Leo Constellation – A Lion in the Stars

In this article, we’ll be sharing with you everything you need to know about the constellation of Leo, including how to find it and deep space objects contained within it. We’ll also show you how to use it to find other night sky objects more easily.

The Constellation of Leo

Leo is the Latin word for ‘lion’ and the constellation of Leo is one of the few constellations that resembles its namesake, as you can see in the SkySafari screengrab below.

The constellation of Leo shown as a lion (from SkySafari 6)

This makes it one of the easiest constellations to identify, which is probably why it’s also one of the oldest. It has been associated with a lion as early as 4000 B.C.

In Greek mythology, Leo was the Nemean lion that Heracles killed during the first of his Twelve Labors. Heracles also appears in the legend behind the constellation of Cancer.

To help you spot Leo in the night sky, here’s how SkySafari shows its brightest stars (to magnitude 5.0). Click the image to open a full-screen version.

The stars in the constellation of Leo
The stars of Leo, click for a full-screen image

As you can see, Leo is a large constellation. It has an area of 947 square degrees and is the 12th largest of the 88 recognized constellations. Leo is bordered by the constellations Virgo, Ursa Major, Coma Berenices, Crater, Hydra, Leo Minor, Linx, and Sextans.

Many of us will recognize its famous asterism, the Sickle, which looks like a backward question mark.

The Sickle asterism
The Sickle asterism in Leo (from SkySafari 6)

If you’ve watched the annual Leonid meteor shower (peaks November 16-17), you may also recognize the sickle as the location of this meteor shower’s radiant point.

The January Leonids (peak ~Jan 4) are Leo’s second annual meteor shower and also have their radiant in the northwest part of Leo.

In the next section we’ll discover how to find Leo.

How To Find Leo In The Night Sky

As one of the zodiacal constellations, Leo is found along the ecliptic. It is visible to observers at latitudes between +90° and -65°.

Observers in the Northern Hemisphere will be able to see it in the evening from February to July, while those in the Southern Hemisphere will see it during their summer and autumn months.

The best views are had in May when the Leo transits due south at 10pm and rides midway between the horizon and zenith. The image below shows the view from mid-USA in May at 10pm.

Leo constellation reaching transit at 10pm in May
Leo transit at 10pm in May, click for a full-screen image

To find Leo, carry out a naked-eye hunt for the Sickle. Remember that it may be leaning on its side, depending upon when you are observing.

Six bright stars form the sickle that represents the lion’s head: Ras Elased Australis, Ras Elased Borealis, Adhafera, Algieba, Al Jabbah, and Regulus (see image further up the page).

If you have trouble spotting Leo’s Sickle, you can star hop to this constellation from the Big Dipper. Connect the two outer stars of the Dipper’s bowl, Merak and Dubhe, with an imaginary line. Extend that line, (shown with the orange arrow in the image below) and the next bright constellation you’ll hit is Leo.

Finding Leo and Regulus using the Big Dipper
Follow the orange line from Dubhe to Merak and you’ll find Leo (click for full-screen)

Similarly, an imaginary line from Megrez to Phecda (the Dipper’s inner bowl stars) points toward Regulus and Algieba, shown as a green arrow in the image. Regulus is Leo’s brightest star at the bottom of the Sickle.

The angular distance between Phecda and the middle of Leo is roughly 40o and you can use your hand at arm’s length to estimate this.

Five hand shapes for measuring angles in the sky
Five hand shapes for measuring angles (click to enlarge)

To find Leo’s exact position for your location on any night, use software such as Stellarium (free) or SkySafari.

Leo’s Brightest Stars

We’ve already named the six stars in the sickle asterism, but Leo contains several other notable stars. The following list details the brightest, and they can all be seen on the SkySafari image below.

The bright stars of Leo
Bright stars of Leo, click for a full-screen version

Alpha Leonis (Regulus) – At magnitude 1.4, it’s the brightest star in Leo and the 22nd brightest star in the night sky. It’s also the bottom star in the sickle.

Located ~77 light-years away, Regulus is a quadruple star system composed of 2 binary star pairs. Regulus A is a blue-white main-sequence star with a white dwarf companion that can’t be observed directly.

Regulus B and Regulus C are 177 arcseconds from Regulus A. These dim main-sequence stars have magnitudes of 8.1 and 13.5 respectively. The ‘B’ star is within easy reach of a backyard scope, but ‘C’ needs a big telescope to reveal it.

Regulus, meaning ‘little king’ in Latin, is the closest bright star to the ecliptic and it is regularly occulted by the moon and occasionally occulted by Mercury and Venus.

Beta Leonis (Denebola) – At the end of the Lion’s ‘tail’ (from which its name is derived), Denebola shines at magnitude 2.1 and is the second brightest star in the constellation and the 61st brightest in the sky.

