Sagittarius

FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy

ABSTRACT: The constellation Sagittarius is a group of stars best seen in the Southern Hemisphere. The stars in this constellation form a shape ancient people thought resembled an archer. Some modern people think part of Sagittarius looks like a teapot. Ancient people studied the constellations for practical and spiritual reasons, and modern astronomers study them to gain more scientific information about the universe. Sagittarius is a large, bright constellation with diverse types of stars as well as galaxies, clouds called nebulae, and possibly a black hole.

Archer in the Stars

Sagittarius is a constellation, a region of space defined by a group of stars that resembles a picture or pattern. In ancient times, people studied the constellations for spiritual reasons or to help keep track of the changing seasons. Since 1930, the International Astronomical Union has recognized eighty-eight constellations, including Sagittarius, which according to many ancient astronomers represented an archer. Many myths even held that this archer was a half-horse, half-human hybrid. Sagittarius is one of about thirteen constellations that comprise the zodiac, the group of constellations that appear to intersect the path of the sun. Ancient people believed the zodiac constellations were especially important.

Attributes of Sagittarius

Sagittarius is a constellation most visible in the Southern Hemisphere between the latitudes of 55 degrees north and 90 degrees south. Its coordinates on the celestial sphere are a right ascension of nineteen hours and a declination of −25 degrees from the celestial equator. It is the largest constellation in the Southern Hemisphere, both in size and in number of planets encompassed. Southern stargazers get the best view of the constellation around August or September in the evenings around 9 p.m. Sagittarius is also visible in the Northern Hemisphere, but it does not rise as high into the sky and is harder to see. In very dark areas, observers can sometimes see the vast reach of the Milky Way, which appears like a whitish haze. Sagittarius is visible in the widest part of this band of stars.

Sagittarius is the fifteenth largest constellation acknowledged by modern astronomers. It is also noteworthy for its inclusion in the zodiac and its position in the center of the Milky Way galaxy, the enormous mass of stars, planets, and other space materials that includes Earth. Some of the deep-sky objects within Sagittarius include the Sagittarius Star Cloud (Messier 24), the globular cluster Messier 22, the Lagoon Nebula (Messier 8), the Trifid Nebula (Messier 20), and the Omega Nebula (Messier 17), all of which can be seen using a telescope or binoculars. During the winter solstice that occurs around December 21–22 each year in the Northern Hemisphere, the sun can be found in Sagittarius. That fact gave the constellation special importance for many groups of ancient people.

There are also many interesting and important stars and other space objects within this constellation. These objects include binary stars, or pairs of stars that orbit a common center of mass, and variable stars, stars whose brightness increases or decreases. Many of these stars are named with letters of the Greek alphabet, such as alpha, beta, gamma, and so on, paired with the name Sagittarii.

Often the brightest star in a constellation is given the designation alpha. In the case of Sagittarius, however, the brightest star is epsilon Sagittarii. Epsilon Sagittarii is the thirty-sixth brightest star anywhere in the night sky. Even though it is 125 light-years from Earth, epsilon Sagittarii is about 375 times brighter than the sun and therefore still casts a powerful brightness.

Epsilon Sagittarii is often paired with three other stars, gamma, delta, and lambda Sagittarii. Together, these stars form the shape of the archer’s bow and arrow. Appropriately, they all have names that reflect parts of the weapon. Epsilon is called Kaus Australis, meaning "southern end of the bow." Lambda is Kaus Borealis, "northern end of the bow." Delta is best known as Kaus Media or Kaus Meridionalis, "middle of the bow." Meanwhile, gamma is known as Nash, Nushaba, Al Nasl, or Alnasl, meaning "tip of the arrow." These stars, although appearing to be near each other, actually range greatly in distance from about 77 to 300 light-years away from Earth.

Other star names also reflect the theme of an archer in the sky. Alpha Sagittarii is also known as Rukbat, "knee of the archer," and beta 1 and beta 2 Sagittarii are called Arkab Prior and Arkab Posterior, which refer to leg parts. The third brightest star in Sagittarius, zeta Sagittarii, is called Ascella, or "armpit of the archer."

One of the most distinguishing features of Sagittarius involves several of the constellation’s brightest stars. The stars delta, epsilon, zeta, phi, lambda, gamma 2, sigma, and tau Sagittarii take on a special shape unrelated to the archer. Many people consider this shape an asterism, or a small pattern that is not quite a constellation, that resembles a teapot with a spout, lid, and handle. Some modern stargazers find the Teapot asterism more appealing and easier to envision than the ancient archer interpretation.

In addition to stars, Sagittarius hosts an array of deep-sky objects. These include a star cloud, star clusters, different kinds of nebulae (clouds of gas and dust in which new stars form), and an irregularly shaped galaxy. Some of the most notable nebulae include the Lagoon Nebula, the Trifid Nebula, and the Omega Nebula (also known as the Swan, Horseshoe, or Lobster Nebula), which features a looped shape and twinkling stars. Some of these objects, though extremely far away, are bright enough to be detected with the naked eye in some areas.

Finally, one of the most mysterious and potentially important features of Sagittarius is known as Sagittarius A*. Located in the center of the Milky Way galaxy, Sagittarius A* is an area of space that emits radio waves. The very unusual behavior of Sagittarius A* has led some astronomers to believe that it may contain a black hole. A black hole is a little-understood space object with such powerful gravity that nothing can escape it.

Monster Myths

Stargazers first discovered the stars of the Sagittarius constellation thousands of years ago. The first ancient astronomers to identify the stars included Sumerians, Babylonians, and Arabs. Each of these peoples created their own interpretations and mythologies relating to the star patterns. In Babylonia, astronomers thought the stars represented the god Pabilsaĝ, a creature with a bull-like body, colorful legs, one human head and one panther head, and wings.

Ancient Greek astronomers also studied Sagittarius with great interest. Their mythical interpretations, first known to have been recorded around the second century CE, have become the basis for the modern view of Sagittarius. According to the Greeks, Sagittarius represented a creature with a human head and torso on the body of a horse. In Greek mythology, there were two monsters with these kinds of features: wild, aggressive centaurs and peaceful, playful satyrs.

Different versions of the Greek myths associate Sagittarius with centaurs or satyrs. In some myths, the constellation represents Crotus. Crotus was a wise and joyful satyr whose benevolence pleased the gods and earned him honor in the stars. In other interpretations, Sagittarius is a more aggressive centaur that is aiming its bow threateningly at its constellation neighbor Scorpio (scorpion). Centaurs were so common in myths that another constellation, Centaurus, was also associated with them.

Modern Astronomical Importance

Today, amateur stargazers enjoy the beauty of stars and planets in constellations. Astronomers study the constellations to gain more scientific knowledge about the changing universe. Sagittarius is a popular constellation for both amateurs and professionals because of its diversity of stars, planets, and other space objects.

New features of Sagittarius are still being discovered. On March 15, 2015, an Australian astronomer discovered a likely nova, a star explosion, within the Teapot of Sagittarius. Experts feel this space object may have originally been a dim, distant star. It only became noticeable when its explosion created a flash of brightness.

PRINCIPAL TERMS

• celestial equator: the imaginary line above Earth’s equator that halves the celestial sphere; it is equally distant from the celestial poles.
• constellation: a region of space defined by a pattern of stars that can be seen in the night sky from Earth.
• declination: a space object’s angular distance north or south of the celestial equator.
• International Astronomical Union: an association of professional astronomers from all over the world who define astronomical constants while promoting research, education, and discussion on important astronomical topics.
• right ascension: a space object’s longitudinal arc along the celestial equator, measured eastward from the vernal equinox.

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