An Etymological Dictionary of Astronomy and Astrophysics
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A quantity which gives the number of sunspots at a given time. It is defined by the relationship R = k(10g + f), where R is the sunspot number, k is a constant depending on the observation conditions and the instrument used, g is the number of the groups and f is the number of individual spots that can be counted. Also called the → Wolf number and → relative sunspot number.

→ sunspot; → number.

The loss of energy by high-energy electrons in a → galaxy cluster, which distorts the → cosmic microwave background (CMB) radiation through → inverse Compton effect. When photons from the CMB radiation travel through a hot plasma (with a temperature of around 10⁸ K), in which bathe a galaxy cluster, they collide with energetic electrons and some of the energy of the electrons is transferred to the low energy CMB photons. If we look at the CMB radiation through such a plasma cloud, we therefore see fewer microwave photons than we would if the cloud were not there.

Named after Rashid Sunyaev (1943-) and Yakov Borisovich Zel'dovich (1914-1987), Russian astrophysicists; → effect.

The Sun as seen in the sky, also referred to as the → apparent Sun; in opposition to → mean Sun.

→ true; → Sun.

A huge wave, caused by undersea earthquakes, volcanic eruptions, or, more rarely, by asteroid or meteoroid impact (as in the case of the K-T extinction).

From Japanese tsunami, from tsu "harbor" + nami "waves."