We think we know our sun. It hangs up there like some blinding, solitary jewel in a plain blue setting.
It's easy to forget that it's really a blazing star, one of millions in our region of the spiraling Milky Way Galaxy. Its ferocious thermonuclear furnace boils and thrashes against a backdrop as black and star-spangled as the winter sky on a cold, clear, country night.
Now, solar physicist Guenter Brueckner of the U.S. Naval Research Laboratory has produced a video that for the first time shows our sun in its natural, starry habitat, flinging ionized gas -- the solar wind -- into space and even swallowing an errant comet.
The hypnotic view, and the remarkable detail of the pictures, were a surprise even to the scientists who produced it.
"No one had ever seen the sun against the backdrop of the galaxy," Brueckner says. Capturing the Milky Way in the pictures was "not even thought about, but also the solar wind -- nobody was prepared for that."
The movie was pieced together from 160 still images captured hourly during a six-day period around Christmas by the Solar Heliospheric Observatory (SOHO)- spacecraft. Just eight seconds long (looped four times, for a 32-second sequence) the Christmas Movie" reveals the sun as it looked from SOHO's outpost, 1 million miles sunward of Earth.
Imagine peering out a circular porthole. The sun is blazing front and center, but the view all around is dominated by the distant cloud banks and dark lanes of the Milky Way galaxy, which are really vast concentrations of stars and dust.
The scene is punctuated by the bright, nearby stars of the constellation Sagittarius. As SOHO moves to the right along its solar orbit, the sun appears to move left relative to the stars shining behind it in the constellation.
Astronomers and astrologers have known since ancient times that the sun -- as seen from Earth -- moved slowly through the "House" of Sagittarius from Nov. 22 to Dec. 22, en route to the stars of Capricorn.
But they couldn't see it directly.
"All of it was deduced from seeing the [night] sky in the summertime," Brueckner says.
No stars in the daytime
That's because our daytime view of the stars is washed out by the sun and blue sky. We can see the Sagittarian star field only in summer, after Earth has moved around to the opposite side of its orbit. From there, the Sagittarian stars are opposite the sun, and shining brightly in our night sky.
If that's hard to visualize, imagine you're standing on second base in a baseball stadium. The pitcher is the sun, and you know there are thousands of fans in back of him in the seats behind home plate. But he shines so brightly you can't see them.
Six months later, you have moved around the bases to home plate. Now you can turn your back on the blazing pitcher and see the fans seated behind the plate.
Brueckner's movie frames were shot with SOHO's visible-light coronograph. The coronagraph is a small telescope with a complex system of light baffles and computer programs that mask the sun's brilliant disk and dim its surrounding glare like an artificial eclipse. It enables scientists to see the sun's faint but turbulent atmosphere, or "corona."
It's similar to the way you would hold your hand up to block the sun in order to see a bird or plane that had flown into its glare.
The Naval Research Laboratory began studying the sun at the end of World War II after learning that eruptions of charged particles from the solar atmosphere could sweep past Earth and disrupt radio communications.
Brueckner, 62, joined the lab in 1967. The German-born scientist has since figured prominently in the development of a series of space-borne coronagraphs. As they have been improved, they have gradually revealed the nature of the sun's corona.
None is better than SOHO's. It was launched on Dec. 2, 1995, by NASA and the European Space Agency and rocketed to a point in space called the L-2 Lagrangian point. That's about a million miles from Earth, where the gravitational pull of the sun and Earth balance out.
From there, SOHO stares unblinkingly at the sun, orbiting once a year with Earth constantly at its back. The view has taught scientists that the sun, even during the present "quiet" period in its 11-year sunspot cycle, is still a busy, violent place.
In Brueckner's speeded-up movie, bright, rippling plumes of ionized gas stream out into space at hundreds of thousands of miles per hour. They are most concentrated near the solar equator, channeled there by the sun's magnetic field.
Suddenly, from the sun's right side, an enormous bubble of gas bursts out in a vast, roiling cloud called a "coronal mass ejection." These are the magnetic storms that race across the solar system from time to time, as happened last week, frying Earth-orbiting satellites, disrupting communications and power grids on Earth.
On Jan. 6, SOHO detected another outburst aimed almost directly at Earth. When it swept by Earth four days later, a $200 million communications satellite was knocked out, and Antarctic aircraft operations were canceled because of radio outages.
"We need much more experience with this," Brueckner says. But "for the first time we are close to predicting [these] communications outages." The real test will come between 2000 and 2002, when the sun's activity cycle is expected to peak: "That will be quite a show."
Death of a comet
During part of Brueckner's movie, a small comet appears. Two days from its rendezvous with the sun, its cargo of gas and dust is swept by the solar wind into a long, graceful tail. But the display ends abruptly as the comet falls into the sun and disappears.
SOHO scientists have discovered seven such "sun-grazing" comets since 1995. "They are all in the same orbit, so close to the sun that they cannot escape its gravity," Brueckner says, "and I would guess they evaporate in the sun's atmosphere."
Scientists suspect they are fragments of a very large comet that cruised too close to the sun 900 or 1,000 years ago and broke into thousands of pieces. "We get them almost regularly, almost one per month," he says.
Brueckner's movie even captures the blizzard of cosmic rays that blow through the solar system. These high-energy particles -- shot across the galaxy by exploding stars -- are recorded as they strike SOHO's sensitive detectors. They appear as randomly flickering dots and streaks of light.
"We have thought, 'Should we remove them electronically?' and we decided no, because they contribute so much to the liveliness of these pictures."
The Christmas Movie and still photos are on the Internet at http: //lasco-www.nrl.navy.mil/lasco.html
Pub Date: 4/13/97