Cetus

By BrightK self-checked Published 2026-09-04 Updated 2026-10-18 Editorial policy

• Constellations
• In the sky now

What is Cetus?

Constellations· Autumn evenings (N) · a sprawling figure south of Pisces, home of Mira. The sea monster sent to devour Princess Andromeda, and the fourth-largest constellation. It holds Mira, the ‘wonderful star’ that appears and vanishes on a cycle of about 11 months, and Tau Ceti, a nearby Sun-like star.

Stars of the figure

The 1 brightest of the 8 stars used in the figure (8 lines). Lower magnitude means brighter.

Deep-sky objects near this constellation

Constellation basics

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Mira's tail — a star trailing 13 light-years

In 2007 the ultraviolet space telescope GALEX photographed Mira and found something unexpected: a tail 13 light-years long streaming behind the star like a comet's. The tail cannot be seen in ordinary light and glows only in ultraviolet, so although people had watched the star for more than 400 years, no one knew it was there.

The tail forms because Mira is moving fast, at 130 km per second relative to its neighbours. As the star swells and shrinks it sheds gas, and that gas runs into the thin gas between the stars and is swept back behind it, much like the wake behind a moving boat. The gas at the far end of the tail left the star about 30,000 years ago.

The tail captures in a single picture how an old star gives its material back to space. Elements such as carbon and oxygen scattered this way later become raw material for new stars and planets. The ‘wonderful star’ that appears and vanishes every eleven months is still racing on, leaving a long trail across the sky.

The constellation in depth

Cetus is the sea monster sent by Poseidon, god of the sea. When Queen Cassiopeia boasted that her daughter was more beautiful than the sea nymphs, Poseidon sent the monster as punishment. The hero Perseus turned it to stone by showing it Medusa's head, and saved Andromeda. Although the name is usually translated as ‘whale’, old star charts drew it as a monster with teeth.

Mira was first recorded by David Fabricius in 1596. About every 332 days it brightens, then fades until it can no longer be seen with the naked eye. It was the first star known to change brightness on a regular cycle, and so it was named Mira, Latin for ‘wonderful’. It is an old, swollen star whose light changes as it expands and contracts.

Tau Ceti, 12 light-years away, is very similar to the Sun in size and colour. Stars that are both this close and this Sun-like are not common, so it has long been a favourite target in searches for radio signals from other civilisations. Several candidate planets have now been reported around it.

On autumn evenings, search the wide area south of the Great Square of Pegasus and east of Fomalhaut for Diphda at the tail and Menkar at the head; the monster's body lies between them. When Mira is bright, an extra reddish star appears at the neck. Set the Sky view to a November night to see the whole figure in the south, and turn on deep-sky objects to mark Mira.

Frequently asked

When can I see Cetus?

Cetus is best seen in autumn evenings, October–December.

How do I find Cetus?

To find Cetus, look for the wide, faint polygon south of the Great Square and east of Fomalhaut.

What is the brightest star in Cetus?

The brightest star in Cetus is Mira, at magnitude 3.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.