This relatively young main-sequence star is thought to be less than 400 million years old and is  ~35.9 light-years away. Denebola is a variable star and changes brightness over just a few hours but the period is unknown and the change can not be visually perceived.

It is 12 times more luminous and 75% more massive than our Sun.

Gamma Leonis (Algieba, Juba) – This double star in the Sickle has a magnitude of 2.0 and is ~130 light-years away.

This star system contains an orange-red giant and a greenish-yellow companion. They are both easily in the grasp of a beginner’s scope because the dimmest star shines at magnitude 3.6. However, they are separated by less than 5 arcseconds, meaning you’ll need decent magnification to split them.

Algieba is derived from the Arabic word for ‘the forehead’ but is also known by its Latin name Juba, meaning ‘mane’.

Delta Leonis (Zosma) – This white, main-sequence star has a magnitude of 2.6 and is ~58.4 light-years away. This star is 15 times more luminous than the Sun.

It is estimated that Zosma will become a red dwarf star in 600 million years. Zosma comes from the ancient Greek meaning ‘the girdle’ and is on the lion’s hip.

Theta Leonis (Chertan, Chort, Coxa) – This white main-sequence star has a magnitude of 3.3 and is ~165 light-years away.

While this star is 2.5 times more massive than the Sun, its estimated age of 550 million years makes it a much younger star.

It is known by various names: Chort is Arabic for ‘small rib’, Chertan is Arabic for ‘two small ribs’ and Coxa is Latin for ‘hip’.

Kappa Leonis (Al Minliar) – This binary star has a magnitude of 4.5 and is ~210 light-years away. The companion star is so faint and close as to not be a backyard astronomer’s target.

Al Minliar means ‘the muzzle of the lion’ in Arabic.

Lambda Leonis (Alterf) – This star has a magnitude of 4.3 and is ~336 light-years away. Alterf means ‘the view (of the lion)’ in Arabic.

Omicron Leonis (Subra) – This double star has a magnitude of 3.5 and is ~135 light-years away.

Subra’s giant primary and main-sequence secondary stars orbit so close together than they can’t be separated with a telescope.

Eta Leonis (Al Jabbah) – This blue-white supergiant is part of the Sickle’s handle. It is an eruptive variable star whose magnitude varies from 3.5 to 3.6 with an irregular period.

A secondary star of unknown orbit is 0.1 arcseconds away from the primary star, but can’t be detected visually.

Al Jabbah is ~2000 light-years away.

Zeta Leonis (Adhafera) -This giant star is part of the curve of the Sickle. It has a magnitude of 3.3 and is ~274 light-years away. Adhafera means ‘the curl’ in Arabic.

Mu Leonis (Ras Elased Borealis, Rasalas) – This is the second star in the sickle. It has a visual magnitude of 4.1 and is  ~133 light-years away.

Epsilon Leonis (Ras Elased Australis, Algenubi) – This bright, yellow, giant is the first star in the sickle and the fifth brightest star in Leo. It has a magnitude of 3.0 and is ~247 light-years away.

Rho Leonis – This binary star has a magnitude of 3.9 and is ~5,000 light-years away. The primary star is a blue giant and its companion is about 0.1 arcseconds away, meaning it can’t be observed in a backyard scope.

Iota Leonis – This binary star has a magnitude of 4.00 and is ~79 light-years away. Its component stars are also too close to together to be resolved.

Sigma Leonis – This blue-white star has a magnitude of 4.0 and is ~210 light-years away.

Fainter Well Known Stars in Leo

While not naked-eye visible, Leo does contain a few other famous stars.

Wolf 359 – This red dwarf has a magnitude of 13.5 and is 7.8 light-years away. Despite being the third nearest star to our Sun, Wolf 359 is too dim to be seen with most small telescopes.

Gliese 436 – This red dwarf has a magnitude of 10.7 and is 33 light-years away. Two extrasolar planets have been discovered in orbit around this star.

R Leonis – This variable, red giant is has a period of 312 days and its magnitude varies from 4.4 to 11.3. R Leonis is ~370 light-years away.

Struve 1496 – This double star has unequal components 20 arcseconds apart and is ~668 light-years away. The primary star is yellow-white and has a magnitude of 8.5, while the secondary star is yellow with a magnitude of 10.6.

88 Leonis – This double star is located ~76 light-years away. The yellow main-sequence primary star has a magnitude of 6.4, the orange-brown secondary star has a magnitude of 8.4 and is ~16 arcseconds away.

Star Hopping From Leo

Leo’s bright stars make it a good starting point for star hopping. Here are a few common targets for beginner astronomers.

Melotte 111 (Coma Star Cluster) – The Coma star cluster is associated with Leo’s tufted tail but it’s actually located in the constellation Coma Berenices.

