Douglas Isbell
Headquarters, Washington, DC             February 14, 1997
(Phone: 202/358-1753)

Public Affairs Office
Naval Research Laboratory, Washington, DC
(Phone: 202/767-2541)

RELEASE:  97-28

THOUGHT-PROVOKING IMAGERY SHOWS SUN'S PLACE IN THE GALAXY

     An action-packed movie assembled from images taken by an 
instrument aboard the NASA-European Space Agency Solar 
Heliospheric Observatory (SOHO) has provided a remarkable 
galactic perspective on the Sun and its place in the Milky Way.

     Taken during Dec. 22-27, 1996, the series of images show 
the Sun drifting in front of the stars of the constellation 
Sagittarius, as the constant solar wind blows outward in all 
directions.  Soon, a comet passes into view from the south 
and disappears behind the Sun.  Finally, in an unrelated event,
a plainly visible giant puff of solar gas is emitted, representing
a large mass ejection in a direction away from the Earth. 

     The remarkable images come from SOHO's visible-light 
coronagraph, LASCO, which is able to mask the intense rays 
from the Sun's surface in order to reveal the much fainter 
glow of the solar atmosphere, or corona.  Operated with its 
widest field of view, LASCO's unprecedented sensitivity 
enabled it to see the thin ionized gas of the solar wind out 
to the edges of the picture, 13 million miles from the Sun's 
surface.  Many stars are brighter than the gas, and they 
create the background scene.

     The results may alter human perceptions of the Sun in 
the same way that the Apollo lunar mission photographs 
revealed the Earth to be a beautiful but isolated planet in 
space, according to leader of the LASCO team, Dr. Guenter 
Brueckner of the Naval Research Laboratory, Washington, DC.

     "I spend my life examining the Sun, but this movie is a 
special thrill," Brueckner said.  "For a moment, I forget the 
years of effort that went into creating LASCO and SOHO, and 
leave aside the many points of scientific importance in the 
images.  I am happy to marvel at a new impression of our busy 
star that gives us life, and which affects our environment in 
many ways that we are only now beginning to understand."

     For many centuries even astrologers knew that the Sun 
was in Sagittarius in December and drifting towards the next 
zodiacal constellation, Capricorn.  This was a matter of 
calculation only, because the Sun's own brightness prevented 
a direct view of the starfield.  The SOHO-LASCO movie makes 
this elementary point of astronomy a matter of direct 
observation for the first time.

     "This is an especially dramatic sample of the data that 
scientists are now starting to gather routinely from SOHO," 
said George Withbroe, director of the Sun-Earth Connection 
science program at NASA Headquarters, Washington, DC.  "It 
really helps drive home the idea that the Sun is a typical 
star, although we certainly have a special relationship with it."

     In the movie, north is at the top of the scene, which 
corresponds with the orientation of the Sun as seen at midday 
in the northern hemisphere of Earth.  SOHO's progress in 
orbit around the Sun remains in step with the Earth's motion.  
SOHO travels towards the right (west) in relation to the 
stars during the period of observation.  As a result, the 
Sun's position appears to shift to the left (eastwards) in 
front of the stars.  In this mode, LASCO observes an area of 
the sky 32 times wider than the visible Sun itself.

     At the time of the observations, SOHO is looking towards 
the heart of the Milky Way Galaxy, which lies in the 
constellation of Sagittarius.  The Milky Way, made by the 
light of billions of distant stars, forms a luminous band 
slanting down and to the right.  Dark lanes seen in the Milky 
Way are real features familiar to astronomers. They are 
created by dust clouds in the disk of the galaxy that obscure 
the distant stars.

     A doomed comet, previously unknown, enters on the left 
of the image on Dec. 22. Its path curves towards the Sun and 
on Dec. 23 it disappears behind the occulting mask of the 
coronagraph.  It fails to reappear on the far side of the 
Sun.  Whether or not its trajectory took it directly towards 
the visible surface, the comet must have evaporated in Sun's 
atmosphere.  It was one of a family of comets known as 
sungrazers, believed to be remnants of a large comet that 
broke up perhaps 900 years ago.  Other fragments were 
responsible for spectacular comet apparitions in 1843, 1882 
and 1965. 

     Called Comet SOHO 6, it is one of seven sungrazers 
discovered so far by LASCO. Analyses of these cometary 
orbits, now in progress, are a prerequisite for their 
inclusion in the official record of comet discoveries.  LASCO 
also provided unique pictures of Comet Hyakutake passing 
behind the Sun in early May 1996. 

     Debris strewn from the tails of many comets makes a disk 
of dust around the Sun, in the ecliptic plane where the 
planets orbit. It scatters sunlight and is sometimes visible 
attwilight on the Earth, known as the Zodiacal Light.  In the 
raw images obtained by LASCO, the Zodiacal Light is brighter 
than the solar corona. Image processing subtracted these 
effects precisely, to bring the solar wind and the Milky Way 
into plain view.

     Random flickers of light in the images are due to cosmic 
rays striking the detector. Cosmic rays are energetic 
particles coming from exploded stars in the Milky Way, and 
variations in the solar wind influence their intensity in the 
vicinity of SOHO and the Earth. Operating beyond the Earth's 
protective magnetic field, which repels many particles, SOHO 
is more exposed to the cosmic rays. 

     In the largest outburst from the Sun seen in the 
December movie, a mass ejection causes billions of tons of 
gas to race out into space on the right-hand (western) side 
of the Sun.  The origin of this event much lower in the Sun's 
atmosphere was evident in an expanding bubble seen in 
processed images from the SOHO extreme ultraviolet imager.

     Coronal mass ejections are the hurricanes of space 
weather. SOHO is ideally placed to report and even anticipate 
their origins in the Sun's atmosphere. Although the Sun is 
supposedly very quiet at present, being close to the minimum 
count of sunspots, LASCO has observed so many outbursts large 
and small -- roughly one a day -- that scientists are having 
to think again about how to define a coronal mass ejection.  
Later LASCO images, from Jan. 6, 1997, revealed a large mass 
ejection directed towards the Earth.

     The movie and still photographs related to it are 
available on the Internet at the Naval Research Laboratory's 
SOHO/LASCO home page:

http://lasco-www.nrl.navy.mil/lasco.html

     A mission of international scientific cooperation 
between NASA and the European Space Agency (ESA), SOHO was 
launched on Dec. 2, 1995.  It is situated near the L-1 
Lagrangian point, where the Earth's and Sun's gravitational 
forces balance, some one million miles sunward from the 
Earth.  This vantage point enables solar astronomers to use 
SOHO's 12 science instruments to observe the Sun 
continuously, with no intervening "night."  Major areas of 
research include studies of the solar interior via 
helioseismology, the solar atmosphere in ultraviolet and 
visible light, and the solar wind and energetic particles.

     SOHO and its instruments are operated from the Goddard 
Space Flight Center, Greenbelt, MD, for the NASA's Office of 
Space Science, Washington, DC.

     Brueckner will discuss the video and its contents in 
detail in a briefing at 6 p.m. EST on Saturday, Feb. 15, 
during the annual meeting of the American Association for the 
Advancement of Science at the Seattle Convention Center.  For 
more information, contact the AAAS press room at 206/624-9087.


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