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Representative lifetimes of stars as a function of their masses The change in size with time of a Sun-like star Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right Chart of stellar evolution
RR Lyrae variable stars can be found in the left of this gap on a section of the diagram called the instability strip. Cepheid variables also fall on the instability strip, at higher luminosities. The H-R diagram can be used by scientists to roughly measure how far away a star cluster or galaxy is from Earth.
For example, Cepheid variables have a characteristic pattern in their lightcurves, the rate of repetition of which is dependent on the luminosity of the star. [9] Since Cepheid variables are a relatively short evolutionary stage in the lifecycle of stars, and knowing the mass of the star allows for the star to be tracked in its evolutionary ...
These stars are fusing hydrogen in their cores and so they mark the lower edge of the main sequence fuzziness caused by variance in chemical composition. [45] A nearly vertical region of the HR diagram, known as the instability strip, is occupied by pulsating variable stars known as Cepheid variables. These stars vary in magnitude at regular ...
In some cases, stars may cross the instability strip for a fourth and fifth time when helium shell burning starts. [citation needed] The rate of change of the period of a Cepheid variable, along with chemical abundances detectable in the spectrum, can be used to deduce which crossing a particular star is making. [15]
Clayton calculated the first time-dependent models of the s-process in 1961 [15] and of the r-process in 1965, [16] as well as of the burning of silicon into the abundant alpha-particle nuclei and iron-group elements in 1968, [17] [18] and discovered radiogenic chronologies [19] for determining the age of the elements.
In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon called the carbon-nitrogen-oxygen cycle. [210] In evolved stars with cores at 100 million kelvin and masses between 0.5 and 10 M ☉, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium: [210]
The simplest commonly used model of stellar structure is the spherically symmetric quasi-static model, which assumes that a star is in a steady state and that it is spherically symmetric. It contains four basic first-order differential equations : two represent how matter and pressure vary with radius; two represent how temperature and ...