Atom & Cosmos
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and more billions
By Ron Cowen
Astronomers studying eight galaxies have
found evidence of a surprising abundance
of faint, low-mass stars — each galaxy has
about 10 times as many as the Milky Way.
The scientists extrapolate that the heavens contain up to three times the total
number of stars previously estimated.
Spectra of the galactic light indicate that
faint red dwarfs account for 80 percent
of stars in elliptical galaxies.
“Extrapolating from the central regions
of these eight galaxies to the entire universe is somewhat hazardous, but if the
galaxies are typical examples of their class
it may well lead to a tripling” of the number of stars in the cosmos, van Dokkum
says. Previous estimates put that number
at roughly 100 billion trillion.
Still, the results might be otherwise
explained by fewer low-mass stars if they
differ in chemical makeup from those in
the Milky Way, says astronomer Richard
Ellis of Caltech.
Demise of a moon, rise of the rings
icy particles surrounding saturn may be remains of satellite
By Alexandra Witze
Saturn’s majestic rings are the remnants of a long-vanished moon that
was stripped of its icy outer layer
before its rocky heart plunged into the
planet, a new theory proposes. The icy
fragments would have encircled
the solar system’s second largest planet
as rings and eventually spalled off
small moons of their own that are
still there today, says Robin Canup, a
planetary scientist at the Southwest
The ice in Saturn’s rings may be remains
of a moon that crashed into the planet.
Research Institute in Boulder, Colo.
“Not only do you end up with the current ring, but you can also explain the
inner ice-rich moons that haven’t been
explained before,” she says. Her paper
appears online December 12 in Nature.
Earlier ideas about how Saturn’s rings
formed have fallen into two categories:
either a small moon plunged intact
toward the planet and shattered, or a
comet smacked into a moon, shredding
the moon to bits. Both scenarios would
produce an equal mix of rock and ice in
Saturn’s rings — not the nearly 95 percent ice seen today.
Canup studied what happened after
Saturn (and the sun’s other planets)
coalesced from a primordial disk of gas
and dust 4. 5 billion years ago. In previous work, she had shown that moon after
moon would be born around the infant
gas giant, each growing until the planet’s
gravitational tug pulled it to its destruction. Moons would have stopped forming
when the disk of gas and dust was used up.
In the new study, Canup calculated
that a moon the size of Titan—Sat-
urn’s largest, at some 5,000 kilometers
across — would begin to separate into
layers as it migrated inward. Saturn’s
tidal pull would cause much of the
moon’s ice to melt and then refreeze as
an outer mantle. As the moon spiraled
into the planet, Canup’s calculations
show, the icy layer would be stripped off
to form the rings.