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"
Car Orbiting Earth
13 February 2018
Last week, a car orbited the Earth. The car, created by humans and robots on the Earth, was launched by the SpaceX Company to demonstrate the ability of its Falcon Heavy Rocket to place spacecraft out in the Solar System. Purposely fashioned to be whimsical, the iconic car was thought a better demonstration object than concrete blocks. A mannequin clad in a spacesuit -- dubbed the Starman -- sits in the driver's seat. The featured image is a frame from a video taken by one of three cameras mounted on the car. These cameras, connected to the car's battery, are now out of power. The car, attached to a second stage booster, soon left Earth orbit and will orbit the Sun between Earth and the asteroid belt indefinitely -- perhaps until billions of years from now when our Sun expands into a Red Giant. If ever recovered, what's left of the car may become a unique window into technologies developed on Earth in the 20th and early 21st centuries.