When we think of the colors of stars, we usually think of a range between red and yellow, with blue and white appearing far more rarely. Other colors were considered extremely uncommon, almost non-existent. However, recent discoveries have uncovered that certain stars have an unusual ability to glow in all sorts of colors and, in doing so, have found the first truly green star.
But first, let’s backtrack: Where does a star’s color come from? The answer lies in the star’s temperature. Think of a flame ignited in regular air; the “cooler” part of the flame is the orange-colored part, whereas the hottest part of the flame is the blue base of that flame. Applying that same logic to stars, the hottest stars would be the ones that appear bluish-white, otherwise known as type O stars; at that point, their surface temperatures can range anywhere from a staggering 30,000 Kelvin to above 50,000 Kelvin.
However, this only includes stars residing within the main sequence, which are stars that fuse hydrogen into helium. Beyond main sequence stars, white dwarfs, Wolf-Rayet stars, and neutron stars take the throne as the hottest types of stars. White dwarfs, in particular, are special because they are the stellar cores of main sequence stars under 8 to10 solar masses after they burn all their outer layers. These stars are given this name because they were determined to be unusually hot and resemble a white color.
That’s how it was assumed for the longest time; however, astronomers from the University of Warwick have recently discovered that these dead star remnants can shine across a broad spectrum of colors. These findings were published in the RAS Techniques and Instruments (RASTI). Dr. Mark Hollands, lead author of these findings and Warwick Prize Fellow, notes that “most stars, including most white dwarfs, behave like ‘perfect radiators,’ so their color is set almost entirely by temperature. But nobody had really looked into what else might be possible for white dwarfs.”
The question naturally arises: Why white dwarfs? That answer lies in their extreme density. Because white dwarf stars are around the Sun’s mass but have a radius comparable to the Earth’s, they naturally have extremely high densities. The coolest white dwarfs have a density of about 1 gram per centimeter cubed, which is about equivalent to water’s density and millions of times denser than the Sun’s photosphere. At this point, trace elements such as carbon, calcium, and sodium interact with the intensely dense plasma and absorb a wider range of wavelengths than in less dense environments.
As such, light coming from these white dwarfs would emit vastly different hues than what we have often observed. Since no white dwarfs are close enough to easily distinguish their color by eye, Hollands and his team analyzed the various spectra of these stars. Through this process, the astronomers discovered that white dwarfs’ colors fit into discrete groups linked to their atmospheric chemical composition: White dwarfs with an abundance of carbon appear pink or purple, whereas those with both hydrogen and helium can appear cyan.
The titular white dwarf, aptly named WD J1330+6435, had a surface temperature of 4000 degrees Celsius and accrued a unique atmosphere consisting of calcium, lithium, and sodium. This gave it that characteristic ghostly green tint, a trait unique to itself to the team’s knowledge as they have been unable to find another like it.
In order to test the scarcity of such stars, the astronomers investigated a sample of 151 white dwarfs within about 65 light-years, finding five individual white dwarfs with unusual tints. They eventually came to the conclusion that approximately one in 30 white dwarfs is differently colored than the normal white or pale blue.
As we continue to learn more about the strange phenomena that appear in our universe, it becomes increasingly clear how little we truly understand about our universe. Simply the fact that this study broke the longstanding belief that white dwarfs are always white or a pale blue proves that we know very little about the universe as we see it, and that we will continue to learn more about it as time goes on.
