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Dudley Observatory
Supporting research and education in astronomy, astrophysics, and the history of astronomy |
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Skywatch July 2007 This is the Skywatch Line for Monday and Tuesday, July
30 and 31. This is Dudley Observatory's Skywatch Line from Friday,
July 27, through
Skywatch line for Wed. and Thurs., July 25 and 26, 2007
by Ray Bogucki
In last week's Skywatch line we described the large
constellation, Cygnus, the Swan, recognized by the well known,
bright asterism, the Northern Cross, shining high in the east these
midsummer evenings. This region is rich in interesting celestial
objects. Just a couple of degrees east of Deneb, the bright, white
star at the top of the cross, is a large cloud of faintly glowing
gas which looks, to the unaided eye, like a slightly brighter patch
of light in the surrounding Milky Way. Photographic time
exposures taken by a telescope show it to be a glowing, red nebula
with a shape remarkably similar to the contours of the North
American continent from which it takes its common name, the North
American Nebula. The fairly sharp edges of the nebula are
delineated by intervening dark clouds of interstellar dust and gas
that obscure most of the distant stars behind them from our view.
This glowing red nebula is a busy, star-forming region where newly
born, but still hidden, stars provide the energy to cause the gas to
fluoresce. Many such "star nurseries" exist, usually in the spiral
arms of galaxies scattered around our observed universe.
Over the past few decades, Cygnus has been the site for the
appearances of several nova stars. The Latin word "nova" means
"new" and these stars are so named because observers looking into
the night sky will see a bright star shining in a spot where, on the
previous night, no star was visible. In their gracious manner,
ancient Chinese astronomers referred to these sudden apparitions as
"guest stars". Novae are not really "new" but rather very old stars
that have run out of nuclear fuel and are now called white dwarfs,
slowly cooling, eventually to become dark cinders. If the white
dwarf happens to be part of a binary system, it can extend its
active life by stealing hydrogen gas from its companion star until a
critical mass of hydrogen has collected on its surface, and is
raised by compression to a temperature of about ten million degrees,
whereupon the entire surface of the white dwarf explodes in a
colossal, hydrogen bomb-type detonation that increases the radiance
of the star by factors of ten thousand to a hundred thousand times.
Such an event occurred on the night of August 30, 1975, very close
to Deneb and not far from the North American Nebula. For one
night the nova shone almost as bright as Deneb, completely altering
the appearance of the Northern Cross and then, within a week, it had
faded to naked eye visibility.
An even mightier blast occurred in a location just south of the
southernmost of the three stars that form the crossbar of the
Northern Cross. About 30,000 years ago a supergiant star blew
itself to bits in a massive blast known as a supernova explosion.
One can only imagine the startled reaction of the cave dwelling
humans at that time to a new star probably bright enough to make the
entire countryside visible at night. The stellar debris from the
explosion was hurled into the cosmos at speeds higher than a million
miles per hour. The still-expanding bubble of gas, known as the
Veil Nebula, has now reached a diameter of 70 light years. It is
easily visible in large telescopes and even in backyard telescopes
with proper filters. It appears as a lacy loop of faint light with
about five times the diameter of the Moon.
It is interesting to note that recent, careful studies of
specific atomic isotopes in the Earth's crust strongly suggest that
2.8 million years ago, a similar supernova explosion destroyed a
star perhaps only a hundred light years away from the Earth. This
would have caused some damage to the Earth's atmosphere but would
not have been strong enough to cause a mass extinction. Astronomers
assure us that there are no supernova candidates close enough to be
a concern so we can sleep well - for now.
This is the
Skywatch Line for Monday and Tuesday, July 23 and 24.
