A nova is a sudden, dramatic explosion on the surface of a dead star called a white dwarf, causing it to brighten by tens of thousands of times in just hours or days. Unlike a supernova, which destroys a star entirely, a nova leaves the white dwarf intact, and the whole process can repeat. At peak brightness, a typical nova shines around 100,000 times more luminously than the Sun.
How a Nova Happens
A nova requires two stars orbiting each other closely enough that one can steal material from the other. The key player is a white dwarf, the dense remnant core left behind after a sun-like star has exhausted its fuel. Its companion is a normal star, often a smaller, cooler one that orbits so close that the white dwarf’s intense gravity pulls hydrogen gas off its outer layers.
That stolen hydrogen doesn’t fall directly onto the white dwarf’s surface. It spirals inward, forming a swirling disk of gas, and gradually accumulates on the surface in a thin but growing layer. As more hydrogen piles up, the pressure and temperature at the base of this layer climb steadily. White dwarfs are extraordinarily dense, so the gravitational compression is extreme. Eventually the base of the hydrogen layer reaches temperatures hot enough to ignite nuclear fusion, the same process that powers the Sun’s core, but here it happens in a sudden, runaway flash rather than a slow burn.
This thermonuclear runaway fuses hydrogen into helium explosively, releasing enormous energy in a very short time. The outer layers of accumulated gas are blasted outward into space, and the system flares to peak brightness. One of the fastest novae ever recorded, V1674 Herculis in 2021, rose more than 10 magnitudes (a factor of 10,000 in brightness) during its climb to peak. Its white dwarf’s outer layers briefly expanded to about 21 times the radius of the Sun, temporarily swallowing its companion star, which orbited only 3.67 hours away.
What Happens After the Explosion
The eruption is powerful but shallow. Only a thin shell of hydrogen on the white dwarf’s surface is involved. The white dwarf itself, and its companion, survive without serious damage. After the blast, ejected material races outward as an expanding shell of gas, and the system gradually fades. Radio emissions from V1674 Her, for example, continued rising for about 44 days after the outburst before beginning to fade, while the visible light decline followed a steady, predictable curve that astronomers use to classify how “fast” or “slow” a nova is.
Because the white dwarf and its companion remain intact, the cycle starts over. Hydrogen begins accumulating again, pressure builds, and another eruption will eventually occur. This is fundamentally different from a supernova, where the star is completely destroyed, with debris flying outward at up to 6% the speed of light. A supernova is also billions of times brighter than the Sun, dwarfing a nova’s peak output.
Recurrent Novae
Most novae are “classical,” meaning they’ve only been observed to erupt once. But a handful go off repeatedly on human timescales, and these are called recurrent novae. The distinction is practical rather than physical: every nova system should eventually erupt again, but for most, the gap between eruptions is tens of thousands to hundreds of thousands of years.
Recurrent novae have much shorter intervals, typically decades. The most prolific known example is U Scorpii, which has erupted at least 11 times since 1863, most recently in 2022. Its gaps range from about 8 to 43 years. RS Ophiuchi has erupted nine recorded times, with its latest outburst in August 2021 reaching a peak magnitude of 4.5, bright enough to see without a telescope. V3890 Sagittarii has been caught erupting in 1962, 1990, and 2019.
What makes recurrent novae erupt so frequently is a combination of a particularly massive white dwarf (close to the maximum possible mass) and a high rate of hydrogen transfer from the companion. A heavier white dwarf needs less accumulated material to reach the critical temperature for a thermonuclear runaway, so the fuse is shorter.
T Coronae Borealis: A Nova You Might See
One recurrent nova has drawn significant public attention in recent years. T Coronae Borealis (T CrB), located in the constellation Corona Borealis, last erupted in 1946 and before that in 1866. Astronomers noticed it entering what appeared to be a pre-eruption phase similar to behavior seen before its 1946 outburst, sparking predictions of an imminent eruption. Observational campaigns at facilities like the University of Kentucky’s MacAdam Student Observatory have been tracking it closely, with current estimates suggesting a possible eruption around 2025.
When T CrB does erupt, it is expected to be easily visible to the naked eye, even from light-polluted cities. For a few days, a “new star” will appear in the night sky where none was visible before. This is exactly the experience that gave novae their name: “nova” comes from the Latin for “new.”
What Novae Contribute to the Galaxy
Nova explosions do more than produce a light show. The material blasted off the white dwarf’s surface carries freshly forged elements into the space between stars, seeding the galaxy with ingredients for future stars and planets.
One of the most significant contributions is lithium. During a nova eruption, nuclear reactions produce beryllium-7, which gets transported to the white dwarf’s surface and ejected before it can be destroyed. Once in space, beryllium-7 decays into lithium-7. Research published in The Astrophysical Journal estimates that roughly 130 novae erupt across the Milky Way each year and that together they have ejected about 110 solar masses’ worth of lithium into the galaxy over its lifetime. That accounts for approximately 73% of all the lithium floating in the gas and dust between stars, making novae the dominant source of this element in the Milky Way.
This matters because lithium is fragile. It’s easily destroyed inside stars, so the galaxy needs an ongoing source to explain the amounts we observe today. Novae fill that role. They also contribute smaller amounts of other elements, including certain isotopes of carbon, nitrogen, and oxygen, though lithium is their signature product.
Nova vs. Supernova at a Glance
- Scale: A nova reaches about 100,000 times the Sun’s luminosity. A supernova reaches billions of times the Sun’s luminosity.
- Mechanism: A nova is surface-level hydrogen fusion on a white dwarf. A Type Ia supernova involves the entire white dwarf detonating, while core-collapse supernovae result from massive stars imploding.
- Survival: The white dwarf and companion survive a nova completely intact. In a Type Ia supernova, the white dwarf is obliterated.
- Recurrence: Novae can and do repeat. Supernovae are one-time events for any given star.
- Ejected mass: A nova blows off a thin shell of hydrogen. A supernova expels the entire stellar mass at extraordinary speeds.
How Often Novae Occur
The Milky Way produces an estimated 130 novae per year, but most go undetected because they occur in distant or dust-obscured regions of the galaxy. From Earth, only a handful are spotted each year, typically by amateur astronomers and survey telescopes scanning the sky for transient events. A nova bright enough to see without optical aid happens roughly once a decade, making T Coronae Borealis a genuinely rare opportunity if it erupts as expected.