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Dudley Observatory
Supporting research and education in astronomy, astrophysics, and the history of astronomy |
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Skywatch October 2007
Skywatch line for Wed. and Thurs., October 31 & November 1,
2007:
by Ray Bogucki
On a typical night of observing, most amateur astronomers make up
a list of objects that they plan to study. These might include star
clusters, nebulas, galaxies, planets, comets and the Moon. The star
clusters and nebulas are fixed in position among the stars and the
astronomer knows precisely where to find them. The other objects
that belong to the Solar System move at various speeds, usually
slow, among the stars, but their paths are well defined, and so,
again, the astronomer knows where to look. It is also uncommon for
the brightness of an object to change suddenly, so surprises in
location or brightness are very rare. It is true that a couple of
times a year a very faint star may suddenly flare up in brightness
by a factor of several thousand. These stars are known as nova
stars, and the mechanisms that cause the sudden eruption in emitted
energy are well known. When one is observed, it causes quite a
stir.
By contrast, all of the objects in the Solar System other than the
Sun are relatively cold and can be seen only by reflected sunlight.
A particularly interesting group are the comets - large chunks of
ice and embedded dust and solid particles that sweep into the inner
Solar System from far out and swing around the Sun. The heat of the
Sun causes the solid ice to sublime into gaseous water molecules,
thus releasing large clouds of dust and sand grain-sized particles
to surround the comet, and reflect sunlight which we see as the
"head of the comet" or coma. The dust particles falling behind the
moving comet forms the usual "tail" of the comet. As the comet
slowly approaches the Sun, the size and brightness of the comet
gradually increases, almost imperceptibly from night to night, over
a period of months.
A comet now approaching the Sun, but still well out between the
orbits of Mars and Jupiter, is designated as Comet Holmes (17P). It
is not well known to the general public, unlike Comets Halley or
Hale-Bop, because even at its brightest, it typically reaches only
magnitude 13, which is too faint to be seen by the naked eye, or
even in binoculars, but it can be seen in backyard telescopes.
Imagine the amazement of the astronomer on the Canary Islands who,
just a week ago, discovered that overnight, Comet Holmes had
brightened by a factor of 10,000, from magnitude 18 to magnitude 8.
A few hours later, it's brightness had increased about a million
fold to a magnitude of 2.7, which is easily visible to the naked
eye. Having found this information in the magazine Sky &
Telescope's website (skyandtelescope.com), this observer brought out
binoculars and telescope to view this phenomenon. The view through
12x50 binoculars was incredible. Instead of a tiny speck of light,
there shone a surprisingly large yellowish ball of diffuse light.
The telescope showed a large, perfectly round coma with a bright,
almost star-like center. No tail was visible, but that may be
because the Sun is almost directly behind us when we look at the
comet. The tail streams away from the Sun so it is pointing almost
directly away from us in space.
Reports and images have been pouring in from astronomers all
around the world. Hypotheses concerning the reason for the sudden
brightening of the comet include a large meteorite striking the
comet and raising a huge cloud of dust, and the possibility of a
sudden fragmenting of the comet, which would greatly increase the
comet's surface area so much more dust could be released. To find
Comet Holmes, first use any star atlas to find the constellation,
Perseus. The brightest star in Perseus is Mirfak, near the center
of the constellation. About 2 degrees east of Mirfak is a bright
starlike object that does not appear on the atlas. This is the
comet and it shows the best image in binoculars or telescope.
This is the Skywatch Line for Monday and Tuesday,
October 29 and 30. This is Dudley Observatory's Skywatch Line for Friday, October 26, through Sunday, October 28, written by Alan French. This weekend's Skywatch Line features news of an unusual and unexpected event now visible in our night sky. On November 6, 1892, English amateur astronomer Edwin Holmes discovered a periodic comet, known since as Comet Holmes or P17. He discovered it while making observations of the Andromeda galaxy. The orbit takes it around the Sun once every 6.9 years. It was observed in 1899 and 1906, but then was lost. It was recovered in 1964, and has been seen every time it has approached the Sun since then. Right now Comet Holmes is well away from the Sun, out between the orbits of Mars and Jupiter. Until Wednesday night, it was far too faint to be seen with any but the largest amateur telescopes. Then, in a major surprise and thrill to astronomers, it suddenly brightened to naked eye visibility, altering the appearance of the constellation Perseus. At about magnitude 2.5, it is now bright enough to be seen even with the competition from a bright waning gibbous Moon. To the eye on Thursday night, it looked like a star of about the same brightness as the star that marks the bottom back of the Big Dipper's bowl. Ten-power binoculars showed it as a slightly bloated, out of focus star, and 15 power binoculars showed a starlike nucleus surrounded by a fainter coma. Through a telescope, it shows a starlight nucleus, surrounded by two regions of differing brightness - looking somewhat like a planetary nebula. Whether you only look with the unaided eye, or have access to binoculars or a telescope, Comet Holmes is well worth a look. It is not often a comet out between Mars and Jupiter becomes bright enough to see with the unaided eye! For more information, and a finder chart, visit either www.skyandtelescope.com or www.spaceweather.com.
