Collinder 135, known sometimes as the Pi Puppis Cluster, is an open cluster

Open cluster

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...

 in Puppis

Puppis

Puppis is a constellation in the southern sky. Its name is the Latin word for the poop deck of a ship, and Puppis represents the deck of the ship and its deckhouses...

 constellation

Constellation

In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

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It consists of four stars visible also to naked eye

Naked eye

The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...

, and a widespread population of stars. It lies in the southern celestial hemisphere near a rich star field. The main component is the star Pi Puppis

Pi Puppis

Pi Puppis is a star in the constellation Puppis.In Chinese, , meaning Bow and Arrow, refers to an asterism consisting of π Puppis, δ Canis Majoris, η Canis Majoris, c Puppis, ZI 674, ο Puppis, κ Puppis, ε Canis Majoris and κ Canis Majoris...

, which gives to the cluster its common name; it is an orange supergiant

Red supergiant

Red supergiants are supergiant stars of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive...

 with a visual magnitude of 2.71. The 5th magnitude stars are all variables

Variable star

A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

: one (NV Puppis) is a Gamma Cassiopeiae variable, and the other (NW Puppis) is a Beta Cephei variable

Beta Cephei variable

Beta Cephei variables are variable stars which exhibit variations in their brightness due to pulsations of the stars' surfaces. The point of maximum brightness roughly corresponds to the maximum contraction of the star. Typically, Beta Cephei variables change in brightness by 0.01 to 0.3...

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