Hubble looks at the cosmic blue ball

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 half a million incredible stars, this 8 billion year old cosmic trinket is one of the largest and brightest globular 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, the 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 main sequence stars that look young because they are more blue and brighter than other stars in the bunch. Since it is thought that all the stars of the globular groups have formed together and are therefore 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.

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