(Every day until December 25, I’m posting a science-related image and description.)

Day 7

Stars seem like permanent fixtures in the sky. We know they move, evolve, and die, but that process is extremely slow on the scale of human history. Their relative stability has allowed for celestial navigation, discovered independently by many cultures, as well as the development of astronomy as a scientific study from its roots in astrology.

For that reason, transient events in the sky are exciting: comets, meteor showers, or even novas. “Nova” literally means “new”, and describes the appearance of a star-like object. But in most respects, novas are not star-like: they appear suddenly, then fade again. Looking at the spectrum of light they emit, it’s very clear these events are not star-like at all. In the case of Nova Centauri 2013 in the photo above, the nova occurred in a binary star system: a red giant star locked in mutual orbit with a white dwarf. Unfortunately for most of us living in the Northern Hemisphere, it’s too far south to be seen, but if you live in the southern part of the world, it’s briefly visible with the unaided eye and very easy to spot with binoculars.

White dwarfs are the dense remnants of the cores of stars like our Sun. Their surfaces are hot enough that material falling onto them can heat up sufficiently to result in uncontrolled nuclear fusion: a thermonuclear explosion. Flare-ups like Nova Centauri 2013 are precisely that: gas stripped from the companion red giant star lands on the white dwarf, a nice bright nuclear explosion takes place, and we on Earth see a new “star” in the sky. Unlike white dwarf supernovas (also known as type Ia), novas don’t destroy the white dwarf. Some novas are even recurring, brightening and fading as material accumulates and explodes.

Historians of astronomy are pretty certain the “star of Bethlehem” in the Christian story of Jesus’ birth wasn’t a nova (astrologers of the day were pretty meticulous in their records). However, novas are things of beauty in themselves. They are a chance to see a special astronomical event, and a reminder that it’s the apparent unchanging character of the sky that’s an illusion. Change, birth, death, and motion are part of the nature of the cosmos, and it’s a rare gift to see that expressed so wonderfully as in a nova.