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

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Orion Glossary

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The shift in the spectrum of a source of electromagnetic radiation that’s approaching the observer. Equal to the shift in wavelength of a particular feature in the spectrum divided by the original (or rest) wavelength of the same feature. A blueshift corresponds to a shortening of the emitted wavelength.

brown dwarf

Brown dwarfs are cool, faint objects, with a temperature too low for hydrogen burning to commence. They are inherently difficult to observe, but a number have been discovered.


A structural component of the Milky Way and other spiral galaxies, consisting of a thick, dense concentration of stars around the centre of the disc. Bulges are a common feature of spiral and lenticular galaxies. In the case of the Milky Way (and many other galaxies), the bulge is elongated to form a bar.


carbon burning

The process by which carbon nuclei undergo nuclear fusion to form a neon nucleus (and a new helium nculeus) in massive stars.


The lower part of the Sun’s atmosphere situated immediately above the photosphere.


A relatively small (typically a few kilometres across) body made of icy materials and rocky materials. Comets are found mainly in the Oort cloud, and also in the Kuiper Belt, and some enter the inner Solar System.


The central very hot region of a star (such as the Sun) where nuclear reactions occur during the Main Sequence lifetime.


The upper part of the Sun’s atmosphere. It is very hot, very tenuous and very extensive.

cosmological constant

A constant (often written as a capital lambda or Λ) introduced into the equations of general relativity by Albert Einstein. It was originally conceived as a property of space and time that counteracts the predicted expansion or contraction of the Universe. It is now sometimes thought of as a content of the Universe, rather than a property of space itself. See also dark energy.


dark energy

Energy of currently unknown origin that has become the dominant influence on cosmic expansion and is currently causing that expansion to accelerate. Dark energy is thought to account for about 70% of all the energy in the Universe. It is not thought to be associated with dark matter. It is related to Einstein’s cosmological constant: this constant can be thought of as an in-built property of space and time to expand, or as a mysterious substance within space and time that causes expansion. If taking this latter view, the cosmological constant can be thought of as a particular (constant) type of dark energy.

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