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.
21 December 2016
This year the December Solstice is today, December 21, at 10:44 UT, the first day of winter in the north and summer in the south. To celebrate, watch this amazing timelapse video tracing the Sun's apparent movement over an entire year from Hungary. During the year, a fixed video camera captured an image every minute. In total, 116,000 exposures follow the Sun's position across the field of view, starting from the 2015 June 21 solstice through the 2016 June 20 solstice. The intervening 2015 December 22 solstice is at the bottom of the frame. The timelapse sequences constructed show the Sun's movement over one day to begin with, followed by traces of the Sun's position during the days of one year, solstice to solstice. Gaps in the daily curves are due to cloud cover. The video ends with stunning animation sequences of analemmas, those figure-8 curves you get by photographing the Sun at the same time each day throughout a year, stepping across planet Earth's sky.