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FM
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Why these blue stars should(n't) exist


 


Blue stragglers are stars that are observed to be
brighter and bluer than we Earthlings would expect, since these
characteristics make the stars appear younger than they actually are.


No one's been able to pin down
how blue stragglers form. But a group of scientists report a theory in
this week's issue of the journal

Nature
based on new
observations.


Most blue stragglers have
a companion star; the two form a binary system. But the companion is
usually in a wider orbit than what's usually found in binaries. Blue
stragglers are hotter than red stars, and are fairly rare.


Aaron Geller and colleagues at Northwestern University
observed a cluster of stars that all formed at about the same time - 7
billion years ago - with 21 blue stragglers. The scientists looked at
companion stars that are about half the mass of our sun. Blue stragglers
tend to be more massive than our sun, on the other hand.


They believe blue stragglers form in a process called
mass transfer. This happens when a red giant, a large star that's
relatively old, has an outer envelope of material that is no longer
gravitationally bound to the star anymore. The red giant transfers mass
onto the blue star in the binary system. Material that comes from the
red giant forms an accretion disk around the blue star.


"It was a pretty happy star living on its own, with
this binary companion, until its companion grew to become a giant and
then started dumping all of this mass onto it," Geller said.


This material can be used as fuel that this bluer star
can use to burn at its core, which keeps it alive longer, Geller said.
Eventually all of the outer layer from the red giant will go to the blue
star, turning the red giant into a white dwarf, and the accretion disk
around the blue star will go away.


"This bluer star will be blue straggler. It will more
massive than it was before, because it's gained all this material, and
all this material is allowing it to stay bluer, brighter, longer than it
should have," Geller said.


Scientists are observing the final state of this mass
transfer process - systems with a blue straggler and white dwarf
companion - but have not actually seen the transformation happen.


Geller and colleagues have not discovered any planets orbiting blue
stragglers, but NASA's Kepler
mission may come up with something in the future, Geller said.


   




Replies sorted oldest to newest

quote:
Originally posted by D2:
Giant blue stars should not exist under our current theory star formation.Rigel ( the bright blue star opposite Betelgeuse, a giant red star )in the famous constellation of Orion is one of these.


Correct. Scientists still use guesswork and theories. Occasionally, their theories are found inconclusive and need enhancements.
FM

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