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Examples: A binary star, e.g., Alpha Centauri (approx. the same mass) A binary asteroid, e.g., 90 Antiope (approx. the same mass) A further simplification is based on the "standard assumptions in astrodynamics", which include that one body, the orbiting body, is much smaller than the other, the central body. This is also often approximately valid.
The equation of hydrostatic equilibrium may need to be modified by adding a radial acceleration term if the radius of the star is changing very quickly, for example if the star is radially pulsating. [9] Also, if the nuclear burning is not stable, or the star's core is rapidly collapsing, an entropy term must be added to the energy equation. [10]
Marginal cases are allowed; for example, a star may be either a supergiant or a bright giant, or may be in between the subgiant and main-sequence classifications. In these cases, two special symbols are used: A slash (/) means that a star is either one class or the other. A dash (-) means that the star is in between the two classes.
A-type star In the Harvard spectral classification system, a class of main-sequence star having spectra dominated by Balmer absorption lines of hydrogen. Stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 times the mass of the Sun, and have surface temperatures of 7,600–10,000 kelvin.
A simple example illustrating relaxation is two-body relaxation, where a star's orbit is altered due to the gravitational interaction with another star. Initially, the subject star travels along an orbit with initial velocity, v {\displaystyle \mathbf {v} } , that is perpendicular to the impact parameter , the distance of closest approach, to ...
In modern physical cosmology, the cosmological principle is the notion that the spatial distribution of matter in the universe is uniformly isotropic and homogeneous when viewed on a large enough scale, since the forces are expected to act equally throughout the universe on a large scale, and should, therefore, produce no observable inequalities in the large-scale structuring over the course ...
Atomic physics plays a key role in astrophysics as astronomers' only information about a particular object comes through the light that it emits, and this light arises through atomic transitions. Molecular astrophysics , developed into a rigorous field of investigation by theoretical astrochemist Alexander Dalgarno beginning in 1967, concerns ...
The Hubble eXtreme Deep Field (XDF) was completed in September 2012 and shows the farthest galaxies ever photographed at that time. Except for the few stars in the foreground (which are bright and easily recognizable because only they have diffraction spikes), every speck of light in the photo is an individual galaxy, some of them as old as 13.2 billion years; the observable universe is ...