Star caught in the act of sucking companion, then exploding

Hah! Got your attention that time, didn’t I?

No, this blog has not devolved into discussing Paris Hilton, who actually to the best of my knowledge has not had a recent public temper tantrum. In fact, the subject of today’s post is a white dwarf star in the zodiacal constellation Ophiuchus (next to Libra).

A team of astronomers has gotten lucky and been able to observe a white dwarf nearing the supernova (explosion) phase, as it sucks matter from its companion star. Usually, supernovae occur unannounced — we see them after the explosion has happened — so finding a star ready to blow up is a rare, exciting find. It will allow astronomers to gain a better understanding of the supernova process.

There are two types of supernovae, imaginatively termed type 1 and type 2. Astronomers subdivide type 1’s into three subclasses, based on their spectral emissions. The star in question, RS Ophiuchi, is nearing a type 1a supernova event, astronomers believe.

A white dwarf is the corpse of a medium-sized star, not unlike our Sun, that has exhausted its usable supply of nuclear “fuel.” Deprived of the outward pressure keeping its normal diameter at about 1.6 million kilometers, the star collapses. Intense gravitational pressures force atoms so close together that electrons are forced to share orbits they would not normally occupy, creating what is termed “degenerate matter.” The star is now the size of an earth-sized planet, about 15,000 km across.

Meanwhile, the pressure on the remaining matter heats it up, so that the surface is now white-hot. You can guess where white dwarfs get their name.

Many stars have partners, so a white dwarf in such a binary system can be so BBC artist conception of WD binary systemclose to a normal companion star that its intense gravitational pull draws off the companion’s outer layers. As the matter builds up on the white dwarf’s surface, the new layer’s mass eventually reaches the point of causing the lighter elements, hydrogen and helium, to undergo nuclear fusion. The result is a bright flash of light, called a nova.

This process happens fairly regularly, so an astronomer can observe the life cycle of the white dwarf. The trouble has been that most type 1a candidates are very far away, astronomically speaking, and none have exhibited RS Ophiuchi’s behavior in modern times.

The last nearby type 1a explosion was in 1572, the observation and description of which made the career of Danish astronomer Tycho Brahe. Science at that time held that the heavens were constant and immutable. Flashes of light in the night sky and the irregular apparitions of comets were believed to be atmospheric phenomena. Brahe, through careful measurement and analysis, deduced that the 1572 supernova was as far away as the stars. The Danish king was so impressed that he gave Brahe an entire island as an astronomical research facility, the best of its kind anywhere.

RS Ophiuchi’s most recent outburst was last February. Reporting in the journal Nature, astronomers were able to track the motion of the matter flying away from the star, and noticed it did not take long at all for it to fall back down.Dr. Jeno Sokoloski

“The explosion is so energetic it actually lifts an envelope of material off the surface of the star and throws it off into space,” lead author Jeno Sokoloski (at left), of Harvard University, told the BBC’s Science in Action programme.

By tracking the plume, she adds, they could learn more about the star that launched it.

“It started slowing down almost immediately, within just two days, and that tells us the white dwarf must be extremely massive, in fact almost massive enough to collapse.”

In 1930, Indian physicist Subrahmanyan Chandrasekhar calculated that a white dwarf can exist only as long as its mass remains below 1.44 times of the mass of our Sun. Once its mass exceeds that limit, its gravitational pull will result in its own collapse, as the inward pressure exceeds the repulsion of the electrons in the degenerate state. The sudden collapse precedes an immense explosion; as the outer layers of the star hurtle toward the superdense core, they rebound and fuse, blasting an envelope of matter and energy into space.

The mass of RS Ophiuchi is nearing the Chandrasekhar limit, Sokoloski’s team believes. Since its nova-burst material falls back down to join new layers of stuff drawn from its companion, RS Ophiuchi is gaining weight at an ever-accelerating rate. That makes it an ideal candidate to test the theoretical model of a type 1a supernova with some hard data.

The question is when. The supernova could occur tomorrow or in a thousand years. Like celebrities, stars in outer space are tempermental and unpredictable, but at least we know where to look. Astronomers are a very patient lot.

The BBC has more details here.

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