Cassiopeia

Cassiopeia A — the youngest known supernova remnant in the Milky Way, imaged by the Hubble Space Telescope ACS instrument in 2006. The star exploded around 1680 CE, about 11,000 light-years away. Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)–ESA/Hubble Collaboration.

The Queen of the North

In Greek mythology, Cassiopeia was an Ethiopian queen, wife of Cepheus and mother of Andromeda. Her vanity — she boasted she was more beautiful than the sea nymphs — earned her a place in the sky, but a humbling one: she circles the celestial pole and is sometimes depicted upside-down, perpetually tumbling around the sky as punishment.

In practice, Cassiopeia is simply one of the easiest constellations to find. Its five main stars form a clear W (or M, depending on orientation) and are circumpolar from most of the Northern Hemisphere — meaning they never set below the horizon. On autumn and winter evenings in the northern United States, Cassiopeia rides high overhead and can be spotted in seconds from even a suburban backyard.

Key Stars

• α Cas — SchedarMagnitude 2.24 (variable). An orange giant ~228 light-years away. The brightest star in the constellation and the bottom-right of the W as it rises.
• β Cas — CaphMagnitude 2.27. A white giant ~54 light-years away. One of the three stars that share the meridian with the vernal equinox — making it useful for timekeeping.
• γ Cas — NaviMagnitude variable (1.6–3.0). The central star of the W. A hot blue Be star with a circumstellar disk — its brightness fluctuates unpredictably as the disk grows and shrinks. Named "Navi" by astronaut Gus Grissom (his middle name reversed) for use in spacecraft navigation training.
• δ Cas — RuchbahMagnitude 2.66. An eclipsing binary ~99 light-years away. The fourth point of the W.
• ε Cas — SeginMagnitude 3.35. A blue-white giant at the fifth point of the W, completing the familiar shape.

Cassiopeia A: A Star's Violent End

The single most spectacular object associated with Cassiopeia is Cassiopeia A (Cas A) — the remnant of a supernova explosion roughly 340 years ago, making it the youngest known supernova remnant in the Milky Way. The progenitor star likely exploded around 1680 CE, though it apparently left no strong historical record of a bright new star (possibly because interstellar dust dimmed it).

Cas A sits about 11,000 light-years from Earth in the direction of Cassiopeia. The expanding shock wave now spans roughly 10 light-years and travels at about 6,000 km/s. At X-ray and radio wavelengths, it is the brightest source in the sky outside the solar system. In visible light, the Hubble image above shows intricate filaments of glowing gas — the shredded outer layers of the progenitor star, colliding with surrounding interstellar material and heating it to millions of degrees.

In 2024, the James Webb Space Telescope delivered its own stunning view of Cas A in infrared, revealing details of the dust and ejecta structure that Hubble couldn't penetrate.

Cas A — Close Detail

A zoomed Hubble ACS view showing the fine structure of ejecta knots and filaments within the Cassiopeia A remnant.

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Milky Way Through Cassiopeia

A Hubble wide-field view of the rich star fields in Cassiopeia, lying along a dense arm of the Milky Way.

Credit: ESA/Hubble & NASA

Deep-Sky Highlights

The Double Cluster (NGC 869 & NGC 884)

Just over the border into Perseus — but best found by sweeping from Cassiopeia — is the Double Cluster, one of the finest naked-eye objects in the sky. Two open clusters separated by just half a degree, each containing hundreds of young, hot stars around 7,500 light-years away. They are genuinely young clusters: NGC 869 is about 5.6 million years old, NGC 884 about 3.2 million years old. In binoculars they look like two sparkling clouds side by side. In a small telescope they are breathtaking.

Messier 52 (NGC 7654)

A rich open cluster of about 200 stars, roughly 5,000 light-years away. Easy in binoculars, it appears as a fuzzy star to the naked eye. A small telescope resolves the cluster well, with a bright orange star near one edge providing a visual anchor.

Messier 103 (NGC 581)

A compact open cluster of about 40 stars, one of the most distant Messier objects at roughly 8,500 light-years. It appears as a small triangular spray of stars in a telescope, with a distinctive reddish giant near the center.

The Heart and Soul Nebulae (IC 1805 & IC 1848)

Two massive star-forming emission nebulae in Cassiopeia, together spanning more than 200 light-years. At roughly 7,500 light-years away, they are faint visually but spectacular in long-exposure photography — especially in narrowband hydrogen-alpha, where the glowing ionized gas clouds take on dramatic sculpted forms. The Heart Nebula surrounds an open cluster, Melotte 15, whose massive young stars ionize the surrounding gas.

NGC 457 — The Owl (or ET) Cluster

An open cluster with a distinctive shape: two bright stars forming "eyes" with chains of fainter stars spreading like wings or arms. Depending on orientation it looks like an owl, or — famously — like the movie character E.T. About 7,900 light-years away, it is easy in binoculars and striking in any telescope.

Historical Supernovae

Cassiopeia has hosted two historically significant supernovae. Tycho Brahe's supernova of 1572 (SN 1572) blazed bright enough to be seen in daylight and helped overturn the idea of an unchanging celestial sphere — Brahe's careful observations demonstrated it was not a comet or atmospheric phenomenon but a genuine new star far beyond the Moon. Its remnant, Tycho's Supernova Remnant, is still visible at radio and X-ray wavelengths.

The second, around 1680 CE, produced Cassiopeia A. Its relative obscurity in historical records has been attributed to interstellar dust absorption that may have dimmed it significantly even at its peak.

Finding and Observing Cassiopeia

Cassiopeia is circumpolar for observers north of about 20°N latitude, meaning it never sets. From the northern United States and Canada, it is visible every clear night of the year. The best approach to finding it: locate the Big Dipper, then draw a line through the two stars at the end of its handle outward past Polaris — Cassiopeia will be on the opposite side of the pole, roughly equidistant.

In autumn evenings, Cassiopeia is nearly overhead. In spring, it sits lower in the north while the Big Dipper is high. Either way, the W shape is unmistakable once you've seen it once.

For deep-sky observing, even a 50mm binocular reveals the Double Cluster, M52, M103, and NGC 457. A 6-inch telescope will show dozens of Cassiopeia's open clusters well. The emission nebulae (Heart and Soul) are targets for astrophotography rather than visual observing, but long-exposure images with narrowband filters produce some of the most dramatic nebula imagery in the sky.

Star Formation in Cassiopeia

A Hubble view of a star-forming region in Cassiopeia, showing young hot stars heating surrounding gas clouds.

Credit: ESA/Hubble & NASA

Cassiopeia — Wide Field

A wide-field ground-based view of the Cassiopeia region, lying along a rich arm of the Milky Way.

Credit: ESO

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