Wolf–Rayet Stars are massive stars showing emission lines from highly ionised helium and nitrogen or carbon, indicating extremely high surface temperatures. Blue Giant, Blue Supergiant and Blue Hypergiant Starsīlue giants and supergiants have a spectral class of O, B or A. The luminosity class 0, or alternatively Ia-0 or Ia+, identifies a hypergiant. The term yellow hypergiant is more widely accepted by astronomers for massive, yellow stars of absolute bolometric magnitude of around -9. However, it has also been suggested that the term should be used only for red supergiant stars with particular spectral signatures. The term red hypergiant is commonly used, however, for the most massive and luminous red supergiants, with an absolute magnitude brighter than -7. Red hypergiant stars are similar to supergiant stars, although the term is not well defined and is not usually used by astronomers. These are some of the largest stars known, with the red supergiant star UY Scuti having an estimated radius of around 1,700 times that of the Sun.īetelgeuse, in the constellation of Orion – the ninth brightest star in the night sky – is a red supergiant of around 1,000 times the radius of the Sun. If a main-sequence star has a mass of around 10 to 30 times the mass of the Sun, it can evolve to become a red supergiant, through a similar process to the evolution of a red giant, described above. The Sun, itself, will become a red giant in around five to six billion years from now, expanding to around the size of the Earth’s current orbit. For example, Aldebaran, the “bull’s eye”, in the constellation of Taurus, is an orange giant of around 1.5 times the mass of the Sun, however, it is around 44 times the Sun’s diameter, and over 400 times the Sun’s luminosity, due to its much larger size. The dimensions of red giants compared to their original main-sequence stars can be enormous. Stars of this type, are known as a red or orange giants. When this happens, the luminosity of the star will increase by anything from 1,000 to 10,000 times, causing the outer layers of the star to expand to enormous distances. With a lower core temperature, the gravitational pressure from the outer layers of the star will cause the star to collapse inwards, until the temperature increases again to a point where hydrogen can start to burn in a layer surrounding the core. Eventually, all of the hydrogen will have been converted to helium and the star’s core will start to cool. Red Giant StarsĪ main-sequence star steadily burns through its supply of hydrogen in its core, via nuclear fusion, creating helium in the process. The Sun is an example of a main-sequence star, unofficially known as a Yellow Dwarf, and classified as a G2V star in the Morgan–Keenan system. Also, the term “white dwarf” is used for a class of older, dying stars that are no longer on the main sequence (see below). However, this is somewhat of a confusing term, as blue stars on the main sequence are among the most massive stars and do not necessarily vary significantly in size when they reach the blue giant stage of their evolution (see below). Main-sequence stars are unofficially (and historically) know as dwarf stars, as opposed to giant stars. However, all main-sequence stars share the luminosity class V in the MK system. temperature), O, B, A, F, G, K, M in the Morgan–Keenan (MK) system. Main-sequence stars can have any spectral class, (i.e. Massive, hot, blue, main-sequence stars, are found on the top left of the Hertzsprung-Russell diagram, while less-massive, cooler, red stars are found on the bottom right.Īround 90 per cent of stars in our galaxy are on the main sequence. The colour of a star is linked to its temperature (see Black-Body Radiation on the Quantum Mechanics page) and the hottest, most massive stars will radiate at a peak wavelength in the blue and ultraviolet end of the spectrum, whilst less massive stars are cooler and will radiate more towards the red end of the spectrum. These types of stars lie within the central diagonal “main sequence” band of the Hertzsprung-Russell diagram – a plot of the colours of stars verses their luminosities. Most stars, will generally remain fairly stable for the majority of their lifespan, gradually burning through their supply of nuclear fuel. A simple Hertzsprung-Russell diagram showing the position of the main sequence, giant, supergiant and white dwarf stars, as well as some example star positions – click to enlarge Main-Sequence Stars – luminosity class VĪfter a star is formed, from a gravitationally condensing cloud of gas and dust, it will start to burn the hydrogen in its core to produce helium via nuclear fusion.
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