Minerals
55 %Organic
material
45 %30 % 30 % 5.5% 1.6%MaS distribution of 67PChemical composition of 67P
30 % 0.6%Hydrogen MagnesiumSilicon2.4%CarbonOxygen Iron OtherASTRONEWS
WWW.ASTRONOMY.COM 13A Cepheid variable star pulsates over
hours or days. As its physical size
changes, its temperature, surface
gravity, and brightness change as
well. In the early 1900s, Henrietta
Leavitt spotted a relationship
between a Cepheid’s brightness and
the time period over which it varies.
This relationship, called the Leavitt
Law, allows astronomers to calculate
the star’s intrinsic brightness and
determine the distance to the star.
Cepheids remain vital distance indi-
cators and tracers of galactic evolu-
tion, and it’s important to ensure these
stars are mapped and measured accu-
rately in large surveys, where a single
image could catch the star at any ran-
dom point during its pulsation period.Recent work to improve Cepheid vari-
able measurements was presented in
January at the 231st Meeting of the
American Astronomical Society in
Washington, D.C.
Kate Hartman, an undergraduate
from Pomona College working with
Rachael Beaton, the NASA Hubble
and Carnegie-Princeton postdoctoral
fellow at Princeton University, exam-
ined Cepheid variables in the Sloan
Digital Sky Survey’s Apache Point
Galactic Evolution Experiment
(APOGEE) catalog to determine
whether APOGEE produces reliable
information about these unique stars.
Hartman studied several images of
the same variable taken over time to
measure the elements in the star.“When we looked at a star’s spec-
trum across its entire pulsation cycle,
we found no significant differences
in the results. ... We’re getting reli-
able results every time we look,” she
said in a press release.
That means APOGEE accurately
measures the chemical composition
of Cepheid variables, regardless of
where or when they’re seen. Now
the catalog’s data can be confidently
paired with other up-and-coming
surveys, such as Gaia, which mea-
sures distances via parallax, to fur-
ther test and calibrate the Leavitt
Law so astronomers can reliably use
Cepheid variables of all compositions
to map and study our galaxy and
others nearby. — A.K.QUICK TAKES
STELLAR STROLL
Walking Among the Stars is a
virtual reality project that lets
users explore the Cassiopeia A
supernova remnant in 3-D.- LIFE LAG
Research suggests the presence
of life can change a world’s
atmosphere enough to alter
how quickly the planet spins.
TABBY’S BELT
A circumstellar disk is the most
likely explanation for the
periodic dimming of Tabby’s Star
— not an alien megastructure.
PULSAR PILOTING
Researchers proved that
spacecraft can navigate
autonomously with a GPS-like
system that tracks narrow
beams of radiation from pulsars.
SPACE SWEATS
Prolonged periods of
weightlessness cause persistent
fevers in astronauts and
significantly increase body
temperatures during exercise.
ASTROVIROLOGY
A January paper in Astrobiology
calls on astronomers to broaden
their search for life in the
cosmos by also combing space
for viruses.
FIRST LIGHT
The Keck Observatory’s newest
instrument, the Near-Infrared
Echellette Spectrometer, saw
first light with an image of the
planetary nebula NGC 7027.
CLAPPED IN IRON
Kepler has revealed that iron-
rich stars are more likely to host
shorter-period planets, while
iron-poor stars tend to have
longer-period planets.
CASTING CALL
The first six hexagonal segments
of the Extremely Large
Telescope’s 130-foot (39 meters)
main mirror have been
successfully cast.
UPPER LIMIT
A Johns Hopkins University
astrophysicist proposed a new
upper boundary for the mass of
a planet: between 4 and
10 Jupiter masses.
NOWHERE TO HIDE
NASA astronaut Peggy Whitson
recently became the first person
to collect unknown organisms
and sequence their DNA while
in space. — J.P.
MARTIAN METHANE. New measurements taken by the Curiosity rover indicate that atmospheric methane — a sign
of life on Earth — changes with the seasons on Mars.New surveys help calibrate a century-old law
A SINGLE BEACON.
A Cepheid variable
identified by Edwin
Hubble in the
Andromeda Galaxy,
designated V1, allowed
him to measure the
galaxy’s distance,
proving that it was not
a nebula inside the
Milky Way. Cepheid
variables change
brightness predictably
with time via a
relationship called
the Leavitt Law. NASA, ESA,
AND THE HUBBLE HERITAGE TEAM
(STSCI/AURA)ASTRONOMY: ROEN KELLYWHAT’S IN A COMET? FAST FAC T
The comet 46P/
Wirtanen was
Rosetta’s original
target, but a
delay in launch
date led the team
to select 67P as
the spacecraft’s
destination.BUILDING BLOCKS. Recent
results from the Rosetta mission’s
COSIMA instrument team have
highlighted the “ingredients”
present in Comet 67P/Churyumov-
Gerasimenko, based on analysis of
dust grains emitted as the comet
neared the Sun. Researchers studied a
small subset of 30 grains (out of over
35,000 collected) that ranged from
0.01–1 millimeter in size, and found
they all have a similar composition,
revealing the makeup of the comet.
About 45 percent of 67P is organic, making it one of the most
carbon-rich bodies currently known in our solar system. The
comet’s minerals are non-hydrated, meaning they lack water
compounds. While the comet does of course include water,
scientists believe the fact that it is frozen as ice prevented the
water from mixing with the minerals.
67P’s material is considered “pristine,” left over from the
formation of our solar system. Studying the composition of
67P and other comets can help astronomers better piece
together the history of the solar system and of Earth, as comets
are thought to have delivered a significant percentage of the
materials that compose the planet — and paved the way for
life to form. — A.K.December 17, 2010December 21, 2010December 30, 2010January 26, 2011Cepheid variable star V1 in M