The skies of July and August offer an excellent view of the constellation Sagittarius. Easy to spot and full of fascinating deep-sky objects, Sagittarius is an ideal subject for study for both astronomers and stargazers.
The constellation Sagittarius is often referred to as the teapot because of its appearance: the main shape of the box is the body of the teapot, from which a handle and spout extend outward. Some observers add that the Milky Way appears to be coming out of the spout as if it were steam.
How to find the Sagittarius constellation
In the northern hemisphere, Sagittarius reaches its highest point in the southern part of the sky during July and August and until early September. Sagittarius is also visible high in the northern part of the sky for regions south of the equator.
Sagittarius has such a distinctive shape that it is not very difficult to spot in the sky. Just look for the teapot shape next to the curved body of Scorpio. These constellations are not only full of fascinating celestial bodies to observe, but they are also on either side of the core of our galaxy, where the black hole Sgr A* lives.
Sagittarius is best known as the figure of a cosmic archer, although the Greeks saw it as a starry representation of a mythical creature called a centaur. Alternatively, some mythology identifies Sagittarius as the son of Pan, the god who created archery.
His name was Crotus, and he was placed in the sky by the god Zeus so that everyone could see how archery worked. (However, most viewers do not see an archer when they look at Sagittarius—the teapot shape is much easier to identify.)
Main stars of the Sagittarius constellation
The number next to each star is its apparent magnitude, its brightness from our viewpoint on Earth; the lower the number, the brighter the star will appear in the night sky.
- Arkab: Also known as Beta Sagittarii or Arkab Prior, Arkab is a system of two stars about 378 light-years from Earth. It appears very close in the sky to another star, as a result both are known as Beta Sagittarri.
- Rukbat: Also known as Alpha Sagittarii, Rukbat is a blue main sequence star with surface temperatures more than twice those of the Sun. It is about 180 light-years from Earth.
- Ascella: Also known as Zeta Sagittarii, Ascella is a binary star system about 90 light-years from Earth. Both stars are white giants that orbit each other every 21 years.
- Kaus Australis: Also known as Epsilon Sagittarii, Kaus Australis is a binary star system about 140 light-years from Earth and the brightest component of the constellation. The primary star is a blue giant about seven times larger in radius than the Sun.
- Kaus Borealis: Also known as Lambda Sagittarii, Kaus Borealis is an orange subgiant star about 80 light-years from Earth. The star is about 11 times larger in radius than the Sun.
- Nash: Also known as Gamma Sagittarii, Nash is an orange giant star about 100 light-years from Earth. The star is 12 times larger in radius than the Sun.
The brightest stars in the Sagittarius constellation
The brightest star in the Sagittarius constellation is called Kaus Australis (or Epsilon Sagittarii). The second brightest is Sigma Sagittarii, commonly known as Nunki. Sigma (Nunki) was one of the stars that the Voyager 2 spacecraft used for navigation while traveling toward the outer solar system to study the giant gas planets.
There are eight bright stars that form the “teapot” shape of the main constellation. The rest of the constellation, according to the IAU boundaries, has a couple of dozen more stars.
Deep sky objects in the constellation Sagittarius
Sagittarius lies right in the plane of the Milky Way, and its teapot spout points almost directly toward the center of our galaxy. Because the galaxy is so well populated in this part of the sky, observers can see many star clusters, including a number of globular clusters and open clusters.
Globular clusters are spherical collections of stars, many of them much older than the galaxy itself. Open star clusters are not as tightly bound by gravity as globular clusters.
Sagittarius also contains some charming nebulae: clouds of gas and dust illuminated by radiation from nearby stars. The most prominent objects to look for in this area of the sky are the Lagoon Nebula, the Trifid Nebula, and the globular clusters M22 and M55.
Nebulae in Sagittarius
Because we are looking at the galaxy from the inside, it is very common to see clouds of gas and dust in the plane of the Milky Way. This is especially true in Sagittarius. The Lagoon and Trifid Nebulae are easy to spot, although they can usually only be seen well with binoculars or a small telescope.
Both nebulae contain regions where star formation is actively taking place. Astronomers see both newborn stars and protostars in these regions, which helps them track the process of star birth.
The Trifid is also known as Messier 20 and has been studied by many ground-based observatories as well as the Hubble Space Telescope. It will look a little faint, but should be easy to spot in a small telescope. Its name comes from the fact that it looks like a small swimming pool next to the brighter regions of the Milky Way.
The Trifid appears to have three “lobes” connected to each other. They are just over four thousand light-years away from us.
Globular clusters in Sagittarius
Globular clusters are satellites of the Milky Way. They often contain hundreds, thousands, or sometimes millions of stars, all closely bound together by gravity. M22 (which is the twenty-second object on Charles Messier’s list of “Faint and Blurry Objects” compiled in the 18th century) was first discovered in 1665 and contains around 300,000 stars, all together in a region of space about 50 light-years in diameter.
Another interesting globular cluster is also found in Sagittarius. It is called M55, and was discovered in 1752. It contains just under 300,000 stars gathered in an area 48 light-years across. It is located almost 18,000 light-years from us. Look for other clusters and nebulae in Sagittarius, especially using a pair of binoculars or a small telescope.
Mythology
In Greek mythology, Sagittarius represents a centaur, half human, half horse, half creature with the torso of a man and the body and four legs of a horse. The centaur is depicted as aiming an arrow at the heart of the neighboring constellation Scorpio, represented by the red supergiant star Antares. Sagittarius is sometimes mistakenly identified as the centaur Chiron, represented by the constellation Centaurus.
The constellation Sagittarius has its roots in Sumerian mythology. Eratosthenes associated it with Crotus, a mythical creature with two legs and a satyr’s tail , who was the nurse of the nine Muses, daughters of Zeus.
Eratosthenes argued that the constellation actually represented a satyr and not a centaur. According to the Roman author Hyginus, Crotus was the son of Pan and the archer who gave his name to the constellation. Crotus invented archery and lived on Mount Helicon. Because he was close to the Muses, they asked Zeus to place him in the sky.
In Babylonian mythology, Sagittarius is associated with the centaur god Nergal, and is represented with two heads, one human and one panther, as well as wings and a scorpion’s sting placed on a horse’s tail.
