Cassiopeia is one of the most recognizable constellations in the northern sky, thanks to the distinctive W-shaped pattern formed by five of its brightest stars. Covering 598 square degrees, it is the 25th-largest of the 88 modern constellations.
One of its brightest stars is Schedar (Alpha Cassiopeiae), an orange giant about 228 light-years from Earth with an apparent magnitude of around 2.24. Cassiopeia is circumpolar from much of the Northern Hemisphere, meaning it can remain above the horizon throughout the year, although autumn is an especially good time to see it high in the evening sky.
The constellation also lies against the rich star fields of the Milky Way and contains numerous star clusters, nebulae and stellar remnants. It was here that Tycho Brahe observed the brilliant supernova of 1572, while several other fascinating deep-sky objects can be found within its boundaries.
Messier 52 (NGC 7654)
Messier 52 is a rich open star cluster discovered by Charles Messier in 1774. Its exact distance has proved difficult to determine because interstellar dust absorbs some of its light, but estimates generally place it several thousand light-years from Earth.
The cluster contains a dense concentration of stars and can be spotted with binoculars under a dark sky, while a small telescope begins to resolve many of its individual members. Modern estimates of its age vary considerably, but it is a relatively young open cluster on astronomical timescales.
M52 also appears close in the sky to the Bubble Nebula (NGC 7635), although the two objects are not physically associated. The Bubble is an emission nebula shaped by the powerful stellar wind from the hot, massive star BD+60°2522.
Messier 103 (NGC 581)
Messier 103 is another open cluster in Cassiopeia, lying roughly 9,400 light-years away. It is a young cluster, probably only around 10–25 million years old, and appears relatively small and compact from Earth.
The cluster contains numerous hot blue stars as well as a conspicuous red giant, creating a striking color contrast in telescopic views. Some of the brighter stars seen in the same field are foreground objects rather than true members of M103.
M103 also has an interesting place in Messier catalog history. It was discovered by Pierre Méchain in 1781 and became the final object included in the original published version of Charles Messier’s catalog. Objects M104 through M110 were added to the catalog much later on the basis of Messier and Méchain’s observations and notes.
Cassiopeia A
Cassiopeia A, usually shortened to Cas A, is one of the youngest known supernova remnants in the Milky Way. It lies about 11,000 light-years from Earth and is the expanding debris of a massive star whose explosion would have reached Earth in the late 17th century.
The remnant was discovered as a powerful radio source in the 1940s and is one of the brightest extrasolar radio sources in the sky. It is also an important target for X-ray astronomy. NASA’s Chandra X-ray Observatory chose Cas A for its first-light observation in 1999.
The expanding shell contains gas heated to tens of millions of degrees, while stellar debris races outward at millions of miles per hour. At the center is a compact neutron star left behind by the explosion. Observations of Cas A continue to give astronomers an unusually detailed view of how massive stars explode and distribute newly formed elements into space.
Pacman Nebula (NGC 281)
NGC 281 is a large star-forming region about 9,200 light-years away. Its informal name, the Pacman Nebula, comes from the shape created by its glowing gas and prominent dark dust lanes in optical images.
The nebula contains the young open cluster IC 1590 and a population of hot, massive stars whose ultraviolet radiation energizes the surrounding hydrogen gas. Dense clouds known as Bok globules are also visible against the bright nebula and may contain stars still in the process of forming.
Observations at different wavelengths reveal different parts of this stellar nursery. X-rays detected by Chandra trace young stars, while infrared observations from the Spitzer Space Telescope penetrate dust that obscures parts of the region at visible wavelengths.
Caroline’s Rose (NGC 7789)
NGC 7789 is a large open star cluster discovered by astronomer Caroline Herschel in 1783. It is commonly known as Caroline’s Rose because the curved chains of stars and darker gaps between them can resemble the overlapping petals of a rose when viewed through a telescope.
The cluster lies about 8,000 light-years away and spans more than 50 light-years. At roughly 1.6 billion years old, it is much older than open clusters such as M52 and M103.
That age is visible in its stellar population. Many of the cluster’s more massive stars have already exhausted the hydrogen in their cores and evolved away from the main sequence, producing the numerous red giants visible among its stars. This makes NGC 7789 particularly useful to astronomers studying how stars evolve as a cluster ages.
NGC 185 (Caldwell 18)
NGC 185, also known as Caldwell 18, is a dwarf galaxy about 2 million light-years away. Although it appears within Cassiopeia from our viewpoint, it lies far beyond the Milky Way and belongs to the Local Group of galaxies.
NGC 185 is a satellite of the much larger Andromeda Galaxy and is often discussed alongside another Andromeda satellite, NGC 147. The two dwarf galaxies appear relatively close together in our sky, although NGC 185 is generally easier to observe because of its higher surface brightness.
NGC 185 is unusual for a dwarf galaxy of its type because its central regions contain gas, dust and evidence of comparatively recent star formation. Its nucleus has also been classified by some astronomers as a possible Type II Seyfert active galactic nucleus, although that interpretation remains debated. If the Seyfert classification is correct, NGC 185 would be the only known Seyfert galaxy in the Local Group and the closest example to Earth.