Astronomers watch a nova go from start to finish for the first time

A nova is a dramatic episode in the life of a binary pair of stars. It’s an explosion of bright light that can last weeks or even months. And though they’re not exactly rare—there are about 10 each year in the Milky Way—astronomers have never watched one from start to finish until now.

A nova occurs in a close binary star system, when one of the stars has gone through its red giant phase. That star leaves behind a remnant white dwarf. When the white dwarf and its partner become close enough, the massive gravitational pull of the white dwarf draws material, mostly hydrogen, from the other star.

That hydrogen accretes onto the surface of the white dwarf, forming a thin atmosphere. The white dwarf heats the hydrogen, and eventually the gas pressure is extremely high, and fusion is ignited. Not just any fusion: rapid, runaway fusion.

When the rapid fusion ignites, we can see the light, and the new hydrogen atmosphere is expelled away from the white dwarf, into space. In the past, astronomers thought these new bright lights were new stars, and the name “nova” stuck. Astronomers now call these types of nova “classical” novae. (There are also recurrent novae, when the process repeats itself.)

This is an enormously energetic event, that produces not only visible light, but gamma rays and x-rays too. The end result is that some stars that could only be seen through a telescope can be seen with the naked eye during a nova.

All of this is widely accepted in astronomy and astrophysics. But much of it is theoretical. Recently, astronomers were fortunate enough to observe the entire process from start to finish, confirming the theory. The explosion of Nova V906 in the constellation Carina is giving researchers some answers and has confirmed some of the theoretical concept behind novae.

Novae like V906 Carinae are thermonuclear explosions on the surface of white dwarf stars. For a long time, astrophysicists thought that a nova’s luminosity is powered by continual nuclear burning after the initial burst of runaway fusion. But the data suggests something different.

In the new paper, the authors show that shocks play a larger role than thought. The authors say that “shocks internal to the nova ejecta may dominate the nova emission.” These shocks may also be involved in other events like supernovae, stellar mergers, and tidal disruption events, according to the authors. But up until now, there’s been a lack of observational evidence.

“Here we report simultaneous space-based optical and ?-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a remarkable series of distinct correlated flares in both bands,” the researchers write. Since those flares occur at the same time, it implies a common origin in shocks.

“During the flares, the nova luminosity doubles, implying that the bulk of the luminosity is shock powered.” So rather than continual nuclear burning, novae are driven by shocks. “Our data, spanning the spectrum from radio to gamma-ray, provide direct evidence that shocks can power substantial luminosity in classical novae and other optical transients.”

Source: “Astronomers watch a nova go from start to finish for the first time” Universe Today, 21 April 2020.<>

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