White dwarf stars can be green, say scientists

We all know how the song goes - twinkle, twinkle bright green star..

Not heard that one before?

Researchers at the University of Warwick believe they have found certain stars which may look pink, purple and even green if you were to see them up close.

They say this is because of the atmosphere that surrounds the stars.

Stars are made up of gases. They are held together by their own centre of gravity and generate their own heat.

They can vary in size, how old they are and how hot they are.

White and blue-white colours often mean that a star is very, very hot. Red stars are a cooler temperature by comparison.

A white dwarf is a very dense star. Its the left over core of a star that no longer produces its own energy through nuclear fusion.

It will cool gradually over billions of years.

We typically see white dwarves as being white, or tinged-blue, because of how hot they are.

But the researchers at the University of Warwick say that if we were to travel closer to one of these white dwarves, they could appear to be some very strange colours indeed.

"It turns out these incredibly dense stars can produce colours nobody expected, and some of them genuinely surprised me," said Dr Mark Hollands who led the study.

The colour of a white dwarf star can be affected by how dense its atmosphere is, say the researchers.

Density is the mass of an object divided by its volume.

For example, a feather-filled pillow could be described as not a very dense object. Whereas a stone like granite is very dense.

Different densities in a white dwarf's atmosphere allow different amounts of light from the light spectrum to travel through.

This is what can make the white dwarf stars appear to be all kinds of colours, say the scientists.

They believe one in thirty white dwarf stars could be a colour other than bright white.

"One in thirty might not sound like a huge fraction," said University of Warwick's Professor Boris Gänsicke, "but across the entire population of white dwarfs in the Universe, that is still a lot of oddly coloured stars out there."