Contents

The Night Sky in February 2011

By Harry J. Augensen
Professor of Physics & Astronomy, Widener University

Moon’s Phases
New Moon on the 2ndFull "Snow Moon" on the 18th                            

Stars and Constellations

 February nights bring with them not only frigid temperatures but also a brilliant tapestry of stars. The constellation Cassiopeia, which represents the throne of the Queen of ancient Ethiopia, can be seen high in the northwest, looking like the letter "M." Even higher in the northwest above Cassiopeia is Perseus, whose brightest stars form a horn shape which opens toward the nearby Pleiades cluster. Nearly overhead on February evenings is Auriga, the Charioteer, with the bright yellow star Capella as its "eye." Just south of Auriga is Taurus, with its bright orange star Aldebaran. Aldebaran is classed as a red giant star, and it stands in the foreground of a more distant loose cluster of stars known as the Hyades. Further to the west of the Hyades is the more famous and compact Pleiades star cluster, looking like a miniature dipper. Following Taurus to the east is Gemini and its two brightest stars Pollux and Castor. Orion the Hunter now stands high in the south, dominating the midwinter night sky. Orion's two brightest stars, Betelgeuse and Rigel, are classed, respectively, as red and blue supergiants, and are among the most luminous stars known. Orion's "sword" contains the Great Orion Nebula, a vast complex of star formation nearly 1500 light years from our solar system.

Just below and to the left of Orion is the brightest star in the night sky, Sirius, in the constellation Canis Major. Sirius, the "Dog Star," is one of the Sun’s nearest neighbors, at only 8.6 light years distance. This distance was first deduced using trigonometric parallax in 1838 by the famous German astronomer and mathematician Friedrich Bessel. In addition, Bessel observed that Sirius was traveling through space not in a straight line but in a wavy fashion, and he inferred that Sirius must have a faint companion star which is gravitationally bound to it. While Bessel could not actually see the faint companion star through his telescope because it was overwhelmed by the brilliance of Sirius, he deduced that it orbited Sirius in a period of 50 years. In 1862, while testing his newly constructed 18 1/2 inch telescope lens, Alvin Clark of Massachusetts was the first to spot the elusive companion star, known today as Sirius B or, more affectionately, the "Pup." Sirius B is now known to be a white dwarf, in which internal nuclear reactions are no longer sustained. The white dwarf possesses a diameter of only one-hundredth that of the Sun, or nearly the same size as planet Earth. My late friend and colleague, Swarthmore College astronomer Wulff D. Heintz, determined that the Sirius B has a mass of 0.92 solar masses, just a little less massive than the Sun. Combined with its miniscule size (by star standards), the white dwarf star must have a density of approximately fifteen thousand times that of lead!

Just a bit further to the east of Sirius is its neighbor Procyon in Canis Minor. Like Sirius, Procyon is also nearby, at 11 light years away, and also like Sirius, Procyon is a binary star system containing a white dwarf. After about 8 pm, you can spot some of the stars of spring mounting the sky in the east. In particular, Regulus in the constellation Leo, lies low in the east-northeast. Looking a little further northward, you may spot the Big Dipper, which is part of the constellation Ursa Major, or Big Bear, rising above the horizon.

 Naked-Eye Planets In the Evening and Morning Sky 

Jupiter looks like a brilliant cream-colored star in the southwest shortly after sunset. Once the sky becomes dark, Jupiter outshines all the stars and planets except for Venus, which won’t rise until the early morning hours. Jupiter’s reign in the evening sky is nearing its end, as it is sinking lower and lower toward the horizon with each passing night. This February, Jupiter sets around 9:30 pm at the start of the month, and by 8:00 pm at month’s end. Jupiter will remain visible in the evening sky through most of March, and will pass through conjunction with the Sun in early April.

Saturn resembles a cream colored star situated to the upper right of the true star Spica in Virgo. Saturn is currently located in the opposite part of the sky from Jupiter, and so as Jupiter prepares to exit the evening sky, Saturn is poised to enter it. Look for Saturn rising in the east after 10:30 pm on the 1st, and after 8:30 pm at month’s end.

Venus blazes like a yellow diamond in the southeast during the pre-dawn hours, easily outshining all other nighttime objects except for the Moon. Venus rises a full three hours before the Sun as February opens, well before the onset of morning twilight, but that interval shrinks to only two hours by month’s end.

Mercury rises only about an hour before sunrise as February begins, making it very difficult to spot just above the southeastern horizon in the pre-dawn twilight. That time interval only dwindles as the month progresses, and Mercury eventually reaches conjunction with the Sun on the 25th. Mars reaches conjunction with the Sun on the 4th, and is too close to the Sun to be seen all month. Mars will eventually reappear in the morning sky in late spring.

Information on lunar phases and rise/set times of Sun and planets is obtained from the US Naval Observatory Data Services at http://www.usno.navy.mil/astronomy/. Times given apply for observers near to the latitude and longitude of Philadelphia, USA: 40 degrees North latitude, 75 degrees West longitude.

For more information on astronomy and weather, visit http://www.widener.edu/stargazing/, then click on Web Links & Resources. A set of free sky maps can be obtained at http://www.skymaps.com/