Sky and Telescope - June 2016 - 11

MILKY WAY I Giant Cloud's Gas Native to Our Galaxy
distant active galactic nuclei (AGN) that lie
behind the cloud from our perspective.
With Hubble, the team observed the AGN
through the cloud to see how well the
gas absorbed certain wavelengths from
the accreting black holes' light. Which
wavelengths the gas absorbs, and how
deeply, reveals how tainted the cloud is
by elements heavier than helium, known

NASA / ESA

A massive cloud falling toward our
galaxy's disk is likely from the Milky Way
itself, not a visitor, a new study shows.
This object, called the Smith Cloud, lies
40,000 light-years away and contains
2 million Suns' worth of gas, primarily
hydrogen. It's falling toward the galactic
disk at nearly 300 km/s (660,000 mph).
The Smith Cloud is one of the best
characterized high-velocity clouds, speedy
wisps that rain down as a light drizzle onto
our galaxy. This shower helps fuel star formation (S&T: Sept. 2015, p. 16). But despite
these clouds' importance, astronomers
know a paltry amount about what they're
made of and where they come from.
Andrew Fox (Space Telescope Science Institute) and colleagues used the
Hubble Space Telescope and the 91-meter
(300-foot) Green Bank Telescope to better
understand the Smith Cloud's makeup.
The astronomers took advantage of three

This artist's conception shows the cometlike Smith Cloud (red) and its current trajectory with respect to the Milky Way's disk. If
visible, the cloud would span 30 full Moons.

as metals in astrospeak. The researchers found that the Smith Cloud contains
about half the concentration of metals
that the Sun has.
But metals come from stars - they're
synthesized in stellar cores or created
when stars die - and astronomers don't
know of a single star in the Smith Cloud.
So it's highly unlikely that this object is
a chewed-up dwarf galaxy or a pristine,
ﬁrst-time visitor from intergalactic space.
However, the heavy-element level is
a close match for the gas in the Milky
Way's own outer disk. The team argues in
the January 1st Astrophysical Journal Letters that the cloud's gas was either spewed
or torn out of our galaxy. And given the
cloud's orbit, it either passed through or
came from the outer disk about 70 million years ago. So the Milky Way might be
feeding on itself.
■ CAMILLE M. CARLISLE

IN BRIEF
Booster Impact Site Found on Moon. In
December, researchers poring over Lunar
Reconnaissance Orbiter images identiﬁed
the impact site of the Apollo 16 S-IVB stage
booster. The booster's impact left a small
crater in Mare Insularum 40 by 30 meters
(130 by 100 ft) wide, festooned with fresh,
brilliant rays, 260 km (160 miles) southwest
of the prominent crater Copernicus. It had
lain unidentiﬁed since the booster crashed
into the lunar surface in 1972. The discovery
completes the search for the impact sites of
Apollo-era rocket boosters. Read more and
ﬁnd out how to scour the lunar surface yourself at http://is.gd/apollo16booster.

ﬁrst successful measurement of a superEarth's atmospheric composition. The team
found tantalizing hints of hydrogen cyanide
(HCN). This molecule would only dominate
in a carbon-rich environment, instead of one
based on oxygen-rich silicates as on Earth.
That supports researchers' suspicion that
this dense world, nicknamed the "diamond
planet," contains crystallized carbon in its
interior. The team also found no trace of water
vapor, which would form easily if oxygen were
widespread on this planet, the astronomers
report in an upcoming Astrophysical Journal.

■ DAVID DICKINSON

AAS Adopts WorldWide Telescope. The
largest national association of astronomers is
now the new home of the virtual observatory
known as the WorldWide Telescope. WorldWide Telescope is an online portal that theoretically allows access to any observation ever
taken, at any wavelength, of a given object or
section of sky (S&T: Apr. 2015, p. 28). By taking charge, the American Astronomical Soci-

Hubble Sheds Light on "Diamond
Planet." New observations are helping
characterize the atmosphere of an exotic
exoplanet. Angelos Tsiaras (University College London) and his team took an infrared
spectrum of the super-Earth 55 Cancri e (now
called Janssen, S&T: Apr. 2016, p. 14). It's the

■ MONICA YOUNG

ety intends to use the portal to make research
papers more interactive, with video abstracts
and the ability to zoom in or out on and superimpose observations. Team members also
hope that, as publishing drives development,
the WWT's outreach programs will continue
to grow with schools and planetariums.
■ MONICA YOUNG

Mirror Done for Hubble's Successor.
NASA has assembled the primary mirror for
the James Webb Space Telescope (JWST),
a big step on the way to the telescope's
October 2018 launch. The mirror comprises
18 hexagonal segments, each 1.3 m (4.2 ft)
across and coated with gold and beryllium.
Each weighs 40 kg (88 pounds). Fully assembled, the primary mirror is 6.5 m in diameter,
making JWST the largest telescope ever to be
ﬁelded in space. The telescope will observe
from the long-wavelength end of the visual
spectrum into the infrared regime, peering
farther back in cosmic time than Hubble can.
■ DAVID DICKINSON

http://www.is.gd/apollo16booster

Sky and Telescope - June 2016

Table of Contents for the Digital Edition of Sky and Telescope - June 2016