Globular clusters are intrinsically beautiful objects, but the subject of this NASA / ESA Hubble Space Telescope image, Messier 3, is generally recognized as one of the most beautiful of all.
Containing an incredible half million stars, this eight billion year old cosmic bauble is one of the largest and brightest evergreen clusters ever discovered. However, what makes Messier 3 so special is its exceptionally large population of variable stars, stars whose brightness fluctuates with time. New, variable stars continue to be discovered in this glittering star nest to this day, but we know so far 274, the highest number of all globular groups. At least 170 of them belong to a special variety called RR Lyrae variables, which pulsate with a period directly related to their intrinsic brightness. If astronomers know how bright a star is in terms of its mass and classification, and how bright it looks from our point of view here on Earth, they can calculate its distance from us. For this reason, RR Lyrae stars are known as standard candles – objects of known luminosity whose distance and position can be used to help us better understand the vast celestial distances and the scale of the cosmos.
Messier 3 also contains a relatively large number of so-called "blue" laggards, which are clearly shown in this Hubble image. These are blue stars in the main sequence that look young because they are more blue and brighter than the other stars in the cluster. As all the stars of the globular clusters are thought to have formed together and are therefore of about the same age. Only a difference in mass can give these stars a different color: an old red star may look bluer when it gains more mass, for example by removing it from a nearby star. The extra mass changes it into a bluer star, which makes us think that she is younger than she actually is.
Credit: ESA / Hubble & NASA, G. Piotto et al.