If we heat an iron bar, it first turns red, then orange, and finally acquires a white glow. Something similar happens with stars: their temperature determines a good part of the color we perceive. Astronomers have long applied this rule to interpret starlight, including in white dwarfs, the compact remnants left behind when stars like the Sun run out of fuel. But a new study shows that these stellar corpses can show a much broader palette, with shades of green, pink, cyan, yellow and purple.
The protagonist of this discovery is called WD J1330+6435. It is a relatively cool white dwarf, with a temperature of approximately 4,000 degrees, whose light has a yellowish-green hue. A team of scientists from the University of Warwick, led by Mark Hollands, consider it the first star for which a genuinely green appearance has been convincingly documented. This is not, however, a direct photograph of its surface: The color has been calculated from its spectrum and models that reproduce how the human eye perceives light. The results have been published in RAS Techniques and Instruments.
“Most stars, including most white dwarfs, behave as a kind of perfect radiators, so their color is determined almost entirely by their temperature. However, no one had really analyzed what other possibilities could exist in the case of white dwarfs – explains Hollands in a statement -. “It turns out that these incredibly dense stars can produce colors that no one expected, and some really surprised me.”
White dwarfs concentrate a large amount of matter in a tiny volume. In the coldest ones, the density of the atmosphere can reach approximately one gram per cubic centimeter, comparable to that of water and millions of times higher than that of the solar photosphere. Under these conditions, elements present in small quantities, such as sodium, calcium or carbon, They interact with the environment in a different way than they would in a less dense atmosphere. The result is that they absorb light in much wider ranges of the visible spectrum and modify the color we perceive.
In WD J1330+6435, the decisive element is sodium. The star has incorporated rocky material from its planetary system, which has enriched its atmosphere with sodium, calcium and lithium. The high density widens the sodium absorption bands, which eliminates a significant part of the orange light and leaves a distribution of colors that the eye interprets as yellowish green.
“The traces of sodium in this white dwarf’s atmosphere absorb the orange part of the spectrum, leaving a ghostly green glow – adds Hollands -. Imagine being on a planet and looking up to see a green star. It seems like something out of science fiction.”
The team did not limit itself to studying this exceptional star. It also analyzed a representative sample of 151 white dwarfs located less than 65 light years from Earth. Five had unusual colors: two yellowish and three pink. From this sample, the authors estimate that approximately one in thirty white dwarfs could emit light of unusual hue.
Among them stands out WD J1011+2845, located about 48 light years away, which could be one of the reddest stars so far identified in this category. Its atmosphere contains carbon from the inner layers of the star. The molecules formed by this element absorb a good part of the visible light and shift the perceived color towards an intense salmon pink.
Other white dwarfs can acquire shades cyan or pale blue when they contain a mixture of hydrogen and helium, while those rich in carbon tend to pink or purple. Those that accumulate remains of asteroids and comets can show yellowish tones. The variety depends on the chemical composition, temperature and extreme conditions of their atmospheres.
The work also corrects a widespread idea: although white dwarfs are usually represented as blue, the average color of the sample studied is white. Its average color temperature is around 6,500 ºC. The hottest and bluest ones are brighter and, therefore, appear more frequently in catalogs based on brightnesswhich can distort our image of the whole.
“One in thirty may not seem like a huge proportion, but if we apply it to the entire population of white dwarfs in the universe, we are talking about many stars with strange colors – says Boris Gänsicke, co-author of the study -. However, if we take the median of a representative population, we find that the average white dwarf is white.” But sometimes, what’s outside the average is more interesting, at least in scientific terms. If we take into account that only our galaxy would have 10 billion white dwarfs, There would be at least 300 million dwarfs that would not be really white… 300 million stars of unexpected colors.
The discovery does not change the laws that govern the stars, but it does expand our way of interpreting them. Temperature remains essential; It just doesn’t tell the whole story. Sometimes, The chemistry of such a dense atmosphere can transform light to produce a color that seemed impossible. And so, among the remains of stars that exhausted their fuel billions of years ago, astronomers have found a new way to be surprised by the universe.