Astro 101 Slide Set:
Binary System Makes
Near Miss
• Developed by the WISE team
Topic:
Close pass of binary system
Concepts:
Solar neighborhood, red dwarfs, brown
dwarfs, cosmic scale, gravitational
interactions.
Missions:
WISE
Coordinated by
the NASA Astrophysics Forum
An Instructor’s Guide for using
the slide sets is available at the
ASP website
https://www.astrosociety.org/e
ducation/resources-for-the-0
higher-education-audience/
The Discovery
The diagram shows the trajectory of WISE
0720-0846 past the sun 70,000 years ago,
as well as the distances of the four nearest
stellar neighbors to the sun (and the years
their distances were determined): the triple
system Alpha Centauri/Proxima Centauri,
Barnard’s Star, and newly discovered
brown dwarfs. (The Oort Cloud boundary
is the outer edge of the gigantic reservoir of
icy leftovers surrounding the sun from
which long-period comets originate. The
distances are to scale, but the sizes are
exaggerated.) Credit: Penn State
University and P. Eisenhardt.
A small binary system called WISE 0720-0846, discovered in 2013 in the
data of the Wide Field Infrared Survey Explorer (WISE), has turned out to
be a daredevil. The system, consisting of a red dwarf star and its brown
dwarf companion, buzzed the sun 70,000 years ago, passing just 0.8 light
years away and dipping into the outer Oort cloud, where icy leftovers from
the solar system’s formation lurk. It is the sun’s closest known stellar
encounter.
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How was the Discovery Made?
Artist’s conception of WISE 0720-0846—also called Scholz’s
Star after its discoverer, Ralf Dieter-Scholz, using WISE
data—during its flyby of the solar system 70,000 years ago.
The brown dwarf—a body too small to fuse hydrogen like a
regular star—is in the foreground, the red dwarf star beyond.
The sun appears as a bright star in the left background.
Credit: Michael Osadciw/University of Rochester.
After the binary was discovered in the WISE data, scientists studying it
noted that although it was relatively close—20 light years away—it showed
very little “tangential” motion—motion across our line of sight. Most of its
motion was “radial”—along our line of sight, and it was moving away.
By extrapolating backward the binary’s trajectory and velocity, they were
able to determine that it had made a close pass of the sun—the closest yet
known.
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The Big Picture
Red and brown dwarfs are so small and dim that they cannot be
detected at large distances. But scientists expect these “small fry” to
be the most common kinds of stars.
WISE’s all-sky infrared survey allows the nearest of these to the sun to
be discovered. Assuming the sun’s neighborhood is typical, this allows
scientists to better characterize these smallest members of our galaxy’s
stellar population.
And if these stars are among the most
common kinds of stars, they are the
most likely to pass near the sun.
Finding all of the nearby stars gives us
a benchmark on how common brown
dwarfs really are, and the longer term
implications for the dynamics of close
encounters with the solar system and
the Earth.
Artist’s conception of a brown dwarf (foreground, with
hypothetical moon.) Credit: NASA/JPL.
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What are the Implications?
Space is so vast that the odds of one star actually colliding with another are
very small. But close encounters can produce gravitational effects. If a
passing star is massive enough and the encounter lasts long enough, its
gravitational pull could dislodge Oort Cloud objects from their positions,
sending some of them toward the sun in a “rain” of comets, risking collisions
with the planets including the Earth. It could take millions of years for these
comets to reach the inner solar system, long after the star has passed.
Scientists calculate that the
WISE 0720-0846 binary
wasn’t massive enough and
the encounter was too brief to
cause such a comet rain—but
we may not know for certain
for two million years!
Artist’s conception of a comet storm. Credit: NASA/JPL-Caltech.
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Resources
Papers:
http://arxiv.org/abs/1502.04655 (Mamajek et al analysis paper)
http://adsabs.harvard.edu/abs/2015ApJ...800L..17M
http://adsabs.harvard.edu/abs/2014A%26A...561A.113S and
http://arXiv:1311.2716 (Sholz discovery paper)
http://adsabs.harvard.edu/abs/2015AJ....149..104B and
http://arXiv:1410.4288 (Burgasser et al measurements paper)
News Stories:
http://www.rochester.edu/newscenter/scholz-star/
http://www.upi.com/Science_News/2015/02/18/Neighboring-star-oncecame-within-a-single-light-year-of-the-sun/3581424273371/
http://www.zmescience.com/space/observations/star-flyby-red-dwarf18022015/
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Binary System Makes Near Miss
BONUS CONTENT
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When Stars Do Collide . . .
The most likely scenario for stellar collisions occurs in binary systems—systems
of two stars gravitationally bound to each other and which are already relatively
close to each other.
For example, astronomers at Keele
University are studying a red giant star
that grew so large it collided with its
companion, lost up to 90% of its mass,
and became a new kind of pulsating
star.
Simulated view of a neutron star collision. Credit:
NASA Goddard.
NASA Goddard astronomers have
simulated the collision of two neutron
stars, which end up ripping each other
apart and forming a black hole.
The Swift spacecraft has recorded the high-energy blasts created when neutron
stars collide, and follow-up studies with Hubble and other telescopes suggest that
in such blasts matter reaches conditions extreme enough for the heaviest
elements to synthesize--such as mercury, lead, and gold!
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