- Asteroid
- A small rocky body orbiting the Sun, typically found in the asteroid belt between Mars and Jupiter. Most asteroids are remnants from the early solar system.
- Astrophysics
- The branch of astronomy that applies physics principles to understand how celestial objects and the universe work. Astrophysicists ask why and how—how do stars form, what happens inside them, what are black holes made of.
- Big Bang
- 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.
- Black Hole
- 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, or when supermassive black holes at galactic centers grow by consuming surrounding material.
- Cosmic Microwave Background (CMB)
- Leftover radiation from the Big Bang that fills the entire universe. It's like the "afterglow" of the universe's birth and provides strong evidence for the Big Bang theory.
- Cosmology
- 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?"
- Dark Energy
- 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, but we know it exists from observing distant supernovae.
- Dark Matter
- 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 the way galaxies move.
- Event Horizon
- 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.
- Exoplanet
- A planet that orbits a star outside our solar system. Thousands of exoplanets have been discovered, some potentially habitable. This is one of the most exciting areas of modern astronomy.
- Galaxy
- 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, which is just one of billions of galaxies in the observable universe.
- Gravitational Lensing
- 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 behind it.
- Light-Year
- 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 or 9.46 trillion kilometers.
- Nebula
- A cloud of gas and dust in space. Some nebulae are regions where new stars are forming (emission nebulae), while others are remnants of dead stars (planetary nebulae).
- Neutron Star
- 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.
- Parallax
- The apparent shift in position of a nearby star as Earth orbits the Sun. Astronomers use parallax measurements to calculate distances to stars. The closer the star, the larger the parallax shift.
- Pulsar
- A rotating neutron star that emits beams of electromagnetic radiation. As it rotates, these beams sweep across Earth like a lighthouse, causing the pulsar to appear to pulse.
- Red Shift
- A shift in light toward the red end of the spectrum that occurs when an object is moving away from us. Used to measure the motion and distance of galaxies and provide evidence that the universe is expanding.
- Spectroscopy
- The study of light from celestial objects split into its component wavelengths. Spectroscopy reveals the composition, temperature, and motion of stars, galaxies, and nebulae.
- Supernova
- The violent explosion of a star at the end of its life. Supernovae are among the brightest events in the universe and can briefly outshine an entire galaxy.
- Supermassive Black Hole
- A black hole with a mass millions or billions of times that of our Sun. Supermassive black holes exist at the centers of most galaxies, including our Milky Way.
Astronomy Glossary
Essential terms for beginner stargazers