Caldwell 42

Globular clusters like Caldwell 42 are relics of our galaxy's youth.


135,000 light-years

Apparent Magnitude




object type

Globular Cluster

NGC 7006 resides in the outskirts of the Milky Way. This roughly spherical region of our galaxy is made up of dark matter, gas and sparsely distributed stellar clusters. Like other remote globular clusters, NGC 7006 provides important clues that help astronomers to understand how stars formed and assembled in the halo. The cluster has a very eccentric orbit, indicating that it may have formed independently in a small galaxy outside our own that was then captured by the Milky Way.
ESA/Hubble & NASA

Caldwell 42, also known as NGC 7006, is one of 18 globular clusters in the Caldwell catalog. If you look at this Hubble image of the cluster closely, you may be able to spot several tiny background galaxies (which appear fuzzier and more extended than the cluster’s stars). Each of those distant island universes likely contains a hundred or more globular clusters of its own. Unlike open star clusters, which are smaller and more loosely bound, globular clusters are densely packed with hundreds of thousands of stars held in a roughly spherical shape by their mutual gravity.

Globular clusters like Caldwell 42 are relics of the galaxy’s earliest years. Since they were born as the nascent galaxy was forming, these clusters provide a glimpse backward in time and provide a hint of what the Milky Way was like billions of years ago. Studying globular clusters allows scientists to learn more about how the first stars formed in our galaxy and the role the clusters played in the galaxy’s development. However, Caldwell 42 has a very elongated orbit around the center of our galaxy, which might hint toward an extragalactic origin. It seems that some globular clusters may have once been independent dwarf galaxies that were later absorbed by the Milky Way.

The cluster is nestled in the diminutive constellation Delphinus and is best viewed in the late summer in the Northern Hemisphere or late winter in the Southern Hemisphere. It was discovered in 1784 by British astronomer William Herschel — the discoverer of dozens of celestial objects in the Caldwell catalog. Caldwell 42 isn’t terribly bright to begin with compared to other star clusters, so at its distance of about 135,000 light-years from Earth it’s really not the most impressive sight to most amateur astronomers. In a moderate-sized telescope, the magnitude-10.5 cluster appears as a dim, circular smudge. It is difficult to pick out individual stars in the cluster, though large amateur telescopes may be able to resolve a few of the cluster’s most vibrant stellar members.

This image, taken by Hubble’s Advanced Camera for Surveys, includes light detected at both visible and infrared wavelengths. When viewed in visible wavelengths alone, Caldwell 42 appears dimmer. This is because intervening dust between us and Caldwell 42 scatters some of the cluster’s visible light but lets the infrared light pass through. Hubble’s multi-wavelength view transforms this otherwise faint smudge into a dazzling spectacle, while helping astronomers analyze the cluster’s stars and investigate Caldwell 42’s history.

For more information about Hubble’s observations of Caldwell 42, see:
A Remote Outpost of the Milky Way

This star chart for Caldwell 42 represents the view from mid-northern latitudes for the given month and time.
Image courtesy of Stellarium


Dwarf Galaxy - A small, faint galaxy with only millions to a few billion stars.

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.

Magnitude - The brightness of an astronomical object, represented by a number; bright objects have low numbers on the magnitude scale, while dim objects have high numbers.

Open Cluster - A group of stars loosely bound by gravity, destined to be short lived because the gravitational interactions between members are weak enough that stars can be drawn away from the cluster by stronger gravitational forces.

Explore Hubble's Caldwell Catalog

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

Stars with four diffraction spikes dot the scene against a black backdrop.

Caldwell 1

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

Red cloud of dust with a bright white star in the center of it. Lots of reddish and orangish stars in the background.

Caldwell 2

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

Large grouping of bright white, blue and red stars. Lightly colored blue dust surrounds the stars.

Caldwell 3

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