page_title: Red-giant branch
section_title: Discovery
hierarchical_section_title: Red-giant branch / Discovery
caption_attribution_description: English: Most of the rich globular star clusters that orbit the Milky Way have cores that are tightly packed with stars, but NGC 288 is one of a minority of low-concentration globulars, with its stars more loosely bound together. This new image from the Advanced Camera for Surveys on the NASA/ESA Hubble Space Telescope completely resolves the old stars at the core of the cluster. The colours and brightnesses of the stars in the picture tell the story of how the stars have evolved in the cluster. The many fainter points of light are normal low-mass stars that are still fusing hydrogen in the same way as the Sun. The brighter stars fall into two classes: the yellow ones are red giant stars that are at a later phase in their careers and are now bigger, cooler and brighter. The bright blue stars are even more massive stars that have left the red giant phase and are being powered by helium fusion in their cores. The stars within globular clusters form at about the same time from the same cloud of gas, making these close families of stars. However, astronomers think that the stellar siblings in low-concentration globular clusters such as NGC 288, which are not so tightly bound together by gravity as richer and denser clusters, may eventually disperse and go their separate ways. NGC 288 is found within the rather obscure southern constellation of Sculptor, at a distance of about 30 000 light-years. This constellation also contains NGC 253, more commonly called the Sculptor Galaxy due to its location, and these two deep sky objects are close enough together on the sky to be observed in the same binocular field of view. William Herschel first spotted NGC 288 in 1785 and also recognised that it was a globular cluster that could be resolved into stars in his telescope. This picture was created from Hubble images taken using the Wide Field Channel of the Advanced Camera for Surveys through four different filters. Light recorded through a blue filter (F435W) is coloured blue, light through an orange filter (F606W) appears as green, light coming through a near-infrared filter (F814W) is red and finally the light from glowing hydrogen (F658N) is orange. The exposure times were 740 s, 530 s, 610 s and 1760 s respectively and the field of view is 3.2 arcminutes across.
context_page_description: The red-giant branch, sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass stars. Red-giant-branch stars have an inert helium core surrounded by a shell of hydrogen fusing via the CNO cycle. They are K- and M-class stars much larger and more luminous than main-sequence stars of the same temperature.
context_section_description: Red giants were identified early in the 20th century when the use of the Hertzsprung–Russell diagram made it clear that there were two distinct types of cool stars with very different sizes: dwarfs, now formally known as the main sequence; and giants.
  The term red-giant branch came into use during the 1940s and 1950s, although initially just as a general term to refer to the red-giant region of the Hertzsprung–Russell diagram. Although the basis of a thermonuclear main-sequence lifetime, followed by a thermodynamic contraction phase to a white dwarf was understood by 1940, the internal details of the various types of giant stars were not known.
  In 1968, the name asymptotic giant branch (AGB) was used for a branch of stars somewhat more luminous than the bulk of red giants and more unstable, often large-amplitude variable stars such as Mira.  Observations of a bifurcated giant branch had been made years earlier but it was unclear how the different sequences were related.  By 1970, the red-giant region was well understood as being made up from subgiants, the RGB itself, the horizontal branch, and the AGB, and the evolutionary state of the stars in these regions was broadly understood. The red-giant branch was described as the first giant branch in 1967, to distinguish it from the second or asymptotic giant branch, and this terminology is still frequently used today.
  Modern stellar physics has modelled the internal processes that produce the different phases of the post-main-sequence life of moderate-mass stars, with ever-increasingly complexity and precision. The results of RGB research are themselves being used as the basis for research in other areas.
caption: The brightest stars in globular clusters such as NGC 288 are red giants
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