Star
A star is a massive, self-luminous sphere of plasma held together by its own gravity. It shines by converting hydrogen into helium through nuclear fusion in its core, releasing energy across the electromagnetic spectrum.
Stars form in large clouds of gas and dust—called nebulae—where regions collapse under gravity, creating protostars. When core temperatures reach millions of degrees, fusion ignites, marking the birth of a star. Observatories like Hubble and missions such as NASA’s Infrared telescopes have imaged this process in action.
The majority (~90%) of stars are main-sequence stars, fusing hydrogen into helium. These include a broad range of masses—from red dwarfs (small, long-lived, faint) to blue giants and supergiants (massive, hot, and short-lived). Our Sun is a middle-aged G-type main sequence star.
As stars exhaust their hydrogen fuel, their evolution depends on mass. Lower-mass stars become red giants then white dwarfs. More massive stars undergo successive fusion stages, end in supernova explosions, and leave behind neutron stars or black holes.
Stars vary in brightness, size, and color. They are classified using spectral types (O, B, A, F, G, K, M) based on surface temperature and absorption lines. For example, O- and B-type stars are hot and blue; M-type are cool and red.
Stellar remnants include white dwarfs (Earth-sized cores of former stars), neutron stars (city-sized remnants of supernovae), and black holes (extreme-density objects from the most massive stars).
Stars are not static—many rotate, exhibit magnetic activity (like sunspots and flares), and broadcast stellar winds. Their lifecycle enriches the interstellar medium with heavier elements, seeding future generations of stars and planets.
Stars often exist in groups—binary or systems within star clusters and galaxies. Their properties are studied via brightness, spectra, parallax, variability, and statistical surveys by missions like Gaia and Kepler.
Sources
APODs including "Star"
R Leporis: A Vampire's Star
31 October 2018
Better known as Hind's Crimson Star, R Leporis is a rare star in planet Earth's night sky. It's also a shocking shade of red. The star's discoverer, 19th century English astronomer John Russell Hind, reported that it appeared in a telescope "... like a drop of blood on a black field." Located 1,360 light-years away in the constellation Lepus the star is a Mira-type variable, changing its brightness over a period of about 14 months. R Leporis is now recognized as a carbon star, a very cool and highly evolved red giant with an extreme abundance of carbon. Extra carbon in carbon stars is created by helium fusion near the dying stellar core and dredged up into the stars' outer layers. The dredge-up results in an overabundance of simple carbon molecules, like CO, CH, CN, and C2. While it's true that cool stars radiate most of their energy in red and infrared light, the carbon molecules strongly absorb what little blue light is left and give carbon stars an exceptionally deep red color. R Leporis is losing its carbon-rich atmosphere into the surrounding interstellar material through a strong stellar wind though, and could be near the transition to a planetary nebula. Oh, and Happy Halloween from the folks at APOD.