Leo is one of the larger constellations in the northern hemisphere. Its name means "lion" in Latin. In Greek mythology, the constellation is identified with the lion of Nemea, killed by Heracles as one of his Twelve Labours. The lion was terrorizing the town of Nemea near Corinth and its skin made it invulnerable. Heracles was unable to kill it with arrows as they bounced off the lion's skin, so he followed the beast into its cave and choked it to death. He kept the pelt and later wore it for protection in battle.
The constellation Leo is usually depicted as a crouching lion, with its head outlined by six stars that form the shape of a sickle, its heart marked by the brightest star in the constellation, 
alpha Leonis (Regulus), and 
beta Leonis (Denebola) lying on the tip of the lion's tail.
Greek scholar Eratosthenes and Roman author Hyginus both say that the lion was placed among the stars because of its status as king of the beasts. The Chinese know most of the stars in the constellation as Xuanyuan, the Yellow Dragon. The Chaldeans associated the constellation with the Sun because, in those times, the Sun was passing through Leo during the summer solstice. Egyptians venerated the lion in the sky because the period of the Sun passing through the constellation coincided with the rise of the river Nile.
The constellation Leo occupies an area of 947 square degrees and contains eight stars with known planets. It can be seen at latitudes between +90° and -65° and is best visible at 9 p.m. during the month of April. The Sun is considered to be in the constellation between August 16 and September 15 in sidereal astrology and between July 24 and August 23 in tropical astrology.
The brightest star in Leo is 
alpha Leonis, also known as Regulus ("prince" or "little king"), Basiliscus ("little king"), Cor Leonis ("the heart of the lion"), Qalb, Kabelaced, Qalb al-Asad ("the heart of the lion"), and Rex ("king"). Lying about 77.5 light-years away, Regulus is one of the brightest stars in the sky along with being the closest of the brightest stars to the ecliptic. It is a multiple system consisting of two binary star systems. Regulus A is a spectroscopic binary with a blue-white main sequence star as the primary component and a white dwarf as the companion. Regulus B and C are faint main sequence stars.
beta Leonis, also known as Denebola ("the tail of the lion"), is the second brightest star in the constellation. It is an an A-class star and a Delta Scuti type variable, with slight variations in brightness every few hours. Along with 
alpha Pictoris, 
beta Canis Minoris and stars in the open cluster IC 2391, Denebola is part of the IC 2391 supercluster, a stellar association also known as the Omicron Velorum Cluster, located in the constellation Vela. The stars in the cluster share a common motion through space, but are not gravitationally bound, which means that they were probably created in the same location.
gamma Leonis, also known as Algieba ("forehead"), is a binary star that consists of a K-class giant and a G-type giant. It appears as a bright double star with orange-red and greenish-yellow components. The primary star has a planetary companion. Together with 
zeta Leonis and 
eta Leonis, Algieba is part of the celestial sickle that marks the lion’s head. 
zeta Leonis, also known as Adhafera ("the braid" or "the curl"), is a yellow-white giant 260 light-years distant. 
eta Leonis or Al Jabhah ("the front" or "the forehead") is a white supergiant, a suspected binary star, 2,100 light-years distant.
Other notable stars in Leo include 
delta Leonis, also known as Zosma ("girdle") and Duhr, a main sequence star located at the lion’s hip, 
epsilon Leonis, also known as Ras Elased Australis or Asad Australis and Algenubi ("the southern star of the lion’s head"), a bright yellow giant, 
theta Leonis or Chertan ("two small ribs"), a white dwarf, and 
mu Leonis, also known as Rasalas and Alshemali (abbreviations of an Arabic phrase meaning "the northern star or the lion’s head"), a K-class, metal-rich giant 133 light-years distant.
The star Wolf 359 is interesting for being one of the closest stars to Earth, at 7.7 light-years’ distance. It is a very faint red dwarf and can only be seen in a telescope. The only stars nearer to Earth are 
alpha Centauri in the constellation Centaurus
and Barnard’s Star in Ophiuchus
. Another red dwarf in Leo, Gliese 436, has one the smallest known extrasolar planets in its orbit.
Leo contains a number of bright galaxies. The best known ones are spiral galaxies Messier 65, Messier 66, Messier 95, Messier 96 and elliptical galaxy Messier 105. Messier 65 (NGC 3623) is 22 million light-years away. It is a intermediate spiral galaxy that forms the famous Leo Triplet group of galaxies along with Messier 66 (NGC 3627), another intermediate spiral galaxy, 95,000 light-years across in size, and NGC 3628, a barred spiral galaxy notable for an obscuring band of dust on the outer edge of its arms.
Messier 95 (NGC 3351) is another barred spiral galaxy, 33 million-light-years distant, with a ring-like star-forming area around the nucleus. Messier 96 (NGC 3368) is an intermediate spiral galaxy, the brightest one in the M96 Group, which also includes M95, M105 and nine other galaxies. Messier 105 (NGC 3379) is an elliptical galaxy with a confirmed supermassive black hole
Another interesting object in the constellation is the Leo Ring, a huge primordial cloud of helium and hydrogen gas orbiting two galaxies, left over from the Big Bang.
Leo belongs to the Zodiac family of constellations, along with Virgo
Constellations directly bordering Leo are Ursa Major
, Leo Minor
and Coma Berenices