Beginner-friendly definitions of key astronomy and cosmology terms
The scientific theory that the universe began as an extremely hot, dense point approximately 13.8 billion years ago and has been expanding ever since. Not an explosion in space, but an expansion of space itself.
A region of space where gravity is so strong that nothing, not even light, can escape. Black holes form when massive stars collapse at the end of their lives.
Leftover radiation from the Big Bang that fills the entire universe. It's like the "afterglow" of the universe's birth and provides evidence for the Big Bang theory.
The scientific study of the universe's origin, structure, and evolution. Cosmologists ask big questions like "How did the universe begin?" and "What is it made of?"
A mysterious force that makes up about 68% of the universe and causes its expansion to accelerate. Scientists don't yet fully understand what it is.
Invisible matter that makes up about 27% of the universe. We can't see it directly, but we know it exists because of its gravitational effects on visible matter and light.
The boundary around a black hole beyond which nothing can escape. Once you cross the event horizon, you cannot return, not even light can escape.
A planet that orbits a star outside our solar system. Thousands of exoplanets have been discovered, some potentially habitable.
A massive collection of stars, gas, dust, and dark matter held together by gravity. Our Sun is one of 100-200 billion stars in the Milky Way galaxy.
The bending of light around massive objects like galaxies or black holes. It acts like a cosmic lens, magnifying and distorting light from distant objects.
Radiation emitted by black holes due to quantum effects near the event horizon. Named after physicist Stephen Hawking who predicted it.
The distance light travels in one year through the vacuum of space. Used to measure distances to stars and galaxies. One light-year equals about 5.88 trillion miles.
A cloud of gas and dust in space. Some nebulae are regions where new stars are forming, while others are remnants of dead stars.
The extremely dense remnant of a massive star that has exploded as a supernova. A teaspoon of neutron star material would weigh as much as a mountain on Earth.
The shift of light toward the red end of the spectrum, indicating an object is moving away from us. Used to measure how fast galaxies are receding from Earth.
A point of infinite density at the center of a black hole where the laws of physics break down. Also refers to the initial state of the universe at the Big Bang.
An extremely bright stellar explosion that occurs when a massive star reaches the end of its life or when a white dwarf pulls material from a companion star.
The ongoing increase in the distance between galaxies. Space itself is expanding, carrying galaxies apart from each other like dots on an inflating balloon.
The dense, Earth-sized remnant of a star like our Sun after it has exhausted its nuclear fuel. Very hot but gradually cooling over billions of years.
The band of constellations along the ecliptic plane that the Sun appears to pass through during the year. Used in astrology but has no scientific significance in astronomy.