Cassiopeia Constellation: The W in the Sky

Cassiopeia is one of those constellations that, once you've seen it, you'll never fail to recognize again. Its five bright stars form a distinct W shape (or an M, depending on its orientation) that stands out against the Milky Way. It's visible year-round from most of the Northern Hemisphere, it's useful for navigation, and it contains a surprising wealth of deep-sky objects hidden among its stars.

Finding Cassiopeia

Cassiopeia is circumpolar from latitudes above about 35°N, meaning it never sets below the horizon. It circles the celestial north pole opposite the Big Dipper, when the Big Dipper is low, Cassiopeia is high, and vice versa.

The quickest way to find it:

1. Locate the Big Dipper (Ursa Major).
2. Find Polaris using the Dipper's pointer stars (Merak and Dubhe).
3. Continue the same distance past Polaris on the opposite side. You'll land right on Cassiopeia's distinctive W.

In autumn evenings, Cassiopeia rides high in the northeast, right in the thick of the Milky Way. This is the best time for deep-sky observing in this constellation because the targets are highest above the atmosphere.

The Stars of the W

The five main stars of Cassiopeia, from west to east as the W appears in autumn:

• Epsilon Cassiopeiae (Segin): Magnitude +3.4. A blue-white giant at the western end of the W.
• Delta Cassiopeiae (Ruchbah): Magnitude +2.7. An eclipsing binary, its brightness varies slightly as one star passes in front of the other.
• Gamma Cassiopeiae (Navi): Magnitude +2.2 (variable). The central and brightest star of the W. It's an unusual variable star that occasionally flares in brightness, and its nickname "Navi" was given by astronaut Gus Grissom (it's "Ivan", his middle name, spelled backward).
• Alpha Cassiopeiae (Schedar): Magnitude +2.2. A cool orange giant star, one of the closest to Polaris in the W pattern.
• Beta Cassiopeiae (Caph): Magnitude +2.3. A yellow-white star at the eastern end, one of the first stars used to define the modern magnitude scale.

Deep-Sky Treasures

Cassiopeia sits right in the plane of the Milky Way, which means it's loaded with star clusters and nebulae. Here are the best targets:

Star Clusters

Nebulae

Cassiopeia hosts some large, photogenic nebulae that are challenging visually but spectacular in photographs:

• The Heart Nebula (IC 1805): A massive emission nebula shaped like a Valentine's heart. Very faint visually but one of the most popular astrophotography targets in narrowband. H-alpha and O-III filters reveal its structure.
• The Soul Nebula (IC 1848): Adjacent to the Heart Nebula, forming the famous "Heart and Soul" pairing. Another excellent narrowband target.
• The Bubble Nebula (NGC 7635): A small but beautiful emission nebula with a spherical shell blown by a massive central star. Best in photographs or through an O-III filter with a large telescope.
• NGC 281, Pacman Nebula: An emission nebula with dark Bok globules (dense dust clouds) that give it a resemblance to the classic video game character. Visible in an 8-inch telescope with an O-III filter.

Cassiopeia A: The Youngest Supernova Remnant

Hidden in Cassiopeia is Cassiopeia A, the remnant of a supernova that exploded around 1680 AD, making it the youngest known supernova remnant in our galaxy. It's one of the strongest radio sources in the sky, but in visible light it's extremely faint, requiring large amateur telescopes (16-inch+) and an O-III filter to detect even a hint of the expanding shock wave.

If you're interested in supernova remnants you can actually see, our Crab Nebula guide covers a much more accessible example.