A spectacular 8-billion-year-old globular cluster containing half a million stars has been imaged by the Hubble telescope. Named Messier 3, it is one of the largest and brightest globular clusters ever discovered.
A globular cluster is an inherently beautiful object, as you’ll see from the gallery at the foot of this article, but the subject of this NASA/ESA Hubble Space Telescope image — Messier 3 — is commonly acknowledged by astronomers to be one of the most beautiful of them all.
The 8-billion-year-old ‘cosmic bauble’ is one of the largest and brightest globular clusters ever discovered — containing half a million stars.
What makes Messier 3 so extraordinary, though, is an unusually large population of variable stars — stars that fluctuate in brightness over time.
New variable stars are being discovered in this sparkling stellar nest but so far we know of 274, the highest number found in any globular cluster to date. Astronomers believe these globular clusters are extremely old and they are generally found orbiting around galactic centres.
At least 170 of these are of a special variety called RR Lyrae variables, which pulse with a period directly related to their intrinsic brightness. If astronomers know how bright a star truly is based on its mass and classification, and they know how bright it appears to be from our viewpoint here on Earth, they can thus work out its distance from us.
For this reason, astronomers refer to RR Lyrae stars as standard candles — objects of known luminosity whose distance and position can be used to measure vast celestial distances and the scale of the cosmos.
Also visible in this stunning new image is Messier 3’s relatively high number of so-called blue stragglers — blue main sequence stars that appear to be young because they are bluer and more luminous than other stars in the cluster.
As all stars in globular clusters are believed to have formed together and thus to be roughly the same age, only a difference in mass can give these stars a different colour. A red, old star can appear bluer when it acquires more mass, for instance by stripping it from a nearby star.
The extra mass changes it into a bluer star, which makes us think it is younger than it really is.
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Hubble’s incredible Messier image catalogue
Messier 3 is featured in Hubble’s Messier catalogue, which includes some of the most fascinating objects that can be observed from Earth’s Northern Hemisphere.
Among them are deep-sky objects that can be viewed in stunning detail using larger telescopes but are also bright enough to be seen through a small telescope. This characteristic makes Messier objects extremely popular targets for amateur astronomers possessing all levels of experience and equipment.
They are so popular, in fact, that they have inspired a special award from the Astronomical League (an organization for amateur astronomers) given to observers who are able to spot each of these objects.
You can visit the catalogue of stunning images here: https://www.nasa.gov/content/goddard/hubble-s-messier-catalog