Skywatch line for Wed. & Thurs., Oct. 24 & 25, 2007:
by Ray Bogucki Early last Sunday morning, right on predicted schedule, the Earth was passing through the stream of particles left in our orbit by Halley's Comet. Between 4:00 a.m. and 5:45 a.m., this observer caught sight of 18 meteors generated by tiny particles of matter streaking through the Earth's upper atmosphere at speeds of near 66 kilometers per second (or 142,000 miles per hour). By the time a particle has penetrated the outer atmosphere to within about 100 miles above the ground, it has been heated to incandescent white heat by violent collisions with atoms and molecules of air. As this white-hot particle passes through the thin atmosphere, it transmits energy to surrounding atoms and molecules of air, which thereupon begin to fluoresce and give off light. It is this trail of fluorescing molecules of air which our eyes see as the meteor streak. The late part of the calendar year is much richer in meteor showers than the early part of the year. We will have five more dependable meteor showers before the end of the year. These will be cited in Skywatch Lines as they occur. This month, Full Moon occurs tomorrow (Thursday) at about midnight, within hours of the Moon's passing through perigee, its closest approach to Earth for this orbit, and also for this year. When Full Moon occurs close to perigee, two effects are immediately noticeable. First, tomorrow's Full Moon, only 214,000 miles away as opposed to the average 238,000 miles, will appear to the eye as larger in diameter than the usual Full Moon. Second, the Moon's gravitational pull on the oceans is quite sensitive to the Moon's distance, and this month's unusually close approach will enhance the Moon's effect which will produce exceptionally high tides. Finally, moving to a more distant object in our Solar System, this Sunday evening, October 28, presents our best opportunity for observing an interesting event known as a double shadow transit on Jupiter. Each time the orbit of one of the four Galilean moons carries it between the Sun and Jupiter, the tiny shadow of the moon can be seen in a telescope moving across Jupiter's surface. Every couple of years, the orbits of the two innermost moons, Io and Europa, fall into synchronization so that for a period of six weeks, each time Europa's shadow falls on Jupiter's surface, it is joined by the shadow of Io and they cross the planet's surface together. This Sunday, shortly after sunset, train your telescope on Jupiter, low in the southwest, but bright, even in the twilight. If it's after 6:30 p.m., Europa's shadow should already be on Jupiter's surface. At 7:34 p.m., Io's shadow will appear. They will then cross together until Jupiter sets about 8:30 p.m., or earlier. Although there will be 16 double shadow transits in the next five weeks, the unfortunate current location of Jupiter makes this the only one we might possibly get to see, if the weather is kind. Good luck! This is the Skywatch Line for Monday and Tuesday,
October 22 and 23.
Skywatch line for Wed. & Thurs., October 17 & 18, 2007:
by Ray Bogucki
In last week's Skywatch line, we compared two bright stars,
orange-tinted Arcturus, setting in the west, and yellow Capella,
rising in the northeast. We decided that, because of its color,
Arcturus is significantly cooler than Capella, and from its radial
and proper motions, we can calculate that Arcturus travels a much
different path from that of most stars in our galaxy. Capella turns
out to be very different from Arcturus in another way. Although
when seen with our eyes, binoculars or a backyard telescope, each
star appears as a single point of light, Capella is actually a
binary star. This fact was uncovered by a spectral study of
Capella's light, which showed subtle changes over time caused by the
orbital motions of the two stars. Analysis of the light revealed
that the two components of this double star are each about 3 times
as massive as the Sun and several times larger in diameter. They
orbit around their common center of mass with a period of 104 earth
days at a distance of separation of only 70 million miles, roughly
the distance from the Sun to the planet, Venus. At a distance from
Earth of 42 light years, this tiny separation would show up as a gap
of about 1 milliarc-second, far too small to be detected by even the
largest amateur telescopes. One of the great satisfactions of star
gazing is that while your eye sees one point of light, in your
mind's eye you can envision the magnificent sight of these two giant
suns whirling around each other in a precise dance that has been
going on for billions of years.
