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"
Galaxies in the GOODS
25 June 2003
This tantalizing view of galaxies scattered near and far is part of the Hubble Space Telescope's contribution to the GOODS - the Great Observatories Origins Deep Survey project. The GOODS' goal is to study galaxy formation and evolution over an unprecedent wide range of cosmic distances, therefore spanning time from the present to the early Universe. Joined by the Chandra X-ray Observatory and soon by the anticipated Space Infrared Telescope Facility along with major ground-based observatories, the project expands greatly on the past Hubble Deep Fields of regions in the northern constellation of Ursa Major and southern constellation Tucana. Across the electromagnetic spectrum, a sample of large nearby galaxies, like the interacting pair at the lower left above, will be compared with distant younger galaxies in a search for clues to the origins of these lighthouses of the cosmos. Preliminary results of the project confirm that the birth rate of stars was higher in the past and that galaxies have indeed been constructed from the "bottom up", growing from mergers and accretion of small infant galaxies to their present day forms.