU, International Year of Astronomy, IYA 2009, stars
add a comment
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.
His 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
Posted by jcconwell in IYA 2009, stars.Tags: Astronomy, EIU, International Year of Astronomy, IYA 2009
add a comment
Our third International Year of Astronomy Talk
Wednesday, April 22
7:00PM Phipps Lecture Hall
Extreme Universe: Hottest White Dwarf! December 25, 2008
Posted by jcconwell in Astronomy, Extreme Universe, stars.Tags: Astronomy, white dwarf
add a comment
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,”
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.add a comment
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
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.
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.
