Deep-Sky Objects in Sagittarius

Terzan 5
Image Credit: Terzan 5 globular cluster by NASA/ESA Hubble Space Telescope

Sagittarius is the 15th biggest of the 88 recognized constellations and the 5th biggest of the zodiac constellations, taking up an area of 867 square degrees of the southern sky. The brightest star in the constellation is Kaus Australis (Epsilon Sagittarii), a binary system that has an apparent visual magnitude of +1.79, which translates into a luminosity 375 times that of the Sun from a distance of 143 light years away. Kaus Australis is also the 36th most luminous star in the entire sky.

Sagittarius is visible from latitudes of between +55° and -90°, but is best seen from the northern hemisphere at about 9 PM Local Time during the month of August, when it is highest in the sky. Note that like the constellation Scorpius, Sagittarius never rises very high above the horizon, even for observers from mid-northern latitudes. Nonetheless, Sagittarius is relatively easy to find, since the brightest stars in the constellation make up the famous Teapot asterism.

The constellation Sagittarius not only contains the galactic centre, but it is also home to many other famous deep sky objects, including two dwarf galaxies, the enormous Sagittarius Star Cloud, and more than a dozen Messier objects, the most notable of which are briefly explored in this list.

Terzan 5

Named after its discoverer, French-Armenian astronomer Agop Terzan, this 2 million-solar mass globular cluster seen in the image above is located about 18,000 light years away, right in the galactic bulge. Although it stretches across 5.4 light years, 50% of it is concentrated in an inner radius of only 1’02”, making it one of the most densely populated cluster cores in the Milky Way.

One of the most notable aspect of Terzan 5 is the fact that it has two distinctly different stellar populations; one population is estimated to be about 12 billion years old, while the other is much younger, at only about 4.5 billion years, which is younger than our Sun. This finding suggests that like Omega Centauri and M54, this cluster may not be true globular cluster, but rather the remains of a dwarf galaxy that was assimilated by the Milky Way during its formation. Terzan 5 also contains a pulsar with the fastest known rotation rate. This pulsar, designated PSR J1748-2446ad, spins at 70,000 km/sec at its equator, which translates into a speed of 25% the speed of light.

Sagittarius A

Sagittarius A
Credit: NASA/CXC/MIT/F.K.Baganoff et al.

Sagittarius A is a radio source that is almost universally accepted as marking the geometric centre of the Milky Way galaxy. Although it cannot be seen in optical light because of the dense intervening gas and dust in the galaxy’s spiral arms, the structure is known to consist of a supernova remnant designated Sagittarius “A” East, and a spiral structure designated Sagittarius” A” West that has the radio source mentioned at its centre.

However, Sagittarius” A” West is not an actual spiral; it only appears to be a spiral due to the huge clouds of dust and gas that orbit Sagittarius A at extremely high velocities before falling into it at around 1,000 km/sec. The supernova remnant Sagittarius “A” East is about 25 light years in diameter, and is believed to have formed when the progenitor star ventured too close to the black hole at the galaxy’s centre, and was literally squeezed to death by the black hole during the fatal interaction that happened between 35,000 and 100,000 years ago.

Lagoon Nebula (M8, NGC 6523)

Lagoon Nebula
Image: Australian Astronomical Observatory

Located about 4,100 light years away, this emission nebula is one of only two nebulae that are both active star-forming regions, and can be seen without the use of an optical aid. Messier 8 also contains a smaller nebula informally known as the Hourglass Nebula, which should not be confused with the much larger Hourglass Nebula in constellation Musca.

The smaller Hourglass nebula is notable for its several Herbig-Haro objects, which are compact knots of nebulosity that contain either proto-stars, or new-born stars that are still enshrouded in the Herbig-Haro object. The presence of these stellar objects are taken as conclusive proof of star forming activity.

Omega Nebula (M17, NGC 6618, Swan, Horseshoe or Lobster Nebula)

Omega Nebula
Image: NASA’s Hubble Space Telescope

M17 is another emission nebula in Sagittarius that is being lit up from inside by the light of a small cluster of about 35 young, but very hot stars. Stretching across 15 light years of space, the nebula contains around 800 stars in total, while towards its outer region a thousand more stars are being formed. The Omega Nebula is so named because it resembles the last leter of the Greek alphabet, with the nebula’s reddish colour a result of hydrogen gas being heated and lit up by newly formed stars.

