A
representation of the green star.
(Courtesy Mark A. Garlick/University of Warwick)
Astronomers analyzing
white dwarf stars have for the first time found a star that would appear green
to human eyes, according to a new study.
While for most stars, color is temperature-dependent
and limited to a few hues, white dwarfs, which are the dense remnants of
sun-like stars that have died, can glow in an incredible range of colors, said
lead study author Mark Hollands, an astrophysicist at the University of
Warwick, England, on Thursday.
“The colors that normal stars can have are pretty much
only orange, white or blue,” Hollands told CNN.
“What we found is, for most white dwarfs, that’s the same as well. But there are some special cases where the physics in the outer layers of these stars are sufficiently strange that it causes actually quite big deviations in terms of their spectrum,” he said.
Simulated images of a number of
white dwarfs of different colors.
(Courtesy Mark
Hollands/University of Warwick)
These deviations are due to the interaction between the helium in their
atmosphere and traces of other elements, which vary between white dwarfs,
depending on what material has interacted with the star over the course of its
lifetime.
For instance, iron, magnesium and sodium, elements commonly found in rocks,
can be found on the surface — potential evidence of a nearby former planetary
system, or from “something like an asteroid that ventures too close to the
white dwarf,” Hollands explained.
“It gets shredded up into dust, and then those rocky bits of dust and
debris fall onto the white dwarf,” he said.
“And then that gets mixed into the outer layers, and then you can actually
use that to measure what the rocky stuff was made off,” Hollands said.
“So we can actually use the white dwarf as a sort of diagnostic tool to
actually measure the chemistry of planetary material,” he added.
“The white dwarf is taking apart the planets, and we’re able to see what
was actually inside that planet — the proportion of the different elements —
and that gives us clues in how these planets were formed,” said Adam Burgasser,
a professor of astronomy and astrophysics at the University of California San
Diego. “It is a crime scene, and we’re kind of piecing through the evidence
that we have left available to us, and that’s allowing us to kind of piece the
story of what the planet was and how it was made way, way, way back in in
history.”
Human connection
For this study, Hollands looked at the 151 known white dwarf stars that are
within 20 parsecs, or around 65 light years, of Earth.
As for the green star, the main elements present are calcium, sodium and
lithium, Hollands said.
“It kind of stimulated interest in me in looking more at that object and
why it has so much material being accreted onto it,” said Burgasser, who was
not involved in the study. “It can excite the public as well as the scientists
in learning more about some of the more, I would say, extreme objects in this
sample.”
Another notable star was “almost like a salmon pink,” due to the presence
of a high density of carbon molecules, Hollands added.
Another white dwarf that would
appear pink to the human eye.
Courtesy Mark A.
Garlick/University of Warwick
While no white dwarf stars can be seen directly from Earth, Hollands was
able to calculate their color using star spectrum data, which a statement from
the researchers defines as “the ‘fingerprint’ produced when starlight is split
apart by wavelength, much like a prism splits sunlight into a rainbow.”
Hollands explained that the research has also brought these space objects
much closer to home.
“It gives you a nice human connection to these very distant things that are
so far away that sometimes even as a researcher you feel quite disconnected
from these objects that you study,” he said. “But then you can sort of add a
human element into it and it feels a lot more tangible.”
The observations about the green star are based on existing data and may
not shed new understandings about white dwarfs, but the study does offer an
intriguing approach and view of these celestial bodies, said Agnès Kim, an
astrophysicist and professor of physics at Penn State Scranton, who was not
involved in the work.
“Doing a study that goes from spectrum to what the human eye would see is a
cool intersection of physics and biology to me,” Kim wrote in an email. “I love
that some would look green to us if they were close enough for us to see them,
and even more fun that some would look pink.”
Burgasser added that the study’s categorization scheme using temperatures
and atmospheric properties can be useful for the field moving forward.
“Some things are kind of purplish pink, and some other things were greenish
and blue — and that kind of classification, which by the way we do all the time
in astronomy, is classify things, allows us to focus on: why are these types of
objects these colors, and are they related to each other based on their
composition?” Burgasser said. “We have that information in other forms, but I
think this is maybe a simplified way of organizing that data.”
Next, Hollands is planning to keep analyzing more white dwarf star spectra,
with advances in scientific techniques meaning that researchers will be able to
look at a new tranche of stars that are even farther away from Earth.
“We know of white dwarfs that we don’t have spectra for,” he said, “but
that’s all about to change because there’s quite a few spectroscopic surveys”
that are going to start delivering data.
“We’ll be getting a tenfold increase in the number of white dwarfs that
have spectra,” Hollands said. “And so maybe if we’re lucky, we’ll go from one
or maybe two green white dwarfs to 10 or 20.”
The study was published in the journal RAS Techniques and Instruments, or RASTI.
PUBLISHED
By
Jack Guy and Lauren J. Young
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