via Astronomy Now.
White dwarf stars are the end products of low mass stars like our own Sun. After a lifetime of nuclear fusion, the stars run out of fuel and their cores contract. The outer layers escape into space, forming planetary nebulae and the cores end up as crushed glowing debris – white dwarfs. Their mass is around sixty percent of the Sun’s and is crushed into a sphere the size of the Earth.
Now, when white dwarfs form, a large stellar core is crushed into a small ball. The problem is, angular momentum must be maintained. For this to happen, stuff spinning round that has now been compressed must spin faster. Alternatively, the angular momentum could be passed on to the stuff that creates the planetary nebula. Observations of white dwarfs show a relatively slow rotation rate – days, weeks, years – rather than the minutes or seconds expected from the white dwarf holding onto its angular momentum. Trouble is, this was just a surface measurement – could the insides be running round faster?
Step in asteroseismology. Some stars pulsate and in the same way that geologists determine the structure of the Earth from how waves propagate through it, astronomers can pick up on subsurface activity through measurements of the modes of pulsations. This has been used extensively on the Sun, but a team of researchers led by Gilles Fontaine of the University of Montreal decided to apply the method to white dwarfs.
Using the romantically named PG 1159-035, one of a number of white dwarfs in a period of evolution that means it is unstable and so exhibits pulsations, Fontaine found that the star rotates with a period of 34 hours – not just at the surface but throughout at least ninety percent of its depth. The rotation is slow and rigid, suggesting that PG 1159-035 transferred angular momentum over a period of 10,000-1,000,000 years to the planetary nebula around it.