This image displays
a bow shock around the very young star, LL Ori. It is located in the star-forming
region called the Great Nebula in the constellation Orion. It illustrates a
phenomenon also present around planet Earth, a phenomenon recently discovered
to possess properties previously only imagined in theory.
A bow shock
is created in space when two streams of gas collide. LL Ori emits a stellar wind,
a stream of charged particles. This stellar wind collides with gas evaporating
away from the center of the Orion
Nebula, to the lower left in this image. The surface where the two winds
collide is the crescent-shaped bow shock seen in the image. A second, fainter
bow shock can also be seen around a star near the upper left-hand corner of the
image. A bow shock may be thought of as similar to the way water is pushed out
of the way by the forward movement of a ship.
A bow shock
also occurs above the Earth, where the Sun's stellar wind contacts the Earth's magnetosphere
(the magnetopause). ESA's
Cluster spacecraft, approaching the Earth's bow shock, discovered in 2001 a
shock wave that kept breaking and reforming – predicted only in theory.
On 24
January 2001, the four Cluster spacecraft were flying at an approximate
altitude of 105,000 kilometers, in tetrahedron formation, separated
by about 600 kilometers. With this spacing, scientists expected that every
spacecraft would record a similar signature of the passage through this region.
Instead, the readings they got were highly contradictory.
They showed large fluctuations in the magnetic and electric field surrounding
each spacecraft. They also revealed marked variations in the number of solar
wind protons that were reflected by the shock and streaming back to the Sun.
Vladimir Krasnoselskikh, of the Centre National de la
Recherche Scientifique, Orléans, France, had predicted the shock reformation
model theoretically in 1985. It is a little similar to the way waves in the
ocean build up and then break onto the shore, only to reform again, some way
out to sea.
The
detection will affect the way astronomers investigate larger bow shocks around
distant celestial objects. Bow shocks are related to some of the most energetic
events in the universe. Exploding stars and strong stellar winds from young
stars both produce them. Reforming bow shocks can also accelerate particles to
extremely high energies and throw them across space.
Cluster provided the first opportunity ever to observe such
an event, the details of which have been published in a paper on March 9 of this
year.
--ESA and SPACE.com Staff
Credit: NASA/ESA and The Hubble Heritage Team
STScI/AURA
