Sky & Telescope - October 2020 - 11

PRO-AM

Huge Circular Arc Spotted
near the Big Dipper
A 30-DEGREE-LONG ARC lies splashed
just outside the bowl of the Big Dipper,
likely a pristine shock front expanding
from a star that exploded some 100,000
years ago.
The thin, circular, and extremely
faint trace of hydrogen gas spans a third
of the northern sky, a record holder for
sheer size. But it can't be seen visually;
ultraviolet and narrowband photography captured the arc, as study coauthor
Robert Benjamin (University of Wisconsin, Whitewater) reported in June
at the virtual meeting of the American
Astronomical Society.
Andrea Bracco (Ru¯er Boškovi© Institute, Croatia) and colleagues happened
upon the Ursa Major Arc when poring
over the ultraviolet images archived
by NASA's Galaxy Evolution Explorer
satellite. The researchers were looking
for signs of a straight, 2.5° ﬁlament
that had been observed two decades ago
- but they found out that that length
of gas was less straight than they had
thought, forming instead a small piece
of a much larger whole.
The team found unexpected conﬁrmation of the feature in a hydrogen-

The circular ﬁlament as seen in
the MDW Sky Survey. See more
stunning images of the H-alpha
sky at mdwskysurvey.org.

alpha survey conducted by amateur
astronomers David Mittelman (who
died in 2017) and Sky & Telescope's own
Dennis di Cicco and Sean Walker. With
the MDW Sky Survey, the amateurs are
imaging the whole sky at 656.3 nanometers, the wavelength of light emitted
by ionized hydrogen. In the process,
they captured a circular arc that lines
up "pretty much perfectly" with the
ultraviolet data, Benjamin says.
Benjamin describes the curved
feature as a thin, rippling shock front
typical of supernovae, expanding away
from a collapsed star at its center. Based
on the gas's interaction with its surroundings, Benjamin and colleagues
estimate that the arc is roughly 600

light-years away. The researchers report
observations of the arc in the April issue
of Astronomy & Astrophysics.
Interestingly, there are two wellknown "holes" in interstellar material,
termed the Lockman Hole and the
Extended Groth Strip, that appear to
lie within the bubble. These windows
have given astronomers exceptionally unobscured views of intergalactic
space. Given that the arc is so perfectly
shaped, it's likely that some other explosive event had already cleared out the
area when this supernova shock front
passed through it, Benjamin says. Nevertheless, the explosion may have played
a role in keeping these windows open.
■ MONICA YOUNG

A RC OF G AS: DAVID MIT TELM A N, DENNIS DI CICCO
& SE A N WA LK ER; V ENUS PASSAG E: A LFRED DE
WIJN & GUN THER KÖNNEN

Venus Transits the Solar Corona
Sixteen years after Venus passed in front of the Sun in 2004,
and eight years after its 2012 transit, Venus again passed near
the Sun on the sky. This time, while Venus did not pass directly
across the Sun's disk from our point of view, it did pass over the
solar corona, coming within 13 arcminutes of the disk on June 3rd
at 18:48 UT (2:48 p.m. EDT). Alfred de Wijn (National Center for
Atmospheric Research) and Gunther Können (Royal Netherlands
Meteorological Institute) recorded Venus's passage in front of the
corona using the COSMO K-coronagraph (K-Cor) instrument at
the Mauna Loa Solar Observatory in Hawai'i. K-Cor blocks the
Sun's light to observe the corona, then further separates the corona's polarized light from the much more intense, but only weakly
polarized, sky background. The brightness of the corona drops
rapidly with distance from the solar disk, so the image here is
scaled with a radially graded filter to improve the contrast. Venus
won't pass so close to the Sun again until December 2109, eight
years before the next transit of Venus in 2117.
■ ALFRED DE WIJN & GUNTHER KÖNNEN

View the video at https://is.gd/VenusCorona.
sk yandtelescope.org * OCTOBER 2 02 0

http://www.mdwskysurvey.org https://www.is.gd/VenusCorona http://www.skyandtelescope.org

Sky & Telescope - October 2020

Table of Contents for the Digital Edition of Sky & Telescope - October 2020