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"
Frosted Leaf Orion
17 November 2010
Sometimes, you can put some night sky in your art. Captured above Japan earlier this month, a picturesque night sky was photographed behind a picturesque frosted leaf. The reflecting ice crystals on the leaf coolly mimic the shining stars far in the background. The particular background sky on this 48-second wide angle exposure, however, might appear quite interesting and familiar. On the far left, although hard to find, appears a streaking meteor. Below and to the right of the meteor appears a longer and brighter streak of an airplane. The bright star on the left is the dog-star Sirius, the brightest star on the night sky. To Sirius' right appears the constellation of Orion, including the three linear belt stars below the red giant Betelgeuse. The bright patch of light further to the right is the Pleiades open star cluster. Similar views including the constellation Orion can be seen above much of the northern hemisphere for the next several months, although you might have to provide your own leaf. Challenge: Can you identify what type of leaf was imaged?