Fact Files

Stars are massive spheres of plasma generating energy by nuclear fusion. The Sun is our nearest star.

• Classified by spectral type (O, B, A, F, G, K, M).
• End states: white dwarf, neutron star, or black hole.
• Massive stars live fast and die young.

Galaxies are huge systems of stars, gas, dust and dark matter. The Milky Way is our home galaxy.

• Types: spiral, elliptical, and irregular.
• Typical sizes: tens of thousands to hundreds of thousands of light-years across.
• Galaxies grow by mergers.

Nebulas are clouds of gas and dust — often star-forming regions or remnants of dead stars.

• Types: emission, reflection, planetary, and dark nebulas.
• Orion Nebula is one of the brightest star-forming regions we can see.
• Supernova remnants are also nebulas.

Planets are celestial bodies orbiting stars, large enough to be rounded by gravity but not massive enough for fusion.

• Our solar system has eight planets.
• Terrestrial planets: Mercury, Venus, Earth, Mars.
• Gas/ice giants: Jupiter, Saturn, Uranus, Neptune.

Moons are natural satellites orbiting planets. They come in many shapes and sizes.

• Earth has one moon — simply called “the Moon.”
• Jupiter has over 90 moons, including Ganymede, the largest in the solar system.
• Moons can have atmospheres and even oceans beneath their surfaces.

Black holes are regions of spacetime with gravity so strong that nothing, not even light, can escape.

• Formed by the collapse of massive stars or through mergers.
• Supermassive black holes lie at the centers of galaxies.
• They can power energetic phenomena like quasars.

Comets are icy bodies from the outer solar system. When they approach the Sun, they develop glowing comas and tails.

• Short-period comets come from the Kuiper Belt.
• Long-period comets originate in the distant Oort Cloud.
• Famous example: Halley’s Comet.

Exoplanets are planets that orbit stars outside our solar system.

• First confirmed in the 1990s.
• Thousands have been discovered using telescopes like Kepler and TESS.
• Some may lie in habitable zones where liquid water could exist.