This is Dudley Observatory's Skywatch Line for Friday, July 20, through Sunday, July 22, written by Alan French. The Sun now sets just before 8:30 PM and the last vestiges of evening twilight are gone from the western sky by 10:30. The Moon will be at first quarter at 2:29 AM Sunday morning, so the hours before midnight this weekend will be dominated by moonlight. Nightfall will find the Moon rather low toward the southwest. One of the most familiar star patterns is the Big Dipper. If you look toward the northwest around 10:30, you'll find it. Four bright stars outline the bowl of the dipper, and three stars extending from the upper left side of the bowl mark the handle. The bowl of the dipper is slanted downward toward the horizon and the handle extends upward into the sky. Look closely at the star in the bend of the handle, Mizar – the second star from the end. If you have good eyesight, you'll notice a second, fainter star right next to it. This is Alcor. To the Iroquois Indians, the four stars of the bowl represented a bear, and the stars of the handle and some of the stars in the neighboring constellation Boötes were Indians chasing the bear across the sky. Alcor was known as Chickadee, and the second, fainter star was a pot he was carrying to cook the bear in. The Big Dipper is an asterism – a pattern of bright stars, but it is part of the constellation Ursa Major, the Great, or Big Bear. In this version of the bear, the handle is a long tail. A triangle of stars to the right of the bowl marks his face. Lines of stars from the rear of the bowl mark his two hind legs, each ending in a pair of stars. Another line of stars going from the back of his face, also ending in a pair of stars, marks one of his front legs. Can you picture the bear standing in the sky? On Saturday, the Albany Area Amateur Astronomers will host a public star party in Glenville's Indian Meadows Park beginning at 9:30 PM. The park is on the west side of Droms Road, which runs between Swaggertown and Charlton Roads. The park entrance is about 1/2 mile south of the Parkside YMCA and is marked by a large wooden sign. After passing the park buildings on your right, follow the sign at the fork in the road. If there is no night baseball, it will direct you to bear left at the fork in the road. If there is night baseball, it will direct you to bear right. Telescopes will be aimed at the Moon, approaching first quarter, and other celestial wonders. Guests of all ages are always welcome at star parties and there is no charge. The star party is cancelled if the skies are mostly cloudy. If in doubt about the weather, call 374-8460 after 8:00 PM Saturday.
Skywatch line for Wed. and Thurs., July 18 and 19, 2007:
by Ray Bogucki
As we approach midsummer, the familiar asterism known as the
Summer Triangle is climbing high in the eastern sky at nightfall.
The triangle is formed by three bright stars from three different
constellations. Vega, at zero magnitude the brightest of the three,
shines in the constellation Lyra, the Lyre, near the zenith. About
30 degrees to the right, or south of Vega, is the second brightest
star, Altair, in the constellation Aquila, the Eagle. The third
star, Deneb, lies about 20 degrees below or northeast of Vega in the
constellation Cygnus, the Swan. Cygnus is most easily identified by
a collection of six stars forming a very credible shape of a cross
lying on its side, with first magnitude Deneb at the top. This
asterism is known as the Northern Cross.
Both Altair and Vega are a little larger and more luminous than
our Sun. Altair is the closest to us. At 17 light years distant,
the light we see tonight left Altair about the time the Berlin wall
came down. The light from Vega began its voyage to Earth about the
time of the premier of the film E.T. Deneb is in an entirely
different league. This supergiant white star is 82,000 times more
luminous than our Sun, and tonight's light left Deneb about the year
500 AD.
Cygnus lies along the Milky Way, the faint combined light of the
billions of stars that make up the spiral arms of our home galaxy.
Many historical measurements have been made on stars in Cygnus. In
1792, astronomer Guiseppe Piazzi noticed that a faint star,
designated as 61 Cygni, was moving at a relatively fast rate against
background stars, measured over a period of years. This suggested
to F.W. Bessel that the star was relatively close to the Sun, and in
1838, he set about to calculate the distance to this star by its
parallax, the slight back and forth shifting of the star's apparent
location as the Earth swung the 186 million miles from one end of
its orbit to the other. His success in this first calculation of a
star's distance opened the door to the quantitative investigation of
the Universe.
In 1970, NASA launched an early satellite telescope named Uhuru,
to study puzzling X-ray emissions from various sources in the night
sky. One of the strongest sources, designated Cygnus X-1, lies very
close to the third star in the long bar of the Northern Cross.
Radio astronomers found that the X-ray source coincided with a giant
but distant blue star. Sudden changes, or flickers, in the X-ray
energy occurred in time periods as short as a thousandth of a
second. This clearly indicates that the body emitting the X-rays is
very small -- perhaps no more than 200 miles across. Spectroscopic
measurements show that the giant star and its tiny companion orbit
around their common center of mass in a 5 1/2 day period, and that
the tiny invisible object has an amazing mass of about five times
the mass of our Sun. Many astronomers have proposed that the tiny
object is a black hole whose powerful gravitational field is pulling
gaseous matter from the giant star to itself at dizzying speeds that
raise the gas temperature to millions of degrees, which would
explain the X-ray emission. While the behavior of this distant
binary system provides some of the strongest evidence to date for
the existence of black holes, their precise physical nature remains
a profound mystery.
