Any spiral or elliptical galaxy that does not have unusual structure, a disturbed or active nucleus or non-thermal radio emission.

For a surface, the fraction of light reflected straight back from a beam of light falling on that surface in a direction perpendicular to it.

A complex region of nebulosity in the constellation Cygnus. It includes emission nebulae, reflection nebulae and absorption nebulae. It was discovered by William Herschel in 1786 and first photographed in 1890 by Max Wolf. The photographs revealed the shape, reminiscent of the North American continent. It is about 1° across and is just detectable with the naked eye under ideal conditions. Its estimated distance is 2,300 light years.

A region of radio and X-ray emission extending northwards from the plane of the Galaxy. It appears to be the most prominent segment of a huge ring of gas, probably a very old supernova remnant.

The angular distance on the celestial sphere between an object and the north celestial pole, measured along a great circle.

A list of about a hundred stars, all lying within two degrees of the north celestial pole and covering a wide range of brightness, which are used as standards for the astronomical magnitude scale. They were selected so they can be observed for comparison purposes at any time of the year from northern observatories.

A popular name for the star Polaris, which lies within one degree of the north celestial pole.

A name sometimes given to the constellation Cygnus and, in particular, the cross formation made by the stars Alpha α, Beta β, Gamma γ, Delta δ, Epsilon ε and Eta η.

English name for the constellation Corona Borealis.

A popular name for an aurora when observed from northern latitudes.

A star that suddenly increases in brightness by about ten magnitudes, then declines gradually over a period of months. The word nova is a shortening of the Latin nova stella - a new star. Observations have demonstrated that novae are close binary stars of which one component is a white dwarf. When the companion star evolves and expands to fill its Roche lobe, material streams towards the white dwarf, forming an accretion disc around it. The accepted theory of nova outbursts is that material accumulates in a layer on the surface of the white dwarf until the temperature and pressure at the base of the layer become high enough for the carbon cycle nuclear reactions to be initiated. The energy produced is unable to escape as more material is deposited in the overlying layers. The temperature may rise to 100 million degrees and, at some point, explosive nuclear reactions are triggered, producing the observed nova outburst. Some novae have been seen surrounded by an expanding envelope of gas. At speeds of up to 1,500 km/s, the envelope soon disperses into space. It is estimated that the mass of material lost is about one ten-thousandth 10-4 the mass of the Sun, and the energy released is only a millionth of that released in a supernova. The ejected material is rich in the elements carbon, nitrogen and oxygen, and the observed ratios of the isotopes 13C / 12C and 15N/14N are consistent with the theory. Classical novae are observed to erupt only once, though it is believed that outbursts may recur every 10,000 to 100,000 years. Recurrent novae, such as P Cygni, have been observed to repeat their outbursts on timescales of ten to a hundred years. In any one galaxy, typically a few tens of novae occur in a year. See also: dwarf nova.

Abbreviation for north polar distance.

Abbreviation for National Radio Astronomy Observatory.

Abbreviation for National Solar Observatory.

Abbreviation for New Technology Telescope.