File:Caldwell 58 (50193004227).jpg

Caldwell 58, also known as NGC 2360 or Caroline’s Cluster, was discovered by and named after the German astronomer Caroline Herschel in 1783. The younger sister of famed astronomer William Herschel, Caroline was the first woman to win the prestigious Gold Medal of the Royal Astronomical Society. She earned this award for her work verifying her brother’s astronomical observations and compiling a catalog of nebulae to aid other astronomers.

Caldwell 58 is an open cluster — a group of stars loosely bound together by gravity. It is located in the constellation Canis Major, roughly 3,700 light-years from Earth. The cluster has an apparent magnitude of 7.2 and can be spotted with a pair of binoculars in dark, moonless skies.

Caldwell 58 is most easily observed during the Northern Hemisphere’s winter and the Southern Hemisphere’s summer.

Astronomers used Hubble to study white dwarfs in Caldwell 58 and better understand the age of our galaxy. After a Sun-like star has exhausted its supply of nuclear fuel and ejected its outer layers of gas, what is left behind is the hot core of the star — a white dwarf. These objects cool over a period of billions of years and are some of the oldest stars in our galaxy. Some white dwarfs pulse regularly as they cool. The time between these pulsations changes over the white dwarf’s lifetime, so the time between pulses can be used to estimate how quickly the white dwarf is cooling, and thus how long it has been cooling. This information is useful to astronomers because it means pulsating white dwarfs can be used as chronometers, or “clocks,” that constrain the age of our galaxy. These observations of Caldwell 58 were made with Hubble’s Advanced Camera for Surveys to help astronomers calibrate white-dwarf chronometers.

Credit: NASA, ESA, and T. von Hippel (Embry-Riddle Aeronautical University); Processing: Gladys Kober (NASA/Catholic University of America)