NASA's Hubble Spots Star-Spangled Cosmic Scene

More than 500,000 stars blaze red, white, and blue in this image from NASA's Hubble Space Telescope, released in celebration of the United States' 250th anniversary. The image showcases Messier 3 (M3), one of the Milky Way galaxy's most massive globular clusters, or spherical collections of gravitationally bound stars. Globular clusters are made up of ancient stars that formed at roughly the same time from the same cloud of gas, giving those stars similar ages. Around 150 known globular clusters are sprinkled around the outer regions of the Milky Way.

In addition to its significant mass, M3 is unusual because it lies relatively far from the galactic center and has more than 240 RR Lyrae variable stars, the most of any globular cluster in our galaxy. RR Lyrae variables are some of the galaxy's oldest stars and are of special interest to astronomers, due to their age and because their light fluctuates over time in a way that tells us their intrinsic brightness. This true brightness can be used to measure distances in the cosmos, just as knowing the brightness of car headlights on a dark road can help estimate the distance to an oncoming vehicle.

The M3 globular cluster also contains around 70 identified "blue straggler" candidates, which are stars that shine with a bright, blue light that makes them look like younger stars than the typical, redder residents of globular clusters. This was the first cluster in which these oddball stars were located. These stars are thought to have gravitationally pulled mass from companion stars, rejuvenating them and making them appear bluer and younger despite their true age.

The unusual characteristics of M3 may arise from its origins. The globular cluster, which contains two distinct populations of stars, may be the result of a merger of two globular clusters. These two clusters were members of the same dwarf galaxy, which was later swallowed up by the Milky Way.

Hubble has taken several images of M3, also known as NGC 5272, documenting its complicated and intriguing characteristics. In this image, blue indicates the shorter wavelengths that are visible light, while red depicts the longer wavelengths of visible light, as well as some near-infrared light. Colors in Hubble images are chosen based on standard image processing techniques to best represent the wavelengths of light that pass through the filters used in the observation. Because the color and temperature of stars are directly related, we know that the blue stars in this image are hotter and the red stars are cooler.

This image is part of a Hubble Treasury program survey designed to observe approximately half of the Milky Way's globular clusters to construct a detailed chronology of how the Milky Way galaxy formed. With over 30 years of observations, Hubble is one of NASA's flagship observatories and works in complement with its sibling space missions, including the infrared-detecting Webb Space Telescope and the upcoming Nancy Grace Roman Space Telescope, to weave together a comprehensive picture of our vast universe.