The name Capella means "she goat" and this mother goat has three
"kids" close by in the sky, just a couple of degrees to the
southeast. The three stars form a sharply pointed triangle, with
the third magnitude star at the sharp point designated as epsilon
Aurigae. This ordinary-looking star is one of the most unusual and
mystifying objects in the sky. To begin with, it stands at a
distance of over 2000 light-years away. To appear as a third
magnitude star from this incredible distance, it must be one of the
largest, most luminous stars known, at least 130 thousand times more
luminous than our Sun. Like Capella, it seems to be a binary system
with its orbital plane close to our line of sight so that the system
is an eclipsing variable star. Once in each orbital period of 27
years, the primary star begins to grow fainter, indicating that its
companion is passing between us and the primary star and blocking
some of its light. The mystery arises from the fact that the
eclipsing object is invisible and it requires two years to complete
its passage in front of the primary star. At an orbital speed of
perhaps 40 kilometers per second, this invisible object must be
truly immense in size, about the diameter of Saturn's orbit.
Speculations suggest that it might be a huge dust cloud surrounding
a newly forming star or perhaps a black hole which would absorb some
of the light from the primary as it passes in front of it. The next
eclipse should begin in 2009, and many astronomers with the latest
equipment will be studying this event.
A final note to early risers -- about 3 a.m., a couple of dark
hours before morning twilight begins on Saturday and Sunday, watch
for the Orionid meteor shower generated by the Earth's passage
through the stream of debris left in our orbit by Halley's Comet.
This is the Skywatch Line for Monday and Tuesday,
October 15 and 16.
Skywatch line for Wed. & Thurs. Oct. 10 & 11, 2007:
by Ray Bogucki
The splendid gathering of four bright objects; Venus, Saturn,
Regulus and the Moon, undoubtedly occurred early last Sunday morning
as we predicted in last week's Skywatch line, but in the Albany
area, the view was blocked by a sky full of soggy gray clouds. All
is not lost however, because, although the Moon has left the scene,
the other three bright objects will remain closely grouped for the
next couple of weeks. First magnitude Regulus is a fixed star,
while distant Saturn appears to move very slowly among the stars.
Over the next two weeks, the apparent distance and angle between
Saturn and Regulus will remain essentially constant, while much
faster-moving Venus will shift its position relative to the other
two noticeably each day, forming a triangle with ever changing
shape. Check this moving panorama in the east any morning around
5:30. With a telescope, it is even more interesting to watch the
daily change in the size and visible shape of Venus. Having passed
between Earth and the Sun last August, Venus is now receding rapidly
from the Earth, its disk becoming slightly smaller each day even as
its phase waxes from crescent to half illuminated to gibbous. If
Copernicus had possessed a good optical telescope, this visual
behavior of Venus would have provided him with
incontrovertible evidence for his theory that the planets revolve
around the Sun.
Moving further out into our galaxy we might look for two very
bright stars, one departing and one arriving to replace it.
Arcturus, the bright orange-tinted star low in the west, will soon
be setting shortly after sunset and will be lost to our evening
skies until next Spring. Capella, just about as bright as Arcturus
and with the same yellow color as our Sun, currently rises in the
constellation Auriga in the northeast at nightfall. Each of these
two stars is relatively close to us, each roughly 40 light years
away. Because they show roughly the same brightness from roughly
the same distance, they must be about the same in absolute
luminosity -- in fact, about 130 times more luminous than our Sun.
In that respect they are quite similar but in other ways they are
very different. The reddish hint of color in Arcturus tells us that
it is significantly cooler than Capella.
Arcturus is also quite different in its galactic motion. Capella
and the Sun are relatively close together, revolving in a 200
million-year orbit around the central bulge of our galaxy in the
same spiral arm and in the galactic plane - like two cars traveling
the same direction on the same highway. Arcturus, on the other
hand, is a member of a small number of stars that orbit the center
of the galaxy in a plane set at a large angle to the galactic
plane. In other words, Arcturus is like a car on a galactic road
that meets our road at an intersection, crosses in front of us, and
after a while, disappears from our view as it travels away from us
down the crossroad. Capella, traveling the same direction as we
are, but at a little different speed, will remain visible for a very
much longer time.
