Small but Mighty: KPD 0005+5106, the 200,000K White Dwarf

Sirius B1 - one of the more famous white dwarf stars (Frank Gregorio)

A white dwarf called KPD 0005+5106 has been identified as the hottest star observed, ever. KPD 0005+5106 lives in the globular cluster M4, 7,200 light years away, and astronomers have always been intrigued by this stellar lightweight as its emissions have previously hinted it was quite toasty. Now, astronomers using data from the defunct NASA Far-Ultraviolet Spectroscopic Explorer (FUSE), have studied the white dwarf in more detail. KPD 0005+5106 emits radiation in the far-ultraviolet, indicating that its surface has a temperature of 200,000K. This is an unprecedented discovery, far-ultraviolet emissions are usually reserved for superheated stellar coronae. It may be small, but it’s a record-breaker

Our Sun has a surface temperature of around 6000K. As you track the temperature of solar plasma as it exits the Sun, coronal heating processes will heat the tenuous gas very rapidly to over 1 million Kelvin. Our Sun only starts to emit strong ultraviolet emissions in the solar corona, just above a point called the “transition region”; ultraviolet emissions simply are not attainable at the surface as the solar plasma is too cool.

So it may be a surprise that another stellar object, a white dwarf, is generating temperatures on its surface capable of producing emissions usually associated with the temperatures of stellar atmospheres.

White dwarfs are known as being very hot, and temperatures of around 100,000K are not uncommon, but this dwarf star is outshining all the competition. KPD 0005+5106 is the hottest white dwarf, and the hottest star observed to date with this degree of precision. Perhaps even more interesting is the fact that white dwarfs are very small, of Earth-sized proportions, after evolving from a larger star of 1-8 solar masses.

Composition of a white dwarf
Composition of a white dwarf

The white dwarf is what remains of a star after fusion has ceased in the parent stars core, and in the case of our Sun, a white dwarf will be left over after the Sun has puffed up into a red giant and blown apart into a planetary nebula. It is like the pearl left behind after breaking an oyster shell apart; a tiny, shiny sphere. White dwarfs are not maintained by nuclear fusion either; all the fuel has been spent, it is maintained by a balance between degenerate matter and gravity. Gravitational pressure compresses the stellar matter to such an extent, the quantum Pauli Exclusion Principal takes over, disallowing electrons to occupy the same energy levels. The matter therefore becomes degenerate, preventing the matter from being compressed any further.

For a short time after formation, white dwarfs are expected to be very hot. Although the white dwarf phase of stellar evolution is very stable (it can last for billions of years), astronomers will be very lucky to observe this hot period as, statistically speaking, young white dwarfs are rare. In the case of KPD 0005+5106, it would appear that it is a very young white dwarf with a very hot surface.

Fortunately for the astronomers who made this discovery, the FUSE observatory had a lot of data on KPD 0005+5106 as the object was used as a calibration target to track the telescope’s performance. This is archived spectroscopic data as FUSE was decommissioned in 2007 after eight years in space. However, the data was put to good use.

Spectroscopic data of the far-ultraviolet wavelength range, in particularly the emissions from a calcium ion, revealed just how extreme the white dwarf’s temperature was. CaX, or nine-times ionized calcium (nine electrons stripped from the calcium nuclei), was detected, indicative of the 200,000K stellar environment.

My love for spectroscopic observatories just got stronger…

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