Scientists
looking at three rare and radiant pulsating stars have found they each are
surrounded by a fairly bright layer of matter, a "cocoon," that has never
before been detected around stars of this kind.
The
astronomers think the cocoons form as the stars shed huge amounts of mass at a
tremendously faster rate than normal stars like the Sun.
The cocoons
are about two to three times larger than the stars and about 4 percent as
bright, very bright considering that these pulsating stars themselves are
incredibly radiant. One of the enveloped stars is 400 times brighter than the
Sun.
'We
don't know'
Most stars
have well-defined, much thinner surfaces. For instance, the Sun is 870,000
miles (1.4 million km) in diameter, with a surface that's only 620 miles (1,000
km) thick.
Now
astronomers are baffled by why the newly discovered cocoons, or envelopes,
don't burn up despite their proximity to these exceptionally hot-burning stars.
"Are they
replenished by the pulsation? Currently, we don't know," said Pierre Kervella
of the Observatoire de Paris, who is the lead author of the report in the
journal Astronomy and Astrophysics of an envelope around a star called L
Carinae.
The
envelopes were found around a type of pulsating star called Cepheid variables,
which are part of a class of stars called supergiants. Some supergiants are
larger than our entire solar system.
Most stars have a life cycle that starts with some 5 billion years or more
of hot thermonuclear fusion, converting hydrogen into helium. As they age and
run out of fuel, they expand into giants
or supergiants,
and then explode to become a supernova
or planetary
nebula. Later they turn into white
dwarfs, neutron
stars or black holes.
Widespread
phenomenon
Cepheids
pulsate regularly every few days. This makes them useful as distance
benchmarks for objects in distant galaxies. A mathematical relationship between
their intrinsic brightness and pulsation allows for the calculation of their
distance from Earth.
L Carinae
is the brightest of Cepheid in the sky, with about 10 times the mass of the Sun
and measuring 180 times larger across than the Sun. Another team, led by
Antoine Merand of the Observatoire de Paris and CHARA Array at Mt. Wilson
Observatory in California, found similar
cocoons around Polaris (the North Star) and Delta Cephei.
The finding
for all three stars, which have very different properties, "seems to imply that
envelopes surrounding Cepheids are a widespread phenomenon," Kervella said.
Astronomers
had found hints that some Cepheids had envelopes, including around one called
RS Pup. But no one had looked carefully at more Cepheids, fairly close to the
surface, to see if they also had envelopes.
Incredibly sensitive instruments are required to
detect Cepheids. Kervella, Merand and their colleagues combined the light from
several telescopes to see the new Cepheids and their envelopes. The achievement
is comparable to observing an Apollo lunar module on the Moon.
Now that
astronomers know many Cepheids have cocoons or envelopes, they will study them
further to learn their shape and other properties.
"There are
certainly interesting (and unknown) physics inside the envelope, possibly shock
waves. Our first detection is just a glimpse," Kervella told Space.com. "Now we
will observe these stars further and try to understand better how their
envelopes work."
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