Space.com
04-09-07
New Thinking on the Death of Sun-Like Stars
By Charles Q. Choi
Special to SPACE.com
When stars like our Sun die, they bloat to become red giants and then eject
gigantic clouds of gas and dust into space. Increasingly, however, scientists
found themselves at a profound loss to explain how exactly dying stars could
blow away these clouds.
Now astrophysicists propose that unexpected chemical reactions during the
formation of stardust could help solve this mystery.
"In the long run, the new scenario may have an important impact on our
understanding of the chemical evolution of our galaxy, and the origin of elements
crucial for the existence of life," astrophysicist Anja Andersen at the University of
Copenhagen in Denmark told SPACE.com.
The puzzle
Stars smaller than the Sun and up to eight times as massive die by swelling up
into red giants before shedding most of their mass to shrink into very compact,
dying embers called white dwarfs. Two kinds of red giants exist—those with lots
of carbon, and others richer in oxygen than in carbon.
Carbon-rich stars release carbon particles such as soot and graphite during their
death throes. Scientists proposed these pitch-black grains absorb rays from the
dying star and get shot into space by starlight, a theory that fit both the
observations and computer models.
Increasingly, however, researchers could not explain how oxygen-rich stars like
our own Sun could propel their clouds away during the final stages of mass loss,
"what we here call 'the death zone,'" said Iowa State University astronomer
Lee Anne Willson. Oxygen-rich stars create large quantities of water vapor and
silicates, such as quartz or sand. These are transparent, meaning starlight
should go right through them.
This mystery began roughly five years ago and reached a head last year when
the calculations of two independent teams of scientists "showed that it seems
impossible" for oxygen-rich stars to drive their transparent clouds away,
explained astrophysicist Susanne Höfner at the University of Uppsala in Sweden.
Some dead ends ...
Andersen and Höfner contemplated possible solutions. They first pondered
whether the silicates might have iron in them, which would render them opaque
instead of transparent. But calculations showed the dust grains would have
evaporated if they had iron in them.
They next wondered if enough molecules surrounded the core of a dying oxygenrich star to block out its light and thus create a wind that blew the star's outermost
layer into space. But their models suggested these molecules could not block off
enough light and create strong enough winds.
Given that stars pulsate as they die, the scientists then suggested these
pulsations could perhaps force a star's matter out, but this idea did not match
with astronomers' observations.
... And one good idea
But inspiration then dawned upon Andersen and Höfner. Perhaps some of the
carbon in the oxygen-rich stars could help force the outer layers of the stars into
space. They believe shock waves from the pulsations of dying stars could make
carbon in oxygen-rich stars form pitch-black dust.
"The theory fits with all our subsequent model calculations," Andersen said, and
it matches observations from dying oxygen-rich red giants. Moreover, Höfner
explained "this mechanism strongly favors the presence of magnesium silicates
over iron silicates in the interstellar medium," in agreement with recent findings
from NASA's comet-sampling Stardust space probe.
Andersen and Höfner detailed their findings in the April issue of the journal
Astronomy & Astrophysics.
"The calculations needed to make sure this is right will be hard to do," Willson
said. "They make a good argument, but there will be quite a bit more work
needed to check that it really does work this way."
If proven correct, the beauty of the new scenario is that it suggests a common
driving mechanism for many dying stars shedding their mass via dusty winds,
"with possible long-reaching consequences for the origin of chemical elements
relevant for life," Höfner said.