An international team of astronomers, studying the left-over remnants of
stars like our own Sun, have found a remarkable object where the nuclear
reactor that once powered it has only just shut down. This star, the
hottest known white dwarf, H1504+65, seems to have been stripped of its
entire outer regions during its death throes leaving behind the core
that formed its power plant.

Scientists from the United Kingdom, Germany and the USA focused two of
NASA’s space telescopes, the Chandra X-ray observatory and the Far
Ultraviolet Spectroscopic Explorer (FUSE), onto H1504+65 to probe its
composition and measure its temperature. The data revealed that the stellar
surface is extremely hot, 200,000 degrees, and is virtually free of
hydrogen and helium, something never before observed in any star. Instead,
the surface is composed mainly of carbon and oxygen, the ‘ashes’ of the
fusion of helium in a nuclear reactor. An important question we must
answer is why has this unique star lost the hydrogen and helium, which
usually hide thestellar interior from our view?

Professor Martin Barstow (University of Leicester) said. ‘Studying the
nature of the ashes of dead stars give us important clues as to how stars
like the Sun live their lives and eventually die. The nuclear waste of
carbon and oxygen produced in the process are essential elements for life
and are eventually recycled into interstellar space to form new stars,
planets and, possibly, living beings.’

Professor Klaus Werner (University of TUbingen) said. ‘We realized that
this star has, on astronomical time scales, only very recently shut down
nuclear fusion (about a hundred years ago). We clearly see the bare, now
extinct reactor that once powered a bright giant star.’

Dr Jeffrey Kruk (Johns Hopkins University) said: ‘Astronomers have long
predicted that many stars would have carbon-oxygen cores near the end of
their lives, but I never expected we would actually be able to see one.
This is a wonderful opportunity to improve our understanding of the
life-cycle of stars.’

The Chandra X-ray data also reveal the signatures of neon, an expected
by-product of helium fusion. However, a big surprise was the presence of
magnesium in similar quantities. This result may provide a key to the
unique
composition of H1504+65 and validate theoretical predictions that, if
massive enough, some stars can extend their lives by tapping yet another
energy source: the fusion of carbon into magnesium. However, as magnesium
can also be produced by helium fusion, proof of the theory is not yet
ironclad. The final link in the puzzle would be the detection of sodium,
which will require data from yet another observatory: the Hubble Space
Telescope. The team has already been awarded time on the Hubble Space
Telescope to search for sodium in H1504+65 next year, and will, hopefully,
discover the final answer as to the origin of this unique star.

This work will be published in July in the ‘Astronomy & Astrophysics’
journal.

IMAGE
An artist’s impression of the young, extremely hot white dwarf star
H1504+65, as viewed from a distance similar to that of the Earth from the
Sun is available at www.star.le.ac.uk/~mab/h1504art2.jpg
(Credit: University of Leicester).

CONTACTS

Professor Martin Barstow, University of Leicester, UK
Tel: +44 116 252 3492 (work), +44 116 2862330 (home);
+44 776 62 333 62 (mobile)
Fax: +44 116 252 3311 Email: mab@star.le.ac.uk

Professor Klaus Werner and Dr Thomas Rauch, University of Tubingen,
Germany
Tel: +49 7071 2978601 (work), +49 7072 921465 (home)
Fax: +49 7071 2973458
Email: werner@astro.uni-tuebingen.de

Dr Jeffrey Kruk, Johns Hopkins University, Baltimore, USA
Tel: +01 410 516 8744 (work), +01 301 270 2974 (home)
Fax: +01 410 516 5494
Email: kruk@pha.jhu.edu

NOTES

H1504+65 and white dwarf stars

White dwarfs are the dying remnant cores of stars similar to the Sun. In
fact, the formation of a white dwarf is the ultimate fate of most stars up
to about 8 times the mass of the Sun. Stars more massive than this explode
as supernovae. During their lives stars generate energy from the process of
fusing hydrogen into helium and, later on, fusing helium to form carbon and
oxygen. As they begin to run out of fuel they become unstable and shed
their
outer layers. When all possible sources of thermonuclear fusion are
exhausted the remnant core of the star collapses under its own gravity to
form a white dwarf. At the same time it becomes extremely hot, up to 200000
degrees or so. As it has no internal source of energy the white dwarf will
gradually cool down and fade.

H1504+65 is important because it is the hottest, and therefore the youngest
white dwarf known. It is a faint, inconspicuous object in visible light but
is among the brightest objects in the sky in X-rays. It has a surface
temperature of 200,000 degrees, more than 30 times that of the Sun.

Chandra and FUSE

The Chandra X-ray observatory and the Far Ultraviolet Spectroscopic
Explorer
(FUSE) were both launched into orbit by NASA in 1999. Their instruments
make
use of a technique called spectroscopy, which spreads the light obtained
from astronomical objects into its constituent X-ray and ultraviolet
‘colours’, in the same way visible light is dispersed into a rainbow
naturally, by water droplets in the atmosphere, or artificially, by a
prism.
When studied in fine detail each spectrum is a unique ‘fingerprint’ which
tells us what elements are present and reveals the physical conditions in
the object being studied.