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Electromagnetic radiation

energy that travels through space in the form of waves

Visible light

electromagnetic radiation that can be seen with the unaided eye


The distance between crests of waves, such as those of the electromagnetic spectrum.


the range of wavelengths of electromagnetic waves

Optical telescope

a telescope that uses lenses or mirrors to collect and focus visible light

Refracting telescope

a telescope that uses a curved mirror to collect and focus light

Convex lens

a piece of transparent glass curved so that the middle is thicker than the edges

Reflecting telescope

a telescope that uses a curved mirror to collect and focus light

Radio telescope

A device used to detect radio waves from objects in space


An imaginary pattern of stars in the sky


an instrument that separates light into colors and makes an image of the resulting spectrum

Apparent brightness

The brightness of a star as seen from Earth.

Absolute brightness

the brightness a star would have if it were at a standard distance from earth


light distance travels in one year (9.5 trillion km)


the apparent change in position of an object when seen from different places

Hertzsprung-Russell diagram

a graph relating the surface temperatures and absolute brightnesses of stars

Main sequence

A diagonal area on an H-R diagram that includes more than 90 percent of all stars


a building that contains one or more telescopes

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