The constellations best seen in May are Canes Venatici, Centaurus, Coma Berenices, Corvus, Crux, Musca and Virgo. Canes Venatici and Coma Berenices are northern constellations, while Centaurus, Virgo, Corvus, Crux and Musca lie south of the celestial equator. Virgo and Centaurus are the 2nd and 9th largest constellations in the sky, while Crux is the smallest.
May is the best time of year to observe the many interesting deep sky objects located in these constellations. The most popular telescope targets include the Virgo Cluster and Coma Cluster of galaxies, the Whirlpool Galaxy (M51), the Sunflower Galaxy (M63), the Black Eye Galaxy (M64), the Sombrero Galaxy (M104), the Antennae Galaxies, Omega Centauri, Centaurus A and the Jewel Box Cluster.
The northern constellations Canes Venatici and Coma Berenices are easy to find because they are located under the handle of the Big Dipper, one of the most recognizable asterisms in the sky. However, neither constellation is particularly conspicuous because the only star brighter than magnitude 4.00 in this region is Cor Caroli, the brightest star in Canes Venatici. With a combined visual magnitude of 2.9, the binary star marks one of the vertices of the Great Diamond, a prominent spring asterism also formed by Arcturus in Boötes, Spica in Virgo and Denebola in Leo.
Canes Venatici is also home to La Superba (Y Canum Venaticorum), one of the reddest stars in the sky. The red giant is a carbon star classified as a semi-regular variable. Its visual magnitude varies from 4.86 to 7.32 over a period of about 160 days.
Canes Venatici hosts five Messier objects: the Whirlpool Galaxy (M51), Sunflower Galaxy (M63), Cat’s Eye Galaxy (M94), the spiral galaxy Messier 106, and the bright, large globular cluster Messier 3. In addition to these, the constellation is home to the Whale Galaxy (NGC 4631), the Hockey Stick Galaxies (NGC 4656 and NGC 4657), the Cocoon Galaxy (NGC 4490), and a number of other stunning deep sky objects.
Coma Berenices is slightly smaller than its neighbour, but just as abundant in deep sky objects. Home to the Coma Cluster of galaxies, as well as to the northern portion of the Virgo Cluster, the constellation is particularly rich in galaxies.
The supergiant elliptical galaxies NGC 4874 (Coma A) and NGC 4889 (Coma B) dominate the central region of the Coma Cluster. The cluster contains at least 10,000 galaxies, mostly elliptical. The brightest spiral galaxy in the cluster is NGC 4921 and other notable members include the Mice Galaxies (NGC 4676), an interacting pair of spiral galaxies, and Dragonfly 44, an ultra diffuse galaxy believed to consist almost entirely of dark matter.
Coma Berenices contains a total of eight Messier objects. The most famous one, the Black Eye Galaxy (M64), is a spiral galaxy known for its bright nucleus surrounded by a dark band of dust. It is sometimes also known as the Sleeping Beauty or Evil Eye galaxy. Other Messier objects in the constellation are the globular cluster Messier 53 and the Virgo Cluster members Messier 85, Messier 88, Messier 91, Messier 98, Messier 99 (Coma Pinwheel), and the grand design spiral galaxy Messier 100.
Coma Berenices also contains the Coma Star Cluster, a large open star cluster, the famous edge-on spiral galaxy known as the Needle Galaxy (NGC 4565), and the spiral galaxies NGC 4314, NGC 4414 and NGC 4651, the last of which is known as the Umbrella Galaxy.
Occupying an area of 1294 square degrees south of Coma Berenices, Virgo is smaller only than Hydra. The constellation is visible from most inhabited places on Earth and its brightest star, Spica, is the 16th brightest star in the sky, with a visual magnitude of 0.98.
Spica, Alpha Virginis, is a binary system consisting of a blue giant and a very close companion. It is one of the nearest massive binary systems to Earth. The star marks one of the vertices of the Spring Triangle (the other two being Arcturus and Regulus or Denebola) and it is also part of the Great Diamond, along with Cor Caroli, Regulus and Arcturus.
