"Star" - what is it, definition of the term
A star is a massive, self‑gravitating sphere of plasma that sustains continuous nuclear fusion of hydrogen into helium in its core, releasing vast amounts of electromagnetic radiation; its intrinsic properties—mass, temperature, composition—govern its luminosity, spectral class, and evolutionary pathway from main‑sequence stability to eventual end states such as white dwarfs, neutron stars, or black holes.
Detailed information
A stellar body is a self‑gravitating sphere of plasma that generates energy through nuclear fusion in its core. Fusion converts hydrogen into helium, releasing radiation across the electromagnetic spectrum. The balance between outward pressure from this radiation and inward gravitational force determines the object's stability.
Formation begins in a molecular cloud where density fluctuations cause collapse under gravity. As the protostellar core contracts, temperature rises until fusion ignites, marking the entry onto the main sequence. Classification relies on surface temperature and luminosity, producing categories ranging from hot, blue‑white objects to cool, red dwarfs. Spectral types O, B, A, F, G, K, and M delineate this range.
Lifecycle stages depend on mass. Low‑mass objects exhaust core hydrogen slowly, expand into red giants, shed outer layers as planetary nebulae, and leave behind dense remnants called white dwarfs. High‑mass objects burn fuel rapidly, evolve through supergiant phases, and end in core‑collapse supernovae, producing neutron stars or black holes.
Key physical parameters include:
- Mass: determines gravitational pressure and fusion rate.
- Radius: influences surface gravity and escape velocity.
- Luminosity: proportional to the fourth power of surface temperature (Stefan‑Boltzmann law).
- Metallicity: fraction of elements heavier than helium, affecting opacity and evolution.
In biological terminology, words such as tick, bug, louse, and flea denote distinct arthropods. They differ in taxonomy, morphology, and host interaction:
- Tick – arachnid, blood‑feeding ectoparasite of vertebrates.
- Bug – informal term for various insects; in entomology, refers to true bugs (order Hemiptera).
- Louse – wingless insect, obligate parasite of mammals and birds.
- Flea – small, wingless insect, capable of jumping great distances, parasitizing mammals and birds.
These terms share no etymological connection with the astronomical concept, yet all appear in scientific literature requiring precise definition.