page_title: Globular cluster
section_title: Morphology
hierarchical_section_title: Globular cluster / Morphology
caption_attribution_description: English: 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 Hubble’s images it can be quite hard to tell them apart – 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 isn’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 — 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. Hubble’s image shows a wide range of colours and brightnesses in the cluster’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 Hubble’s Wide Field Camera 3. This filter set lets the telescope “see” colours slightly further beyond red and the violet ends of the spectrum.
context_page_description: A globular cluster is a spherical collection of stars that orbits a galactic core. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes, and relatively high stellar densities toward their centers. The name of this category of star cluster is derived from the Latin, globulus—a small sphere. Occasionally, a globular cluster is known simply as a globular.
  Globular clusters are found in the halo of a galaxy and contain considerably more stars, and are much older than the less dense open clusters, which are found in the disk of a galaxy. Globular clusters are fairly common; there are about 150 to 158 currently known globular clusters in the Milky Way, with perhaps 10 to 20 more still undiscovered. Larger galaxies can have more: The Andromeda Galaxy, for instance, may have as many as 500. Some giant elliptical galaxies, such as M87, have as many as 13,000 globular clusters.
  Every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, and almost every large galaxy surveyed has been found to possess a system of globular clusters.
context_section_description: In contrast to open clusters, most globular clusters remain gravitationally bound for time periods comparable to the life spans of the majority of their stars. However, a possible exception is when strong tidal interactions with other large masses result in the dispersal of the stars.
  After they are formed, the stars in the globular cluster begin to interact gravitationally with each other. As a result, the velocity vectors of the stars are steadily modified, and the stars lose any history of their original velocity. The characteristic interval for this to occur is the relaxation time. This is related to the characteristic length of time a star needs to cross the cluster as well as the number of stellar masses in the system. The value of the relaxation time varies by cluster, but the mean value is on the order of 10⁹ years.
  Although globular clusters generally appear spherical in form, ellipticities can occur due to tidal interactions. Clusters within the Milky Way and the Andromeda Galaxy are typically oblate spheroids in shape, while those in the Large Magellanic Cloud are more elliptical.
caption: NGC 411 is classified as an open cluster.[61]
exif: {"Image ImageDescription": "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 Hubble\u2019s images it can be quite  hard to tell them apart \u2013 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 isn\u2019t 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 \u2014 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. Hubble\u2019s  image shows a wide range of colours and brightnesses in the cluster\u2019s  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 Hubble\u2019s Wide Field Camera 3. This filter set lets  the telescope \u201csee\u201d colours slightly further beyond red and the violet  ends of the spectrum.", "Image Artist": "", "Image Copyright": ""}