Physics Photo of the Week
February
27, 2009
Comet Lulin
 |
Photo and telescope assistance
by Melanie Kemp.
|
About every
year a telescopic or binocular comet visits the inner
Solar System. Two dedicated astrophotographers got up at 4:00 am
on February 20, 2009 to set-up the telescope to obtain these
photographs. The temperature was about 16 degrees F with a strong
north wind making observing very difficult and
unpleasant.
This comet is moving quite rapidly between the constellations Virgo and
Leo. This week the comet can be viewed at a more appropriate time
in the
late evenings after 10:00 pm approaching the bright star Regulus in
Leo, and is visible in binoculars.
The
animated picture at right spans a 25 minute time frame. From the
scale of the picture (about 1/2 degree) the comet is moving at about
0.003 deg/minute. This amounts to about 4.1 deg/day - 8 times the
diameter of the full Moon. The rapid apparent speed is enhanced
because
the comet is orbiting the Sun in the opposite direction from the Earth
and it is now relatively close to the Earth - about 40% of the
Sun-Earth distance. As a result of Newton's laws of gravity and
orbital dynamics, the comet's velocity is about the same as the
Earth's speed, but in the opposite direction.
In order to view the structure of the comet (a thin tail to the left),
several 30-second exposures were stacked (aligned and digitally
added). The brightness was also
transformed to a logarithmic display (similar to the way our eyes
respond). Otherwise the central part of the comet would appear to
be
highly overexposed.
The comet is mostly a ball of diffuse gas and dust that evaporates
from the small (kilometer-sized) nucleus of the comet due to the heat
of
the Sun. The gas and dust are driven away from the comet by the
radiation pressure from the Sun forming a tail. A comet's tail is
always directed away from the Sun. The comet is now situated such
that the Earth is between the comet and the Sun. As a result the
tail points mostly toward the far side of the comet. As the tail
particles are pushed further from the Sun, they still preserve the
angular momentum around the Sun, but travel more slowly at the greater
distances. As a result the tails of comets are usually
curved. We cannot see the curve, because our vantage point is in
the same plane as the curve. The fact that the tail
curves to the left allows us to see the tail at all. Thus the
particles in the left end of the tail in these photos are actually
considerably further from the Earth and Sun.
On Monday 
night (February 23, 2009) Pengye Su, George Pilzer, and Libba Miano, helped align the
telescope and photograph the comet in color. The weather again
was very cold. The monochrome CCD camera makes color photos by
recording images through three color filters: red, green, and
blue.
The three images are later assembled by digital superimposition using
red, green, and blue-colored pixels respectively. Because the
whole
process required about a half-hour the comet moved considerably during
the imaging session. For the alignment, stacking, and color
assembly, the images were aligned to "follow the comet" rather than the
stars. As a result the stars left trails in the opposite
direction.
In fall 2007 students helped photograph Comet Holmes (PPOW
Dec. 7, 2007 and Nov.
1, 2007). Other comets are featured in PPOW
for Nov. 3, 2006 and Jan.
10, 2005.
Physics
Photo of the
Week is
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
Click
here to see the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
