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.
5 April 2016
Why might you want to visit super-earth Cancri 55 e? Its extremely hot climate would be a deterrent, and fresh lava flows might be common. Discovered in 2004, the planet Cancri 55 e has twice the diameter of our Earth and about 10 times Earth's mass. The planet orbits its 40 light-year distant Sun-like star well inside the orbit of Mercury, so close that it is tidally locked, meaning that it always keeps the same face toward the object it orbits -- like our Moon does as it orbits the Earth. Astronomers have recently measured temperature changes on this exoplanet using infrared observations with the Spitzer Space Telescope. Given these observations, an artist created the featured video with educated guesses about what one revolution of Cancri 55 e might look like. Depicted are full phase, when the planet is fully illuminated, and new (dark) phase when it passes near the line of sight to Earth. The illustrated red bands on the Cancri 55 e indicate bands of lava that might flow on the planet. A recent density determination for 55 Cancri e show that this exoplanet is not made primarily of oxygen, as are the inner planets in our Solar System, but rather of carbon. Therefore, one reason to visit Cancri 55 e might be to study its core, because this planet's great internal pressure might be sufficient to make the carbon found there into one huge diamond. New for Programmers: NASA Open API for APOD