Caldwell 51

This irregular dwarf galaxy is in the constellation Cetus.

Distance

2.3 million light-years

Apparent Magnitude

9.9

constellation

Cetus

object type

Dwarf Galaxy

Caldwell 51, also known as IC 1613, is an irregular dwarf galaxy located in the constellation Cetus.
Caldwell 51, also known as IC 1613, is located just 2.3 million light-years from Earth. Despite its relative proximity, Caldwell 51 is very difficult to spot and is one of the most elusive Caldwell objects. It appears as an extremely faint and diffuse smudge in the sky, even when viewed through moderately sized telescopes.
NASA, ESA, and J. Holtzman (New Mexico State University); Processing: Gladys Kober (NASA/Catholic University of America)

Caldwell 51, also known as IC 1613, is an irregular dwarf galaxy located in the constellation Cetus. This member of our Local Group of galaxies is located just 2.3 million light-years from Earth and has an apparent magnitude of 9.9. Despite its relative proximity, Caldwell 51 is very difficult to spot and is one of the most elusive Caldwell objects. It appears as an extremely faint and diffuse smudge in the sky, even when viewed through moderately sized telescopes. The galaxy is visible from the Northern Hemisphere in the autumn and from the northern regions of the Southern Hemisphere in the spring. It was discovered in 1906 by the German astrophotographer Max Wolf, who also found many other astronomical objects over the course of his life, including comets, supernovae, and asteroids.

This infrared image of an outer region of Caldwell 51 includes some even more distant background galaxies.
This infrared image of an outer region of Caldwell 51 includes some even more distant background galaxies. It was captured by Hubble’s Wide Field Camera 3.
NASA, ESA, and M. Malkan (UCLA); Processing: Gladys Kober (NASA/Catholic University of America)

Caldwell 51 is notable for its role in improving distance measurements to more remote objects. Unlike many galaxies, Caldwell 51 is largely unobstructed by dust, and our relatively clear view of the RR Lyrae variable stars it hosts allows astronomers to make precise distance measurements. RR Lyrae variables are pulsating stars that have pulsation periods directly related to their luminosity. So, how fast they pulse tells astronomers how bright the stars truly are; then, how bright the stars actually appear tells astronomers how far away they must be. This relationship allows the stars to act as “standard candles” that can be used to determine the distances to objects within our Local Group of galaxies.

This image of a region near the center of Caldwell 51 is a composite of ultraviolet, infrared, and visible-light observations made by Hubble’s Wide Field Camera 3. The observations were taken to determine the galaxy’s chemical composition. Since most elements in the universe are formed by stars and distributed into their galaxy when the stars die, this information can help researchers learn more about the evolution of the galaxy and its star formation history.

This context image shows two regions of Caldwell 51 (IC 1613) imaged by Hubble’s Wide Field Camera 3 (WFC3).
This context image shows two regions of Caldwell 51 (IC 1613) imaged by Hubble’s Wide Field Camera 3 (WFC3). The wider-field image of the whole galaxy in the bottom right was taken by the Very Large Telescope’s survey telescope at the European Southern Observatory (ESO). The orange box shows the location of Hubble’s infrared observations in the galaxy’s outskirts (shown at left), while the blue box shows its observations of the galaxy’s center (top right).
Ground-based image: European Southern Observatory, acknowledgment: VST/Omegacam Local Group Survey; Hubble WFC3 IR image: NASA, ESA, and M. Malkan (UCLA); Hubble WFC3 UVIS image: NASA, ESA, and J. Holtzman (New Mexico State University); Processing: Gladys Kober (NASA/Catholic University of America)
Star chart for Caldwell 51
This star chart for Caldwell 51 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.

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

Standard Candle - An astronomical object with a known absolute magnitude, allowing astronomers to precisely calculate its distance from Earth by measuring its apparent brightness.

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.

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.