Mystery of Explosive Star Solved

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A artist's impression of the nova explosion in RS Ophiuchi,a binary star system located 5,000 light-years away. The red giant is dumping gas onto the surface of the white dwarf, setting off thermonuclear explosions much more powerful than those that occur on the sun.
Credit: David A.Hardy/www.astroart.org & PPARC.

 

In
February, a faint star a few thousand light-years away flared suddenly, beaming
so brightly that for a few days it was visible to the naked eye.

The star is
a stellar corpse the size of Earth, known as a white
dwarf, and it is paired in a binary system with a red
giant, a dying, bloated star that once resembled our Sun. The red giant has
been dumping gas onto the surface of the white dwarf, and every few years,
enough matter accumulates to set off a giant thermonuclear explosion.

It was one
of these explosions, called a "nova,"
that astronomers and stargazers detected earlier this year.

The
two-star system, called RS
Ophiuchi, is known as a recurrent nova because five similar eruptions have
been detected before. The first observation occurred in 1898; the last eruption
prior to this latest one happened in 1985.

The new
observations, made using advanced radio and X-ray telescopes not available
during the last outburst, reveal the explosion to be more complex than was
previously assumed.

Standard
computer models had predicted a spherical explosion with matter ejected in all
directions equally. The latest observations instead showed that the explosion
evolved into two lobes, confirming suspicions that the nova outburst produces twin
jets of stellar material that spews out from the white dwarf in opposite
directions.

"The
radio images represent the first time we’ve ever seen the birth of a jet in a
white dwarf system. We literally see the jet ‘turn on,’" said Michael
Rupen, an astronomer at the National Radio Astronomy Observatory who studied RS
Ophiuchi using the Very Long Baseline Array (VLBA).

As
impressive as the nova are, they might just be precursors for a more violent
supernova explosion that will occur in the future, scientists say.

 

Like the
Sun, Only More Powerful

The white
dwarf’s thermonuclear blasts are similar to those that occur on the surface of
the sun, but they can be over 100,000 times more powerful. During each
outburst, an amount of gas equal to the mass of the Earth is flung into space.
Some of this ejected matter slams into the extended atmosphere of the inflated
red giant, creating blast waves that accelerate electrons to nearly the speed
of light. As the electrons travel through the stars’ magnetic fields, they emit
radio waves that can be detected by telescopes on Earth.

The blast
waves move at over four million miles (about 6.4 million km) per hour. For a
few weeks during each outburst, the white dwarf becomes a red giant.

"After
the [thermonuclear explosion], the white dwarf will puff up into a red giant
for a few weeks as the hydrogen that has been blasted into space fuses into
helium," explains Richard Barry of the NASA Goddard Space Flight Center in
Maryland.

All eyes
on Ophiuchi

Japanese
astronomers first detected signs of RS Ophiuchi’s latest nova on the night of
Feb. 12. Follow-up observations by radio telescopes revealed an expanding blast
wave whose diameter was already the size of Saturn’s orbit around the Sun.

In the
weeks following, several radio and X-ray telescopes around the world tracked RS
Ophiuchi closely, including the MERLIN array in the UK, the European EVN array,
the Very Long Baseline Array (VLBA) and Very Large Array (VLA) in the United
States, and NASA’s Swift
and Rossi
X-ray Timing Explorer satellites.

Findings
from the Rossi X-ray Timing Explorer and the VLBA/EVN observations are detailed
in two separate studies published in the July 20 issue of the journal Nature.

The red
giant and white dwarf stars making up RS Ophiuchi
are separated by about 1.5 astronomical units, or one and a half times the
distance the Earth is from the sun. The binary star system is located in the
constellation Ophiuchus, about 5,000 light-years away–very close by
astronomical standards.

"We
have a ringside seat for this very important event," Barry told SPACE.com.
Barry is a co-author on another study on RS Ophiuchi that will appear in an
upcoming edition of Astrophysical Journal.

Supernova
precursor?

 

When the
outburst is over, gas will once again build up on the white dwarf and the
explosions will begin anew, perhaps in some 20 years time. It’s unknown whether
the white dwarf casts off all of its accumulated matter during each eruption,
or whether some of the material is being hoarded and slowly increasing the mass
of the dead star.

"If
the white dwarf is increasing in mass then it will eventually be ripped apart
in a titanic supernova explosion and the cycle of outbursts will come to an
end," said Tim O’Brien of the University of Manchester, a co-author on one
of the Nature studies.

White
dwarfs must attain a critical 1.4 solar masses before they can explode in what
scientists call a Type
1a supernova. The white dwarf in RS Ophiuchi is near this critical limit
now, but it will still probably need hundreds of thousands of years to accumulate
the final bit of mass, scientists say.

Because all
Type 1a supernovas emit the same amount of light at their peak, they serve as
important "standard
candles" which astronomers use to calculate cosmic distances.

"Our
understanding of these objects is exceedingly important as any miscalculation
or uncertainty in the total light of output of supernovae could have a dramatic
effect on our calculations of the scale and size of the entire universe,"
Barry said.

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