Caldwell 1

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

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Distance

5,400 light-years

Apparent Magnitude

8.1

constellation

Cepheus

object type

Open Cluster

Caldwell 1
NASA, ESA, and L. Dressel (STScI); Processing: Gladys Kober (NASA/Catholic University of America)

This collection of stars is part of Caldwell 1, the first object in the Caldwell catalog. Also known as NGC 188, it is an open cluster, a group of stars that were all formed from the same large cloud of gas and are therefore all roughly the same age. Scientists are interested in such clusters because they can study how the chemicals in each star vary from one star to another.

Two images of Caldwell 1. Lower left is a wide-field image that shows the location of the Hubble image. Right two-thirds is the Hubble image, which shows a scattering of stars against a black backdrop.
An image in the lower left, taken with the 0.9-meter telescope at Arizona’s Kitt Peak National Observatory, provides a full view of star cluster Caldwell 1 (NGC 188). The inset shows Hubble’s Wide Field Camera 3 (WFC3) close-up image of stars near the center of the cluster.
Ground-based image: NOAO/AURA/NSF; Hubble image: NASA, ESA, and L. Dressel (STScI); Processing: Gladys Kober (NASA/Catholic University of America)
Three images of Caldwell 1. Upper-left is a wide-field image that shows the locations of the other two Hubble images. Bottom-left and bottom-right are Hubble images of a scattering of stars against a black backdrop. Stars in left image are bluish. Stars in right image are reddish.
Hubble’s Wide Field and Planetary Camera 2 (WFPC2) captured images of stars in other parts of Caldwell 1 (NGC 188). Two close-up views from WFPC2 appear at the bottom, while a ground-based image in the upper right shows where those fields reside in the cluster.
Ground-based image: NOAO/AURA/NSF; Hubble images: NASA, ESA, STScI, M. Bolte (University of California, Santa Cruz), and K. Williams (Texas A&M University); Processing: Gladys Kober (NASA/Catholic University of America)

Many open clusters, like Caldwell 1, are made up of stars that aren’t very close together and therefore have only a loose gravitational connection. For this reason, these stars have the tendency to slowly drift apart over millions of years. But Caldwell 1 is different. One of the oldest open clusters known, it sits far enough away from our galaxy’s core that it hasn’t yet been ripped apart by the galaxy’s gravitational influences. Caldwell 1 is estimated to be at least 6.8 billion years old.

Caldwell 1 was discovered in 1831 by John Herschel, an English polymath who invented the blueprint and named seven of Saturn’s moons along with four moons orbiting Uranus, the planet his father, William Herschel, discovered. This ancient cluster, which boasts an apparent magnitude of 8.1, contains a good number of middle-aged stars called main sequence stars. Such stars often pique the interest of researchers because they tend to be the best candidates to host exoplanets that could support life. However, Caldwell 1 is over 5,400 light-years away from Earth, making it slightly more difficult to study than closer stars when searching for planets.

Caldwell 1 is located in the constellation Cepheus and is circumpolar, meaning it is so close to the North Celestial Pole that it is above the horizon at all times from anywhere in the Northern Hemisphere. Because of this, Caldwell 1 is easily viewed year-round for northern observers. It is not far from Polaris, the North Star, located at the end of the “handle” of the Little Dipper. Use a telescope to find the soft, dim glow from this cluster of stars, and on especially dark nights, you may be able to see some of the brightest of the bunch resolved within.

The image above, showing just a portion of Caldwell 1, was captured by Hubble’s Wide Field Camera 3. Astronomers have studied Caldwell 1 with that camera as well as its predecessor, the Wide Field and Planetary Camera 2, to better understand the age of the cluster’s stars and of the cluster itself. Using Hubble’s Advanced Camera for Surveys, others have investigated unusual stars called “blue stragglers” in Caldwell 1, finding signs that these stellar oddballs are actually pairs of stars that have shared material between them, causing one member of the pair to appear hotter and bluer than it should for its age. Caldwell 1 was also the first target of one of Hubble’s original science instruments, the Faint Object Camera, which imaged a pair of stars in the cluster during an engineering test soon after Hubble’s launch in 1990.

For more information about Hubble’s observations of Caldwell 1, see:
Hubble Helps Solve Mystery of ‘Born Again’ Stars
ESA’s Faint Object Camera First Images

Line drawings of constellations pinpoint the location of Caldwell 1.
This star chart for Caldwell 1 represents the view from mid-northern latitudes for the given month and time.
Image courtesy of Stellarium

Glossary

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.

Blue Straggler - A blue star in a star cluster that appears to be far younger than its neighbors, likely resulting from collisions between stars or by other stellar interactions.

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.

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.