Stars have an image. Most people think about blue, white, yellow, orange, and red when thinking of stars.
Green is mostly where the conversation goes dark. Astronomers were mostly in the same place.
Then one dead star challenged that assumption.
A team of scientists studying the true colors of white dwarfs came across a white dwarf that researchers calculated would appear green to human eyes.
The calculated green appearance appears to be linked to rocky debris from a former planetary system.
The story began with an ordinary white dwarf
White dwarfs are the extremely high-density remnants of stars (like our Sun) once the fuel runs out of these stars and the outer layers are shed off.
Temperature explains most of what we see when it comes to determining the colors of stars.
Hotter stars tend to be bluer. Cooler stars tend to appear towards warmer colors.
Because this is such a commonly occurring phenomenon, there weren’t too many people anticipating finding a white dwarf that would resemble anything remotely close to being green. Until the data told otherwise.
The clue came from a dead planetary system
When astronomers were attempting to determine what white dwarfs truly would look like to human eyes, researchers at the University of Warwick weren’t searching for a green colored star.
At some time prior to the current state, rocky materials from a planetary system fell into the atmosphere of WD J1330+6435.
These materials contained elements including sodium, calcium, and lithium.
Under normal circumstances, these elements would not be expected to make a star appear green. White dwarf atmospheres are far from ordinary.
Sodium absorbs large swaths of orange light in the white dwarf’s extraordinarily dense atmosphere. By removing sufficient amounts of orange light, the remaining radiance shifts in a completely different manner.
This ultimately resulted in a color which astronomers rarely even considered possible for a star.
One strange star led to another surprise
Examining more white dwarfs revealed unexpected colors.
Some were pink. Others were purple. Some were yellow. Others were cyan.
The various colors were related to chemistry and not solely temperature, according to findings published in RAS Techniques and Instruments.
Carbon rich white dwarfs frequently exhibited pink or purple hues. A nearby white dwarf became so heavily polluted with carbon brought up from its core that researchers described it as appearing salmon pink.
For decades, stars followed a somewhat predictable color scheme. Suddenly, the number of colors in the palette increased dramatically.
The discovery changes how astronomers look at white dwarfs
Light emanating from a star provides information. Researchers did not identify the green appearance because they built a new telescope.
Instead, the green appearance emerged because they asked themselves a different question. Rather than focusing exclusively upon physical attributes, they analyzed what the stars would actually appear like if viewed through human vision.
This relatively minor change in focus led to the discovery of something that was hidden in plain sight. The information existed in the spectra all along.
Nobody had translated it into color in quite the same way.
Astronomers found more colors than expected
For generations, white dwarfs seemed to follow a predictable sequence. Hotter stars tended to be bluer. Cooler stars tended to be more reddish.
The color survey revealed that the pattern was far less rigid than astronomers once believed.
Then researchers calculated that WD J1330+6435 would appear green to human eyes. It was not alone for long.
White dwarfs could appear in shades that extended well beyond the familiar stellar palette.
Astronomers began by asking what color white dwarfs truly are. The answer proved far more diverse than they originally anticipated.
Green turned out to be only the beginning.
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