Stars shine as a result of nuclear fusion reactions taking place inside their cores. Fusion involves hydrogen molecules fusing (smashing together) to form helium molecules, releasing enormous energy in the form of heat and light. This “cooking” within the star creates an intense outward pressure, as the gases struggle to shoot out into space. Gravity acts as a counter force. The delicate balance between gravity and the outward pressure emanating from the core holds the star together throughout its lifetime.
A star does not have endless supply of hydrogen, so once it fuses all of its hydrogen into helium, it will reach the end of its life and start to “die.” How a star dies depends on its mass. For a medium-mass star like our sun, when it has fused all of its hydrogen into helium, the outward pressure weakens, and gravity wins out. This causes the star to start collapsing in on itself. The collapsing re-ignites the star, and any hydrogen residue remaining in the core undergoes the fusion reaction. This causes the outward layers of a star to expand. Soon, the star will become a red giant. Red giants are cooler because heat is more spread-out throughout their wider, less-compacted surface area. Gradually, the star starts to “cook” every other element remaining in its core until it completely exhausts them. At this point, gravity once again wins the game, and the star implodes until all that remains is the sweltering core. This core is now known as a “white dwarf.” One well-known white dwarf is Sirius B from the Sirius star system. White dwarfs will eventually cool off and become dark.
Formation of a Nova
The word “nova” means “new” in Latin. The term was first thrown out by 16th century Danish astronomer Tycho Brahe, who wrote a book about the supernova SN1572 in the constellation Cassiopeia titled De Stella Nova, or “concerning the new star.” The publication of this book led to the term “nova” being used to refer to new stars in the sky.
Novae are referred to as “new stars” because they are white dwarf stars that suddenly become bright again after lying dim and dormant for so long.
Some white dwarf stars live in a “binary” system. A binary system occurs when a pair of stars orbit around each other. If the white dwarf star’s partner becomes a red giant and starts expanding, the white dwarf’s gravity may start to “eat” some of the hydrogen from the red giant’s atmosphere. Now remember, the white dwarf’s surface is extremely hot. So the hydrogen gas quickly ignites upon contact with the surface, creating a brilliant nuclear explosion — bright enough to be seen light years away. This is what we call a “nova”, a seemingly “new” star in the night sky.
A nova’s brightness can last anywhere from several weeks to several years, after which the white dwarf gradually dims. The cycle then repeats weeks or years later.
All images from the Wikimedia Commons.