How space works
How stars form, shine, and die
A star's mass controls much of its life. Its light, temperature, composition, and eventual remains also shape the history of the planets around it.
Updated
Gravity builds a star
Stars form within cold concentrations of gas and dust. As gravity contracts a dense region, the material heats up. A growing protostar may be surrounded by a disk, while jets and outflows remove some material and angular momentum. A star joins the main sequence when sustained hydrogen fusion becomes its central energy source.
The balance between gravity and internal pressure allows a star to remain stable for a long time. Stability does not mean a quiet surface. Convection, rotation, magnetism, and stellar winds continue to move matter and energy. Observers infer internal properties from the star's light, oscillations, and evolutionary models rather than seeing directly into its core.
Color is a temperature clue
Hotter stellar photospheres emit a larger fraction of their light at shorter wavelengths. Cooler stars appear redder. The common spectral sequence O, B, A, F, G, K, M broadly runs from hot to cool, with additional classifications needed for unusual objects and cooler substellar bodies.
Effective temperature describes the temperature of an ideal radiator with the same total surface flux. It is not the central temperature. Luminosity also depends on size: a large cool giant can emit far more total energy than a small hotter star. For exoplanets, the host's radius and temperature help determine planetary radius and the energy received at an orbit.
Mass sets the fuel budget and the pace
A massive star contains more fuel but consumes it much faster. Low-mass red dwarfs can remain on the main sequence far longer than Sun-like stars. High-mass stars evolve rapidly and can end their lives in core-collapse explosions, leaving neutron stars or black holes under suitable conditions.
A Sun-like star takes a different route. After exhausting central hydrogen, it evolves through giant phases, sheds its outer layers, and leaves a white dwarf. A white dwarf is a compact stellar remnant, not an ordinary hydrogen-burning star. Predicting the exact fate of a particular object requires its mass, composition, interactions, and evolutionary history.
A planet inherits the star's history
A star's luminosity and activity change with age. A planet's current irradiation therefore gives only one point in its history. Early radiation, winds, giant phases, or close stellar companions can affect atmospheres and orbits. A presently mild energy input does not establish that a planet retained water.
The Codex lists stellar age only when the selected archive record includes it. Ages can depend on models and may be less precise than a neatly formatted number suggests. Solar metallicity comparisons also need care: in astronomy, metals means elements heavier than helium, and a logarithmic abundance such as [Fe/H] describes a ratio relative to the Sun, not the percentage of a star made of iron.