Astronomers can use Wilhelm Wien's displacement law to estimate the surface temperature of a distant star by measuring its intensity at different wavelengths.
In 1863, William Huggins suggested that stellar spectra indicated that the stars are made of exactly the same elements as can be found on Earth.
In 1920, Harkins and Arthur Eddington suggested that the fusion of hydrogen to form more massive atoms and release energy could be the energy source of stars.
In 1924, Eddington developed the main-sequence mass-luminosity relationship, that stars of similar composition and energy are brighter when they have more mass.
In 1938, Hans Bethe, Critchfield, Carl von Weizsäcker worked out that stars were powered by nuclear fusion, involving the carbon-nitrogen cycle.
In 1939, Hans Bethe and Carl von Weizsäcker proposed the proton-proton chain as the thermonuclear energy source for the sun, where four protons form helium.
In 1942, J. J. L. Duyvendak, Nicholas Mayall, and Jan Oort deduce that the Crab nebula was a remnant of the 1054 supernova observed by Chinese astronomers.
In 1967, Jocelyn Bell and Anthony Hewish discovered radio pulses from a pulsar, the first of four that Bell found. Some 350 pulsars are now known.
Pulsars are rotating bodies that emit directional radio signals, so that in certain positions, the signals are intermittent, forming a series of pulses.