Nine objects of the first magnitude or brighter, not counting Moon, illuminate the early evening sky near the beginning of May. The brightest of the nine, by far, is the planet Venus, beginning the month at -4.5 magnitudes and fading to -4.1 by month's end. The remaining eight, from brightest to faintest, are the stars Arcturus in Bootes the Herdsman, Vega in Lyra the Harp, Capella in Auriga the Charioteer, Procyon in Canis Minor the Little Dog, Betelgeuse in Orion the Hunter, Spica in Virgo the Maiden, Pollux in Gemini the Twins, and Regulus in Leo the Lion.

    You probably recognize Capella, Procyon, Betelgeuse, and Pollux as winter stars. Vega is the western-most star in the Summer Triangle, making it more properly a summer star. Only Regulus, Arcturus, and Spica are truly stars of the spring sky.

    Arcturus is the brightest of these three, and the fourth brightest in the night sky. Only Sirius is brighter in our sky. ( Canopus and Alpha Centauri are also brighter than Arcturus, but they are not visible from our latitude.) Arcturus, from the Greek for Guardian of the Bear, is the brightest star in the northern half of the sky. In his book Star Names, R.H. Allen says that this star was one of the first to be given a name and was well known in ancient times. It is a giant star, about 25 times the diameter of Sun, and emits 115 times as much radiant energy as does Sun. It has a surface temperature of 7100 degrees F, a mass four times that of Sun, and is located about 37 light years away.

    Despite it's being relatively close to Sun, Arcturus is an interloper in our part of the Galaxy. It is moving quite rapidly with respect to Sun and most of our other neighbors, and is really a member of the family of stars making up the halo of the Galaxy, just passing through our region of the Galaxy on a highly inclined orbit around the center of the Milky Way. It first became visible tothe naked eye about half a million years ago, and will become invisible after another 500,000 years have passed.

    We find Arcturus by using the handle of the Big Dipper as it curves away from the bowl to "follow the arc to Arcturus." If we continue the rhyme we can "spin on to Spica" in Virgo, the second brightest of the "true" spring stars and the 16th brightest in our sky. Spica shines at 1.00 magnitude (with some small variations) and lies a little more than 2 degrees from the ecliptic, Sun's annual path through the stars. The name is from Latin for the spike of wheat that Virgo is often pictured holding in her left hand. Spica is about 2300 times as luminous as Sun, with a mass 10.9 times Sun's, a surface temperature of more than 36,000 degrees F, and a diameter 8.1 times Sun's. It is located at a distance of 275 light years, and has a companion that is so close that its presence is detected only by variations in the spectral lines emitted by Spica and the companion. Studies of the spectral variations yield a center-to-center separation between the two of about 11 million miles.

    The third spring star, Regulus, has an apparent magnitude of 1.36, making it the 21st brightest star. Its name means the Little King, and its location at the end of the handle of the "Sickle of Leo," a backward question-mark pictured as the head and mane of the Lion, often leads to its identification as the Heart of Leo the Lion. Regulus is about 85 light years from Sun and is 160 times as luminous as Sun with a surface temperature of 23,400 degrees F and a diameter about 5 times Sun's. Regulus lies less than a degree from the ecliptic. In 130 B.C. the Greek astronomer Hipparchus compared the positions of Regulus and Spica, measured along the ecliptic from Sun's location at the beginning of spring, with positions recorded by Babylonian observers in 2100 B.C. and found a large increase. He deduced that the orientation of Earth's rotation axis is slowly changing with respect to the stars, a motion we now call precession.