Key Terms Explained Simply
A beginner-friendly reference for astronomy and cosmology terms you'll encounter while learning about space. All definitions are written without heavy jargon.
The event approximately 13.8 billion years ago when the universe began expanding from an incredibly hot, dense point. Not an explosion in space, but an expansion of space itself.
A region in space where gravity is so strong that nothing, not even light, can escape once it crosses the event horizon (the point of no return). Black holes form when massive stars collapse.
A period of extremely rapid expansion in the early universe (fractions of a second after the Big Bang) that smoothed out the universe and spread matter evenly.
The scientific study of the origin, structure, and evolution of the entire universe. It combines physics, astronomy, and mathematics.
A mysterious force that makes up about 68% of the universe and causes its expansion to accelerate. We don't yet know what it is, but we know it exists from observations.
Invisible matter that makes up about 27% of the universe. We can't see it, 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 it, you're doomed to fall into the black hole's center (singularity).
A planet that orbits a star outside our solar system. As of 2024, over 5,000 exoplanets have been discovered, and many may harbor life.
A massive collection of stars, gas, dust, and dark matter held together by gravity. Our galaxy, the Milky Way, contains about 100-200 billion stars.
The bending of light around massive objects (like galaxies or black holes) due to gravity. This acts like a lens, magnifying distant objects and allowing us to see things we couldn't otherwise.
The region around a star where conditions are right for liquid water to exist on a planet's surface. This is considered necessary for life as we know it.
Radiation emitted by black holes due to quantum effects near the event horizon. Predicted by Stephen Hawking in 1974, it means black holes can eventually evaporate.
The distance light travels in one year through the vacuum of space (about 5.88 trillion miles). Used to measure distances between stars and galaxies.
A cloud of gas and dust in space. Some nebulae are where new stars form, while others are the remnants of dead stars. Plural: nebulae.
The incredibly dense remnant left behind after a massive star explodes as a supernova. A teaspoon of neutron star material would weigh as much as a mountain.
The portion of the universe we can see from Earth, limited by the distance light has traveled since the Big Bang. It has a radius of about 46.5 billion light-years.
The curved path an object takes around another object due to gravity. Planets orbit stars, moons orbit planets, and galaxies orbit each other.
The apparent shift in position of a nearby star against distant stars as Earth orbits the sun. Used to measure distances to nearby stars.
The shift of light toward the red end of the spectrum when an object moves away from us. Used to measure how fast galaxies are moving away and their distance.
The point at the center of a black hole where all matter is crushed to infinite density. Physics breaks down here, and we don't fully understand what happens.
The study of light from stars and galaxies to determine their composition, temperature, and motion. One of astronomy's most important tools.
The explosive death of a star. A supernova can briefly outshine an entire galaxy and scatter elements throughout space, seeding new stars and planets.
Everything that exists—all matter, energy, space, and time. We don't know if it's infinite or finite, but we know it's been expanding for 13.8 billion years.
These terms are explained in detail on many of the channels in our YouTube Channel Directory. Start with beginner channels like Kurzgesagt or Crash Course Astronomy for visual explanations of these concepts.