Incredible Comet Bigger than the Sun
Robert Roy Britt
Senior Science Writer
SPACE.com Thu Nov 15, 10:45 AM ET
A comet that has delighted backyard astronomers in recent weeks
after an unexpected eruption has now grown larger than the
sun.
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The sun remains by far the most massive object in the solar system,
with an extended influence of particles that reaches all the
planets. But the comparatively tiny Comet Holmes has released so
much gas and dust that its extended atmosphere, or coma, is larger
than the diameter of the sun. The comparison is clear in a new
image.
"It continues to expand and is now the largest single object in the
solar system," according to astronomers at the University of
Hawaii.
The coma's diameter on Nov. 9 was 869,900 miles (1.4 million
kilometers), based on measurements by Rachel Stevenson, Jan Kleyna
and Pedro Lacerda of the University of Hawaii Institute for
Astronomy. They used observations from the Canada-France-Hawaii
Telescope. The sun's diameter, stated differently by various
sources and usually rounded to the nearest 100, is about 864,900
miles (1.392 million kilometers).
Separately, a new Hubble Space Telescope photo of the comet reveals
an intriguing bow-tie structure around its nucleus.
The comet's comamostly microscopic particlesshines by reflecting
sunlight.
See for yourself
Holmes is still visible to the naked eye as a fuzzy star anytime
after dark, high in the northeast sky. You can find it by using
this sky map. It is faintly visible from cities, and from dark
country locations is truly remarkable.
"Right now, in a dark sky it appears as a very noticeable circular
cloud," said Joe Rao, SPACE.com's Skywatching Columnist. Rao
advises looking for the comet this weekend, before the moon becomes
more of a factor. The comet will likely diminish in brightness yet
remain visible for the next two to three weeks, he said.
"Over the next few weeks and months, the coma and tail are expected
to expand even more while the comet will fade as the dust
disperses," Stevenson and her colleagues write.
On Monday, Nov. 19, the comet will create a unique skywatching
event with its see-through coma, according to the Web site
Spaceweather.com: "The comet will glide by the star Mirfak [also
called Alpha Persei] and appear to swallow ita sight not to be
missed."
A small telescope will reveal the fuzzy coma. Lacking a long tail
characteristic of some great comets, however, Holmes is not the
most dramatic object in the sky for casual observers.
Mystery outburst
Nobody knows why Holmes erupted, but it underwent a similar
explosive brightening in 1892. The recent display, which began Oct.
24, brought the comet from visual obscurity to being one of the
brighter objects in the night sky. It has since dimmed somewhat as
the material races outward from the nucleus at roughly 1,100 mph
(0.5 km/sec).
The Hawaiian astronomy team writes in a press statement: "This
amazing eruption of the comet is produced by dust ejected from a
tiny solid nucleus made of ice and rock, only 3.6 kilometers
(roughly 2.2 miles) in diameter."
The new image from the Hawaiian observatory also shows a modest
tail forming to one side, now just a fuzzy region to the
lower-right. That's caused by the pressure of sunlight pushing on
the gas and dust of the coma.
But the comet is so far away149 million miles (240 million
kilometers), or about 1.6 times the distance from Earth to the
sunthat even Hubble can't resolve its nucleus.
The offset nature of the coma, seen in ground-based images,
suggests "a large fragment broke off and subsequently disintegrated
into tiny dust particles after moving away from the main nucleus,"
Hubble astronomers said in a statement today. The comet's distance,
plus all the dust, prevent Hubble from seeing any fragments,
however.
