![]() ![]() The radius of a neutron star is upheld by neutron degeneracy, an outward pressure that prevents total collapse. The incredibly high temperatures of a neutron star means that they are not visible on the HR diagram. These can be detected using radio telescopes on Earth, provided that the plane of the magnetic field intersects with that of the detector. This gives neutron stars a density of approximately 10 17 kgm -3.Īn example of a neutron star is a pulsar with a periodically rotating magentic field. Neutron stars (after a supernova of 10 to 29 solar masses) have a mass up to two or three times that of the Sun and a radius in the order of 10 km (roughly a city!). The Chandrasekhar limit is 1.4 solar masses. These vary from white dwarfs because their mass exceeds the Chandrasekhar limit, above which the electron degeneracy pressure in the star's core is insufficient to balance the inward force of gravity. The outcome of a supernova depends, once again, on the mass of the material remaining. A supernova is the only cosmological event that is sufficiently energetic for the fusion of elements heavier than iron. A supernova's luminosity is too great to be displayed on the HR diagram. When fusion of iron ceases in a super giant, the star collapses once and then explodes in a supernova, releasing mass outwards. However, the image shows the scale of a blue supergiant in comparison to Jupiter's orbit in the Solar System. The Sun will never become a super giant, because of its limted mass. Super giant stars are both massive and luminous, placing them at the top of the HR diagram. The temperature range of supergiant stars spans 3000 K to over 20 000 K with any spectral class possible. They come from main sequence stars of spectral class O and B with masses over 8 times that of the sun. Super giant stars are large enough to fuse nuclei to produce elements as large as iron, the nucleus of highest stability according to binding energy. This places white dwarf stars to the bottom right of the HR diagram, in spectral classes O, B and A.Īpproximately 97% of stars in the Milky Way will become a white dwarf at the end of their lives. White dwarfs are hot and small, on a similar scale to the Earth, but with no fusion taking place to produce light. From red giants with mass under 10 times that of the Sun, having cast off a planetary nebula of dust and gas due to the weaker gravitational field at the outer layers of the star.Directly from the main sequence, for stars of mass under 0.3 times that of the Sun.White dwarf stars can form in one of two ways: This electron degeneracy creates an outward pressure that prevents total collapse. The contractions under gravity retain a radius sufficient to keep the electrons from entering the same quantum mechanical state. The appearance of the red giant is from yellow-orange to red, including the spectral types K and M. With their large radius (the diagram shows the scale that the Sun will reach as a red giant), the surface temperature of a red giant is lower than in the main sequence. This process will repeat for as long as the temperature will allow, with the star contracting and expanding as each new type of nucleus, including carbon, is born. Depending on the mass of the star, the temperature rise may be sufficient to recommence fusion of hydrogren from the outer layers or to commence fusion of helium. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a surface temperature around 5,800 K.When no hydrogen remains in the core, the outer layers of the star collapse under gravity, increasing the temperature of the core. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for subgiants, class V for main-sequence stars, class sd (or VI) for subdwarfs, and class D (or VII) for white dwarfs. This is based on the width of certain absorption lines in the star's spectrum, which vary with the density of the atmosphere and so distinguish giant stars from dwarfs. In the MK system, a luminosity class is added to the spectral class using Roman numerals. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the classical system, such as class D for white dwarfs and classes S and C for carbon stars. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g., A8, A9, F0, and F1 form a sequence from hotter to cooler). Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest ( O type) to the coolest ( M type). ![]()
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