English: 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.

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.

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

For more information about Hubble’s observations of Caldwell 1, see:

hubblesite.org/contents/news-releases/2015/news-2015-43.html

hubblesite.org/contents/news-releases/1990/news-1990-06.html

For Hubble's Caldwell catalog site and information on how to find these objects in the night sky, visit:

www.nasa.gov/content/goddard/hubble-s-caldwell-catalog