Caldwell 27

Also called the Crescent Nebula or NGC 6888, Caldwell 27 holds a massive, incredibly hot star at its heart.


4,700 light-years

Apparent Magnitude




object type

Planetary Nebula

colorful streams of yellow, blue, and green gas and dust extend from the lower left to the center right.
Also called the Crescent Nebula or NGC 6888, Caldwell 27 holds a massive, incredibly hot star at its heart.
NASA, ESA, Brian D. Moore and J. Jeff Hester (Arizona State University)

Commonly called the Crescent Nebula, Caldwell 27 looks more like a prehistoric dinosaur egg. Hubble’s vibrantly colored image zooms in on a small region of the nebula that is “only” 3 light-years across, or about 17.6 trillion miles. The black-and-white inset is a ground-based image that displays nearly the full nebula, which spans roughly 16 by 25 light-years. The cloudy, mottled shell surrounds an extremely hot and short-lived type of star called a Wolf-Rayet. Dubbed WR 136, this colossal star is unleashing a powerful stellar wind of charged particles from its surface, which is tearing apart the shell of surrounding material that the star blew off 250,000 years ago.

colorful streams of yellow, blue, and green gas and dust extend from the lower left to the center right. At the bottom right is a black and white image from a ground based telescope.
NASA, Brian D. Moore, Jeff Hester, Paul Scowen (Arizona State University), Reginald Dufour (Rice University)

WR 136 created this web of luminous material during the late stages of its life. As a bloated, red supergiant, WR 136 puffed away some of its bulk, which then settled around it in a vast, roughly spherical cloud. When the star evolved from a supergiant to a Wolf-Rayet star, it developed an even fiercer stellar wind and began expelling mass at a furious rate. The stellar wind collided with the material around the star and swept it up into a thin shell. That shell broke apart into the network of bright clumps seen in the Hubble image.

Hubble’s close-up of the nebula reveals with unprecedented clarity that the shell of matter is a network of filaments and dense knots, all enshrouded in a thin “skin” of gas (seen in blue). Hubble’s sharp vision allows scientists to probe the intricate details of the complex system. Such details are crucial to understanding the life cycle of stars and their impact on the evolution of our galaxy. The observations were taken in June 1995 with Hubble’s Wide Field and Planetary Camera 2. Scientists selected the image’s colors to correspond with the ionization state of the gases (how many atoms have been lost or gained in the atoms of each gas), with blue representing the highest and red the lowest observed ionization.

Also cataloged as NGC 6888, Caldwell 27 was discovered by William Herschel in 1792. This stellar demolition zone lies within our own galaxy, about 4,700 light-years from Earth in the constellation Cygnus, the Swan. For best viewing, observe Caldwell 27 with a moderate to large telescope equipped with a light-pollution filter during the late summer from the Northern Hemisphere (or during the winter in the Southern Hemisphere). With a magnitude of 8.8, the Crescent Nebula is not visible to the naked eye — but if it were, it would appear in the sky as an ellipse one-quarter the size of the full moon. In the future, the nebula’s shell may become compressed and begin glowing again, this time as a powerful blast wave moves outward from the Wolf-Rayet star when it completely destroys itself in a supernova explosion.

For more information about Hubble’s observations of Caldwell 27, see:
Hubble Watches Star Tear Apart Its Neighborhood

This star chart for Caldwell 27 represents the view from mid-northern latitudes for the given month and time.
Image courtesy of Stellarium


Ionization - The process by which particles become electrically charged; radiation from various astronomical sources, such as stars, can charge surrounding gases with electromagnetic radiation, causing these clouds of gas to glow.

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.

Supernova - The explosion of a massive star at the end its life, which ejects material into space and causes the star to temporarily brighten in our sky.

Wolf-Rayet Star - A massive, incredibly hot star that blasts substantial winds of particles out into space and can shed mass equal to that of our Sun in just 100,000 years.

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.