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"
A Backward Sunspot and the New Solar Cycle
30 August 2006
Why is sunspot 905 backwards? Perhaps it is a key marker for the beginning of a new magnetic cycle on our Sun. Every 11 years, our Sun goes through a magnetic cycle, at the end of which its overall magnetic orientation is reversed. An 11-year solar cycle has been observed for hundreds of years by noting peaks and valleys in the average number of sunspots. Just now, the Sun is near Solar Minimum, and likely to start a long progression toward the most active time, called Solar Maximum, in about 5.5 years. An indicator that the sun's magnetic field is reversing is the appearance of sunspots with the reverse magnetic polarity than normal. A few weeks ago, one small candidate reverse sunspot was sighted but faded quickly. Now, however, a larger sunspot with negative polarity is being tracked. This sunspot, numbered 905, appears as the unusual white spot in the above magnetic image of the Sun taken with the SOHO spacecraft a few days ago. In the past few days, Sunspot 905 has actually begun to break apart and might also become the source of coronal mass ejections and explosive solar flares. Solar astronomers predict that the coming Solar Maximum will be unusually active.