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"
Launch and Landing
13 January 2018
A composite of three consecutive exposures, this night skyscape follows the January 7 launch and first stage landing of a Falcon 9 rocket from a beach on planet Earth's space coast. With the launch from Cape Canaveral Air Force Station, the bright streak beginning farthest left traces the initial phase of the rocket's flight. A visible upward hook marks the first stage beginning its return trajectory with a "boostback burn" near the top of the arc, while the second stage separates and continues toward orbit. Above the top of the launch arc due to perspective, a bright streak shows the returning first stage slowing and descending toward the Cape. Centered below, the streak at the horizon is a 17 second burn finally slowing the first stage to a successful vertical landing about 8 minutes after launch at Landing Zone 1. During the scene's effective long exposure time, the background stars leave short trails in the night sky of the rotating planet.