Caldwell 106

Hubble observations of Caldwell 106 revealed a dynamic conveyor belt of star motion within the cluster.


16,700 light-years

Apparent Magnitude




object type

Globular Cluster

A sphere of blue-white stars fills the scene. They are concentrated at image center and taper off as toward the image's edges. Orangish, yellowish, and pinkish stars dot the periphery.
This NASA/ESA Hubble Space Telescope image shows a globular cluster known as NGC 104 — or, more commonly, 47 Tucanae, since it is part of the constellation of Tucana (The Toucan) in the southern sky. After Omega Centauri it is the brightest globular cluster in the night sky, hosting tens of thousands of stars. Scientists using Hubble observed the white dwarfs in the cluster. These dying stars migrate from the crowded centre of the cluster to its outskirts. Whilst astronomers knew about this process they had never seen it in action, until the detailed study of 47 Tucanae.
NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration Acknowledgment: J. Mack (STScI) and G. Piotto (University of Padova, Italy)

This Hubble image of Caldwell 106 displays a glittery spray of ancient stars that are held together by their mutual gravity. Globular clusters are isolated star cities, home to hundreds of thousands of stars. And like the fast pace of cities, there's plenty of action in these stellar metropolises. The stars are in constant motion, orbiting around the cluster's center. Past observations have shown that the heaviest stars tend to crowd into the “downtown” core area, while lightweight stars reside in the less-populated suburbs.

A mass of yellow and white stars fill the upper-left corner and taper off toward the lower-right corner. Reddish-orang stars dot the scene, all on a black background.
This image shows the Hubble telescope's close-up look at a swarm of 35, 000 stars near the cluster's central region. The stars are tightly packed together: They are much closer together than our Sun and its closest stars. The picture, taken by the Wide Field and Planetary Camera 2, depicts the stars' natural colours and tells scientists about their composition and age. For example, the red stars denote bright red giants nearing the end of their lives; the more common yellow stars are similar to our middle-aged Sun.
NASA, ESA, and Ron Gilliland (STScI)

As mid-sized stars age, they run out of hydrogen, the “fuel” they use in the nuclear process, and the star’s core collapses under its own weight. The star’s outer layers swell outward and cool, with its stellar wind blowing away much of this gas. After the purge, only the stars' bright, superhot cores, called white dwarfs, remain. This weight-loss program causes the now lighter weight white dwarfs to be nudged out of the downtown area through gravitational interactions with heftier stars. At each encounter, the white dwarfs' orbits begin to expand outward from the cluster's packed center.

Until these Hubble observations of Caldwell 106, astronomers had never seen this dynamic conveyor belt in action. This Hubble image, taken in visible, infrared, and ultraviolet light using the Wide Field Camera 3, reveals young white dwarfs amid their leisurely 40-million-year exodus from the bustling center of the cluster.

Also known as 47 Tucanae and NGC 104, this globular cluster is about 16,700 light-years away from Earth toward the southern constellation Tucana. Containing at least half a million stars, the cluster was discovered by astronomer Nicolas-Louis de Lacaille in 1751 and is highest above the horizon in the spring from the Southern Hemisphere. The sparkling magnitude-4 stellar collection appears near the Small Magellanic Cloud (a nearby dwarf galaxy) and is the second brightest globular cluster in the sky, visible to the naked eye under dark skies and easily viewed using a pair of binoculars.

Left: dark red to black background with bright-red gas clumps dotted with yellow stars. Right: red background dotted with yellow stars
These comparison images of the core of the globular cluster 47 Tucanae (NGC104) were taken with the COSTAR Corrected Faint Object Camera to show the improvement in performance when compared to images taken with the uncorrected camera.
NASA, ESA, Dr. R. Jedrzejewski (STScI)
Line drawings of constellations pinpoint the location of Caldwell 106.
This star chart for Caldwell 106 represents the view from mid-southern latitudes for the given month and time.
Image courtesy of Stellarium


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.

White Dwarf - The core of a dead Sun-like star whose outer layers have been expelled into space.

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.