Two other stars in Virgo are brighter than magnitude 3.00: Porrima (Gamma Virginis), a binary system consisting of almost identical white main sequence stars, and Vindemiatrix (Epsilon Virginis), a yellow giant star used to find the center of the Virgo Cluster of galaxies, which is located about halfway between Vindemiatrix and Denebola in Leo.
Virgo contains a total of 11 Messier objects. The famous Sombrero Galaxy (M104), a magnificent unbarred spiral galaxy located near the border with Corvus, is the only one that isn’t a member of the Virgo Cluster. The remaining ones are: Messier 49, Messier 58, Messier 59, Messier 60, Messier 61, Messier 84, Messier 86, Messier 87, Messier 89, and Messier 90. Messier 87, also known as Virgo A, is the largest galaxy in the cluster. The supergiant elliptical galaxy is also one of the most massive galaxies in the Local Supercluster. The brightest member of the Virgo Cluster is the elliptical galaxy Messier 49.
Messier 84 and Messier 86 are part of Markarian’s Chain, a majestic stretch of galaxies near the center of the cluster that also includes the Eyes Galaxies, a famous interacting pair catalogued as NGC 4438 and NGC 4435.
Located between Virgo and Hydra, the constellation Corvus, the Raven, occupies an area of only 184 square degrees, which makes it one of the smaller constellations, only the 70th in size. Its four brightest stars – Gienah (Gamma Corvi, mag. 2.58), Kraz (Beta Corvi, mag. 2.65), Algorab (Delta Corvi, mag. 2.94) and Minkar (Epsilon Corvi, mag. 3.02) – form a quadrilateral asterism not far from Spica that makes Corvus easy to identify even if the stars themselves aren’t particularly bright. The asterism is known as the Sail or Spica’s Spanker.
Corvus is visible from any location south of the equator and from northern latitudes south of 60°N. Unlike its northern neighbours, the constellation does not have any Messier objects, but is home to several interesting deep sky objects that can be seen in amateur telescopes. The Antennae Galaxies, a famous pair of interacting galaxies catalogued as NGC 4038 and NGC 4039, are members of the NGC 4038 Group, a group of galaxies found in Corvus and the neighbouring Crater. The Ringtail Galaxy (NGC 4027), a peculiar barred spiral galaxy, is another well-known member of the group. Corvus also contains NGC 4361, a large planetary nebula located near the centre of the constellation.
Occupying an area of 1060 degrees, Centaurus is the ninth largest constellation in the sky. It contains 10 stars brighter than magnitude 3.00, including Alpha Centauri, the third brightest star in the sky, and Beta Centauri (Hadar), the 11th brightest star. Alpha and Beta Centauri are known as the Southern Pointers, as they point toward Crux, the Southern Cross, which is used in navigation to find true south. Centaurus has a total of 281 stars brighter than magnitude 6.5, more than any other constellation.
Alpha Centauri is a multiple star system consisting of two Sun-like stars and a small, faint red dwarf. The binary star Alpha Centauri AB has an apparent magnitude of -0.27, which makes it fainter only than Sirius and Canopus. Proxima Centauri (Alpha Centauri C), the third component of the Alpha Centauri system, is the nearest star to the Sun. The red dwarf is located at a distance of only 4.24 light years.
Not far from Hadar lies one of the largest stars known. V766 Centauri (HR 5171) is a triple star system consisting of a contact binary, a star sharing an envelope of material with a smaller star, and a third component. The larger component of the contact binary system is believed to be either a red supergiant or a yellow hypergiant with an estimated diameter 1,300 to 1,500 times that of the Sun. The smaller component is about one third the size of the primary. The third component in the system is a class B supergiant, a highly luminous star, but considerably fainter than the primary component.
