The Double Cluster in Perseus, the Heart Nebula, the Soul Nebula, and nearby star clusters
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Perseus is one of the larger constellations of the northern sky, ranking 24th in size among those recognized today. It is part of the autumn constellations group and is most prominently visible from mid-latitude regions such as Poland during autumn. The number of stars visible to the naked eye in this constellation is estimated to be around 90. Perseus is also one of the 48 constellations described by Ptolemy.
According to ancient Greek mythology, Perseus was the son of Zeus and Danaë, and one of the greatest heroes in Greek lore.
The king of Argos, Acrisius, had been told by an oracle that he would die at the hands of his own grandson. After Perseus was born, the king had his daughter Danaë and her child sealed inside a chest and cast into the sea, intending to prevent the prophecy from coming true. However, Zeus ensured the chest washed ashore on the island of Seriphos, where Diktys, brother of King Polydectes, took them in. As Perseus grew up, Polydectes sent him on a seemingly hopeless quest for the head of the fearsome Medusa (the youngest of the Gorgons, whose sisters were Stheno and Euryale), whose gaze turned anyone who looked at her into stone. Thanks to the gods, Perseus received useful gifts—including winged sandals, a magic pouch, and the Helmet of Invisibility from the nymphs, a steel sickle from Hermes, and a shield from Athena—and succeeded in beheading Medusa. From the blood of this monster sprang the famous winged horse Pegasus, later ridden by Bellerophon. On his way back through Ethiopia, Perseus saved the beautiful princess Andromeda from a sea monster called Cetus (the Whale). He then took her to Seriphos as his bride. Upon returning to the island, he discovered Polydectes’s treachery and used Medusa’s head to turn the entire court, including the king, to stone. Perseus then traveled with his wife and mother back to his hometown of Argos. During some athletic games, an accidental throw of a discus struck his grandfather’s head, fulfilling the prophecy. After his death, Perseus was placed among the stars by the gods, alongside Andromeda, her parents Cassiopeia and Cepheus, as well as Cetus and Pegasus.
Within the boundaries of the constellation Perseus lie many intriguing deep-sky objects. I will discuss several of them in the course of the following observations.
Observations
September 15, 2020, around 11:30 PM – Jaworzno (Poland)
suburban conditions, high level of light pollution
That night, the sky was occasionally a bit hazy, and at times there were some passing clouds. Undeterred, I went out to my garden to enjoy the view of the night sky. After setting up my equipment, I noticed that the region of Perseus appeared to be free of clouds, haze, or any atmospheric disturbances that could hamper observations. Consequently, I managed to gather enough data, which resulted in Photo 1.
The photograph covers the area of the sky near the boundary between Perseus and Cassiopeia (as confirmed by the presence of both Eta (η Persei), known as Miram, and Epsilon (ε Cassiopeiae, or Segina), where the hero’s hand holding a sword was traditionally imagined. So many interesting objects are visible here that I can hardly decide which one to start with.
NGC 663 (Caldwell 10) is a young open cluster formally located within the boundaries of Cassiopeia. It was discovered by William Herschel on November 3, 1787, and lies about 8,000 light-years from the Sun. It contains around 400 stars, roughly 20–25 million years old. The cluster includes at least five so-called blue stragglers, which appear significantly hotter and bluer than other stars of similar brightness, making them seem much younger.
Another comparable object is NGC 1027, which is closer—just over 3,000 light-years away. It, too, was discovered by Herschel. NGC 1027 is an open cluster in Cassiopeia, found in 1787. It sits between two emission nebulae but is not physically associated with them. The cluster’s brightest star has a magnitude of about 9.3m.
A similarly intriguing object is the open cluster Stock 2. Its stars are spread out rather loosely, and the cluster’s angular dimensions can reach up to 1°, about twice the size of the full Moon. The cluster is about 1,050 light-years away. Its overall brightness is usually pegged at around 4.4m, but with a good pair of binoculars (10×50 will do, though 15×70 is better), one can discern about 20 stars of ~8m. Many observers find that this cluster, sometimes dubbed the Muscle Man or the Ballerina cluster, resembles a stylized human figure (shown schematically in Photo 2).
This collection is relatively understudied. The cluster’s age is estimated at 450 ± 150 million years, and the average mass of its stars is around 2.8 M⊙.
A delightful surprise was that, in addition to star clusters, the simple photographic technique I used also captured as many as three nebular objects: the Heart Nebula, the Fish Head Nebula, and the Soul Nebula.
The Heart Nebula (IC 1805) is both an H II region and an emission nebula located in the Perseus Arm (one of the spiral arms of our Galaxy). This nebula, which resembles the human heart symbol (Photo 3), lies about 7,500 light-years from Earth and spans roughly 200 light-years.
Its vivid red light is primarily emitted by hydrogen (H2), the most abundant element in the Universe. At the Heart Nebula’s center is the open cluster Melotte 15—a very young assembly of stars merely 1.5 million years old. This cluster contains a few bright OB-type stars with masses up to 50 times that of the Sun, along with numerous fainter stars with only a fraction of the Sun’s mass.
Right next to the Heart Nebula lies the Fish Head Nebula (IC 179), another emission nebula. It is part of a complex star-forming region situated along the edge of a large molecular cloud in the Perseus Arm of the Milky Way.
The Soul Nebula (IC 1848) is in many ways similar to the Heart Nebula—like it, this nebula is an H II region and an emission nebula. However, it is slightly closer to us, at around 6,000 light-years away. The designation IC 1848 also refers to the associated open cluster discovered by Edward Barnard in the late 1890s, which illuminates the nebula and has an approximate magnitude of 6.5m.
The Double Cluster in Perseus occupies the 14th position (Caldwell 14) in the catalog compiled by the renowned amateur astronomer and science communicator Alfred Patrick Caldwell-Moore. It is also known as the Sword Handle or h+χ Persei.
The Double Cluster comprises two open clusters: NGC 869 and NGC 884.
The Double Cluster in Perseus was first recorded by the Greek astronomer Hipparchus, but we can be certain it was observed much earlier, since it is easily visible to the unaided eye under suitable conditions.
Both clusters lie over 7,000 light-years from Earth but are located very close to each other—only a few hundred light-years apart. Another similarity is their approximate age, which can be deduced from the properties of their member stars. They are relatively young groups—NGC 869 is roughly 5.6 million years old, and NGC 884 about 3.2 million years old. Both clusters are moving toward Earth at a speed of just over 20 km/s.
Each cluster occupies an angular size corresponding to the apparent diameter of the full Moon. NGC 869 is brighter, more compact, and contains more stars than NGC 884. Most stars in these clusters are blue-white, but in and near NGC 884, there are several red giants. Through binoculars, one can see a curved tail of stars running toward the previously mentioned cluster Stock 2.
The Double Cluster in Perseus has been featured in literature. In the novels of the People as Gods (Люди как боги) cycle by Soviet writer Sergey Snegov, this region was the home of the malevolent Demiurges who threatened humanity.
Photos 1, 2, 3 Parameters: Photo 4 Parameters:
Marek Ples