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Images of
Comet Holmes by DAS Members |
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Joe Gafford
November 5th at EGK
Dark Site
Mosaic of 5/5/4/5 Minute LRGB, 10" f4.5 with SBIG
ST-2000XM CCD camera. (Click photo for larger image) |
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Brad Gilman at Chamberlin
Observatory
Afocal,
digital camera image taken at the University of
Denver's historic Chamberlin Observatory on the
evening of October 25th, 2007. Telescope: Alvan
Clark-Saegmuller 20-inch refractor (1894) |
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Phil Good
Stellarvue SV115 at
f14 with Televue 2X Powermate. Mountain Instruments
MI-250 mount. SBIG ST-10XME with Astrodon LRGB
Filters. 5 minutes each LRGB in 15 second subframes.
Denver, CO. October 26, 2007, at 4 am MDT. See
Phil's
Holmes Animation. |
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Dr. Robert Stencel
Negative of Brad
Gilman's image above, processed by Dr. Robert
Stencel (shows an extremely faint ring beyond the
central coma.) |
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David Wolf
ST-2000XM 10X20 sec
LRGB SV102 Televue 2" 2X Powermate |
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Joe Gafford
17p Holmes combined
photo 10/25/07
Two different images, aligned and stacked to show
difference in size and position. 3 hours difference.
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Joe Gafford
10-25-07 7:04:58
UT. 10" f4.5 with barlow to f8.1. Series of 10/6/6/6
0.5 second exposures averaged. |
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October 24th, 2007,
astronomers in
Japan, Persia,
and Europe reported
that Comet
17P/Holmes was
undergoing a
spectacular
eruption. The 17th
magnitude comet
brightened almost
one million-fold during the 24
hour period from
October 23-24,
becoming a naked-eye
object in the
evening sky. In Late
October the comet
could be seen as a yellow 2.5th
magnitude "fuzzball"
in the constellation
Perseus (see
Finder Chart),
which is rising
around sunset and
high in the
Northeastern sky by
midnight. The coma
of the comet doubled
in size on October
25th. The comet had
acquired a blue ion
tail by November 4th
and looked as big as
the full moon a few
days later.
Comet Holmes
Time Line:
The comet was easy
to observe from
Denver city and
suburbs on the nights
of October 24-25th,
even though the moon
was nearly full.
Visually, the comet
looked yellowish and
was visibly less
sharp than nearby
stars. In
binoculars, the disk
was readily apparent
and looked very
yellow. The comet
was
truly spectacular in even
a small telescope.
By Sunday, October 28th, the comet had become an
obviously non-stellar object to the naked eye and was
readily seen as a comet with a nucleus and coma in 10x50
binoculars. A dim background star was seen through the
diaphanous coma. As the coma has expanded, it has lost the
yellow hue that was noted earlier in the eruption.
Over the weekend of November 2-4, the comet's
visible coma had increased to half the size of the full moon
and imagers started to pick up detail of a blue ion tail in
the direction of the small fan-like dust tail within the
coma. The coma in the direction of the ion tail clearly
showed feathering and extension to visual observers.
The tail question has been resolved. There was a minor controversy about the comet's apparent
lack of a tail. However, because the comet is moving away
from Earth and the Sun, a long tail crossing our line of sight
would not be expected. A tail would be
streaming out away from Earth-bound observers.
That is clearly shown by Joe Gafford's remarkable image at
the right. This image is probably one of the best now
available. It clearly shows the yellow in the coma, the
green haze around the coma, and the ion tail streaming out
in front of the comet's path as it moves away from the Sun
and Earth.
Over November 8th and 9th, the blue ion tail
(which is visible only in long exposure photographs taken
through fairly large telescopes) "broke away" from the coma,
probably due to an interruption of the venting of the gases
that were generating it, although other possibilities have
been entertained by some observers.
On the night of Sunday, November 11th, the comet
was still bright in Perseus and easily visible to the naked
eye, although it no longer had noticeable color. The central
coma looked very large in binoculars and virtually filled
the field of view in an 11-inch telescope -- about as large
as the full moon. Through a smaller instrument, the
"downwind" side of the coma showed increased feathering by
the solar wind. According to Chris Peterson of the
Cloudbait Observatory
in Guffey, Colorado, the central coma of Holmes now has a
diameter of more than 1.6 million kilometers, making it the
largest "object" in the solar system. The outer coma, which
has a distinctly green color, is visible in long-exposure
photographs.
November 14th: The comet was a massive
object near Mirfak. The coma was so extensive it was best
viewed in binoculars or low power telescopes. Although
fading, the coma was still visible to the naked eye as a
large ball in dark Suburban skies.
See or download Phil Good's animation of the orbit of
Comet Holmes
HERE. (Requires QuickTime viewer.)
See a movie of the comet on
Spaceweather, showing movement in the emerging ion
tail.
This is the second eruption of Comet Holmes that has been
observed. The first was in November 1892, when the comet was
discovered by Edwin Holmes.
- Darrell Dodge, DAS
Exploring the Significance of Comet Holmes
The bright comet in the sky and in the news appears in
Denver's northeastern sky just after sunset. This is a well
known comet, discovered back in the late 1800's. We lost
track of it for several decades. Dr. Marsden of the USNO
tracked it down in the 1960's and astronomer's have been
watching it ever since.
This comet is a pretty average, run-of-the-mill comet. A
few weeks ago it was merrily moving away from the Sun. It is
so small and at that time it was so far away that it was
17th magnitude or 17 - 6 = 15 magnitudes too faint to be
seen with the unaided eye! It is now 2nd magnitude, or 2.512
to the (17 - 2 =) 15th power or 1,000,678 time brighter than
it was!
The cause of the brightness in unknown. It appears to be
spewing gas, which has expanded to make a pretty bright
little fuzz ball.
Astronomers like math, so lets play with the numbers and
the data.
On the evening of 26 October, 17P/Holmes will be about
2.44 astronomical units (AU) or about 2.34 million miles
from the Sun, and 1.63 AU or 152 million miles from Earth.
One astronomical unit is equal to 149,597,870.691 km. These
data were adapted from the Jet Propulsion Laboratory's
Horizon's site at
http://ssd.jpl.nasa.gov/horizons.cgi#top
The apparent angular size of the fuzz ball as seen from
Earth is about 1 arc minute. This is 1/(360 x 60) of a
circle, or 0.0000463th of a complete circle!
Now for a little "Law of small angles". An angle is a
fraction of the circumference of a circle. Circles have
curvature, so the distance along a part of the circumference
of a circle will have curvature and be longer than the
distance between the two points on the circle that define
the angle. However, if the angle is very small, that
curvature is essentially flat!
Given that the apparent angle of the fuzz ball is so
tiny, that fraction of the full circle's circumference can
let us estimate the actual size of the fuzz ball itself!
We know the radius of the circle ( or the distance from
Earth to the comet) is 152 million miles. A circle with a
152 million mile radius has 2 x PI x 152,000,000 or
955,044,167 miles! HUGE!
But we are only interested in that small fraction of the
circle where the stuff of the comet is located. This is
0.0000463th of 955,044,167 miles or 44,218 miles.
So the diameter of the gas around the comet is about
1/5th the distance to the Moon!
Pretty good for a comet that was estimated to have a
diameter of only 2.11 miles to start with!
- Wayne Green, DAS (October 25th)
Comet Holmes
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