Sky & Telescope - December 2019 - 14

NEWS NOTES
MILKY WAY

Untangling Stellar Strings
ASTRONOMERS HAVE IDENTIFIED

thousands of new groupings of stars
within 3,000 light-years of the Sun, and
hundreds of them appear to be surprisingly string-like in shape. These stellar
threads may trace the evolution of the
Milky Ways' spiral arms, Marina Kounkel (Western Washington University)
and colleagues explain in the September
issue of the Astronomical Journal.
Kounkel and colleagues started with
almost 20 million stars for which the
European Space Agency's Gaia satellite
had measured distances, positions on
the sky, and velocities toward or away
from Earth. Using a type of machine
learning (S&T: Dec. 2017, p. 20), the
researchers sorted through the immense
amount of data to determine whether
and how stars are clustered.
While less than 1.5% of the 20 million stars belong to a group, of those
stars that do move through space
together, roughly half belong to long,
string-shaped groups. The other half
divide themselves among more than
1,300 smaller, more spherical groups.

The stellar strings are on average 30
light-years across and 650 light-years
long, which falls into the typical size
and shape range of star-forming clouds.
Kounkel says it's likely that a single
cloud would make a single string.
The astronomers determined the
strings' ages to explore their evolution,
ﬁnding that the structures are surprisingly long-lived. "We generally thought
young stars would leave their birth
sites just a few million years after they
form," Kounkel explains. "But it seems
that stars can stay close to their siblings
for as long as a few billion years."
At the same time, the strings appear
to trace something ephemeral: The
Milky Way's spiral arms. The youngest
strings - those younger than 100 million years old - together form a narrow
structure that follows along the Local
Arm, the spiral arm that houses the
Sun (S&T: Nov. 2019, p. 16). The strings
themselves lie perpendicular to the arm.
Older strings appear to align in ways
that don't relate to the Local Arm, but
the team argues they might mark other
spiral arms that have since dissipated.
Coauthor Kevin Covey (also at Western
Washington University) notes, "Older

Sun
To
galactic
center

1,600 light-years

p As a group the youngest strings (less than
100 million years old, shown here) follow the
Local Arm of the Milky Way, which runs along
the vertical line that crosses this plot. However,
individual strings line up perpendicular to this
arm. The Sun is located at the center.

strings are an important 'fossil record'
of our galaxy's spiral structure."
■ MONICA YOUNG

* Explore an interactive 3D version of
the map at https://is.gd/stellarstrings.

p This image comes from the Yutu 2 rover as it looked back over
its tracks across the lunar surface. As it drives, instruments aboard
Yutu 2 investigate the regolith for clues to the Moon's past.

D EC E M B E R 2 019 * SK Y & TELESCOPE

China's Chang'e 4 mission has completed its ninth lunar day on
the farside of the Moon, during which the Yutu 2 rover has driven
284.7 meters (933.9 feet). The mission is due to enter its 10th lunar
day exploring Von Kármán Crater in the South Pole-Aitken impact
basin on September 22nd. First results came in the May 16th Nature,
in which mission scientists reported in situ observations of materials that might have originated from below the Moon's crust. Additional results are forthcoming from radar measurements of the lunar
subsurface. Meanwhile, the Queqiao relay satellite, which facilitates
communications between Chang'e 4 and Earth, will soon be starting
a science experiment of its own. It carries the Netherlands-China Low
Frequency Explorer (NCLE), a pathfinder experiment for low-frequency
radio astronomy. In the coming months, NCLE will begin observations
between 80 kHz and 80 MHz. (Frequencies below 30 MHz are only
accessible from space.) Finally, a tiny, 47-kilogram (100-lb) satellite
that launched with Queqiao ended its mission on July 31st by crashing into the Moon. Longjiang 2 lasted more than a year as a technology verification and amateur radio and imaging satellite.
■ ANDREW JONES

* See more images at https://is.gd/change4.

STELL A R STRINGS: M. KOUNK EL / ASTR ONOMICAL JOUR NAL
2019; M OON VIE W: CLEP / CNSA

On the Farside: Chang'e 4 Update

https://www.is.gd/stellarstrings https://www.is.gd/change4

Sky & Telescope - December 2019

Table of Contents for the Digital Edition of Sky & Telescope - December 2019