Why are stars different colors? At a casual glance they may all appear to be a uniform sparkling white but astronomers classify them differently. If you look at the constellation of Orion that dominates the night sky in February, Betelgeuse in the giant’s shoulder is classified as a red giant, but may appear yellow-orange, while Rigel diagonally across from Betelgeuse is classified as blue-white. What’s the difference?

In a nutshell, it is their temperature. Cooler stars are a different color from hotter ones. Just as a piece of iron first glows red, then yellow and finally white as it is heated, stars have a characteristic color based on their temperatures.

In fact, astronomers have a “spectral class” that they use to assign stars a letter of the alphabet based on their color and temperature. Cooler red stars, such as Betelgeuse or Aldebaran, are given the designation “M.” Our sun, a moderately hot star that glows yellow is “G,” and the hotter stars such as Rigel that are blue to bluish-white are “B.”

There are several intermediate grades in this generalized scale and a star may appear colored somewhat differently from listed because of atmospheric conditions. The spectral class is nonetheless a convenient means of giving information about the star’s temperature and, as a consequence, some idea of its size and its stage in the stellar life cycle. By the way, the hottest star discovered thus far is, at 360,000 degrees Fahrenheit, more than 30 times hotter than the sun.

Focus on the planets

Mercury will become visible on the western horizon around Feb. 10 and then gives one of its best appearances of the year until early March when it once again sinks from view. Look to the western horizon about 45 minutes after sunset on Feb. 28 for Mercury to the upper right of a thin sliver of crescent moon.

Venus blazes low in the southeast about three-quarters of an hour before dawn for much of the month. On Feb. 24, the waning crescent moon and Venus rise together in the pre-dawn sky.

Mars is high in the south-southwest at nightfall during the month. On Feb. 5, Mars, the moon, and Pleiades occupy the same region of the sky. Reddish orange Mars makes its way slowly toward Aldebaran, the “red eye of the bull,” during February.

Jupiter rises in the southeast shortly after midnight as February opens and by 11:30 p.m. by month’s end. The best time to view Jupiter and its four major moons is during the early morning twilight hours. About an hour before sunrise on Feb. 20 look for Jupiter with the moon to its immediate lower left. Farther to the left is the red-orange star Antares.

Saturn is well up in the east at nightfall situated near the southern edge of M44, the Beehive Star Cluster, early in the month. Saturn’s ring system is tilted for favorable viewing; a half-dozen of its moons will be dancing around the planet to delight viewers who have telescopes. The best time to see the moons are the nights of Feb. 18 and 19.

Uranus can be spotted about one-half degree from Mercury on Feb. 14. For viewers in some parts of the world, such as Russia, the separation will be only one minute of arc, the closest conjunction of any two planets until 2013.

Neptune is too close to the sun to observe in February.

Pluto will not be placed for viewing for several months even for those with the skill and equipment to find the tiny distant planet.

February events

1 Sunrise, 6:55 a.m.; sunset, 4:43 p.m.

2 Candlemas or Groundhog Day, the cross-quarter day marking the midpoint between the winter solstice and vernal equinox. Saturn is less than a degree from the Beehive cluster tonight.

5 Moon in first quarter, 1:28 a.m. The moon passes just to the upper right of Mars shortly after nightfall.

13 Full moon, 11:44 p.m. The full moon of February is called the Snow Moon, Hunger Moon or Wolf Moon.

14 Happy Valentine’s Day! The moon is at apogee, or farthest distance from Earth today. Mercury is only 0.02 degree from Uranus, the closest planet-planet approach this year.

16 The Sun enters Aquarius on the ecliptic.

18 The Sun enters the astrological sign of Pisces but astronomically has only just entered Aquarius. Tonight is a good chance to see six of Saturn’s moons about the ringed planet.

20 Jupiter is well up on the southern horizon at dawn with the moon just to its lower left. The red star Antares is well to Jupiter’s lower left.

21 Moon in last quarter, 2:17 a.m.

23 Look for Mercury low in the west after sunset.

24 The thin crescent moon is to the lower right of Venus on the southeast horizon about an hour before dawn.

28 The moon is at perigee, or closest approach to the Earth, today. New moon, 7:32 p.m. The combination of these two events could lead to abnormally high tides. Sunrise, 6:15 a.m.; sunset, 5:22 p.m.

Clair Wood taught physics and chemistry for more than a decade at Eastern Maine Technical College in Bangor.