The Constellation Leo

The constellation Leo is one of the few star patterns on which ancient Greek, Egyptian, and Indian astronomers agreed: they all saw a great celestial Lion ushering in the start of spring.  With its bright stars and unmistakable sickle-shape, Leo is a magnificent sight with nearly no end of delights for stargazers in both hemispheres.

Visible from March through June, Leo is the dominant constellation of the northern spring sky.  It’s easy to find.  Just look for the sickle-shaped group of stars high in the southern sky after sunset.  The sickle spans about 12 degrees… just a little wider than your fist at arms length.

The west-facing sickle, which makes up the head and mane of the lion, consists of the bright stars epsilon Leonis, Rasalhas, Adhafera, Algieba, eta Leonis, and, at the bottom, the icy-white star Regulus, the brightest star in the constellation.  About 15 degrees east, look for the little triangle of stars Zosma, Chertan, and Denebola that make up the haunches and tail.

Note: This year, it’s a little trickier to spot the sickle because the planet Mars is just to the west… the map below will help you sort it out.

The constellation Leo

The constellation Leo

As a constellation, Leo dates back to ancient times.  The Greeks beheld Leo as the fearsome Nemean Lion, which came to Earth as a shooting star from the Moon and terrorized the inhabitants of Nemea, south of Corinth.  The Lion lived in a nearby valley, which he littered with the parched bones of his unfortunate victims.

As with the serpent Hydra, it fell upon brave Hercules to dispatch the beast as the first of his twelve labors to redeem himself in the eyes of wrathful Hera.  Hercules knew the lion’s fur was too thick for mortal weapons to penetrate.  So by night, he snuck into the beast’s cave while it slept and strangled it with his powerful arms.  Zeus placed the beast in the sky in a place of honor.

While most western cultures associate the constellation with a lion, ancient Chinese astronomers saw the sickle and other nearby stars as a yellow dragon they called Xuanyuan.

Bright Regulus is a quadruple star system consisting of two pairs of two stars, all of which are gravitationally entwined and lie about 78 light years away.  Most of the light you see comes from Regulus A, a young B-type main sequence star that’s flattened to an egg-shape by its rapid rotational period of just 16 hours.  A second star lies too close to resolve in any telescope.  But even the smallest scope or a good pair of binoculars can resolve the other pair of stars, which at 8th magnitude appear as a single point of light about 3 arc-minutes away.

The area around gamma Leonis (called Algieba) is lovely in binoculars.  The 5th magnitude star 40 Leonis lies 20 arc-minutes (1/3 degree) to the southwest.  The two are not physically associated.  But in a telescope at 100x or more, you’ll see Algieba’s true companion star about 4 arc-seconds away.  The color contrast of the pair is striking: the brighter star looks orange-red and the fainter appears yellow-green.

For galaxy hunters, Leo offers fine quarry.  The easiest and prettiest grouping is the Leo Triplet, consisting of the galaxies M65, M66, and NGC 3628.  You can see them in a single low-power field of view in a telescope just a couple of degrees southeast of Chertan.  Dark sky is essential to see the dimmer NGC 3628.  But the other two can be seen even from the city.  They look like eyes staring out of the depths of intergalactic space, some 20 million light years away.

Look for the fine spiral galaxy NGC 2903 just off the snout of the lion (see map above).  Messier missed this fine galaxy while compiling his list, though it’s a challenge to see it in a telescope as small as Messier used.  A 3-4″ telescope shows the galaxy as a faint oval patch of light.  A bigger scope and long-exposure photograph shows the intricate structure of the galaxy’s spiral arms.

In an upcoming issue, we’ll tell you how to find a variable star in Leo that’s blasting off its outer layers and may be incinerating a giant planet in its orbit.  But that’s a tale for another day…