SHARON SHANKS | The Cosmos Spring's return is marked by star

It's nice to see Spica peeking above the eastern horizon again. By the time the sun is fully set, this sure sign of the return of spring is rising, bringing with it the renewal of life.
The brightest star in the constellation of Virgo, Spica is a first magnitude star and the 16th brightest in the nighttime sky. It is supposed to represent the ear of wheat that Virgo holds in her left hand. It is about 260 light years away and 2,300 times more luminous than our sun.
It's a good star for comparisons, sort of the perfect "one" of magnitude.
When astronomers talk about the magnitude of a star, they are describing its apparent brightness to our eyes. The brightness, of course, depends on how far away the star is and its actual luminosity. A star's apparent magnitude can be quite different from its absolute magnitude -- how bright it would appear from a standard distance of 10 parsecs (32.6 light years) away.
Brightness scale: The numeric scale of brightness starts with first magnitude. Stars of second magnitude are dimmer. People with good eyesight and dark viewing locations can see stars of sixth magnitude; beyond that, the stars are too dim to see with the unaided eye.
On the logarithmic scale of magnitude, a sixth magnitude star is 100 times dimmer than a first magnitude star.
Stars that are even brighter than first magnitude are assigned smaller number. A star of zero magnitude is brighter than a first magnitude star, and stars assigned negative numbers are brighter yet.
Let's use Sirius, the brightest star in our nighttime skies, and Rigel, a bright star in the nearby constellation of Orion, as examples. Sirius has an apparent magnitude -- how bright it looks to us on Earth -- of 1.4. Rigel's apparent magnitude is .12, or just barely above zero.
But when we factor in their distances and their actual luminosity, Rigel far outshines Sirius. Sirius is only about 9 light years away; Rigel is 900 light years away. Sirius' absolute magnitude at 10 parsecs is a respectable 1.4; Rigel's absolute magnitude is 7.1. If Rigel were as close to us as Sirius, it would far outshine any other star in the nighttime sky and would be even brighter than the planet Venus, which shines at 4.2 or so at its brightest.
Another example of distance vs. brightness is close by our example: Betelgeuse, the brightest star in Orion. The monstrous Betelgeuse is about 450 light years away and is nearly 10,000 times brighter than our sun, which is an ordinary yellow star. Its apparent magnitude, however, is only about .7. Sirius, remember, is only 9 light years away and seems to be brighter than Betelgeuse, but it is only 20 times brighter than our sun.
Benchmark: When looking at stars and trying to place their magnitude, Spica is a convenient star to use as the standard for apparent magnitude. Both Sirius and Rigel appear to be brighter, so their apparent magnitudes must be smaller numbers.
In mythology, Virgo is sometimes associated with the goddess Astraea, the deity of justice. Others link her with Persephone, the daughter of Ceres, the goddess of the harvest.
Persephone, according to Greek and Roman mythology, was abducted by Hades, the god of the underworld, who was enchanted by her beauty and wanted to make her his wife. When Ceres discovered her daughter was missing she searched the ends of the earth for her, neglecting her duties and causing famine to spread across the world. The Olympian gods intervened and ordered Hades to return Persephone to her mother so Ceres would resume her vital work. Hades agreed, on one condition: Persephone could return if she had consumed no food while in the realm of the underworld.
Fanciful explanation: Alas, Persephone had eaten three seeds from a pomegranate and was apparently doomed to spend the rest of eternity in the darkness of the underworld. Ceres mourned the loss of her daughter and the world continued to starve -- until the gods stepped in again. They convinced Hades to allow Persephone to return to her mother for half of the year and spend the other half with him.
This arrangement suited everyone and also conveniently explains the changing seasons: that after the harvest there is no growth because Ceres is mourning again the return of her daughter to Hades. In the spring, life and growth return as Ceres rejoices once more.
Romantically, the reappearance of Spica, the bright marker for Virgo, signals the return of the spring growing season; Ceres is happy again (and so are we).