This is the Skywatch Line for Monday and Tuesday, July
16 and 17. This is Dudley Observatory's Skywatch Line for Friday,
July 13, through
Skywatch line for Wednesday and Thursday, July 11 & 12, 2007
by Ray Bogucki
Astronauts, orbiting a couple of hundred miles above the Earth's
surface during a total solar eclipse, have reported watching the
dark, round, umbral shadow of the Moon racing across the Earth's
surface. Reading these fascinating accounts brings to mind the many
ways that shadows play a part in astronomical observing. Probably
the most spectacular event is the aforementioned total solar eclipse
which is observed by anyone fortunate enough to be in the path of
the Moon's shadow. It has been described by some observers as the
appearance of a perfect round black hole punched in the sky with
faint white rays of light, known as the corona, streaming out for
millions of miles from the hidden surface of the Sun. The Earth is
plunged into the darkness of late twilight, bright stars and planets
appear in the sky, the temperature drops about ten degrees, flowers
close, and birds revert to nocturnal behavior. If you ever have an
opportunity to witness a solar eclipse, don't miss it! The next
total solar eclipse occurs on August 1, 2008.
The Earth also casts a shadow, much larger than the Moon's shadow,
but it is not noticeable until the Full Moon moves into it during a
total lunar eclipse. Unlike the Moon, the Earth casts an umbral
shadow that is not completely dark because some sunlight is
refracted through the Earth's atmosphere. Instead of disappearing
completely, the Full Moon usually drops from bright white to a pale,
coppery red. When fully immersed in the umbra, the Full Moon
presents a remarkably 3-dimensional binocular view which clearly
shows the Moon to be a spherical ball of rock rather than the round
disk-like appearance it usually presents when fully illuminated by
the Sun. At our location, the next lunar eclipse will occur in the
early morning of August 28 when the fully eclipsed Moon will set in
the west as morning twilight begins to brighten the sky.
We can also see shadows of more distant planets and their moons.
Currently, the angle of the Sun's light on the ringed planet Saturn
throws a shadow of the spherical planet onto the rings behind,
giving a marvelous 3-dimensional view. A little closer in,
Jupiter's four large moons routinely cast their shadows on Jupiter's
surface. While Galileo's optics 400 years ago may have been too
primitive for him to see their shadows, it is a favorite pastime of
today's amateur astronomers with backyard telescopes to watch the
tiny black dots of their shadows sweeping across Jupiter's surface.
For example, tomorrow (Thursday) the shadow of Io, Jupiter's
innermost moon will begin to cross the planet's surface at about
8:30 p.m. in bright twilight shortly after sunset and leave
Jupiter's disk about 10:30 p.m. in full darkness. On Saturday
evening, the shadow of Ganymede, Jupiter's third moon, will enter
Jupiter's disk about 11:45 p.m. and finish its crossing about 2 a.m.
It is a mundane expectation that we will see our own shadows cast
on the ground by the Sun or bright Moonlight, but for a more
uncommon experience, try this experiment. At a dark sky location,
far from city lights, and with an unobstructed view of the western
horizon, face brilliant Venus, low in the west, which has now
reached its maximum brightness, and moving your finger back and
forth in front of a piece of white cardboard, try to catch the faint
shadow of your finger cast by Venus light. But do it soon -- Venus
will be setting earlier each evening as it prepares to pass between
Earth and the Sun in mid-August.