This is the Skywatch Line for Monday and Tuesday,
October Eighth and Ninth. The Sun sets at 6:25; night falls at 7:58 PM. Dawn breaks at
5:25 and ends with sunrise at 7 AM. This is Dudley Observatory's Skywatch Line for Friday, October 5, through Sunday, October 7, written by Alan French. The Albany Area Amateur Astronomers will take advantage of the dark skies to host public star parties at 8:00 PM on Friday, October 5, and Saturday, October 6. Both will be held at Landis Arboretum in Esperance. Landis Arboretum offers dark skies away from city lights, and good horizons, with a fine view of the night sky. At a Star Party a variety of telescopes are pointed at celestial objects for your enjoyment. All are welcome and there is no admission charge. Star parties last at least an hour and generally go much longer if the sky conditions are good. Entering Esperance from the east on Route 20, take the first right after crossing Schoharie Creek. There is a large "George Landis Arboretum" sign at this intersection. Follow the signs for 1.5 miles until you reach the arboretum, and then continue up Lape Road past the main parking area on your left and farmhouse on your right. At the top of the small hill, turn right into the Meeting House drive. There will a reflective "Star Watch" sign at the entrance to the driveway. The star party will be held in the field by the Meeting House. Star Parties are canceled if the skies are mostly cloudy. Call 374-8460 if the skies are uncertain, or for more information. People frequently ask me if everything we see by eye in the night sky is in our own galaxy, or do we see some celestial objects outside our galaxy? The answer is that everything we see is, indeed, in our galaxy – with one exception. The Andromeda Galaxy, known to amateur astronomers as M31, is clearly visible to the unaided eye from a location with dark, moonless skies free of significant light pollution. If you look toward the east at 8:00 PM in the evening, you'll find the large square of stars that marks Pegasus lying on one corner. Starting with the leftmost star in the Great Square of Pegasus, and counting it as "one," count three stars to the left. Then count three stars upward, again counting the starting star. Just above the third star, you should see a little hazy cloud. This is the Great Andromeda Galaxy. While the fuzzy patch that is the Andromeda galaxy may not appear impressive, keep in mind that you are seeing the combined light of one trillion stars and it took two-and-a-half million years for that light to reach your eye. Our own Milky Way galaxy contains 200 to 400 billion stars. Andromeda is the largest galaxy in the Local Group – essentially the galactic neighborhood our Milky Way Galaxy belongs to. The Triangulum galaxy also belongs to the Local Group, as do about 30 other smaller galaxies. You may wonder why it is also called "M31." Comet hunter Charles Messier compiled a list of objects he encountered while comet hunting – many of which could have been mistaken for a comet in his modest, early telescope. The Andromeda galaxy is the thirty-first object on his list, which contains 110 objects. Most of them are popular targets for amateur astronomers, and you will likely hear a lot of "M" numbers if you attend one of the star parties. Early risers will enjoy Venus and a crescent Moon in the eastern sky around 5:30 AM. On Saturday morning, the Moon will be above Venus. The pair will be closest together on Sunday morning when the Moon will be below and to the left of Venus, with Saturn just below and left of the slender Moon
Skywatch line for Wed. & Thurs., October 3 & 4, 2007
by Ray Bogucki
For Solar System watchers, the Moon and all five planets will be
visible at some time through the night this week. The first planet
to catch is Mercury, the planet closest to the Sun. You'll have to
search the western horizon shortly after sunset because even at its
farthest separation from the Sun in our sky, it sets a scant 45
minutes after sunset, in a sky still bright with evening twilight.
It still shines close to zero magnitude, however, so with binoculars
and a little luck, you should be able to spot it.
Next turn your gaze left to the southern sky, where the giant
planet Jupiter will easily be visible even in the twilight. The
next few weeks will provide the last opportunity for Jupiter -
watching before it gets lost in the Sun's glare. With a telescope,
you might try to watch the Great Red Spot transit Jupiter's central
meridian about 7:50 p.m. next Monday evening at the end of evening
twilight. Jupiter will reappear next January as a morning object,
rising before the Sun.
The next object to appear, rising in the east about 11 p.m., is
the Red Planet Mars, followed closely by the Last Quarter Moon a
little below and to Mars' left. Mars has brightened to greater than
zero magnitude and has grown to 10 arc-seconds in diameter. Amateur
astronomers have reported being able to see the polar ice cap and
other markings on the planet's surface. The view of Mars will
continue to expand and improve throughout the fall and early winter
as Earth continues to close the distance between us and Mars.
Finally, the last two objects appear in the early morning eastern
sky, Venus, the most brilliant planet, rising about 3:30 a.m., with
Saturn, the ringed giant, following a half hour later. Venus and
Saturn have been playing tag since last June when they formed a very
close pair after sunset at the end of June. They drew somewhat
apart in July but continued to set at the same time. In August,
they were lost in the Sun's glare. In mid August they both were in
conjunction with the Sun at about the same time, with Venus passing
in front of the Sun and Saturn passing behind the Sun. They were
reunited in the morning sky. It's important to note that Saturn
requires over 29 years to complete one orbital revolution around the
Sun. This means it will appear to move very slowly through the
fixed background stars. Saturn spends about 2.5 years in each of
the twelve Zodiac constellations before it returns to its starting
point. It has been in the constellation, Leo the Lion, all year and
for the past several months, it has been very close to Regulus, the
brightest star in Leo. The trio, Venus, Saturn and Regulus, have
been making slowly shifting patterns in the early morning sky which
will come to a culmination about 5:30 Sunday morning when they are
joined by the waning crescent Moon to form a splendid close
gathering of the four celestial bodies. This grouping presents a
great challenge for astrophotographers to catch the crescent shape
of Venus, the rings of Saturn, and the craters of the Moon, all in
one image.
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