SpaceClass

The distance light travels in one year — about 9.46 trillion km (5.88 trillion miles). Used to measure vast cosmic distances between stars and galaxies.

The average distance from Earth to the Sun — about 149.6 million km. Used to describe distances within our solar system.

A unit of distance equal to about 3.26 light years. Derived from parallax measurements; commonly used in professional astronomy.

When light from distant objects stretches to longer (redder) wavelengths, indicating they are moving away from us. Key evidence for the expanding universe.

When light from objects compresses to shorter (bluer) wavelengths, indicating they are moving toward us. The Andromeda galaxy is blueshifted.

The full range of electromagnetic radiation: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Telescopes observe across multiple bands.

The change in frequency of a wave relative to an observer moving relative to the source. Used to detect exoplanets and measure star velocities.

The minimum speed needed to break free from a celestial body's gravitational pull without further propulsion. Earth's is about 11.2 km/s.

A technique where a spacecraft uses a planet's gravity to change speed and direction, saving fuel. Voyager missions famously used this.

A star in the longest phase of its life, fusing hydrogen into helium. Our Sun is a main sequence G-type star about halfway through its 10-billion-year lifespan.

The most common type of star in the universe — small, cool, and long-lived. Proxima Centauri (nearest star to our Sun) is a red dwarf.

The dense remnant core of a low-to-medium mass star after it exhausts its fuel. About Earth-sized but with a mass comparable to the Sun.

An incredibly dense remnant of a massive star's supernova. A teaspoon of neutron star material weighs about 6 billion tons.

A rapidly rotating neutron star that emits beams of electromagnetic radiation. Can spin up to 716 times per second, detected as periodic radio pulses.

A neutron star with an extraordinarily powerful magnetic field — a trillion times stronger than Earth's. The most magnetic objects in the known universe.

A region of space where gravity is so strong that nothing, not even light, can escape. Formed from the collapse of massive stars or found at galaxy centers.

The explosive death of a massive star, briefly outshining entire galaxies. Seeds space with heavy elements like iron, gold, and uranium.

Two stars that orbit around their common center of mass. Over half of all star systems are binaries or multiples.

A star whose brightness changes over time, either due to internal pulsation or eclipsing by a companion. Cepheid variables are key distance markers.

A young star still gathering mass from its surrounding molecular cloud. Not yet hot enough for hydrogen fusion — the stage before becoming a main sequence star.

A rocky planet with a solid surface — Mercury, Venus, Earth, and Mars. Distinguished from gas and ice giants by composition.

A large planet composed mainly of hydrogen and helium — Jupiter and Saturn. They lack a well-defined solid surface.

A giant planet composed mainly of elements heavier than hydrogen/helium, like water, ammonia, and methane — Uranus and Neptune.

A celestial body orbiting the Sun, massive enough for gravity to make it round, but not clearing its orbital neighborhood. Examples: Pluto, Ceres, Eris.

A region between Mars and Jupiter containing millions of rocky objects — remnants of the early solar system that never formed into a planet.

A region beyond Neptune extending from 30 to 55 AU, containing icy bodies including Pluto. Much larger than the asteroid belt.

A theoretical sphere of icy objects at the very edge of the Sun's influence, extending up to 100,000 AU. Source of long-period comets.

Five special positions where gravitational forces create stable "parking spots" for spacecraft. The James Webb Space Telescope orbits the L2 point.

A stream of charged particles (plasma) released from the Sun's upper atmosphere at speeds of 400-800 km/s. Causes auroras on Earth.

The bubble of space dominated by the solar wind, extending far beyond Pluto. Voyager 1 crossed its boundary (heliopause) in 2012.

The region around a planet dominated by its magnetic field. Earth's magnetosphere deflects solar wind and protects life from radiation.

Any asteroid or comet with an orbit that brings it within 1.3 AU of the Sun. NASA tracks over 30,000 NEOs for planetary defense.

A vast system of stars, gas, dust, and dark matter bound together by gravity. The Milky Way contains an estimated 100-400 billion stars.

A giant cloud of gas and dust in space. Emission nebulae glow from ionized gas; reflection nebulae scatter starlight; dark nebulae block light behind them.

