The observatory at Mount Abu

French astrophysicist Olivier Chesneau prefers to call them “stellar vampires.” Professional star gazers knew for a while that a nova (meaning “new star” in Greek) sucks material from a companion star, and uses it for a cataclysmic nuclear explosion that makes it suddenly much brighter. The star subsequently returns to its original brightness weeks or years later.

Now, for the first time, researchers — including some from an Indian lab — have successfully tracked a nova and found how it commences the build-up for this eruption. “We have been able to track the evolution of a nova for more than 100 days,” says Dipankar Banerjee, an astrophysicist at the Physical Research Laboratory (PRL) in Ahmedabad, who led the Indian team.

The eruption of a nova typically happens when a very dense star has as a close companion a low-mass normal star called a red dwarf. Such dense stars are called white dwarfs and are considered to be the final evolutionary state of all stars. The mass of a typical white dwarf may be comparable to that of the sun, but in size it may resemble the earth. The two stars get so close to each other that the red dwarf cannot hold itself together and loses its mass to the white dwarf. Occasionally the shell of matter that has fallen on the white dwarf becomes unstable, leading to a thermonuclear reaction that renders the system brighter. While the heavier mass is thrown out into the universe, the lighter particles — such as dust and gases, which constitute nearly 1 per cent of the matter — move towards the white dwarf.

The nova V1280 Scorpii, located close to the constellation Scorpius, was discovered by Japanese amateur astronomers on February 4 last year. Over the next few days it became brighter and brighter, reaching its maximum brilliance nearly two weeks later. Eleven days after this, astronomers witnessed dust formation around the star.

“Immediately after an eruption, a nova begins the build-up for the next eruption, which may take a few thousand years to recur. It is the early stages of this accrual and outburst that we observed,” says Banerjee.

Astronomy is perhaps the only field of science where both professionals and amateurs have equally important roles to play. “While professional astronomers, who use more expensive and sophisticated telescopes, cannot scan the sky all the time, amateur astronomers spend most of their time gazing at the sky,” he explains. “And whenever they find something interesting, they alert all of us.”

According to Banerjee, this is what happened following the eruption of V1280 Scorpii, too. The PRL team, which is already collaborating with Chesneau, the lead author of the study that appeared in the journal Astronomy and Astrophysics, decided to use its infra red telescope in Mount Abu for observing the nova.

Infra red telescopes are important for such studies. This is because the dust and gases accruing on the star make it dimmer, and telescopes working in the visible range of the spectrum fail to pick up faint signals from them, says Banerjee.

More importantly, large telescopes — like the one used for this study, the Very Large Telescope Interferometer owned by the European Space Organisation — are booked by various teams of astronomers from all over the world much in advance. Although the managers of these telescopes can use their discretionary power to allot “telescope time” for such “unanticipated but extremely important observations”, they need to know the precise timeframe of the use. “This is because telescope time is very precise,” says the PRL scientist.

“We were really lucky to go into a collaboration with our Indian colleagues at the Mt Abu observatory,” says Chesneau. “Nowadays, finding telescopes for monitoring fast varying objects (such as a nova that becomes dimmer and dimmer by the day) is difficult, and they do a great job,” Chesneau told KnowHow.

According to the scientists, the initial observations made 23 days after the discovery showed that the shell of dust around the star was compact and thin. But a few days later, they found that it had grown by 13 times, indicating that the dust ring around the star was expanding. The novel techniques employed by the scientists also helped them understand the daily production of dust and infer the total mass ejected from the red dwarf.

“Overall, V1280 Scorpii probably ejected more than an equivalent of 33 times the mass of the earth, a rather impressive feat if one considers that this was ejected from a star not larger than the earth,” says Chesneau.