Astronomers spot a never-before-seen type of white dwarf star

Its discovery could change our understanding of star death

By Loren Grush

For the first time ever, researchers have spotted a white dwarf surrounded by an atmosphere of mostly oxygen. A star of this kind, a super-dense dead star with an oxygen atmosphere, had never been seen before, though astronomers had speculated that one might exist. Such a unique finding could change how we think about the evolution of stars and what happens when these stellar objects die.


To find this unique zombie star, an international team of researchers looked through data from the Sloan Digital Sky Survey — a project that measures the colored lines of light coming off of objects throughout the universe. These lines, called spectral lines, can tell astronomers what types of elements make up a star’s atmosphere. Using this data, the researchers found that one particular white dwarf, with the eloquent name ­­SDSS J124043.01+671034.68, didn’t have any hydrogen or helium in its atmosphere; its surrounding air was instead almost pure oxygen.

“It was completely not expected for a star with a low mass like our star,” said study author Kepler Oliveira, an astronomer at the Federal University of Rio Grande do Sul.

An image of SDSS J124043.01+671034.68. (Kepler Oliveira)

The finding is so surprising because it doesn’t quite fit with our current understanding of what stars look like when they die. Typically, when a star like our Sun runs out of fuel, it starts collapsing. As the star becomes more compact, it heats up, causing its outer layers to expand more than 100 times the star’s original size. Eventually those outer layers are lost and only the core of the star remains — the faint white dwarf.

Most of the star’s hydrogen and helium get lost with those outer layers, but a little bit of them are left over in the white dwarf’s atmosphere. The hydrogen and helium float to the top of the star’s surface, because they’re relatively light; the heavier elements, like oxygen and carbon, remain below.

“It’s the same reason that panning for gold works,” said Andrew Vanderburg, an astronomy graduate student at Harvard University, who was not involved in the study. “If you have gold and sediments in water, the gold is heavier so it’ll sink to the bottom, but the sediments are lighter, so they’ll stay at the top.”


The fact that no hydrogen and helium are seen in the atmosphere of the white dwarf in question is puzzling. It means some kind of event has caused the two elements to disappear, making oxygen the lightest element in the star’s atmosphere. But the researchers aren’t sure what kind of event that was, as they’ve never considered it before. “We don’t make models of things we don’t know exist,” Oliveira said. “But now that we know this star exists, we have to calculate the model for it.”

One possible explanation for the lack of helium and hydrogen is that the star experienced a giant thermal pulse when the object was a red giant, and that intense explosion stripped away all the lighter elements. Another possible scenario is that the star was actually part of a binary system. The stars may have merged together, causing an explosion that ejected the hydrogen and helium. These ideas are only loose theories, though. “We don’t have a calculation that shows [a binary merger] happened, but that’s the only explanation that I can think of,” Oliveira said. “It must have come from a binary system.”

The researchers will work to figure out what happened to this star, but in the meantime, the white dwarf’s discovery is a significant find for the astronomy community. “It’s a new class of star,” said Vanderburg. “We don’t understand how it formed, but this is the kind of thing that pushes our field forward, and who knows where it will take us.”


NASA Plans to Light a Fire Inside a Spacecraft, Then Watch What Happens

Relax, it’s being done for science.


A flame in space, as photographed during a BASS (Burning and Suppression of Solids) experiment. (NASA)


For the past couple of weeks, on and off, astronaut Tim Kopra has been playing with fire on the International Space Station—part of an experiment called Burning and Suppression of Solids—Milliken (BASS-M), to test how flame-retardant cotton fabrics burn in microgravity.

Why? Because fire behaves differently in space than it does on Earth. In normal gravity, hot gas rises, drawing in cool, fresh air at the base of the flame. That’s what gives flames their familiar teardrop shape. In microgravity, hot gas doesn’t rise, so flames tend to be wider, shorter, and rounder than on Earth. As a result, flames in space radiate heat differently than they do on Earth, which in turn affects how fires spread. That means materials may be more or less flammable in orbit than they are on Earth, even with the same mix of atmospheric gases.

When it comes to flammability tests, size matters. On Earth, NASA uses 5 cm by 25 cm samples of flammable material. But pieces that big aren’t allowed on the station (with some exceptions when there is no practical alternative, such as the crew’s clothing). So experiments like BASS-M (which follows up on earlier BASS combustion experiments carried out on the station from 2011 to 2013) make do with small samples, about one centimeter by three centimeters.

“The problem with small samples is that a lot of aspects of the fire don’t scale linearly, so you can’t look at a tiny, one-centimeter fire and extrapolate that to one that’s a foot wide or something,” said David Urban, a combustion researcher at NASA’s Glenn Research Center.

Scientists would like to know exactly how large-scale fires would grow and spread in microgravity, but it’s too dangerous to conduct that kind of experiment on a spacecraft with astronauts on board. Instead, safety engineers have to rely mostly on models based on how flames spread in Earth’s gravity, and on a few small combustion experiments in space.

Sometimes you just need a bigger fire. So Urban and co-investigator Gary Ruff designed the Spacecraft Fire Experiment (Saffire), a series of six tests that will ignite and study contained fires aboard returning Cygnus cargo ships (the next of which is scheduled to depart the station on Friday). When they leave the ISS, the Cygnus ships contain only trash, and they burn up during re-entry. They’re expendable, which makes them the perfect place to set a fire.

When the next Cygnus (number OA-6) launches on March 20, it will carry, along with new supplies for the station,  the experimental hardware for Saffire-I. A metal box with fans at either end houses a 0.4- by 0.94-meter sheet of SIBAL cloth, a blend of 75 percent cotton and 25 percent fiberglass. Cotton is used in crew clothing, towels, and other cloth items aboard the station, and the fiberglass blend keeps the sample material from ripping and tearing as it burns. The fans will regulate airflow into and out of the fire.

After Cygnus detaches from the station in mid-May, a ground team will turn on power to the Saffire hardware and activate an electronic igniter at one corner of the SIBAL fabric. As the sample burns, instruments will measure temperature, pressure, and concentrations of oxygen and carbon dioxide near the fire. Video cameras will record the shape, growth, and spread of the flames.

A Cygnus cargo vehicle on its way up to the space station last December. This one comes home on Friday.

A Cygnus cargo vehicle on its way up to the space station last December. This one comes home on Friday. (NASA)

The fire should consume the sample fabric and burn itself out in about two hours, but Cygnus will spend another four days in orbit, downlinking to stations on Earth so the researchers can retrieve Saffire’s experimental data before the resupply ship re-enters Earth’s atmosphere and breaks up over the Pacific.

Saffire-II is scheduled to launch on OA-7 in October. With nine smaller samples—including more SIBAL cloth, Nomex, and plexiglass—it will replicate the flammability tests that NASA conducts on Earth. That should help researchers determine how well those tests predict the materials’ flammability in microgravity.

In 2017, Saffire-III will repeat Saffire-I’s large-scale fire, but this time with a stronger airflow. Since airflow is the main factor that influences the size of flames in space, researchers expect to see larger flames in Saffire-III.

The recent BASS-M experiments have helped lay the groundwork for these first three fire experiments, just as they will prepare the way for Saffire-IV through Saffire-VI. These later missions will study how heat and pressure from large fires could affect the rest of the spacecraft cabin, and will give NASA a chance to demonstrate fire suppression technologies that it has spent the last several years developing.


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