Tonight – May 25, 2018 – the bright waxing gibbous moon, passes 7o north of the bright star Spica, a key star of the zodiac. That much brighter starlike object to the east of the moon and Spica tonight is no star at all but the brilliant planet Jupiter.
Spica, though a 1st-magnitude star, may appear puny in the glare of tonight’s moon – and in contrast to Jupiter. In fact, Jupiter is better than 20 times brighter than Spica right now. But Jupiter’s brightness, of course, stems from its location in our own solar system, while Spica is a distant star, some 262 light-years away. Spica ranks as the 15th or 16th brightest star to illuminate our nighttime sky.
Spica’s magnitude is virtually the same as that of Antares, another key star of the zodiac. Want to compare them, but don’t know how to recognize Antares? The moon will travel close Antares on the sky’s dome on May 28, as shown on the sky chart below.
Because Spica and Antares both vary somewhat in magnitude, chances are that Spica and Antares occasionally trade places as the 15th and 16th brightest stars.
You might have difficulty seeing the blue-white color of Spica or the red color of Antares in the moonlit sky over the next several days. But once the moon drops out of the evening sky – sometime in the first week of June – the color difference between these two stars will be obvious in a dark sky. It’s said that the human eye tends to see red stars as brighter than blue or white-colored stars of the same magnitude. Red stars appear to grow brighter as you stare at them, and they also appear brighter than their blue and white-colored counterparts in a light-polluted sky, due to something called the purkinje effect.
A blue-white star such as Spica is in the heyday of youth and has a high surface temperature of 22,400 degrees Kelvin (22,127oCelsius or 39,860o Fahrenheit). In contrast, red-colored Antares is in the autumn of its years, having a low surface temperature of 3,600 degrees Kelvin (3,327oC or 6020oF).
Bottom line: On May 25, 2018, let the moon guide your eye to blue-white Spica, the brightest star in the constellation Virgo the Maiden.