The Omega Nebula was discovered by Jean-Philippe Loys De Chéseaux in 1745, and was included by Messier as item #17 in his catalogue some twenty years later. It has an apparent visual magnitude of 6.0, and is located between 5,000 and 6,000 light years from Earth.

Trifid Nebula (M20, NGC 6514)

Trifid Nebula
Image: Hewholooks at

The Trifid Nebula is an easy target for small telescopes, which makes it a favorite object among amateur observers. Measuring about 28 minutes of arc along its long axis, this nebula is a sort of composite object, consisting as it does of a reflection nebula (shown in blue), an emission nebula (shown in pink), and an open cluster of stars.

Much of the Trifid Nebula, which incidentally, means, “divided into three lobes”, is an active star-forming region that contains hundreds, if not thousands of young and embryonic stars. The nebula has an apparent visual magnitude of 6.3, and is located about 5,200 light years away.

Sagittarius Cluster (M22, NGC 6656)

Sagittarius Cluster
Image: Hewholooks at

With an apparent visual magnitude of 5.1, the Sagittarius Cluster is one of the most luminous globular clusters in the entire sky. It is also one of only four globular clusters that are known to contain a planetary nebula, with the other three being Messier 15 in the constellation Pegasus, Palomar 6 in Ophiuchus, and NGC 6441 in Scorpius. Designated GJJC1, the planetary nebula in M22 is only about 6,000 years old, and surrounds a blue star.

Unlike most other globular clusters that have spherical shapes, M22 is distinctly elliptical, which is thought to be a result of the cluster’s close proximity to the galactic bulge. Located about 10,600 light years away, M22 is among the closest globular clusters to Earth.

Arches Cluster

Arches Cluster
Image: Chandra X-ray Observatory, NASA/CXC/UMass Amherst, Q.D.Wang et al.

Located about 25,000 light years from Earth, and only 100 or so light years away from the galactic centre, Arches Cluster is one of most densely populated open clusters known. However, it is not visible in optical light frequencies, but only in radio, infrared and X-ray wavelengths. Note that much of the x-ray illumination shown in the this x-ray image derives both from neutron stars and black holes, as well as from collisions between stars and cooler gas in the various clusters that make up the larger structure.

While this cluster appears to be one object, it is in fact three closely related clusters that are packed tightly together. Collectively the three clusters, Arches (upper right), Quintuplet (upper centre), and the GC cluster (bottom centre), contain some of the most massive and luminous Wolf-Rayet and O-type super giant stars known to exist in the galaxy. The cluster is estimated to be only 2.5 million years old, and has a total mass of about 10,000 Suns, which is about ten times more massive than the average open cluster in the Milky Way.

Other DSOs

Other deep-sky objects in the constellation includes Sagittarius B2, a huge molecular cloud spanning 150 light years of space; the Sagittarius Star Cloud (Messier 24), which is 600 light years across and 10,000 light years distant; and Westerhout 31, a vast H II star forming region. Sagittarius also contains numerous other star clusters, nebulae, and galaxies, including the following:

– Open clusters: Messier 18, Messier 21, Messier 23, Messier 25, NGC 6530, NGC 6520, and the Quintuplet Cluster, Ruprecht 147 (NGC 6774), and NGC 6558.

– Globular clusters: Messier 28, Messier 54, Messier 55, Messier 69, Messier 70, Messier 75, NGC 6522, NGC 6528, NGC 6723, NGC 6544, Terzan 7, NGC 6440, NGC 6638, NGC 6624, NGC 6717, 2MASS-GC02, NGC 6553, and NGC 6540.

– Nebulae: NGC 6565, NGC 6578, NGC 6589, Henize 3-1475, Bubble Nebula, Little Gem Nebula (NGC 6818), Eye of Sauron Nebula (M 1-42), Red Spider Nebula (NGC 6537), and The Box Nebula (NGC 6445).

– Galaxies: The Sagittarius Dwarf Elliptical Galaxy, Sagittarius Dwarf Irregular Galaxy, and Barnard’s Galaxy (NGC 6822).

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