1. White dwarfs are dense, cooling remnants of stars like our Sun that have masses around 60% of the Sun's mass and radii similar to Earth's. Their extreme density means that heavy elements in them should sink quickly. 2. However, some cooler white dwarfs still show evidence of carbon, nitrogen, oxygen, silicon, and iron in their atmospheres, which comes from the stars absorbing material from shredded planets orbiting them. 3. By measuring the spectrum of light from white dwarfs, the chemicals present can be identified by their absorption features, and their velocities and amounts determined. Extra absorption features in some stars could mean chemicals exist both in the star and between it and Earth.

1. X-Ray and Observational Astronomy,
Department of Physics and Astronomy
The Chemical Content of Hot White Dwarf
Observations: Grave-digging in Space
N.J. Dickinson (njd15@le.ac.uk)*, M.A. Barstow*, B.Y. Welsh (University of California, USA) , M. Burleigh*, S.L.
Casewell*, J. Farihi*, R. Lallement (Observatoire de Paris, France); *University of Leicester.
1. What is a white dwarf? 2. Why study the chemicals in these stars?
White dwarfs are the cooling end products of the lifecycles Given their huge density, white dwarfs have a massive gravitational pull. On a white
of stars like the Sun. They have masses around 0.6 times dwarf, a 1kg object would feel 100,000 times weightier than on the Earth, so heavy
that of the Sun, and a radius around that of the Earth. This chemicals should sink quickly and not be seen. However, in some cooler white
gives these stars a phenomenal density, 1 teaspoon of dwarfs (with a temperature less than 25,000 degrees) chemicals such as carbon,
‘white dwarf material’ has a mass of around 5 tonnes! They nitrogen, oxygen, silicon, iron iron are seen. This material comes from the
sometimes exist in a cloud, or ‘nebula’ (below), made from star ‘hoovering’ up the remains of shredded planets (background graphic), and tells
material lost from the star during it’s life. us what might ultimately happen to our solar system[1,2]. In all hotter stars, (above
50,000 degrees) other physical processes put material into the atmosphere[3,4].
However, more material than predicted is often observed[3,5]. Could this extra
material be around the star[6], and be evidence of a solar system on its death bed?
A
B
3. How is this studied? C
• All chemicals emit and absorb light at specific colours. Light bulbs filled with a particular gas
produce light of a specific colour, such as a neon light.
• By measuring all of the colours of a star with the Hubble Space Telescope (diagram A), we
obtain a ‘spectrum,’ effectively recording the star’s ‘rainbow’ (diagram B).
•Examining which colours (wavelengths of light) are dimmer, gives the chemicals present (species), In the star
their amount and their velocity through space; we get a ‘barcode’ we can read to tell us how much of
which chemicals are present, and what their velocity is (diagram B). Between the
• This barcode can be made into a graph of the brightness of each colour (diagram star and
C). ‘Absorption features,’ or dimming of the star light, shows carbon is present. In the telescope
upper panel, the 2 features show only 1 carbon population is present (in the star).
• In the lower panel, 2 sets of 2 carbon absorption components are seen, blended I In the star
into one another. This indicates that as well as being present in the star (red),
carbon also exists somewhere between the star and the telescope (blue).
4. What is this extra material?
Planetary nebulae: Material around the star (circumstellar Clouds between the star and telescope
material): (interstellar medium):
As previously mentioned, sometimes white In 1 of the white dwarfs, the material may Nearby clouds (interstellar medium) are
dwarfs exist in a cloud (nebula) of material be around the star, and may have been between us and many of the observed
lost from the star earlier in its life. 2 of the ejected from the star, or may be ground stars. 7 of our star’s extra material has a
10 stars with extra material are in nebulae. up planets. More analysis is velocity consistent with these clouds,
needed. implying they are likely to be related.
References:
[1] Farihi et al. (2010), [2] Melis et al (2010), [3] Chayer et al. (1995), [4] Marsh et al. (1997), [5]
Barstow et al. (2003). [6] Bannister et al. (2003). Background by M. Garlick.