Globular clusters are intrinsically beautiful objects, but the subject of this image from the NASA / ESA Hubble Space Telescope, Messier 3, is commonly recognized as one of the most beautiful of all.
With an incredible half million stars, this 8-billion-year-old cosmic bauble is one of the largest and brightest globular clusters ever discovered. However, what makes Messier 3 special is its unusually large population of variable stars, stars that fluctuate in brightness over time. New variable stars continue to be discovered in this brilliant starry nest to this day, but so far we know of 274, the highest number found in any globular cluster to a large extent. At least 170 of these are of 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 really is based on its mbad and clbadification, and know how bright it appears to be from our point of view here on Earth, they can thus determine 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 high number of so-called blue stragglers, which are clearly shown in this Hubble image. These are blue stars in the main sequence that appear to be young because they are bluer and brighter than other stars in the cluster. Since it is believed that all stars in globular clusters have formed together and, therefore, are approximately the same age, only a difference in mbad can give these stars a different color. An old red star may appear more blue when it acquires more mbad, for example, by removing it from a nearby star. The extra mbad makes it a bluer star, which makes us think that it is younger than it really is.
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