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"
SLIM Lands on the Moon
30 January 2024
New landers are on the Moon. Nearly two weeks ago, Japan's Smart Lander for Investigating Moon (SLIM) released two rovers as it descended, before its main lander touched down itself. The larger of the two rovers can hop like a frog, while the smaller rover is about the size of a baseball and can move after pulling itself apart like a transformer. The main lander, nicknamed Moon Sniper, is seen in the featured image taken by the smaller rover. Inspection of the image shows that Moon Sniper's thrusters are facing up, meaning that the lander is upside down from its descent configuration and on its side from its intended landing configuration. One result is that Moon Sniper's solar panels are not in the expected orientation, so that powering the lander had to be curtailed and adapted. SLIM's lander has already succeeded as a technology demonstration, its main mission, but was not designed to withstand the lunar night -- which starts tomorrow.