Celestial Lights
The brilliant and colorful stars of winter
By Phyllis Burton Pitluga January 4, 2008 San Miguel de Allende

PHOTO CAPTION Earthrise over the Moon: a recent photo from the Japanese Kaguya orbiting spacecraft

We are fortunate here in the northern hemisphere that our longer winter nights are adorned with some of the brightest and most colorful stars visible from Earth. Look high overhead at 11pm in early January, at 10pm in mid-January or at 9pm in late January (the time difference is because we are orbiting the Sun, causing the stars to appear to drift westward). 

Star identification. Follow this sequence to identify the stars:

Overhead is the bright red planet Mars.

Underneath Mars is the red star Betelgeuse (shoulder of Orion, the Hunter). 

Beneath Betelgeuse are the three stars in a row that create Orion’s belt.

Beneath the three stars, to the lower right, is the bright star Rigel (hip of the Hunter).

The three “belt stars” point up to the red star Aldebaran (eye of Taurus, the Bull).

These three stars point down to the brilliant star Sirius (nose of the dog, Canis Major).

Star colors come from the temperature of their uppermost gases. Aldebaran and Betelgeuse are both a reddish hue, indicating a cooler exterior temperature (2200ºF, 4000ºC). Both stars are swelling as they undergo a “fuel shortage” deep in their interiors. The swelling stretches and cools the outer atmosphere of the stars to a reddish color. By contrast, Rigel and Sirius both emit white light from hotter exterior gases (6100ºF, 11,000ºC). Rigel is much bigger yet much farther away. Sirius is the closest star beyond our star, the Sun. The Sun is eight light minutes away and Sirius is eight light years away whereas Rigel is some 800 light years from us. Thus Sirius is much brighter than Rigel. 

Planet colors come from sunlight reflecting off of their uppermost layer. Mars is red because of sunlight reflecting off of its rusty iron-rich surface rocks and sand. Venus, Jupiter and Saturn are yellow-white because the sun reflects off of their cloud tops. 

Phyllis Burton Pitluga is Astronomer Emerita, Adler Planetarium & Astronomy Museum, Chicago.


Sky Calendar, January 2008

By following the Moon as the biggest and brightest “pointer” in the sky, during the month you can identify different planets and bright stars. On following nights you can relocate them but without the Moon—the Moon moves about 25 times its own diameter from one night to the next. The Moon is much closer than the planets of our solar system and the stars are even farther. So, when the Moon appears close to a celestial light they are truly separated by millions, billions or trillions of miles.

In January, Mars is high in the southeast at sunset, crosses the sky all night and sets at dawn. Saturn rises late in the evening and is overhead around 3am. Venus is brilliant in the dawn sky. Jupiter rises just before sunrise in late January.

Jan. 2, Wednesday: Earth is closest to the Sun at 91.4 million miles in its yearly journey around our star (average distance is 93 million miles). Paradoxically, this is also the time of year when our days are shorter and cooler than when we are farthest in early July. This is because the Northern Hemisphere is tipped away from the Sun now. 

Jan. 4, Friday: the Quadrantid meteor shower rains meteors into our atmosphere between 1am and 5am low in the northern sky. Because of our more southerly latitude, we will miss about half of the 120 meteors per hour as they shoot down below the northern horizon.

Jan. 8, Tuesday: New Moon (= no Moon because the dark side faces Earth) 

Jan. 15, Tuesday: First Quarter Moon (looks like half a moon; half is turned away from us; the half we see is half lit = Quarter Moon)

Jan. 19, Saturday: Waxing Gibbous Moon passes above Mars

Jan. 21, Monday: Mercury is separated farthest from the setting Sun and can be seen tonight setting above the glow of twilight about 6:45pm. 

Jan. 22, Tuesday: the Full Moon rises at sunset; for the next two weeks the Moon rises about 50 minutes later each night, making it also visible in the morning sky as the Moon orbits toward the direction of the Sun. 

Jan. 24, Thursday: the Waning Gibbous Moon passes below the planet Saturn.

Jan. 29, Tuesday: Last Quarter Moon.