The Herschel Space Observatory has uncovered
a weird ring of dusty material while obtaining one of the sharpest
scans to date of a huge cloud of gas and dust, called NGC 7538. Image
credit: ESA/NASA/JPL-Caltech/Whitman College. ›Full image and caption
The Herschel Space Observatory has uncovered a weird ring of dusty
material while obtaining one of the sharpest scans to date of a huge
cloud of gas and dust, called NGC 7538. The observations have revealed
numerous clumps of material, a baker's dozen of which may evolve into
the most powerful kinds of stars in the universe. Herschel is a European
Space Agency mission with important NASA contributions.
"We have looked at NGC 7538 with Herschel and identified 13 massive,
dense clumps where colossal stars could form in the future," said paper
lead author Cassandra Fallscheer, a visiting assistant professor of
astronomy at Whitman College in Walla Walla, Washington, and lead author
of the paper published in The Astrophysical Journal. "In addition, we
have found a gigantic ring structure and the weird thing is, we're not
at all sure what created it."
NGC 7538 is relatively nearby, at a distance of about 8,800
light-years and located in the constellation Cepheus. The cloud, which
has a mass on the order of 400,000 suns, is undergoing an intense bout
of star formation. Astronomers study stellar nurseries such as NGC 7538
to better learn how stars come into being. Finding the mysterious ring,
in this case, came as an unexpected bonus.
The cool, dusty ring has an oval shape, with its long axis spanning
about 35 light-years and its short axis about 25 light-years. Fallscheer
and her colleagues estimate that the ring possesses the mass of 500
suns. Additional data from the James Clerk Maxwell Telescope, located at
the Mauna Kea Observatory in Hawaii, further helped characterize the
odd ovoid. Astronomers often see ring and bubble-like structures in
cosmic dust clouds. The strong winds cast out by the most massive stars,
called O-type stars, can generate these expanding puffs, as can their
explosive deaths as supernovas.
But no energetic source or remnant of a
deceased O-type star, such as a neutron star, is apparent within the
center of this ring. It is possible that a big star blew the bubble and,
because stars are all in motion, subsequently left the scene, escaping
detection.
The observations were taken as part of the Herschel OB Young Stellar
objects (HOBYS) Key Programme. The "OB" refers to the two most massive
kinds of stars, O-type and B-type. These bright blue, superhot,
short-lived stars end up exploding as supernovas, leaving behind either
incredibly dense neutron stars or even denser black holes.
Stars of this caliber form from gassy, dusty clumps with initial
masses dozens of times greater than the sun's; the 13 clumps spotted in
NGC 7358, some of which lie along the edge of the mystery ring, all are
more than 40 times more massive than the sun. The clumps gravitationally
collapse in on themselves, growing denser and hotter in their cores
until nuclear fusion ignites and a star is born. For now, early in the
star-formation process, the clumps remain quite cold, just a few tens of
degrees above absolute zero. At these temperatures, the clumps emit the
bulk of their radiation in the low-energy, submillimeter and infrared
light that Herschel was specifically designed to detect.
As astronomers continue probing these budding O-type giants in NGC
7358, the follow-up observations with other telescopes should also help
in solving the puzzle of the humongous, dusty ring. "Further research to
determine the mechanism responsible for creating the ring structure is
necessary," said Fallscheer.
Herschel is a European Space Agency mission, with science instruments
provided by consortia of European institutes and with important
participation by NASA. While the observatory stopped making science
observations in April 2013, after running out of liquid coolant as
expected, scientists continue to analyze its data. NASA's Herschel
Project Office is based at the Jet Propulsion Laboratory in Pasadena,
California. JPL contributed mission-enabling technology for two of
Herschel's three science instruments. The NASA Herschel Science Center,
part of the Infrared Processing and Analysis Center at the California
Institute of Technology in Pasadena, supports the U.S. astronomical
community.
Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov
