Leo is the 12th biggest of the 88 recognized constellations, and the 3rd biggest zodiacal constellations, taking up an area of 947 square degrees of the northern celestial hemisphere. The brightest star in Leo is Regulus, (Alpha Leonis), a complex four-star system located about 80 light years away. With a combined apparent visual magnitude of +1.35, the Regulus system is also the 21st brightest “star” in the entire night sky.
The constellation can be seen from latitudes of between +90° and -65°, and in northern hemisphere becomes visible around the time of the March Equinox, subsequently remaining visible for most of the night until late in May. The best time to observe Leo, though, is at about 9 PM Local Time in April, when it is highest in the sky.
Apart from two meteor showers, the Leonids and the January Leonids that have their radiants in Leo, the constellation contains a large number of galaxies that are relatively easy targets for amateur observers, the most notable of which are explored in this list.
Leo Triplet Group (M66 Group)
Seen at the top of this page, Messier 65 (top right), Messier 66 (bottom right), and NGC 3628 (left) together form the famous Leo Triplet Group of galaxies, otherwise known as the M66 Group, which is a trio of interacting spiral galaxies located about 35 million light-years away.
Messier 65 (M65, NGC 3623)
This large magnitude 10.25 intermediate spiral galaxy is 90,000 light years across, and located about 35 million light years away. It does not contain an over abundance of gas and dust, and mostly consists of ancient stars. However, some recent star formation activity and the fact that the galaxy’s disc is slightly distorted suggest that M65 is currently engaged in a gravitational tug-of war with its large neighbours.
Messier 66 (M66, NGC 3627)
This magnitude 8.9 intermediate spiral galaxy is located about 36 million light years away, with the galaxy’s 95,000-light year wide disc showing prominent dust lanes and very luminous star clusters in its spiral arms. The most notable aspects of M66 is the very high mass concentration in the galactic core, as well as the fact that the interaction with its neighbors is currently stripping an enormous chunk of non-corotating H1 material from one of its spiral arms, which can be seen in the distorted shape of its lower spiral arm.
NGC 3628 (Arp 317, Hamburger Galaxy)
This unbarred spiral galaxy, also referred to as the Hamburger Galaxy or Sarah’s Galaxy, is around 100,000 light years wide, 35 million light years distant, and consists mostly of young open star clusters. It is also the faintest member of the Leo Triplet Group, and has a warped disk believed to be a result of its tidal interactions with M65 and M66.
NGC 3628 is notable for the prominent dust lane that encircles it, and its 300,000 light year-long tidal tail formed from material being tidally stripped from it. Another notable aspect that can be seen in this image is the faint shape that rises above the plane of the galaxy. This represents the boxed-shaped central bulge that extends beyond the galactic plane as a result of the violent churning motions of dust, gas, and young hot stars in, and immediately around, the central bulge.
Leo I Group (M96 Group)
Located near to The Leo Triplet is a collection of between 8 and 24 galaxies collectively known as the M96 Group, which also includes three Messier objects located around 32 million light years from Earth, namely M95, M96, and M105.
Messier 95 (M95, NGC 3351)
This magnitude 11.4 barred spiral galaxy is 46,000 light years across, with its central ring about 2,000 light years in diameter. It contains around 40 billion stars, with the bulk of the galaxy’s star formation activity taking place within this region. Seen fully face-on from a distance of 38 million light years away, this view offers a clear view of the central ring around the core, which is formed by the spiral arms before they start to unwind.
Messier 96 (M96, NGC 3368)
At about 100,000 light years in diameter, M96 is the largest and brightest member of the Leo 1 Group of galaxies, with an apparent visual magnitude of 10.1. It is classified as a double-barred spiral, since it has a small inner bulge through the centre along with a larger, outer bulge. The pronounced distortions of the spiral arms are thought to be the result of continual gravitational interactions with its neighbors, and its high luminosity in UV frequencies suggests that the galaxy contains a super massive black hole in its core. M96 is located about 31 million light years away.
This elliptical galaxy is 54,000 light years across, and has an apparent magnitude of 10.2 from a distance of 32 million light years away. The galaxy contain around 40 billion stars, and is believed to have a supermassive black hole at its center weighing between 140 and 200 million solar masses.
Other Notable DSO Objects
Located about 30 million light years away, this barred spiral galaxy has an apparent visual magnitude of 9.7, but dark skies and excellent seeing conditions are required to observe it.
NGC 3626 is a magnitude 10.9 medium-tightness spiral galaxy that is located about 70 million light years away. It is a member of the NGC 3607 Group of Galaxies, which in turn, is one member of the larger Leo 11 cluster of galactic groupings. Look for NGC 3626 close to the bright star Zosma (Delta Leonis).