StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

NOVEMBER 2013

NOVEMBER STAR MAP | MOON PHASE CALENDAR | STARWATCH INDEX | NIGHT SKY NOTEBOOK

Print Large Sky Charts For 9 p.m. EST: NORTH | EAST | SOUTH | WEST | ZENITH

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898 NOVEMBER 3, 2013: Autumn Fills the Sky: Last week, my Moravian College astronomy students visited Shooting Star Farm, north of Quakertown, for their dark sky field experience. It was one of those iffy days where the clarity of the night was in jeopardy. Eastern PA remained partly cloudy while the rest of the state, including NJ, became overcast. My instincts to hold the event were correct because after sunset the orange tinged, purplish clouds began to dissipate, and the sky turned denim lapis, revealing diamond-like Venus in the southwest. West of the zenith in the twilight were the holdouts of last season’s sky, the stars of the Great Summer Triangle, Vega (brightest), Deneb (faintest), and Altair with a pale hint of the Milky Way passing in between them. Already high in the east were the bright stars of the Great Square of Pegasus, the flying horse, revealed like the bases in a ballgame with the seductive Andromeda trailing behind. She had been chained to a rock as a sacrifice to Cetus, the Sea Monster. In the northeast was Andromeda’s rescuer, winged-footed Perseus the Hero, and above Perseus, the sideways “W” of Cassiopeia, Queen of Ethiopia and mother of Andromeda. Cassiopeia had boasted about her beauty and angered the gods, precipitating the near death of her daughter. “Mama Cass” sat on a crooked backed chair, being swung upside-down, as punishment for her vanity. Low in the north, the Big Dipper was starting to the scrape the horizon, and low in the east the gossamer Pleiades of Taurus the Bull and Capella of Auriga the Charioteer were rising. The seasonal stars of the autumn sky were in full view after sundown with hints of winter in the east. By 9 p.m. my students were numbed from the cold and with frosty exhalations begged to be dismissed. I consented with a warm smile, still cozy in my multiple layers of fleece. Check “this week’s StarWatch” at astronomy.org for maps.

899 NOVEMBER 10, 2013: Looks can be Deceiving: Although we are moving towards the icy grip of winter, some of the vestiges of the summer sky can still be seen high in the west during the early evening hours. The Great Summer Triangle (GST) is my favorite pattern of the fall sky, for as much as I enjoy those surprise winter snow events, I mentally try to hang onto the aura of summer as long as possible. The GST is not really a constellation. It is derived from the brightest luminaries found in three constellations: Vega, the GST’s brightest star of Lyra the harp; Deneb, the faintest of the triad in Cygnus the Swan; and middle of the road, Altair of Aquila the Eagle. After showing my Moravian astronomers the Great Summer Triangle at Shooting Star Farm several weeks ago, one of my students, Sophia Osbourne, posed the question of brightness vs. nearness. “Are the three stars of the GST bright because they are near?” As any good politician might respond, my answer was “yes and no.” In order to answer that query, astronomers had to be able to calculate stellar distances. That problem was not solved to any high degree of accuracy until the early 1990s with the European Space Agency’s Hipparcos satellite, even though the first parallax (triangulation) measurements of stars occurred in the 1830s. Now we know that Altair is 17 light years distant, Vega 25 ly, and Deneb, a staggering 1550 ly away. Astronomers mathematically move stars to a standard distance from the sun to compare their brightnesses. When we do this for the Great Summer Triangle’s stars, Altair turns out to be the wimp, but still 11 times brighter than our sun, Vega, about 50 times Sol’s luminosity; but Deneb, which is visually the faintest star of the GST, turns out to be the real powerhouse. It is nearly 60,000 times the sun’s brilliance. In fact, Deneb is the brightest star for its distance in the entire sky, proving again that in astronomy, looks can be deceiving.

900 NOVEMBER 17, 2013: Comet ISON: Hit or Miss?: The next few weeks hold the promise of becoming one of the great observational periods for astronomy in the 21st century. That may seem too great a claim to be made this early in just 2013, but astronomers have been watching Comet ISON rapidly brighten as it heads inward towards a Thanksgiving Day encounter with the sun. ISON stands for the International Scientific Optical Network located near Kislovodsk, a spa city between the Black Sea and Caspian Sea. Using a 16-inch reflector on September 21, 2012, Vitali Nevski of Belarus and Artyom Novichonok of Russia first imaged the interloper, but prediscovery photos were found to exist as far back as late November of 2011. What makes Comet ISON so interesting is that it is a sungrazer. It will pass the sun at only 724,000 miles from its surface, causing the sun’s energy to “boil” away a substantial amount of itself in the days prior to and after its closest approach. There are two possible scenarios to the ISON story. One is the comet will simply evaporate in the hellish million degree environs of the sun’s corona because the nucleus of ISON, the origin point for the tails and other features which a comet possesses, is not very big, only three quarters of a mile in diameter at its greatest, according to Hubble and Spitzer Space Telescope measurements. If the comet disintegrates, we see nothing. Situation two is that ISON makes it around the sun and into our northern hemispheric morning sky. On November 29 around dawn, the ESE may have a long gossamer spike projecting up from the horizon, the tail of a great comet. The head of ISON should become completely visible about an hour before dawn between the 4th and 6th of December. By Christmas, ISON will be visible all night, much higher in the sky, but much diminished in brightness; but still probably an object visible to the unaided eye. More about ISON next week…

901 NOVEMBER 24, 2013: Safe Journey, Comet ISON: Comet ISON continues to brighten the dawn sky, but now astronomers are wondering whether several new outbursts, one that was first seen on November 12, and another that was noticed on November 19, aren’t really the harbingers of a crumbling comet breaking apart as it plunges towards the sun. Comets are some of the smallest and most ephemeral objects of the solar system. They are mostly water ice, with the firmness of a well-packed snowball. Dry ice (frozen carbon dioxide) and methane ice are also part of the solid constituents of a comet, as well as small grains of dust, which in total make a comet a conglomeration of some of the most primordial “stuff” found in our solar system. As a comet approaches the sun, Sol’s energy begins to vaporize the ices on its surface, as well as beneath its surface too. Pressures build, the surface cracks, and jets of gas containing dust are released into space. The gases are ionized and fluoresced by the ultraviolet light from the sun, then sculpted by the sun’s magnetic field and solar wind to form a bluish ion tail which points directly away from the sun. The very smallest pieces of dust are less vigorously blown back by light pressure from the sun to form an arcuate yellow dust tail that backscatters sunlight to become visible. Larger dust particles remain in the comet’s path to turn into the shooting stars that we see at night if the comet’s orbit intersects the Earth’s orbit. That will not be happening with Comet ISON. If ISON makes it around the sun on November 28, get ready for what will become an easy naked eye target in the predawn skies of early December. If the sun breaks it apart, get ready for some fantastic space photos, but not much else in the way of anything spectacular to see from Earth. As a friend of my once said, “Comets are like cats. They have tails and do exactly as they want.” Safe journey, Comet ISON! See you in December.