Big Bang: The moment, over 13 billion years ago, when the universe began to expand from an almost infinitely dense and hot state.

Big Bang nucleosynthesis: The fusion of hydrogen into helium when temperatures were high enough throughout the universe, from three seconds to twenty minutes after the Big Bang.

black hole: An object so dense that nothing can escape its gravity, not even light. Black holes are formed by the most massive stars at the ends of their lives.

brown dwarf: An object less massive than a star but more massive than a planet. Nuclear fusion of deuterium (heavy hydrogen) occurs within its core for a brief period of millions of years after its birth.

chromosphere: A hot outer layer present in many stars, lying between the photosphere and the corona.

convection: The rising of heated material and falling of cooled material in a region simultaneously heated from below and cooled from above, such as a pot of water about to boil or the interior of a star.

convective zone: A layer of a star, where convection occurs, producing turbulence. This turbulence generates the Sun's magnetic field.

core: The center of a star, where nuclear fusion generates intense energy.

corona: The million-degree outermost layer of many stars, which is so hot that gas escapes the star's gravity and flows out into space as a stellar wind.

dark matter: An invisible substance making up most of the mass in the universe that is detected by its gravitational influence. It has existed since the Big Bang.

element: A substance containing only atoms that all have the same number of protons.

gravity: The force of attraction between any two masses.


astrophysicist or astronomer: A scientist who studies the physical laws of the universe and the physical properties of celestial objects such as stars and galaxies. Today these titles are used interchangeably.

heliophysicist: A scientist who studies the Sun. Also called a solar physicist.

heliosphere: The extent of space affected by the Sun's magnetic field, which reaches past Pluto.

hydrostatic equilibrium: In a star, the balance achieved between the enormous outward pressure of gas heated by fusion and the inward pull of its own gravity.

luminosity: The total amount of light that a star emits. Luminosity is not the same as brightness, which drops off with distance.

magnetic field: The forces produced by moving, charged material, such as the turbulent, ionized gas in a star's convective zone.

mass: The amount of matter contained within a given object.

neutron star: A stellar remnant formed by a massive star when it explodes as a supernova at the end of its life. They are extremely dense and about the size of a city.

nuclear fusion: The combination of light atoms such as hydrogen and helium into heavier ones, such as helium, carbon, and oxygen. This process releases intense energy.

photosphere: The layer of a star where it becomes transparent, and where light escapes into space.

radiation: Energy that travels in the form of rays or waves (e.g. electromagnetic waves such as light, radio, X-rays, and gamma rays), or in the form of subatomic particles.

radiative zone: The layer of a star just above the core, where energy produced by nuclear fusion in the core is diffused outward by radiation.

red giant: The form that most stars take near the end of their lives, after they use up their fuel and their outer layers swell. High mass stars become red supergiants, or even yellow supergiants.

solar flare: A magnetic explosion on the Sun that produces storms in the solar wind and generates dangerous radiation.

star: A huge luminous ball of hot gas in hydrostatic equilibrium.

star cluster: A group of many stars orbiting each other tightly.

stellar wind: A flow of high-speed gas ejected by stars. It is called the solar wind when referring to our Sun.

sunspots: Darker, cooler areas on the Sun's photosphere that form where the magnetic field is strongest.

supernova: An explosion that occurs when a high mass star uses up its fuel and is unable to maintain hydrostatic equilibrium.

white dwarf: The final state of 99% of all stars after they evolve into red giants. White dwarfs are very dense and about the size of Earth.

Also at the Museum Beyond Planet Earth


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Journey to the Stars Online Educator's Guide