Novae are stars which suddenly become very bright and give the appearance of being "new" stars which they are not. All novae have the same kind of light curve.
The sudden brightening, averaging 8 or 9 magnitudes i.e. an increase in brightness of 22000 times is followed by a sharp decline in brightness and later by a slow decline in magnitude, running into hundreds of days.
The fastest novae brighten by 11 to 13 magnitudes. The greatest range in brightening was shown by V476 Cygni, which brightened through 15 magnitudes from 17,2 to 2,0.
Some novae repeat, e.g. RS Ophuici which displayed maxima in 1898, 1933, 1958 and 1967 and U Scorpii in 1863, 1906, 1936 and 1979.
Slower novae range in brightness from 8 to 11 magnitudes and remain at maximum for longer periods than the fast novae; sometimes for months on end. RT Serpentis, 1915, remained at maximum of 10,5 for 10 years and then very slowly dimmed to magnitude 14.
The spectra of novae are typified as Q. Novae whose spectra were known before their brightening had spectra of types A and B. They were thus bright giants before they became novae. Stars that are F and G giants become pulsating Cepheid variables. Ordinary F and G types of which the Sun is one, being G2, may have a different fate in store for them.
At maximum the spectra are continuous and very bright in the ultraviolet. After maximum, lines of hydrogen appear and become stronger. Then these lines become dimmer and molecular bands appear, showing that a drop in temperature takes place. This is because the overlying layers of the stars are cast off as nebulae which form shells around the stars. Ultimately strong Wolf-Rayet bands appear in the spectra, showing ionised atoms of Helium, carbon, oxygen and nitrogen. Some of the atoms lose up to five electrons. These bands indicate temperatures in excess of 35000°K,
The novae then take on the appearance of a ring because there is less gas in the line of sight than in the sides of the nebulae, e.g. the Ring Nebula in Lyra situated at 18 53,6 +33 02 and the Helix nebula at 22 29,6 -20 48. Time exposure photographs are needed to show the nebulae in the Helix. The star in the centre is a white dwarf, which shows that its overlying layers of gas have been cast off, thus giving the ring appearance. In a small telescope these nebulae have the appearance of the disc of a planet, so they were originally called "planetary nebulae" and the name stuck although they have nothing whatsoever to do with planets.
The remaining star is usually a shrunken white dwarf about 2% of the Sun's size and a temperature as high as 100 000 degrees. The high temperatures are responsible for the ultraviolet radiation which gives a typical greenish-blue appearance of doubly ionised oxygen (OIII).
Photographs taken through a red filter show the outward movement of the gases in the nebulae.
Novae usually occur near the plane of the Milky way and a world-wide patrol has been established, consisting of amateur astronomers who each have a certain area of the sky allotted to them, which they photograph repeatedly. In this way novae will not easily escape detection.
The casting off of shells of gas is due to the star reaching a stage where it has consumed a certain percentage of its hydrogen fuel. The radiation from the centre then suddenly decreases so that the gravitational force of the overlying layers causes them to collapse catastrophically on to the nucleus of the star, crushing it to a density of a white dwarf, 8000 times the density of lead, i.e. 90 000 grams per cubic centimetre and a size of 30 000 kilometres or thereabouts. This tremendous crushing raises the temperature to anything from 35000 to 100 000 degrees Kelvin which is responsible for the ultraviolet radiation from the remaining star. Then a rebound takes place and the gases are hurled out.
Jan Eben van Zyl