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White Dwarf Star System Exceeds Chandrasakar’s Mass Limit!? March 15, 2010

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Type 1a Supernova

Richard Scalzo of Yale, as part of called the Nearby Supernova Factory, a collaboration of American and French physicists, has measured the mass of the white dwarf star that resulted in one of these rare supernovae, called SN 2007if, and found it exceeded the Chandrasekhar limit.

It is thought that white dwarfs could not exceed what is known as the Chandrasekhar limit, a critical mass of about 1.4 times that of the Sun, before exploding in a type Ia supernova. This assumption is crucial in measuring distances to supernovae, and using them as standard candles

Using observations from telescopes in Chile, Hawaii and California, the team was able to measure the mass of the central star, the shell and the envelope individually, providing the first conclusive evidence that the star system itself did indeed surpass the Chandrasekhar limit. They found that the star itself appears to have had a mass of 2.1 times the mass of the Sun (plus or minus 10 percent), putting it well above the limit.

More information: Paper: http://arxiv.org/abs/1003.2217

Provided by Yale University

Rare Outburst of the Recurrent Nova U Scorpii Begins January 28, 2010

Posted by jcconwell in Nova, stars, supernova, white dwarf.
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Artists rendition of the recurrent nova RS Oph Credit: David Hardy/PPARC

(From Universe Today): Today, two amateur astronomers from Florida detected a rare outburst of the recurrent nova U Scorpii, which set in motion satellite observations by the Hubble Space Telescope, Swift and Spitzer. The last outburst of U Scorpii occurred in February of 1999. Observers around the planet will now be observing this remarkable system intensely for the next few months trying to unlock the mysteries of white dwarfs, interacting binaries, accretion and the progenitors of Type IA supernovae.

One of the remarkable things about this outburst is it was predicted in advance by Dr. Bradley Schaefer, Louisiana State University, so observers of the American Association of Variable Star Observers (AAVSO) have been closely monitoring the star since last February, waiting to detect the first signs of an eruption. This morning, AAVSO observers, Barbara Harris and Shawn Dvorak sent in notification of the outburst, sending astronomers scrambling to get ‘target of opportunity observations’ from satellites and continuous coverage from ground-based observatories. Time is a critical element, since U Sco is known to reach maximum light and start to fade again in one day.

Subrahmanyan Chandrasekhar October 20, 2009

Posted by jcconwell in Astronomers, Astronomy, white dwarf.
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One Hundred years ago, yesterday, October 19, 1910, Subrahmanyan Chandrasekhar was born.

Subrahmanyan Chandrasekhar

Subrahmanyan Chandrasekhar

Arguably the greatest astrophysicist of the twentieth century, his name is in every astronomy book. From the upper mass limit of a white dwarf, Chandrasekhar’s limit, to the orbiting Chandra X-ray telescope, he left his mark on the very concepts and vocabulary that physicists and astronomers use today.

Chandrasekhar was the nephew of Sir Chandrasekhara Venkata Raman, who won the Nobel Prize for Physics in 1930. Chandrasekhar was educated  at the University of Madras, and at Trinity College, Cambridge. From 1933 to 1936 he held a position at Trinity.

By the early 1930s, scientists had concluded that, after converting all of their hydrogen to helium, stars lose energy and contract under the influence of their own gravity. These stars, known as white dwarf stars, contract to about the size of the Earth, and the electrons and nuclei of their constituent atoms are compressed to a state of extremely high density. Using the new theory of Quantum Mechanics, Chandrasekhar determined what is known as the Chandrasekhar limit—that a star having a mass more than 1.44 times that of the Sun does not form a white dwarf but instead continues to collapse. Later it was found that more massive stars cores collapse blows off its gaseous envelope in a Type II supernova explosion, leaving a neutron star. An even more massive star continues to collapse leaving a black hole. Type Ia supernova use the same mechanism in a different way.If a binary star system has a white dwarf stealing matter from its companion, and it exceeds Chandrasekhar limit, the white dwarf will collapse and detonate. For this contibuttion he was awarded the 1983 Nobel Prize in Physics

Chandra  receives Nobel prize (1983)

Chandra receives Nobel prize (1983)

These calculations contributed to the eventual understanding of supernovas, neutron stars, and black holes, and the production of the elements in the periodic table.

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.