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The observable universe contains as many as an estimated 2 trillion galaxies [36] [37] [38] and, overall, as many as an estimated 10 24 stars [39] [40] – more stars (and, potentially, Earth-like planets) than all the grains of beach sand on planet Earth. [41] [42] [43] Other estimates are in the hundreds of billions rather than trillions.
But of course, other stars also produce radio emission and may produce much more intense radiation in absolute terms than is observed from the Sun. For "normal" main sequence stars, the mechanisms that produce stellar radio emission are the same as those that produce solar radio emission. [ 16 ]
Each individual reaction produces only a tiny amount of energy, but because enormous numbers of these reactions occur constantly, they produce all the energy necessary to sustain the star's radiation output. In comparison, the combustion of two hydrogen gas molecules with one oxygen gas molecule releases only 5.7 eV.
Once stars begin to form, emit radiation, and in some cases explode, the process of galaxy formation becomes very complex, involving interactions between the forces of gravity, radiation, and thermal energy. Many details are still poorly understood. [192] Within a billion years of a galaxy's formation, key structures begin to appear. [193]
The nature of both dark energy and dark matter is unknown. Dark matter, a mysterious form of matter that has not yet been identified, accounts for 26.8% of the cosmic contents. Dark energy, which is the energy of empty space and is causing the expansion of the universe to accelerate, accounts for the remaining 68.3% of the contents. [8] [92] [93]
The more massive star explodes first, leaving behind a neutron star. If the explosion does not kick the second star away, the binary system survives. The neutron star can now be visible as a radio pulsar, and it slowly loses energy and spins down. Later, the second star can swell up, allowing the neutron star to suck up its matter.
The study of stars and stellar evolution is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior. [100] Star formation occurs in dense regions of dust and gas, known as giant molecular clouds.
During a star's evolution, convective mixing episodes moves material, within which the CNO cycle has operated, from the star's interior to the surface, altering the observed composition of the star. Red giant stars are observed to have lower carbon-12/carbon-13 and carbon-12/nitrogen-14 ratios than do main sequence stars, which is considered to ...