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"
Mira Over Germany
21 February 2007
What's that new star in the sky? The star might appear new, but it's actually just the variable star Mira near its brightest. Rolling your cursor over the above vertically compressed image will identify the unusual star Mira, a star that can change from practical invisibility to one of the brighter stars on the sky over the course of a year. Pictured above a castle in Stuttgart, Germany, last week, red giant star Mira appeared near its maximum brightness of magnitude 2. Although similar in mass to our Sun, Mira tenuous and cool atmosphere could extend out past the orbit of Mars, and achieve a luminosity over 10,000 times greater than our Sun. Mira is near the end of its life and its variability is somewhat erratic. Details of Mira's variability are still being researched, but the reason for Mira's pulsations are thought related to periodic changes in the thickness of parts of Mira's atmosphere. Recent high resolution images show that Mira is not even round. Mira lies 420 light years distant toward the constellation of the Monster Whale (Cetus). Mira will fade over the next 200 days, but climb back to naked-eye visibility early next year.