Caldwell 39

A dying star at Caldwell 39's heart began shedding its outer layers 10,000 years ago.


5,000 light-years

Apparent Magnitude




object type

Planetary Nebula

This stellar relic was first spied by William Herschel in 1787. The fuzzy outer region is a disk of material embellished with a ring of comet-shaped objects, with their tails streaming away from the central, dying star. The inner nebula also contains some fascinating details. Although this bright central region resembles a ball of twine, it is, in reality, a bubble of material being blown into space by the central star's intense "wind" of high-speed material.
NASA, Andrew Fruchter, and the ERO Team [Sylvia Baggett (STScI), Richard Hook (ST-ECF), Zoltan Levay (STScI)]

This nebula began forming about 10,000 years ago when a dying star began flinging material into space. When Sun-like stars exhaust their nuclear fuel, they become unstable and expand as they blast their outer layers of gas away into space (bad news for any planets in the area). Hubble’s image of Caldwell 39 shows a snapshot of the unworldly process, captured with the Wide Field and Planetary Camera 2 just a couple weeks after the successful Hubble servicing mission of December 1999.

This composite image of Caldwell 39 (NGC 2392) contains X-ray data from NASA's Chandra X-ray Observatory in purple, showing the location of million-degree gas near the center of the planetary nebula. Data from the Hubble Space Telescope — colored red, green, and blue — show the intricate pattern of the outer layers of gas that have been ejected from the star.
X-ray: NASA/CXC/IAA-CSIC/N. Ruiz et al.; Optical: NASA/STScI

Streams of high-energy ultraviolet radiation cause the expelled material to glow, creating a beautiful planetary nebula — a term chosen for the similarity in appearance to the round disk of a planet when viewed through a small telescope, not because of any actual relation to planets. The nebula’s glowing gases produce the colors in this image: nitrogen (red), hydrogen (green), oxygen (blue), and helium (violet).

Caldwell 39's outer disk of material is embellished by a ring of comet-shaped objects with their tails streaming away from the central, dying star. In the central region of the nebula is a bubble of material that is being blown outward by the star’s intense “wind” of high-speed material. Hubble’s observations provided clues about how the strange structures in the nebula formed and evolve, though scientists are still puzzled about the origin of the comet-shaped features. It seems they may form from a collision of slow- and fast-moving gases. Hubble also helped scientists determine the nebula’s distance and the mass of the star that lurks in its center.

Discovered in 1787 by famed astronomer William Herschel, Caldwell 39 (also known as NGC 2392) is about 5,000 light-years away in the Gemini constellation. In the Northern Hemisphere, it is best viewed in the late winter through a large telescope. (In the Southern Hemisphere, look for it in the late summer.) With a magnitude of 9.2, the nebula can be found with a smaller telescope — but you’re unlikely to see much of the detail in the center. The nebula exhibits a blinking effect similar to that of Caldwell 15 (the “Blinking Planetary”). When looking directly at the central star, the surrounding nebulosity fades; but as one looks away from the center, the nebula pops back into view.

Editor’s Note: As of Aug. 1, 2020, NASA no longer refers to nebula NGC 2392 as the “Eskimo Nebula,” as it can be considered an insensitive and offensive term. More information.

For more information about Hubble’s observations of Caldwell 39, see:
NGC 2392
A Beautiful End to a Star’s Life

This star chart for Caldwell 39 represents the view from mid-northern latitudes for the given month and time.
Image courtesy of Stellarium


Comet - A relatively small, icy body orbiting the Sun (or another star), often producing a tail when approaching the Sun (or star).

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.

Nebula - An interstellar cloud of dust and gas; either a location where new stars are being forged or a cloud of material ejected into space by a dying star.

Planetary Nebula - An expanding shell of gas around an aging or dying Sun-like star, cast off by the star.

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.