An invisible substance making up ~27% of the universe. It doesn't emit or absorb light but exerts gravitational pull, holding galaxies together.

A mysterious force making up ~68% of the universe, driving its accelerating expansion. Discovered in 1998 through supernova observations.

Ancient thermal radiation left over from the Big Bang, about 380,000 years after the universe formed. Fills all of space at 2.7 Kelvin.

A planet orbiting a star outside our solar system. Over 5,600 have been confirmed as of 2024, found via transit and radial velocity methods.

The region around a star where conditions could allow liquid water on a planet's surface. Also called the "Goldilocks zone" — not too hot, not too cold.

The bending of light by massive objects, predicted by Einstein's general relativity. Used to study dark matter and magnify distant galaxies.

An extremely luminous active galactic nucleus powered by a supermassive black hole consuming matter. Some are visible billions of light years away.

A sudden burst of electromagnetic radiation from the Sun's surface. Classified by X-ray intensity: A, B, C, M, X (strongest). Can disrupt radio communications.

A massive burst of solar wind and magnetic fields rising above the solar corona. When directed at Earth, can cause geomagnetic storms.

A disturbance in Earth's magnetosphere caused by solar wind. Measured on the G1-G5 scale. Can affect power grids, GPS, and cause auroras at lower latitudes.

A 0-9 scale measuring geomagnetic activity. Kp ≤ 2 is quiet; Kp 5+ indicates a geomagnetic storm. Used in Celestia's AuraWatch for aurora predictions.

Luminous displays caused by charged particles from the Sun interacting with Earth's magnetic field and atmosphere. Visible near the poles, or globally during strong storms.

A dark region on the Sun's surface caused by magnetic activity. Sunspot count follows an 11-year solar cycle and correlates with space weather activity.

An approximately 11-year cycle of solar magnetic activity. Solar maximum brings more sunspots, flares, and CMEs. We're currently in Solar Cycle 25 (peaked ~2024).

Zones of energetic charged particles trapped by Earth's magnetic field. The Van Allen belts can pose hazards to satellites and astronauts.

Orbits between 160-2,000 km altitude. The ISS orbits at ~408 km. Most Earth observation and communication satellites operate in LEO.

An orbit at ~35,786 km altitude where a satellite matches Earth's rotation, appearing stationary. Used for weather and communication satellites.

The change in velocity needed for orbital maneuvers. A key measure for mission planning — determines fuel requirements for reaching destinations.

A propulsion system that creates thrust by accelerating ions using electricity. Very fuel-efficient but low thrust — ideal for deep space missions like Dawn.

The most fuel-efficient way to transfer between two circular orbits using two engine burns. Standard approach for missions to Mars and other planets.

Using a planet's atmosphere to slow down a spacecraft and reduce its orbit, saving fuel. Used by Mars orbiters like Mars Reconnaissance Orbiter.

Non-functional objects in orbit — defunct satellites, rocket stages, fragments. Over 36,000 objects larger than 10cm tracked; poses collision risks (Kessler syndrome).

A Martian day lasting about 24 hours 39 minutes. Mars rover missions count mission progress in sols. Curiosity has operated for over 4,000 sols.

Moderate Resolution Imaging Spectroradiometer — a key instrument on NASA's Terra and Aqua satellites. Observes land, oceans, and atmosphere in 36 spectral bands.

Fire Information for Resource Management System — NASA's near-real-time fire detection system using MODIS and VIIRS satellite data. Powers Celestia's fire hotspot map.

Earth Polychromatic Imaging Camera aboard DSCOVR at the L1 Lagrange point. Captures full-disc Earth images daily from 1.5 million km away.

Deep Space Climate Observatory — a NOAA satellite at the L1 point monitoring solar wind, space weather, and Earth's radiation budget.

Visible Infrared Imaging Radiometer Suite — a sensor on Suomi NPP and NOAA-20 satellites providing high-res imagery for weather, fire detection, and ocean monitoring.

Acquiring information about Earth from sensors on satellites or aircraft without physical contact. Includes optical, radar, and thermal imaging.