Thousands of years ago Leo was the constellation of high summer. The ancient Egyptians worshiped the celestial lion, and its bright star Regulus heralded the summer solstice and the life-giving Nile floods. Today Regulus no longer marks the June solstice, though it will be the star of the September equinox in another two thousand years or so. This is because of the wobble in the Earth's axis called precession. ("Ecliptic and Equinoxes" explains this – there's a link below this article.)
Leo is a Zodiac constellation, one of the band of constellations along Earth's annual path around the Sun. There have been changes to the shape of Leo, but essentially the stars have represented a lion for more than four thousand years, back to the times of the oldest civilizations of what is now the Middle East.
However the story associated with Leo in Greek mythology doesn't echo the Egyptian reverence for the lion. This lion was a fearsome monster, invulnerable to weapons, and eating the people of Nemea. The first of the twelve labors of Heracles (Hercules to the Romans) was to eliminate the Nemean lion. It was easier said than done, even for a strong, heroic son of Zeus, since arrows and spears just bounced off the creature. In the end, Heracles trapped the lion in its cave and in wrestling with it, grabbed it by the throat and strangled it.
Leo hosts an annual meteor shower, the Leonids, in mid-November. Around November 17 or 18 Earth crosses the path of Comet Tempel-Tuttle. The comet isn't around, but the debris it leaves behind collides with our atmosphere creating meteors, also commonly known as shooting stars.
The constellation is most prominent in the spring sky in the northern hemisphere and the autumn sky in the southern hemisphere. Most constellations don't really resemble what they represent, but I can imagine Leo as a crouching lion. Here is a diagram of Leo – what do you think? The head and mane are part of an asterism known as "the sickle." It looks to me like a backwards question mark with Regulus – the heart of Leo – as the dot.
Leo's brightest star is Regulus, which means "little king". It's also known as Alpha Leonis. Like the Sun, it's using hydrogen for fuel, but it's bigger than the Sun, blue-white in color and over three hundred times brighter. It also spins on its axis at 1.1 million km (700,000 miles) per hour which makes it pumpkin-shaped.
Regulus is misshapen because a spinning object bulges at the equator. Earth's diameter is somewhat bigger at the equator than from pole to pole, and the effect is even more pronounced in the giant planets which spin faster. As for Regulus, the diameter at its swollen equator is about a third bigger than the polar diameter. When the gas expands it gets cooler, so Regulus is five times brighter at the poles than at the equator.
Regulus is also the main star of a four-star system. A faint binary orbits it and there is a white dwarf companion. A white dwarf is a star that has used up its nuclear fuel, collapsed under gravity, and is slowly cooling. No one has seen this white dwarf, but it has been detected from its spectrum.
At the tip of Leo's tail is a young bluish white star called Denebola (Arabic for "lion's tail"). It's only 36 light years away from us, so is considered a near neighbor! Astronomers are interested in the debris disk around Denebola because planets form from such disks. If any planets were to be found, it wouldn't be a first for Leo. There are known extrasolar planets in Leo, including two orbiting the star 83 Leonis.
Leo is popular with backyard astronomers because of the large number of galaxies bright enough to be seen in small telescopes. The most popular is probably the Leo Triplet. [Photo by Daniel Verschatse] All three objects are spiral galaxies, only seeming different because we see them at different angles. You can see some spiral structure in M66 at the bottom left and M65 just above it, because they are tilted somewhat in our direction. However at the top left, NGC 3628 – discovered by William Herschel in 1784 – is seen edge-on.
Let's finish our tour of Leo with a star that is close to Regulus in our view of the sky. It's Wolf 359 and is actually about seventy light years from Regulus.It's so faint that we only notice it because it's so close to us: 7.8 light years. It's about the size of Jupiter with 1/63,000 of the Sun's luminosity. Star Trek: The Next Generation fans may remember that in 2367 Wolf 359 was the scene of a battle between the Federation and the Borg.
David Darling, Encyclopedia, http://www.daviddarling.info/encyclopedia
Gies et al, "A Spectroscopic Orbit for Regulus" http://arxiv.org/PS_cache/arxiv/pdf/0806/0806.3473v1.pdf
Ian Ridpath, "Star Tales: Leo the Lion," http://www.ianridpath.com/startales/leo.htm