More than 500,000 stars shine red, white and blue in this image from NASA’s Hubble Space Telescope, released to celebrate the 250th anniversary of the United States. The image shows Messier 3 (M3), one of the Milky Way’s most massive globular clusters, or spherical collections of gravitationally bound stars. Globular clusters are made up of ancient stars that formed around the same time from the same cloud of gas, giving them similar ages. About 150 known globular clusters are scattered across the outer regions of the Milky Way.

In addition to its significant mass, M3 is unusual in that it is located relatively far from the galactic center and has more than 240 RR Lyrae variable stars, the most of any globular cluster in our galaxy. The RR Lyrae variables are some of the oldest stars in the galaxy 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 actual brightness can be used to measure distances in the cosmos, just as knowing the brightness of a car’s headlights on a dark road can help estimate the distance to an approaching vehicle.

The M3 globular cluster also contains about 70 candidates identified as “blue stragglers,” which are stars that shine with a bright blue light that makes them appear younger stars than the typical redder residents of globular clusters. This was the first cluster in which these strange stars were located. These stars are believed to have gravitationally attracted mass from companion stars, rejuvenating them and making them appear bluer and younger despite their true age.

M3’s unusual features may be due to 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 features. In this image, blue indicates the shorter wavelengths of visible light, while red represents 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 passing 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 study designed to look at about half of the Milky Way’s globular clusters to build a detailed timeline of how the Milky Way formed. With more than 30 years of observations, Hubble is one of NASA’s flagship observatories, working in tandem with its sister space missions, including the infrared-sensing Webb Space Telescope and the upcoming Nancy Grace Roman Space Telescope, to weave a complete picture of our vast universe.

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Media contact:

Claire Andreoli
from NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
claire.andreoli@nasa.gov