First green star found: why this white dwarf glows a different colour

Researchers at the University of Warwick say they have identified the first known green star, a white dwarf called WD J1330+6435. According to the German tech publication t3n, the study also suggests that many of these old, dense stars are more colourful than astronomers have long assumed.

What happened

A team at the University of Warwick in the United Kingdom has published research in the journal RAS Techniques and Instruments. It challenges a basic assumption about how stars get their colour. The long-standing view, as t3n summarises it, is that temperature decides colour: cool stars look orange, stars like the Sun look white, and the hottest stars look pale blue. The new study argues that other factors play a part too.

White dwarfs are small, old stars. They form when a star with roughly the mass of the Sun runs out of fuel and its core collapses under its own gravity. They got their name because they appeared white or pale blue when they were first discovered. The researchers suggest this may come down to distance: from Earth, astronomers simply could not make out their true colours.

Density is what makes the difference. After the collapse, the matter in a white dwarf is millions of times denser than the Sun’s photosphere, its visible outer layer. Under these conditions, elements such as carbon, calcium and sodium interact with the dense helium plasma and absorb light across a wider range of colours. That shifts the star’s appearance away from the familiar white and blue.

The study describes a whole palette. White dwarfs with a high carbon content shimmer pink or violet. Those enriched with debris from asteroids or comets they have swallowed look more yellowish. Cooler white dwarfs with mixed hydrogen and helium atmospheres often glow cyan or pale blue, because hydrogen molecules collide with helium atoms.

The most striking colours appear when one of these “living dead” stars, as t3n calls them, starts feeding on planets around it. WD J1330+6435, about 326 light years away, is the example the researchers highlight. It has taken in rocky material from a planetary companion, which enriched its atmosphere with calcium, lithium and sodium and gave it a green tint. No other star has been credibly reported as green, so WD J1330+6435 is considered the first known green star.

Team leader Mark Hollands of the University of Warwick told the science site Space that nobody had really looked at what else might be possible with white dwarfs. He said these incredibly dense stars can produce colours no one would have expected, and that some of them had genuinely surprised him. On the green star, he explained that traces of sodium in its atmosphere absorb the orange part of the spectrum and leave behind a ghostly green glow.

Why it matters if you live in Germany

This is a science story, and the source describes no direct change for residents, workers or consumers in Germany. It deserves attention because t3n, one of Germany’s best-known technology and digital business publications, chose to cover it. Science and astronomy reporting of this kind regularly reaches German readers through outlets like t3n, and the study itself is a useful reminder that textbook rules can be revised when researchers look more closely.

For amateur astronomers, the research comes with a caveat. The study suggests the true colours of white dwarfs have been hard to see from Earth because of their distance. Anyone hoping to spot a green star through a backyard telescope should keep that in mind. The colours described here come from detailed analysis of how these stars absorb light, not from a casual look at the night sky.

The finding also matters for students and anyone interested in physics. It shows that a star’s colour can say something about its history, including whether it has consumed material from planets or smaller bodies such as asteroids and comets.

What to watch next

The team is careful not to overstate the case. Despite all this variety, white remains the most common colour for white dwarfs, according to the researchers. Only five of the 151 dead stars within about 65 light years of Earth show an unusual colour.

Boris Gänsicke of the University of Warwick’s physics department put that into perspective. One in thirty may not sound like a large share, he said, but across the total number of white dwarfs in the universe it still adds up to a lot of strangely coloured stars. He added that if you take the median of a representative population of white dwarfs, the average white dwarf really is white.

The question now is how many more unusually coloured white dwarfs exist beyond this nearby sample, and whether others will turn out green like WD J1330+6435. Because the colours reveal what a star has absorbed, they could also become another way to study what happens to planetary systems after their stars die. Readers who want the technical detail can find the full findings in RAS Techniques and Instruments.

Source: t3n