stjerne

Stjerne is the Danish and Norwegian word for a star. In astronomical use, a stjerne is a luminous sphere of hot plasma held together by gravity, emitting light and heat produced by nuclear fusion in its core. The term also appears in cultural and navigational contexts as a symbol or object of study.

Most stars form from cold molecular clouds. Gravity concentrates gas and dust into a protostar, and when the core temperature reaches roughly 10 million kelvin, hydrogen fusion begins. The outward pressure from fusion balances gravity, and the star enters a long, stable phase known as the main sequence, during which hydrogen is fused into helium.

Stars are classified by mass and surface temperature, reflected in the spectral sequence O, B, A, F, G, K, M (hot to cool). The Sun is a G-type main-sequence star. After exhausting hydrogen in the core, stars evolve into giants or supergiants. Low- to intermediate-mass stars end their lives as white dwarfs, while more massive ones may explode as supernovae, leaving neutron stars or black holes.

Luminosity and distance are central to stellar study. Brightness is measured as apparent magnitude, while intrinsic brightness is quantified as luminosity or absolute magnitude. Distances are often determined by parallax. The Hertzsprung-Russell diagram plots stars by luminosity and temperature, illustrating evolutionary paths and stellar populations.

Stars vary in size, age, chemical composition, and activity. They are key drivers of galactic evolution, producing heavy elements that enrich the interstellar medium and enable planet formation and the emergence of life.