Surprising Cocoons Found Enveloping Giant Stars
Two model images of the Cepheid star L Carinae as deduced from the interferometric observations: in the near-infrared from VINCI measurements (left) and in the mid-infrared from MIDI (right). In both cases, an envelope is found to surround the star. The contribution from the envelope is about 5 percent in the near-infrared and significantly more in the thermal infrared. As L Carinae is 17,000 times brighter than the Sun, this means that its envelope alone is several hundred times brighter than our Sun.
Credit: ESO

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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