What do we see in this magnificent photo of a “fireball” in space?

The short and powerful eruption of a white dwarf was observed for the first time by an X-ray instrument. The capture of this “nova” confirms a 30-year-old prediction.

The eROSITA X-ray instrument was aimed at the sky on 15 July 2020 and tirelessly scanned the sky. In 22 sweeps, the room was at its most peaceful. Then came the 23rd scan: In the X-ray spectrum and in the so-scanned region, one could see an intensely luminous flash. On the 24th sweep, the flash was gone, everything was quiet again.

8,250 light-years away, near the southern constellation Reticulum, a star suddenly lit up. In a stroke of luck that only science has a secret about (because the instrument only pointed to this cosmic region for 35 seconds), the X-ray observatory recorded the extremely short and intense moment of its eruption. The image, as well as the resulting analysis, has just been published in Nature May 11, 2022

22. swipe left: nothing.  23. center-sweep: nova.  24. swipe right: nothing.  // Source: Nature, Konig & al., 2022
22. swipe left: nothing. 23. center-sweep: nova. 24. swipe right: nothing. // Source: Nature, Konig & al., 2022

What do we see in the picture?

What you see there is not a supernova, but a nova: that’s what the sudden enlightenment of a white dwarf star is called. White dwarfs are small, like the Earth or even the Moon, but extremely dense as they are the contraction of an interstellar star that has burned most of its fuel. They are sometimes described as dead stars.

When a white dwarf integrates into a binary star system that interacts with another star, it absorbs gas from its companion. In the heart of the white dwarf, the heat and pressure increase gradually until the hydrogen is fused to helium. The thermonuclear runaway starts until this storm rises to the surface of the dwarf by expelling matter. It is then the eruption – the “nova” – that we see here with X-rays, and after the moment of explosion T, which we can observe with the naked eye via a telescope.

Why “new”?

This eruption triggers a brief and sudden flash of light where previously nothing special was seen. Previously, astronomers believed that they observed the appearance of a completely new star: this is where the term “nova”, to describe the phenomenon, got its name.

The confirmation of a theory of novas

The detection of the phenomenon with the eROSITA instrument not only makes it possible to obtain a beautiful image. It confirms a prediction, simply because it is the very first time we have captured this specific moment of a nova in the X-ray spectrum. This very special moment is the “fireball”.

Representation of the nova in animation.  // Source: Max Planck Institute (transformation to gif: Nino Barbey / Numerama)
Representation of the nova in animation. // Source: Max Planck Institute (transformation to gif: Nino Barbey / Numerama)

Previously, astronomers had already observed novae in the optical spectrum and during the expansion phase of the materials ejected from the star’s surface. But this time, in the X-ray spectrum, we clearly observe the key moment where the eruption itself occurs – the star then reaches the Eddington brightness, the extreme limit of brightness that a celestial object can reach. And with good reason, the heat on the star’s surface is then about 1 million kelvin.

This very fortunate observation confirms the various stages that a nova follows and gives measurements (such as the exact level of heat released). In particular, this phase had been theorized for literally 30 years without any concrete observation at hand: ” The existence of X-ray flashes [durant ce phénomène] is now confirmed by observation writes the researchers in the newspaper.

Further

One hypothesis in astrophysics is that white dwarfs eventually cool to stay

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