This open star cluster contains 50 stars and was cataloged by Philibert J Melotte in 1915.  With a magnitude of 1.8 and an angular size of 6° squared, this cluster is naked-eye visible in dark sky locations and is a great binocular (or small scope) target in moderately light-polluted areas.

To find Melotte 111, draw an imaginary line from Regulus to Zosma then double its length. Melotte 111’s coordinates are right ascension 12h 26m 07s, declination 25° 59′ 16″.

location of Melotte 111, Coma star cluster
Location of Melotte 111, click for full-screen

The Leo Triplet – Together, M65, M66, and NGC 3628 are the Leo Triplet of galaxies. We’ve provided more information about each galaxy in the next section.

To find the Leo Triplet, imagine a line joining Chertan and Iota Leonis. Almost exactly midway along that line is the M65. Use the image below (click for full-screen) to see the relative positions of M66 and NGC 3628.

Finder chart for the Leo Triplet of galaxies
Where to find the Leo Triplet of galaxies, click for full-screen. Source: SkySafari 6.

Now let’s look at what else can be seen within the constellation boundaries.

Objects To See Within Leo

In addition to its notable stars, Leo has an abundance of deep sky objects and many are suitable for small telescope owners.

M65 (NGC 3623) – Part of the Leo Triplet, this intermediate spiral galaxy has a warped disc. It shines with a magnitude of 10.3 and an apparent size of 7.6 x 2.0 arcminutes.

Virtual Astronomy Club members: click here for detailed guide

It is 42 million light-years away and located at right ascension 11h 20m 00s and declination 12° 58’ 54″.

M66 (NGC 3627) – Also one of the Leo Triplet galaxies, this intermediate spiral galaxy has a magnitude of 8.9 and an apparent size of 10.3 x 4.6 arcminutes.

It is around 37 million light-years away and can be found at right ascension 11h 21m 18s and declination 12° 52’ 49″.

M66 is distorted due to ongoing gravitational interactions with M65 and NGC 3628.

The Leo Triplet with M65 and M66
M65, M66 & NGC3628 in the Leo Triplet (source)

NGC 3628 – The last of the three triplet galaxies is an unbarred spiral of magnitude 9.5 and an apparent size of 11 x 3.4 arcminutes. We view it edge-on, which is why it’s cigar-shaped. This is the hardest of the three to see and is particularly difficult in a small scope.

It is the closest of the three but, at 35 million light-years, ‘close’ is a relative term. Find this galaxy at right ascension 11h 21m 21s and declination 13° 28′ 41″.

M95 (NGC 3351) – This barred spiral galaxy has a magnitude of 11.4 and an apparent size of 7.2 x 4.5 arcminutes. It is 32 million light-years away and is located at right ascension 10h 45m 02s and declination 11° 35’ 48″.

M96 (NGC 3368) – This intermediate spiral galaxy has a magnitude of 10.1 and an apparent size of 8.3 x 5.5 arcminutes. It is 32 million light-years away and is located at right ascension 10h 47m 50s and declination 11° 42’ 46″.

M96 galaxy image from ESO's Very Large Telescope
M96, imaged by the Very Large Telescope (source)

M96 is the brightest galaxy in the M96 group and is a double-barred spiral galaxy. M95, M96, M105, and 9 other galaxies are part of the M96 Group (Leo I Group). Leo I Group galaxies have a diameter greater than 30,000 light-years.

M105 (NGC 3379) – This elliptical galaxy has a magnitude of 10.2 and an apparent size of 4.9 x 4.3 arcminutes. It is 37 million light-years away and is located at right ascension 10h 48m 54s and declination 12° 28’ 27″.

NGC 3384 – This elliptical galaxy near M105 has a magnitude of 10.0 and an apparent size of 2.1 x 1.5 arcminutes. It is located at right ascension 10h 49m 21s and declination 12° 31’ 19″.

NGC 3412 – This barred spiral galaxy has a magnitude of 10.5 and an apparent size of 4.0 x 2.2 arcminutes. It is located at right ascension 10h 51m 58s and declination 13° 18’ 15″.

NGC 3489 – This barred spiral galaxy has a magnitude of 10.2 and an apparent size of 3.4 x 2.0 arcminutes. It is located at right ascension 11h 01m 23s and declination 13° 47′ 31″.

NGC 3607 – This spiral galaxy has a magnitude of 10.0 and an apparent size of 4.6 x 4.0 arcminutes. It is located at right ascension 11h 17m 58s and declination 17° 56′ 30″.

NGC 2903 – This barred spiral galaxy has a magnitude of 8.8 and an apparent size of 11.9 x 5.3 arcminutes. It is 30 million light-years away and is located at right ascension 9h 33m 19s and declination 21° 24’ 41″.

Summary

In addition to being one of the easiest constellations to spot, Leo has an abundance of notable stars and deep sky objects.

Look for the Sickle in spring and enjoy the galactic wonders which this constellation has to offer.


Written by Tanya C. Forde