A nova's new star is actually the burned-out core of a very old one
Nova means new star, and to anyone watching, that is how one looks: a point of light where none shone before. The truth is almost the reverse. Nearly every nova erupts on a white dwarf, the dense leftover core of an ancient star, that has been stealing gas from a companion.
The setup begins with two ordinary stars in a tight orbit. One ages first, swells into a red giant and shrinks to a white dwarf. When the partner later expands too, its outer gas spills across onto the dwarf, building a thin, dense skin of hydrogen. Squeezed and heated, that layer eventually ignites in runaway fusion, converting hydrogen to heavier elements and blasting the remaining gas outward at up to several thousand kilometres per second. The star brightens enormously, then fades over weeks or months.
For all that violence, a nova throws off only about a ten-thousandth of the Sun's mass, and just five percent of the captured gas actually fuses. The dwarf survives and can start refilling. Some erupt again within decades: RS Ophiuchi has flared seven times since 1898, most recently in 2021, and V Sagittae is expected to go off around 2083, give or take about 11 years. Astronomers suspect most novae repeat, just on timescales of 1,000 to 100,000 years. If a dwarf gains too much mass, though, the next blast can destroy it completely as a Type Ia supernova.
The name traces to Tycho Brahe, who in the 16th century described a brilliant newcomer in Cassiopeia in a book titled on the new star. He reasoned it showed no shift against the background stars, so it had to be very far off. His object, SN 1572, later proved to be a supernova, but the word stuck.
The Milky Way probably produces 25 to 75 novae a year, but dust hides most, and only about 10 are spotted, mostly along the band of the galaxy toward Sagittarius. Perhaps one every 12 to 18 months becomes visible without a telescope. Nova Cygni 1975 reached nearly the brightness of Deneb. Novae also seed space with elements such as carbon, nitrogen and oxygen, and may be the main source of the galaxy's lithium.
Source: Nova
