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Cosmic blast announces a future supernova

6 min read

Big booms help measure universal expansion

It’s one thing to theorize about an exploding star the size of our sun, it’s another to look up in the sky and watch one getting ready to blow.

Astronomers are now doing this.

On Feb. 12, a star known as RS Ophiuchi, some 8,000 trillion miles away, erupted in an explosion so bright it could be seen on Earth without a telescope. It was the star’s sixth attention-getting blowout since 1898.

Using satellites and ground-based telescopes, observers from the Harvard-Smithsonian Center for Astrophysics (CfA) and their colleagues from other institutions caught the eruption near its maximum brightness. They measured high-energy X-rays, low-energy radio waves, and heat coming from the outburst.

“Studying such emissions from RS Ophiuchi and stars like it will give us more confidence in measuring distances in the universe, one of the most important topics in astronomy and cosmology,” says Jennifer Sokoloski, a researcher at CfA in Cambridge, Mass.

Certain kinds of supernovae act like measuring posts in the vast emptiness of space. When you look at the brightness of a star, one of the big problems is determining whether it is a faint star that is fairly close, or a bright one very far away. There is no way to distinguish apparent brightness from intrinsic or actual brightness. That type of riddle can be solved with supernovae that all burn with about the same celestial wattage.

Jennifer Sokoloski of the Harvard-Smithsonian Center for Astrophysics describes a violent stellar explosion that lit the sky last Feb. 12. Behind her are radio images showing a sun-sized star ejecting jets of itself. (Staff photo Jon Chase/Harvard news Office)

“These explosions always have about the same brightness,” Sokoloski explains. “Therefore, the supernovae act as ‘standard candles’ in the sense that their apparent brightness indicates how far away they really are.”

The super flashes also shed light on how fast the universe is expanding, which in turn provides clues to how old the universe is and how long it will live. Say you have a standard candle glowing in a distant galaxy. By measuring shifts in its light you can tell how fast it is moving away from us. Getting such information from supernovae at a different distance from Earth tells you how fast everything is expanding.

“Using this technique, astronomers have been surprised to find that the expansion of the universe is actually accelerating,” Sokoloski notes. This finding runs counter to the old idea that gravity will eventually slow the expansion and start a contraction that will cause the universe to eventually end in a colossal crunch. Acceleration, on the other hand, forecasts that the universe will go on expanding forever.

Ready, set…

Getting the best look to date at what goes on in a supernova-in-the- making should give astronomers information on why and how much the flickers of standard candlelight can vary, thus providing more accurate detail about the structure and dynamics of the universe.

“We jumped at the opportunity to study RS Ophiuchi because it occurs in a type of binary [double] star system; that is extremely rare,” Sokoloski says. “There are only four known.”

This system contains a giant red star and what is called a white dwarf star. The red giant is like our sun nearing the end of its life. The white dwarf resembles a sun-type star that has run out of hydrogen fuel. Its outer layers have floated away, leaving a dense core about the size of Earth, according to the National Aeronautics and Space Administration (NASA).

Gases from the red giant blow onto the white dwarf and build up on its surface. From time to time, this activity triggers a runaway thermonuclear explosion. Such galactic hiccups are known as “novae.” But there’s lots more to come.

Instruments on a NASA satellite known as the Rossi X-ray Timing Explorer measured the mass of the white dwarf, which turns out to be about 1.4 times as big as our sun. That’s a critical size for such stars. When one reaches this size, it becomes so unstable it collapses on itself in the stupendous implosion then explosion that is called a “supernova.”

“One day RS Ophiuchi will explode,” says Koji Mukai, a NASA researcher. “What happened last February is just a hiccup, but it allowed us to weight the star [determine its mass] and showed us that it’s almost heavy enough to go off as a supernova” of the standard candle kind.

. . . Go

Interaction between gas exploding from the white dwarf and blowing from the red giant generated X-rays, radio waves, and other types of energetic emissions forceful enough to be easily detected 8,000 trillion miles away. Measurements of this flying debris offered a surprise. Astronomers assume that such debris moves outward in a spherical shell. But that’s not what happened. Seven weeks after the nova, radio waves revealed an expanding ring and two lobes of gas moving in opposite directions. That suggests twin jets of star leftovers spewing out to either side.

The shape of leftover trajectories is important because it helps astronomers understand the process that triggers nova explosions in the first place. Just how does the hot breath of a dying star blowing on its companion create hydrogen-bomb type violence that can be detected trillions of miles away?

Also, ejecta from such stellar violence is important because it contains many of the heavy elements in the universe, from iron through lead, gold, and uranium, which were created in the superheat and superpressure of the nova and supernovae forges. Without this process, life would not exist in the universe.

Sokoloski, Mukai, and their CfA colleagues, Gerald Luna and Scott Kenyon, reported their X-ray findings in the July 20 issue of the journal Nature. Results from ground-based telescopes will be published later by other researchers.

When do scientists think the Ophiuchi supernova will rock the universe? Of course, no one knows enough about what goes on out there to say. But the best guess is it will take thousands of years for the final bit of gas to accumulate and blow the white dwarf away. Meanwhile, these reports should stimulate many more astronomers to focus their attention on the constellation Ophiuchi.