Astronomers Watch a Nova Go From Start to Finish for the First Time
Universe

Astronomers Watch a Nova Go From Start to Finish for the First Time

A nova is a dramatic event in the lives of a binary pair of stars. It’s a bright light explosion that can last for weeks or even months. And, while they’re not precisely rare—about ten occur each year in the Milky Way—astronomers have never seen one from beginning to end.

A nova happens when one of two stars in a close binary star system has completed its red giant phase. That star’s remnant is a white dwarf. When the white dwarf and its partner get near enough, the white dwarf’s massive gravitational attraction takes material, usually hydrogen, from the other star.

That hydrogen accretes onto the surface of the white dwarf, forming a thin atmosphere. The white dwarf warms the hydrogen until the gas pressure becomes exceedingly high, at which point fusion is ignited. Rapid, runaway fusion, not just any fusion.

We can see light as the quick fusion occurs, and the new hydrogen atmosphere is expelled from the white dwarf into space. Originally, astronomers mistook these new bright lights for new stars, and the term “nova” stuck. Astronomers now refer to these novae as “classical” novae. (Recurrent novae occur when the process repeats itself.)

“Suddenly there was a star on our records that wasn’t there the day before.” Rainer Kuschnig, BRITE Operations Manager, Graz Technical University

This is a tremendously energetic event that generates not just visible light but also gamma rays and x-rays. As a result, several stars that were formerly only seen via a telescope can now be seen with the naked eye during a nova.

In astronomy and astrophysics, all of this is widely accepted. However, most of it is theoretical. Astronomers utilizing the BRITE (BRIght Target Explorer) Constellation of nanosatellites recently had the opportunity to observe the entire process from start to finish, confirming the theory.

A statistical summary chart for the BRITE Constellation from 2019. Image Credit: Konstanze Zwintz.

BRITE is a constellation of nanosatellites designed to “investigate stellar structure and evolution of the brightest stars in the sky and their interaction with the local environment,” according to the website. They operate in low-earth orbit and have few restrictions on the areas of the sky they may view. BRITE is a collaboration of Austrian, Polish, and Canadian researchers.

This was the first ever observation of a nova. BRITE had been watching 18 stars in the Carina constellation for several weeks. A new star arrived one day. Rainer Kuschnig, BRITE Operations Manager, discovered the nova during a daily inspection. “Suddenly there was a star on our records that wasn’t there the day before,” he said in a press release. “I’d never seen anything like it in all the years of the mission!”

An image of the starry sky. The NOVA CARINAE 2018 (V906 Carinae) may be seen on the right side of the photo. W. Paech and F. Hofmann, Chamaeleon and Onjala Observatory Namibia are to be acknowledged.

Werner Weiss is a professor at the University of Vienna’s Department of Astrophysics. He emphasized the significance of this observation in a news release. “But what causes a previously unimpressive star to explode? This was a problem that has not been solved satisfactorily until now,” he said. The eruption of Nova V906 in the constellation Carina provides researchers with some answers and confirms several theoretical concepts regarding novae.

“It is fantastic that for the first time a nova could be observed by our satellites even before its actual eruption and until many weeks later.”

Professor Otto Koudelka, Project Manager, BRITE Austria.

The All-Sky Automated Survey for Supernovae discovered V906 Carinae. Fortunately, it appeared in a region of the sky that BRITE had been watching for weeks, thus the data recording the nova is included in the BRITE data. “It is fantastic that for the first time a nova could be observed by our satellites even before its actual eruption and until many weeks later,” says Prof. Otto Koudelka, project manager of the BRITE Austria (TUGSAT-1) satellite at TU Graz.

V906 Carinae is around 13,000 light years away, so the event has already occurred. “After all, this nova is so far away from us that its light takes about 13,000 years to reach the earth,” explains Weiss.

The BRITE team published its findings in a new paper. The paper is titled “Direct evidence for shock-powered optical emission in a nova.” It appears in the journal Nature Astronomy. Elias Aydi of Michigan State University is the first author.

“This fortunate circumstance was decisive in ensuring that the nova event could be recorded with unprecedented precision,” explains Prof. Konstanze Zwintz, head of the BRITE Science Team, from the Institute for Astro- and Particle Physics at the University of Innsbruck. Zwintz immediately realised “that we had access to observation material that was unique worldwide,” according to a press release.

V906 Carinae and other novae are thermonuclear explosions on the surface of white dwarf stars. For a long time, astrophysicists believed that the brilliance of a nova was powered by continuous nuclear burning following the first burst of runaway fusion. However, the BRITE data says otherwise.

The authors of the new paper show that shocks have a bigger influence than previously anticipated. The authors say that “shocks internal to the nova ejecta may dominate the nova emission.” According to the scientists, these shocks may also be implicated in other phenomena like supernovae, stellar mergers, and tidal disruption events. However, there has been a lack of observational data until now.

“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. Because such flares occur at the same time, it suggests a common source 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.”

A bright V906 Carinae is labeled with a white arrow in “A.” “b” and “c” show the star HD 92063 before and after the V906 Carinae nova. Image Credit: A. Maury and J. Fabrega

Shocks, in general, have been shown to have a role in occurrences such as novae. Nevertheless, that understanding is mostly based on the study of timescales and luminosities. This is the first direct observation of such shocks, and it is likely that this is only the beginning of observing and understanding the function such shocks play.

In the conclusion of their paper the authors write “Our observations of nova V906 Car definitively demonstrate that substantial luminosity can be produced—and emerge at optical wavelengths—by heavily absorbed, energetic shocks in explosive transients.”

They go on to say that “With modern time-domain surveys such as ASAS-SN, the Zwicky Transient Facility (ZTF) and the Vera C. Rubin Observatory, we will be discovering more—and higher luminosity—transients than ever before. The novae in our galactic backyard will remain critical for testing the physical drivers powering these distant, exotic events.”

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