Centaurus is also home to the brightest globular cluster in the sky, Omega Centauri. Suspected to be the core remnant of a former dwarf galaxy, Omega Centauri is also the largest and most massive globular cluster in the Milky Way, containing about 10 million stars within a diameter of 150 light years. With an apparent magnitude of 3.9 and an apparent size of 36’.3, it is easily visible without binoculars. When seen from a location with no light pollution, the cluster appears almost as large as the full Moon. It lies at a distance of 15,800 light years from Earth.
Centaurus also contains a couple of open clusters visible to the unaided eye in good conditions: NGC 3766 (mag. 5.3), which contains several dozen fast-rotating pulsating B-type variable stars, and NGC 5460 (mag. 5.6), a slightly older cluster located about 2 degrees east-southeast of the star Zeta Centauri.
The Blue Planetary nebula (NGC 3918), also known as the Southerner, is the brightest planetary nebula in this region of the sky. With an apparent magnitude of 8.5, it can be observed in a small telescope.
Centaurus A (NGC 5128), the fifth brightest galaxy in the sky and one of the closest radio galaxies to Earth, is a popular astrophotography target due to its large apparent size and high surface brightness. It is located about 4 degrees north of Omega Centauri and has an apparent magnitude of 6.84.
Centaurus is also home to the Centaurus Cluster (A3526), which contains hundreds of galaxies located at an approximate distance of 170 million light years from Earth. The elliptical galaxy NGC 4696 is the brightest member. NGC 4622, an unbarred spiral galaxy with a ring structure, nicknamed the Backward Galaxy, is another notable member of the cluster.
Other galaxies in the constellation include the barred spiral galaxy NGC 4945, the polar-ring lenticular galaxy NGC 4650A, the irregular galaxy NGC 5253, the colliding pair NGC 5090 and NGC 5091, and NGC 5291, a system of colliding galaxies surrounded by a collisional ring.
Crux, the smallest of all constellations, occupies an area of only 68.4 square degrees. With the four main stars brighter than magnitude 2.8 and forming a prominent cross-shaped asterism, Crux is probably the best known constellation in the southern sky. The asterism, known as the Southern Cross, has carried cultural significance in the southern hemisphere since ancient times. It appears on the flags of Australia, New Zealand, Papua New Guinea, Samoa, and a number of regions and territories in the countries of South America.
Acrux (Alpha Crucis), Mimosa (Beta Crucis) and Gacrux (Gamma Crucis), the brightest stars in Crux, are the 13th, 21st and 25th brightest stars in the sky respectively. Alpha and Gamma Crucis are used in navigation to find south: an imaginary line drawn from Gacrux to Acrux leads to a point near the southern celestial pole.
Crux contains two notable deep sky objects. One of these, the Jewel Box Cluster (or Kappa Crucis Cluster, catalogued as NGC 4755) is an open star cluster with an apparent magnitude of 4.2. The cluster is visible to the unaided eye, appearing as a blurry star near Beta Crucis in good conditions. It is one of the youngest open clusters in our galaxy. Most of its brightest stars are blue supergiants, including the brightest member, the spectroscopic binary star Kappa Crucis.
Musca, the Fly, is also among the smallest constellations, occupying an area of 138 square degrees. It is invisible to all northern observers except those living in equatorial latitudes. Its brightest star, Alpha Muscae, has a visual magnitude of 2.69 and is the only star in the constellation brighter than magnitude 3.00.
Musca contains several interesting deep sky objects, among them the Engraved Hourglass Nebula (MyCn 18), a young planetary nebula named for its shape, the Spiral Planetary Nebula (NGC 5189), whose S-shape vaguely resembles that of a spiral galaxy in a telescope, the globular clusters NGC 4833 and NGC 4372, and the Dark Doodad Nebula, a dark nebula stretching across almost three degrees of arc in the vicinity of NGC 4372, just south of the better known Coalsack Nebula.
The table below shows the latitudes between which the May constellations are visible.