This is the Skywatch Line for Monday and Tuesday, July 9
and 10. This is Dudley Observatory's Skywatch Line for Friday, July 6, through Sunday, July 8, written by Alan French. Last weekend we had the close conjunction between Venus and Saturn when the pair was less than one degree apart. They are farther apart this weekend, but form a pretty trio in the early evening sky with Regulus, the brightest star in Leo, the Lion. Look for the trio around 10:00 PM. Venus is by far the brightest member of the trio, blazing 9 degrees above the western horizon. On Friday night, Saturn will be just over three degrees to the right of Venus, while Regulus will be four degrees to the upper left of Venus. By Saturday night, Venus will be almost four degrees away from Saturn, with the ringed planet still to the right of Venus. Venus will be somewhat closer to Regulus. By Sunday night, more the four degrees will separate Venus and Saturn, and Venus will be within three degrees of Regulus. The rapidly changing separations clearly illustrate the relatively fast motion of our nearest neighbor, Venus, among the stars. Weather permitting; the Albany Area Amateur Astronomers will host a public star party on Saturday night. At a Star Party a variety of telescopes are pointed at celestial objects for your enjoyment. The star party will be held at Landis Arboretum in Esperance beginning at 10:00 PM. Landis Arboretum offers dark skies and good horizons, with a fine view of the night sky. 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. On Sunday night, two bright satellites will pass over the capital district. Each will be about as bright as the stars in the Big Dipper. The Upper Atmosphere Research Satellite, which was launched from the Space Shuttle Discovery in 1991, will appear above the southwestern horizon at 10:38 PM. It will be highest at 10:41:22 when it will be 73 degrees above the southeastern horizon, and will appear almost directly overhead. It will vanish below the northeastern horizon just after 10:45. Lacrosse 3, a Military reconnaissance satellite with Synthetic Aperture Radar, will move out of the Earth's shadow just before 11:36 PM when 61 degrees above the southern horizon. Look very high in the south to see it appear. It will be highest at 11:36:30 PM when 69 degrees above the southeastern horizon, and will vanish in the northeast just after 11:41.
Skywatch line for Wed. and Thurs., July 4 and 5, 2007:
by Ray Bogucki
When amateur astronomy groups hold public meetings, often called
star parties or star watches, the astronomers are sometimes
surprised and delighted by the questions that arise. For example,
during an observation of a total eclipse of the Moon, when the Full
Moon passes through the Earth's shadow, a visitor may ask, "For a
hypothetical observer on the Moon, what does the Earth look like at
this point?" The answer might be that the dark disk of the Earth,
much larger than the disk of the Sun, is blocking the direct
sunlight from reaching the lunar observer, but the thick layer of
gaseous atmosphere is refracting the sunlight to generate a bright,
often fiery red rim of light around the Earth's disk.
On a day such as today, July 4th, the question might come up,
"Could an observer on the Moon see our fireworks displays, or,
conversely, could we see a similar display of fireworks if it were
set off on the Moon?"
One might answer that if the fireworks were on the dark part of
the Moon, the final mortar round in the finale, which goes off with
a chest-shaking detonation and a brilliant flash, might be picked up
in a large telescope as a momentary pinpoint flash of light.
While we are fairly certain that no one is shooting fireworks on
the Moon, serious Moon watchers have, from time to time, reported
detecting small flashes of light, or sudden color changes or
blurring of distinct objects on the Moon's surface. These
observations are known as Lunar Transient Phenomena or LTPs. Many
astronomers dismiss these as illusory or effects caused within
Earth's own atmosphere. For example, one of the thousands of pieces
of space junk that orbit the Earth as the result of hundreds of
rocket launches, might tumble into an orientation that momentarily
reflects the Sun from a flat, shiny surface just as it is
coincidentally passing in front of the dark surface of the Moon.
Still, many of these reports were made by highly respected observers
well before the beginning of the Space Age. One intriguing
explanation for flashes of light on the Moon proposes that they
could be caused by the high speed impact of particles, such as
cometary debris left in our orbit, with the Moon's surface. We are
all familiar with meteor showers caused when cometary particles
enter Earth's atmosphere at high speeds. Collisions with atoms and
molecules in our upper atmosphere raise the temperature of these
particles to incandescent heat. The high temperature causes the
atmospheric atoms and molecules to fluoresce, much as the electric
discharge in a tube of neon gas causes the familiar red fluorescence
in a neon sign. The traveling bright fluorescence in the atmosphere
provides the streak of light which we call a meteor or "shooting
star".
The Moon, tethered to the Earth by gravity, encounters the same
clouds of particles as the Earth does, but because the Moon has no
atmosphere, the high speed particles collide directly with the
Moon's surface. The high energy impact can raise the temperature at
that point to thousands of degrees with accompanying release of
heat and light. Whether or not these impacts might be visible from
Earth, the frequency and reliability of reports of light flashes,
and other phenomena, indicate that the Moon may not be quite as dead
and inactive a place as we have assumed it to be.
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