Nova

Lore & Background

A nova is a transient astronomical event that appears as a sudden, bright "new" star, fading over weeks or months. All observed novae involve a white dwarf in a close binary system, drawing hydrogen-rich matter from a companion star—often a main-sequence, subgiant, or red giant—onto its surface. This accreted material forms a dense, shallow atmosphere. When the temperature of this layer reaches approximately 20 million Kelvin, runaway fusion ignites via the CNO cycle, explosively expelling the atmosphere into space and producing the visible outburst. The rise to peak brightness may be rapid or gradual; the subsequent decline classifies novae as fast (decaying by 2 magnitudes in under 25 days) or slow (taking over 80 days). Classical novae are the most common type. Recurrent novae, such as T Coronae Borealis, repeat the process on cycles of decades or less as the companion star replenishes the white dwarf’s atmosphere. Novae most often appear along the Milky Way, especially near the Galactic Center in Sagittarius, but can occur anywhere. They average about ten per year in the Milky Way, though only one every 12–18 months is visible to the naked eye, with bright events (first or second magnitude) occurring only a few times per century. The term "nova" derives from Tycho Brahe’s 16th-century book *De nova stella* ("concerning the new star"), though his object was later identified as a supernova. Novae can also emit gamma rays above 100 MeV, as discovered by NASA’s Fermi Gamma-ray Space Telescope in 2010.

Reader's Guide

Novae are transient events that produce a sudden, bright "new" star that fades over weeks or months. They are all cataclysmic variable stars involving white dwarfs in close binary systems. The main sub-classes are classical novae, recurrent novae, and dwarf novae. Classical novae, the most common type, occur when a white dwarf draws hydrogen from a companion star onto its surface, creating a dense atmosphere. This atmosphere is heated until it reaches a critical temperature, triggering runaway fusion that expels the envelope into space, creating the visible light. Recurrent novae repeat this process in cycles of decades or less as the companion continues to feed the white dwarf. Under certain conditions, mass accretion can destroy the white dwarf, resulting in a Type Ia supernova. Novae most often appear along the Milky Way, especially near the Galactic Center, and occur far more frequently than supernovae, averaging about ten per year in our galaxy, though only one every 12–18 months is visible to the naked eye. The term "nova" originated from Tycho Brahe’s description of a supernova in the sixteenth century, though the terms were interchangeable until the 1930s. The rise to peak brightness can be rapid or gradual, and the decay time is used to classify novae into speed classes. Fast novae decay by two magnitudes in under 25 days, while slow novae take over 80 days. The ejected material is only a small fraction of a solar mass, but it is accelerated to thousands of kilometers per second, with luminosity rising to 50,000–100,000 times that of the Sun. Novae also emit gamma rays.

Did You Know?

• The name 'nova' comes from the Latin 'stella nova,' meaning 'new star.'
• Only about five percent of the accreted mass is fused during a nova outburst.