A nova may be a dramatic episode within the lifetime of a binary pair of stars. It's an explosion of bright light that may last weeks or perhaps months. And though they are not exactly rare - there are about 10 every year within the galaxy - astronomers haven't watched one from start to end.
Until now.
A nova occurs in an exceedingly close binary system when one amongst the celebs has had its red giant star phase. That star leaves behind a remnant white dwarf star. When the star and its partner become close enough, the large gravitational pull of the white dwarf star draws material, mostly hydrogen, from the opposite star.
That hydrogen accretes onto the surface of the white dwarf star, forming a skinny atmosphere. The white dwarf star heats the hydrogen, and eventually, the pressure level is extremely high, and fusion is ignited. Not just any fusion: rapid, runaway fusion.
Artist's impression of a nova eruption, showing the star accreting matter from its companion. (Nova_by K. Ulaczyk, Warschau Universität Observatorium)
When the rapid fusion ignites, we will see the sunshine, and also the new hydrogen atmosphere is expelled aloof from the star into space. In the past, astronomers thought these new bright lights were new stars, and also the name "nova" stuck.
Astronomers now call these sorts of nova "classical" novae. (There also are recurrent novae, when the method repeats itself.)
This is an enormously energetic event, that produces not only light but gamma rays and x-rays too. the tip result's that some stars that would only be seen through a telescope are often seen with the oculus during a nova.
All of this is often widely accepted in astronomy and astrophysics. But much of it's theoretical.
Recently, astronomers using the BRITE (BRIght Target Explorer) constellation of nanosatellites were fortunate enough to look at the complete process from start to end, confirming the speculation.
BRITE could be a constellation of nanosatellites designed to "investigate stellar structure and evolution of the brightest stars within the sky and their interaction with the local environment," per the website.
They operate in low-Earth orbit and have few restrictions on the parts of the sky that they'll observe. BRITE may be a coordinated project between Austrian, Polish, and Canadian researchers.
This first-ever observation of a nova was pure chance. BRITE had spent several weeks observing 18 stars within the Carina constellation. One day, a brand new star appeared. BRITE Operations Manager Rainer Kuschnig found 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 very handout. "I'd never seen anything prefer it all together the years of the mission!"
Werner Weiss is from the Department of Astrophysics at the University of Vienna. in a very release, he emphasized the importance of this observation.
A shows bright V906 Carinae labeled with a white arrow. B and C show the star before and after the V906 Carinae nova. (A. Maury and J. Fabrega)
"But what causes a previously unimpressive start to explode? This was an issue that has not been solved satisfactorily so far," he said.
The explosion of Nova V906 within the constellation Carina is giving researchers some answers and has confirmed a number of the theoretical concept behind novae.
V906 Carinae was first spotted by the All-Sky Automated Survey for Supernovae. Fortunately, it appeared in a neighborhood of the sky that had been under observation by BRITE for weeks, therefore the data documenting the nova is in BRITE data.
"It is astounding that for the primary time a nova may be observed by our satellites even before its actual eruption and until many weeks later," says Otto Koudelka, project manager of the BRITE Austria (TUGSAT-1) satellite at TU Graz.
V906 Carinae is about 13,000 light-years away, therefore the event is already history. "After all, this nova is to date far from us that its light takes about 13,000 years to succeed in the planet," explains Weiss.
The BRITE team reported their findings during a new paper. The paper is titled "Direct evidence for shock-powered optical emission in a very nova." It's published within the journal Nature Astronomy. First author is Elias Aydi from Michigan State University.
"This fortunate circumstance was decisive in ensuring that the nova event can be recorded with unprecedented precision," explains Konstanze Zwintz, head of the BRITE Science Team, from the Institute for Astro- and physical science at the University of Innsbruck.
Zwintz immediately realized "that we had access to observation material that was unique worldwide," consistent with an announcement.
Novae like V906 Carinae are thermonuclear explosions on the surface of white dwarf star stars. For a protracted time, astrophysicists thought that a nova's luminosity is powered by continual nuclear burning after the initial burst of runaway fusion. But the info from BRITE suggests something different.
In the new paper, the authors show that shocks play a bigger role than thought. The authors say that "shocks internal to the nova ejecta may dominate the nova emission."
These shocks may be involved in other events like supernovae, stellar mergers, and tidal disruption events, in step with the authors. But up yet, there's been a scarcity of observational evidence.
"Here we report simultaneous space-based optical and gamma-ray observations of the 2018 nova V906 Carinae (ASASSN-18fv), revealing a stimulating series of distinct correlated flares in both bands," the researchers write.
Since those flares occur at the identical time, it implies a standard origin in shocks.
"During the flares, the nova luminosity doubles, implying that the majority of the luminosity is shock powered." So instead of continual nuclear burning, novae are driven by shocks.
"Our data, spanning the spectrum from radio to gamma-ray, provide evidence that shocks can power substantial luminosity in classical novae and other optical transients."
In broader terms, shocks are shown to play some role in events like novae. But that understanding is basically supported by studying timescales and luminosities. This study is that the first direct observation of such shocks, and is probably going only the start of observing and understanding the role that shocks play.
In the conclusion of their paper, the authors write: "Our observations of nova V906 Car definitively demonstrate that substantial luminosity may be produced - and emerge at optical wavelengths - by heavily absorbed, energetic shocks in explosive transients."
They go on to mention that: "With modern time-domain surveys like ASAS-SN, the Zwicky Transient Facility (ZTF) and also the Vera C. Rubin Observatory, we'll be discovering more - and better luminosity - transients than ever before. The novae in our galactic backyard will remain critical for testing the physical drivers powering these distant, exotic events."
