La grappe de jeunes étoiles NGC 411 photographiée par Hubble

$Globular clusters are roughly spherical collections of extremely old stars, and around 150 of them are scattered around our galaxy. Hubble is one of the best telescopes for studying these, as its extremely high resolution lets astronomers see individual stars, even in the crowded core. The clusters all look very similar, and in Hubbleâ€™s images it can be quite hard to tell them apart â€“ and they all look much like NGC 411, pictured here. And yet appearances can be deceptive: NGC 411 is in fact not a globular cluster, and its stars are not old. It isnâ€™t even in the Milky Way. NGC 411 is classified as an open cluster. Less tightly bound than a globular cluster, the stars in open clusters tend to drift apart over time as they age, whereas globulars have survived for well over 10 billion years of galactic history. NGC 411 is a relative youngster â€” not much more than a tenth of this age. Far from being a relic of the early years of the Universe, the stars in NGC 411 are in fact a fraction of the age of the Sun. The stars in NGC 411 are all roughly the same age, having formed in one go from one cloud of gas. But they are not all the same size. Hubbleâ€™s image shows a wide range of colours and brightnesses in the clusterâ€™s stars. These tell astronomers many facts about the stars, including their mass, temperature and evolutionary phase. Blue stars, for instance, have higher surface temperatures than red ones. The image is a composite produced from ultraviolet, visible and infrared observations made by Hubbleâ€™s Wide Field Camera 3. This filter set lets the telescope â€œseeâ€� colours slightly further beyond red and the violet ends of the spectrum.$

NGC 411, l’amas ouvert qui ressemble à s’y méprendre à un amas globulaire.

Contrairement à ce que les apparences suggèrent, NGC 411 photographié ici par le télescope spatial Hubble, n’est pas un amas globulaire. Au nombre de 150 dans la périphérie de notre galaxie, ceux-ci concentrent plusieurs dizaines, voire même pour certains, plusieurs centaines de milliers d’étoiles. La plupart ont le double de l’âge du Soleil soit 10 milliards d’années et ont vu grandir notre Voie Lactée.

NGC 411 est à l’opposé de ces grappes compactes. C’est un amas ouvert qui appartient à la galaxie voisine du Petit Nuage de Magellan. Un essaim de plusieurs milliers d’étoiles très jeunes, nées au sein d’un nuage de gaz et de poussières, il y a plus ou moins 1 milliard d’années. Ses étoiles s’éloignent progressivement les unes des autres, se libèrent des forces qui les retiennent.

Afficher et/ou télécharger l’image en haute résolution (7,4 Mb).

Crédit photo : ESA/NASA/Hubble.

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La grappe de jeunes étoiles NGC 411 photographiée par Hubble

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