Categories: Deep-Sky Objects

Deep-Sky Objects in Scorpius

Messier 4 globular cluster in the constellation of Scorpius

Scorpius is the 33rd biggest of the 88 recognized constellations, and the 9th largest of the zodiacal constellations, taking up an area of 497 square degrees between of the southern sky. The brightest star in Scorpius is Antares (Alpha Scorpii), a red supergiant star that is located about 550 light years away, and has an apparent visual magnitude of +0.96, which makes it either the 15th or the 16th brightest star in the sky, depending on whether the brightest stars in the Capella system are counted as one or two stars.

Scorpius is visible from latitudes of between +40° and -90°, although best seen in the northern hemisphere at about 9 PM Local Time during the month of July, when it is highest in the sky. Note, though, that since Scorpius is the southern-most of the zodiac constellations, it never rises very high above the horizon, and for many observers in far northern latitudes, the constellation only rises partially above the skyline.

Apart from two meteor showers, the Alpha Scorpiids and the Omega Scorpiids that have their radiants in Scorpius, the constellation contains many interesting deep sky objects, including four Messier objects, the most notable of which are explored in this list.

Messier 4 (M4, NGC 6121)

Located about 7,200 light years away, M4 is about 75 light years in diameter, and together with NGC 6397 in the constellation Ara are two of the closest globular clusters to Earth. M4 also has the distinction of being the first cluster in which individual stars were resolved. It has an apparent visual magnitude of 5.9, making it an easy target for amateur equipment, and can be found about 1.3 degrees due west of the bright star Antares. The cluster is about 12.2 billion years old, and the most luminous of its stars have apparent magnitudes of 10.8.

Butterfly Cluster (M6, NGC 6405)

This pretty open cluster was discovered by Giovanni Batista Hodierna in 1654, and it is easy to see why it has become known as the Butterfly Cluster, since it does not take much imagination to see the shape of a butterfly in its appearance. The luminous stars in the grouping are for the most part very hot, blue B-type stars, although the brightest star in the cluster is BM Scorpii, a K-type orange giant. M6 has an apparent visual magnitude of 4.2, and is located about 1,600 light years away.

Ptolemy Cluster (M7, NGC 6475)

This stunning open cluster first recorded by Ptolemy in the year 130 AD is somewhat reminiscent of M45 (Pleiades), and under clear skies can be seen with the naked eye. All told, it contains around 80 stars aged about 200 million years old that stretch across 25 light years of space. The cluster’s apparent visual magnitude is 3.3, with the most luminous star in the group having an apparent magnitude of 5.6. Look for M6 near the Scorpion’s stinger.

Messier 80 (NGC 6093)

M80 has been described as the finest globular cluster in the northern sky, and a worthy rival for Omega Centauri, the biggest globular cluster that is associated with the Milky Way. It contains several hundred thousand stars that are packed into a diameter of only 95 light years, which makes it one of the most densely populated globular clusters in our galaxy.

M80 is notable for the relatively high number of blue stragglers it contains. In simple terms, a blue straggler star is one that appears to be much younger than the rest of the stellar population, and it is believed that these stars form when two aging stars merge or collide, which results in a single star that is hotter than either or both of the stars that collided/merged. M80 has an apparent visual magnitude of 7.87, and is located about 32,600 light years away, making it an easy target for medium-sized amateur telescopes.

Look for M80 at the mid-way point between the stars Antares and Acrab. One supernova, designated Nova 1860 AD, was observed in M80 in the year 1860 when the star T Scorpii exploded. The nova was confirmed when subsequent photographs showed that T Scorpii had disappeared after the event.

Cat’s Paw Nebula (Bear Claw Nebula, NGC 6334, Gum 64)

This striking emission nebula is one vast star-forming region that stretches across about 50 light years. It contains about 10,000 stars in total, many of which are believed to count among the most massive stars in the Milky Way.

The Cat’s Paw Nebula also contains many newly formed stars that are only about 10 million years or so old, but since they are for the most part buried deep within the nebula, it is difficult to determine their exact ages, and number. The nebula appears red in this image because the green and blue light emitted by it is scattered by gas and dust between Earth and the nebula. Nonetheless, the red light is only visible because a specially designed filter allowed the red light from hot hydrogen to be imaged.

Butterfly Nebula (Bug Nebula, NGC 6302, Caldwell 69)

Located about 3,800 light years away, the Butterfly Nebula formed when an ageing red giant blew off its outer layers in a process that lasted for about 2,200 years. NGC 6302 is about two light-years long, and one of the most structurally complex nebula known.

The thick torus of dust and gas at the “waist” of the Butterfly surrounds the central white dwarf, and also constricts the outflow of gas and dust, which is the driving force behind the bi-polar structure of the nebula. While the temperature of the central white dwarf has not been determined directly, most investigators believe it to be as high as 200,000K, which if accurate, will make it one of the hottest stars known to exist in the Milky Way galaxy. NGC 6302 has an apparent visual magnitude of 7.1.

War and Peace Nebula (NGC 6357)

This diffuse nebula got its name from the fact that when viewed in infrared light, the eastern lobe resembles a human skull, while the western lobe somewhat resembles a dove. While the NGC 6357 consists of many new stars, and probably more in the process of forming, it also contains the open cluster Pismis 24 (shown here), which is known for the several extremely massive stars it contains.

One particular star in Pismis 24, designated Pismis 24-1, was at first believed to have at least 300 solar masses, until recent investigations showed that it is in fact a complex, multi-star system. Nonetheless, Pismis 24-1 NE is the brightest star in the system, and one of the most luminous stars known, shining at least 776,000 brighter than the Sun. The other bright component in the system, designated Pismis24-1 SW, is not far behind; it shines with at least 646,000 solar luminosities. The temperatures of the two stars are 42,500K and around 40,000K respectively, which also makes them two of the hottest stars yet discovered.

Peter Christoforou :