In the night sky: Orion
In the night sky: Orion

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Orion Glossary

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A particle of light or other electromagnetic radiation. Monochromatic light consists of photons that each have exactly the same amount of energy, called a quantum of energy. Photons have no mass or electric charge.


The luminous 'surface' of the Sun (or of other stars). The Sun’s photosphere is about 500 km thick.


Large rocky or gaseous bodies in orbit around a star. According to the International Astronomical Union, a planet must meet the following criteria: it is large enough to become spherical under the influence of its own gravity; it orbits a star; it has swept out a clear path on its orbit around the Sun; it is not a satellite of another body.

planetary embryo

A massive body thought to have formed from the accretion of planetesimals, and which themselves combined to form planets, during the formation of the Solar System. A few planetary embryos remain in the form of the largest asteroids and trans-Neptuinan objects.

planetary nebula

The expanding dust and gas thrown out by a star of similar mass to the Sun as it reaches the end of its life.


Small bodies (a few klm in size), that formed from aggregation of dust grains in the early solar nebula and are large enough to attract each other by gravity.


Bodies formed in the early solar system from the accretion of planetary embryos and planetesimals, that are large enough to have undergone differentiation.


A collapsing fragment of a molecular cloud that eventually becomes a star.


A rapidly spinning neutron star. As the neutron star spins on its axis, beams of radio emission sweep around the sky like the light from a lighthouse. On Earth, these beams are detected as pulses of radio emission.


radial velocity

The component of an astronomical object's relative velocity in the line of sight of an observer, i.e. in a radial direction towards or away from the observer.

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