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"
Mysterious Streaks Over Turkey
15 August 2007
What are they? Five streaks near the bottom of the above image taken near Ankara, Turkey on Sunday would be identified at first glance as meteors from the Perseids meteor shower peaking just that night. Unexpectedly, however, these streaks do not point back to the Perseids radiant in Perseus. Their origin is therefore somewhat unclear. The above image was captured over the time span of 40 minutes. Other visible celestial icons include the constellation Orion and the Pleiades star cluster. One hypothesis is that the streaks are part of a microburst from a much less active meteor shower known as the Alpha Ursae Majorids. Another possibility is that they are parts of a satellite that broke up as it re-entered the Earth's atmosphere. A discussion of these possibilities can be found here. This year's Perseids meteor shower was a good one, as it was particularly active and corresponded with the dark skies that come with a new moon.