Sagittarius is the 15th largest of the 88 recognized constellations and the 5th largest of the zodiac constellations, covering 867 square degrees of the southern sky. Its brightest star is Kaus Australis (Epsilon Sagittarii), a binary star system about 143 light-years away with an apparent magnitude of +1.79.
Sagittarius is visible from latitudes between about +55° and -90° and is best seen from the Northern Hemisphere during summer. From mid-northern latitudes it remains fairly low above the southern horizon, but its brightest stars form the distinctive Teapot asterism, making the constellation relatively easy to recognize.
Looking toward Sagittarius means looking toward the heart of the Milky Way. The constellation contains the galactic center and an exceptional concentration of star clusters and nebulae, including more than a dozen Messier objects.
Terzan 5
Terzan 5 lies about 22,000 light-years away in the crowded central bulge of the Milky Way. For decades it was generally classified as an unusually complex globular cluster, but observations with the James Webb and Hubble space telescopes have revealed a much more interesting history.
Terzan 5 contains multiple generations of stars with distinctly different ages and chemical compositions. Astronomers have now identified as many as four stellar populations, evidence that the system repeatedly formed stars and enriched itself with heavier elements.
This behavior is unlike that of a normal globular cluster, whose stars generally formed at roughly the same time. Terzan 5 is now interpreted as a surviving fossil fragment from the early formation of the Milky Way’s central bulge.
Terzan 5 is also exceptionally rich in millisecond pulsars. Among them is PSR J1748-2446ad, the fastest-spinning pulsar currently known, rotating 716 times every second.
Sagittarius A and Sagittarius A*
Sagittarius A is a complex radio-emitting region at the center of the Milky Way, roughly 26,000 light-years from Earth. Hidden from visible-light observations by intervening dust, the region can be studied at radio, infrared and X-ray wavelengths.
At its heart is Sagittarius A* (pronounced “Sagittarius A-star”), the supermassive black hole marking the physical center of our galaxy. It contains about four million times the mass of the Sun.
The surrounding Sagittarius A complex also includes Sagittarius A West, a region of ionized gas moving around the galactic center, and Sagittarius A East, a supernova remnant. X-ray studies suggest that Sagittarius A East may have resulted from an unusual Type Iax supernova involving a white dwarf, rather than from a star being destroyed by the central black hole.
Lagoon Nebula (M8, NGC 6523)
The Lagoon Nebula is a huge emission nebula and active stellar nursery located about 5,200 light-years away. With an apparent magnitude of around 6, it can be glimpsed without optical aid from a sufficiently dark location and is an impressive target through binoculars and small telescopes.
Ultraviolet radiation from hot young stars causes the surrounding hydrogen gas to glow, while dark clouds of dust cut across the brighter nebulosity. The nebula also contains the young open cluster NGC 6530.
Within M8 is a bright region commonly called the Hourglass Nebula, not to be confused with the separate Hourglass Nebula in the constellation Musca. The Lagoon contains numerous young stellar objects and Herbig-Haro objects produced when fast jets from newly forming stars collide with surrounding gas.
Omega Nebula (M17, NGC 6618)
Located about 5,500 light-years away, the Omega Nebula is one of the Milky Way’s most prominent star-forming regions. It is also known as the Swan, Horseshoe and Lobster Nebula.
A very young cluster of massive stars embedded within M17 produces intense ultraviolet radiation that ionizes the surrounding gas and causes the nebula to glow. Dense clouds of gas and dust within the region continue to collapse and form new stars.
M17 was discovered by Jean-Philippe Loys de Chéseaux in 1745 and later entered by Charles Messier as the 17th object in his catalog. With an apparent magnitude of about 6, it is readily visible through binoculars under favorable conditions.
Trifid Nebula (M20, NGC 6514)
The Trifid Nebula is one of the most distinctive nebulae in Sagittarius. Its name means “divided into three lobes,” referring to the dark dust lanes that cross its bright central region.
M20 combines several different astronomical features in one object. Its reddish region is an emission nebula energized by young stars, while the blue region is a reflection nebula where starlight is scattered by interstellar dust. Dark clouds of dust produce the prominent lanes across its center, and the region also contains a young open cluster.
The Trifid is an active star-forming region containing numerous young and still-forming stars. It has an apparent magnitude of around 6.3 and is a popular target for amateur telescopes, although its subtle nebulosity becomes much easier to see under dark skies.
Sagittarius Cluster (M22, NGC 6656)
Messier 22 is a large globular cluster located about 10,000 light-years away, making it one of the nearer members of its class. With an apparent magnitude of about +5.1, it can even be detected with the unaided eye under sufficiently dark skies.
M22 contains hundreds of thousands of old stars packed into a roughly spherical region of space. Through a modest telescope, its brighter outer stars begin to resolve individually, making it one of the finest globular clusters available to amateur observers.
The cluster also contains the planetary nebula GJJC1, an extremely unusual discovery because planetary nebulae are rarely found inside globular clusters.
Arches Cluster
The Arches Cluster lies about 25,000 light-years from Earth and only around 100 light-years in projection from the center of the Milky Way. It is one of the densest young massive star clusters known in our galaxy.
The cluster is only a few million years old but contains thousands of stars, including exceptionally massive and luminous O-type and Wolf-Rayet stars. Its total mass is estimated at tens of thousands of times that of the Sun.
Heavy dust toward the galactic center hides the Arches Cluster at visible wavelengths, so astronomers study it primarily using infrared and X-ray observations.
The nearby Quintuplet Cluster is a separate young massive cluster rather than another component of the Arches Cluster. Together with the Milky Way’s nuclear star cluster, these systems demonstrate the extraordinary concentration of young and massive stars found close to the galactic center.
Other Deep-Sky Objects in Sagittarius
Sagittarius contains far more deep-sky objects than can be covered individually here. Messier 24, the Sagittarius Star Cloud, is an especially rich patch of the Milky Way where a gap in foreground dust gives us a view toward a dense field of distant stars.
Other notable objects include:
Open clusters: Messier 18, Messier 21, Messier 23, Messier 25, NGC 6520, NGC 6530 and the Quintuplet Cluster.
Globular clusters: Messier 28, Messier 54, Messier 55, Messier 69, Messier 70, Messier 75, NGC 6522, NGC 6528, NGC 6544, NGC 6553, NGC 6624, NGC 6638, NGC 6717 and Terzan 7.
Nebulae: the Little Gem Nebula (NGC 6818), Red Spider Nebula (NGC 6537), Box Nebula (NGC 6445), NGC 6565 and NGC 6578.
Galaxies: the Sagittarius Dwarf Spheroidal Galaxy, Sagittarius Dwarf Irregular Galaxy and Barnard’s Galaxy (NGC 6822).
