Home > Positioning > Subjects > Evolution > Stellar evolution

Stellar Evolution

“Stellar evolution” is the life course of a star: its formation, its long stable phase, and its end as a white dwarf, a neutron star or a black hole. No one lives long enough to watch a star change, so the course has always been read off many stars observed at once at different stages. It is the morphology reading of the unfolding sense: one course per instance, found by comparing instances.

The garden

William Herschel, cataloguing nebulae in 1789, described the heavens as resembling “a luxuriant garden, which contains the greatest variety of productions, in different flourishing beds”, and drew the method from it: “is it not almost the same thing, whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering, and corruption of a plant, or whether a vast number of specimens, selected from every stage through which the plant passes in the course of its existence, be brought at once to our view?” His later papers read the nebulae as stages in the condensation of stars.

Lockyer’s inorganic evolution

The astronomer Norman Lockyer took the word from biology on purpose. His meteoritic hypothesis (1890) held that stars form from condensing swarms of meteoritic dust, heating as they contract and cooling afterwards, so that all celestial bodies “are due to an exquisitely simple evolution of matter”. Inorganic Evolution as Studied by Spectrum Analysis (1900) extended the idea to the chemical elements, which he took to be built up from simpler units as stars cool, and opened its argument by borrowing the meaning of evolution from its organic use, presenting his own work as foreshadowed in the way Darwin’s had been.

The giant-to-dwarf sequence

Ejnar Hertzsprung and Henry Norris Russell’s diagram of stellar brightness against colour, presented by Russell in 1913, was read at first as a single track. Russell proposed that a star begins as a cool red giant, heats as it contracts, and then cools as a dwarf, so that the giants “represent successive stages in the heating up of a body” and the dwarfs “successive stages in later cooling”, taking up Lockyer’s idea that a star is hottest in the middle of its history.

The nuclear track

Arthur Eddington’s mass–luminosity relation (1924) raised the first serious objection to the giant-to-dwarf sequence, and Hans Bethe (1939) identified the fusion of hydrogen as the source of stellar energy. The modern account, built between the 1930s and the 1950s, gives each star a single irreversible path through stages of nuclear burning, fixed mainly by its mass: a long residence on the main sequence, expansion into a giant as the core’s hydrogen runs out, and an end determined by how much mass remains. The course is still read off many stars at once, now with the help of models that follow a single star through time.

Stellar populations

Walter Baade divided the stars of the Andromeda galaxy and our own in 1944 into Population I, young and rich in heavy elements, found in the disks of galaxies, and Population II, old and poor in them, found in globular clusters and galactic halos. The populations are classes of stars grouped by age and composition. Later generations of stars are richer in heavy elements because they form from gas enriched by earlier ones, and astronomers call that history galactic chemical evolution.

---

See also: Evolution (the subject landing) · Cosmic evolution · Civilisation as morphology · Eddington