Caldwell 66

Also known as NGC 5694, Caldwell 66 was discovered by William Herschel in 1784 and is one of the oldest, most remote globular clusters in our galaxy.

Read More


100,000 light-years

Apparent Magnitude




object type

Globular Cluster

Bright-white spherical cluster of stars in the upper left corner. Stars taper off down and out  to the right of the cluster against a black background. Reddish-orange stars dot the scene.
NASA, ESA, G. Fahlman (Dominion Astrophysical Observatory), R. Ibata (Université de Strasbourg), and F. Ferraro (Università di Bologna); Processing: Gladys Kober (NASA/Catholic University of America)

Caldwell 66, also known as NGC 5694, was discovered by William Herschel in 1784 and is one of the oldest, most remote globular clusters in our galaxy. Located roughly 100,000 light-years from Earth in the constellation Hydra, this collection of stars appears very faint in the sky with an apparent magnitude of 10.2. A small telescope is needed to spot the distant cluster. It can be seen best from southern locations in the Northern Hemisphere in the late spring and from the Southern Hemisphere in the autumn.

Most globular clusters reside on the outskirts of a galaxy and are gravitationally bound to their galaxy. This means that the clusters will remain in orbit around the galaxy’s gravitational center. However, astronomers suspect that Caldwell 66 is traveling on an unbound, hyperbolic (or U-shaped) path around our Milky Way galaxy. The cluster is located far enough away from the galactic center that it is only loosely bound by gravity, and it is traveling through space so quickly that it will likely escape from our galaxy’s gravitational grip.

This image of Caldwell 66 is a composite of observations made in visible and ultraviolet light by Hubble’s Wide Field and Planetary Camera 2. The observations were taken to help astronomers better understand the evolution of globular clusters and to more precisely determine the age of the cluster. A faint background galaxy appears near the right edge of the image.

Star chart for Caldwell 66
This star chart for Caldwell 66 represents the view from mid-northern latitudes for the given month and time.
Image courtesy of Stellarium


Apparent Magnitude - The brightness of an astronomical object as seen from Earth, influenced by the object's distance from Earth, its absolute magnitude, and even gas and dust that lie between the object and Earth.

Globular Cluster - A spherical group of stars that are gravitationally bound to each other, with most of the stars concentrated at the cluster’s center.

Explore Hubble's Caldwell Catalog

The following pages contain some of Hubble’s best images of Caldwell objects.

Caldwell 1

Also known as NGC 188, this group of stars formed from a large cloud of gas making the stars roughly…

Caldwell 2

This shell of gas is expanding outward, away from the dying star within.

Caldwell 3

This barred spiral galaxy was first spotted by British astronomer William Herschel in April 1793 in the constellation Draco.