Sky and Telescope - March 2016 - 12

News Notes

NASA / ESA / HUBBLE HERITAGE TEAM

GALAXIES I Astronomers Take M87's Pulse

The elliptical galaxy M87 dominates the center of the Virgo galaxy cluster. Astronomers have
now detected the "heartbeats" of old, red supergiant stars within M87.

Astronomers have measured the shimmer in a galaxy's light as individual, old
stars in the galaxy brighten and fade.
To detect the shimmer, Charlie Conroy (Harvard University) and colleagues
took a careful look at the giant elliptical
galaxy M87, which is fi lled with old stars.
First, the team compiled information
about the masses, luminosities, and such
for stars of different ages, focusing especially on asymptotic giant branch (AGB)
stars. These are worn-out, red supergiant stars. They have unstable interiors

and brighten and fade over the course of
several hundred days.
Second, the researchers combined
those data with how different stars'
brightnesses would vary. Third, given the
population of stars expected to exist in
M87, and given how those stars' pulsations would add together in the light we
see, they predicted how the brightness
of individual pixels in an image of the
galaxy would change over time.
The astronomers then looked at a
series of more than 50 images of M87

from the Hubble Space Telescope and
sought out these variations. In a quarter of the pixels, the team found that
the brightness wavered with an average
"beat" of 270 days. Although the detected
fluctuations are small - a mere 0.1% to
1% of a pixel's brightness - they do seem
to be real, the team reports in the November 26th Nature.
"This paper is really very interesting," says Claudia Maraston (University
of Portsmouth, UK), who specializes in
AGB stars and uses them to understand
galaxies. She adds that she would like
to see this technique applied to other,
younger galaxies with larger populations
of these variables.
For even though M87's stars are old,
they're really too old to shimmer strongly.
The AGB phase is a short one, only lasting a million years. The stars pulse more
brightly the more massive they are, and
since bigger ones evolve more quickly,
old AGB stars (say, 10 billion years old,
like those in M87) are going to be much
fainter than young (roughly 1 billion-yearold) ones, Maraston explains. So the pulsations should show up better in younger,
star-forming galaxies. They should also
be stronger in certain zones of a galaxy,
such that we would see spiral disks
shimmering more than galactic bulges or
halos, which mostly have old stars.
■ CAMILLE M. CARLISLE

MILKY WAY I Young Stars in an Old Bulge
A disk of young stars in the Milky Way
Galaxy's central regions confirms our
galaxy's nonviolent past.
At the center of our galaxy's spiral disk
sits a Twinkie-shaped bulge of old stars.
And like a Twinkie, it turns out, the bulge
contains a "cream filling": a thin plane of
very young stars that cuts right through
the bulge and lines up with the galaxy's
disk. István Dékány (Pontifical Catholic University of Chile) and colleagues
found the plane when they discovered 35
Cepheid variable stars, each no older than
100 million years, as part of the VISTA
Variables in Vía Láctea project.
12

March 2016 sky & telescope

"We think that this is essentially a
smooth inner continuation of the thin
disk that has been known to surround
the inner galaxy," Dékány says. The team
will report the discovery in Astrophysical
Journal Letters.
The research shows that, deep within
the aging bulge, some process is forming new stars. That's unsurprising, says
Melissa Ness (Max Planck Institute for
Astronomy, Germany), if the bulge isn't
as separate a component as was once
thought. Simulations suggest that our
galaxy started out as a smooth disk, then
formed its bulge as instabilities wracked

it and threw stars onto skewed orbits. If
that's the case, the bulge got its feedstock
from the disk. "That the stars in the bulge
are mostly old just reflects that [they] are
stars of the disk that formed a long time
ago," says Ness. "But this certainly does
not prohibit new star formation."
Paola Di Matteo (Paris Observatory)
adds that the discovery adds to evidence
that mergers played little role in shaping
the Milky Way's bulge: "This is another
strong indication of the fact that our bulge
has been mainly, if not entirely, formed
via disk evolution and instabilities."
■ MONICA YOUNG



Sky and Telescope - March 2016

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Sky and Telescope - March 2016 - Cover1
Sky and Telescope - March 2016 - Cover2
Sky and Telescope - March 2016 - 1
Sky and Telescope - March 2016 - Contents
Sky and Telescope - March 2016 - 3
Sky and Telescope - March 2016 - A
Sky and Telescope - March 2016 - B
Sky and Telescope - March 2016 - 4
Sky and Telescope - March 2016 - 5
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