Leo is one of the most familiar constellations in the sky. It has been recognized as a lion since ancient times. Look for a pattern of stars resembling a backwards question mark; this represents the head of the lion. The bright star Regulus is at the bottom of the figure. For northern observers, Leo is best viewed from February through July.
The Leonid meteor shower occurs around November 17th of each year. Normally, the Leonids only produces about 12 meteors per hour, but in certain years (most recently 1933 and 1966) this shower has become a "meteor storm" producing thousands of meteors per hour. This is believed to be due to the Earth's passage through a debris cloud from the comet which produced the material that generates the Leonid meteors.
History and Mythology
Leo is the fourth constellation of the Zodiac, and one of the oldest star groups. The Greeks believed that Leo was the lion who had battled Hercules in the first of his twelve labors.
The lion was associated with midsummer by the Egyptians. Lions were associated with hot weather because they would come from the desert to the Nile Valley for relief from the heat of the desert. Their appearance coincided with the time when the Sun was in the stars of Leo. The head of a lion with open jaws was carved on the gates of the canals which were used to irrigate the Nile Valley with flood water.
Notable Stars
Alpha Leonis, or Regulus, is a first magnitude star; its name is Latin for "Prince". It marks the handle of a group of stars forming the "sickle" or reversed question mark which marks the lion's head. Regulus is the 21st brightest star in the sky. It is a blue-white star with 240 times the luminosity of the Sun, shining from 78 light years away. It spins extremely rapidly, with a rotation period of 16 hours; this causes it to have a highly oblate shape.
Denebola, or Beta Leonis, is the second brightest star in Leo and is located at eastern end of the lion's tail. Its name is shortened from the Arabic phrase meaning "tail of the Lion." It is a white 2nd-magnitude star physically similar to Sirius, but 34 light years away, as opposed to Sirius' 8.7.
At the lion's shoulder is the impressive 3rd magnitude double star Gamma Leonis, or Algieba. Algieba's name means "forehead" in Arabic. It consists of an orange 2nd-magnitude primary with a 3rd-magnitude yellow companion, separated by 4 arcseconds. The system is 125 light years away and has an orbital period over 500 years.
Delta Leonis, or Zosma, has a Greek name which means "girdle". At magnitude 2.6, it is Leo's 4th-brightest star; it is 58 light years away.
Leo also contains the 13th-magnitude red dwarf star Wolf 359. Much to dim to see except with large telescopes, this star is only 7.8 light years away, and is the third-closest star system to Earth. In the Star Trek universe, the Battle of Wolf 359 takes place with the Borg Collective in the year 2367.
Clusters, Nebulae, and Galaxies
The galaxies M 95 and M 96 form a pair near the middle of Leo. M 95 is a barred spiral, and M 96 has a dust lane which extends through the nucleus. Near them is M 105, a giant elliptical galaxy of magnitude 9.3. All three are members of the Leo I group of galaxies about 35 to 40 million light years away.
The most impressive objects in Leo are M 65 and M 66. They are both spiral galaxies on the eastern end of the constellation. M 65 is a highly inclined spiral with a prominent dust lane extending across the disk of the galaxy.
M 66 is a spiral with two bright arms which loop outward from the nucleus. This galaxies are best seen with an 8 inch or larger telescope. Along with the nearby 10th-magnitude spiral galaxy NGC 3628, M 65 and M66 make up the famous "Leo Triplet".
Other noteworthy galaxies in Leo include NGC 2903, a 9th-magnitude spiral viewed at an oblique angle from 20 million light years away; the elliptical galaxy NGC 3384, about 35 million light years away; and the 9th-magnitude spiral galaxy NGC 3521, also 35 million light years away.