Sky and Telescope - July 2015 - 16

News Notes

Astronomers are using a famous class
of pulsating star to map out hidden
reaches of the Milky Way.
Classical Cepheid variables are aging,
massive stars trying to avoid gravitational
collapse. Having run out of hydrogen to
fuse in their cores, they struggle to burn
shells of helium instead, puﬃng up and
deﬂating at a rate directly tied to their
intrinsic luminosity.
In a study last year, Michael Feast
(University of Cape Town, South Africa)
and colleagues found ﬁve "classical"
Cepheids in the outer reaches of the
Milky Way's disk, between 42,000 and
72,000 light-years from the galaxy's center. Since classical Cepheids are massive
and young, they typically appear in spiral
arms. Feast's team suggested that these
Cepheids lie in an outer arm of molecular
gas, which would have to ﬂare to account
for the Cepheids' spread above and below
the galactic plane.
Two recent studies build oﬀ these
results. In the ﬁrst, István Dékány (Millennium Institute of Astrophysics, Chile)
and colleagues announced the discovery
of two Cepheids 37,000 light-years from
Earth and 11,000 light-years from our galaxy's center. The pair is remarkably tight,
separated by only 3 light-years, and both

are between 45 million and 51 million
years old, the team reports in the January
20th Astrophysical Journal Letters.
These Cepheids were likely born in
the same (as-yet unseen) star cluster in
the "Far 3-kiloparsec Arm," a spiral arm
thought to encircle the far side of the
galaxy's star-packed bulge. The team will
need deeper observations to conﬁrm.
Another study appearing in Astrophysical Journal Letters reports the detection
of four other Cepheids in data from the
VISTA Variables in Vía Láctea (VVV)
survey, being undertaken at Paranal
Observatory in Chile.
But the pulsing stars found by Sukanya
Chakrabarti (Rochester Institute of
Technology) and colleagues appear to be
almost 300,000 light-years from the Sun
- far beyond the Milky Way's disk, which
is roughly 100,000 light-years across.
Six years ago, Chakrabarti predicted
the existence of a dwarf galaxy at this
location, to explain why the outer part
of the Milky Way's gas disk looks perturbed. She and Leo Blitz (University of
California, Berkeley) ran simulations that
showed a dwarf 1/100 the Milky Way's mass
could have passed through our galaxy and
created ripples. The dwarf would have
escaped detection until now because it's

NASA / JPL-CALTECH

MILKY WAY I Cepheids Map Galaxy & Beyond

Three studies detected Cepheids in and around
the Milky Way: Dékány et al. (pink), Chakrabarti et al. (arrow, since they lie far oﬀ the galactic disk), and Feast et al. (green). Distances are
measured from Earth (below bulge, not shown).

dim and lies right behind the galactic
plane from our perspective.
It's still possible that the variables
Chakrabarti's team detected are a dimmer, rarer type of Cepheid. If so, they're
only 160,000 light-years away and don't
match the dwarf galaxy's predicted
location. Spectroscopy would cinch the
matter, but it will be a challenge for these
faint stars. If the team can get radial
velocity observations, it can see whether
the clump of Cepheids moves together
and in the way predicted for the dwarf's
passage through the Milky Way.
■ MONICA YOUNG

IN BRIEF
Milky Way Has Ripples. Astronomers have
detected four undulations in our galaxy's
disk. Previous observations had revealed
clumps in the density of stars at the disk's
outermost edge, part of two structures now
called the Monoceros Ring and, beyond it,
the Triangulum Andromeda Stream. But
a second look at Sloan Digital Sky Survey
(SDSS) observations suggests that the disk
actually has four ripples, with two more lying
between us and the Monoceros Ring. The
closest is about 6,000 light-years out from the
Sun, with each subsequent structure lying
roughly 6,000 light-years beyond the previous
one. The lumps vary between being above
and below the disk, like the ripples in cor-

July 2015 sky & telescope

rugated cardboard. There also appears to be
a vertical ripple in the disk, Yan Xu (National
Astronomical Observatories of China) and
colleagues report in the March 10th Astrophysical Journal. The two types of ripples
look like the waves a dwarf galaxy would create if it passed through the Milky Way's disk,
according to computer simulations by various
teams. More info at http://is.gd/mwripples.
■ CAMILLE M. CARLISLE

Stars Have Out-of-plane Experience. The
Milky Way Galaxy is a thin spiral, with 85% of
its stars in a disk only 3,000 light-years thick.
A fatter, sparser disk of older stars extends
up to 16,000 light-years above and below the

galactic plane. Young stars generally only
occupy the thin disk. But two newly discovered clusters of stars, still embedded in their
natal clouds of dust and gas, are ﬂoating
16,000 light-years above the pancake-shaped
disk, Denilso Camargo (Military School of
Porto Alegre, Brazil) and colleagues report
in the April 1st Monthly Notices of the Royal
Astronomical Society. The star clusters
themselves are only 2 million years old. The
cloud containing them crossed the thin disk
between 45 and 50 million years ago, an
event that likely caused the clouds to condense and form stars. The clusters will cross
the disk again in another 50 million years.
■ MONICA YOUNG

http://www.is.gd/mwripples

Sky and Telescope - July 2015

Table of Contents for the Digital Edition of Sky and Telescope - July 2015