Astronomers discover micronovae, a new kind of stellar explosion

Date:
April 20, 2022
Source:
ESO
Summary:
A team of astronomers has observed a new type of stellar explosion
-- a micronova. These outbursts happen on the surface of certain
stars, and can each burn through around 3.5 billion Great Pyramids
of Giza of stellar material in only a few hours.



FULL STORY ==========================================================================
A team of astronomers, with the help of the European Southern
Observatory's Very Large Telescope (ESO's VLT), have observed a new
type of stellar explosion -- a micronova. These outbursts happen on the
surface of certain stars, and can each burn through around 3.5 billion
Great Pyramids of Giza of stellar material in only a few hours.


==========================================================================
"We have discovered and identified for the first time what we are
calling a micronova," explains Simone Scaringi, an astronomer at Durham University in the UK who led the study on these explosions published
today in Nature. "The phenomenon challenges our understanding of how thermonuclear explosions in stars occur. We thought we knew this, but
this discovery proposes a totally new way to achieve them," he adds.

Micronovae are extremely powerful events, but are small on astronomical
scales; they are much less energetic than the stellar explosions known
as novae, which astronomers have known about for centuries. Both types
of explosions occur on white dwarfs, dead stars with a mass about that
of our Sun, but as small as Earth.

A white dwarf in a two-star system can steal material, mostly hydrogen,
from its companion star if they are close enough together. As this gas
falls onto the very hot surface of the white dwarf star, it triggers
the hydrogen atoms to fuse into helium explosively. In novae, these thermonuclear explosions occur over the entire stellar surface. "Such detonations make the entire surface of the white dwarf burn and shine
brightly for several weeks," explains co-author Nathalie Degenaar,
an astronomer at the University of Amsterdam, the Netherlands.

Micronovae are similar explosions that are smaller in scale and faster,
lasting just several hours. They occur on some white dwarfs with strong magnetic fields, which funnel material towards the star's magnetic
poles. "For the first time, we have now seen that hydrogen fusion can
also happen in a localised way.

The hydrogen fuel can be contained at the base of the magnetic poles of
some white dwarfs, so that fusion only happens at these magnetic poles,"
says Paul Groot, an astronomer at Radboud University in the Netherlands
and co-author of the study.

"This leads to micro-fusion bombs going off, which have about one
millionth of the strength of a nova explosion, hence the name micronova,"
Groot continues.

Although 'micro' may imply these events are small, do not be mistaken:
just one of these outbursts can burn through about 20,000,000 trillion
kg, or about 3.5 billion Great Pyramids of Giza, of material.* These new micronovae challenge astronomers' understanding of stellar explosions
and may be more abundant than previously thought. "It just goes to show
how dynamic the Universe is. These events may actually be quite common,
but because they are so fast they are difficult to catch in action,"
Scaringi explains.



==========================================================================
The team first came across these mysterious micro-explosions when
analysing data from NASA's Transiting Exoplanet Survey Satellite
(TESS). "Looking through astronomical data collected by NASA's TESS,
we discovered something unusual: a bright flash of optical light lasting
for a few hours. Searching further, we found several similar signals,"
says Degenaar.

The team observed three micronovae with TESS: two were from known white
dwarfs, but the third required further observations with the X-shooter instrument on ESO's VLT to confirm its white dwarf status.

"With help from ESO's Very Large Telescope, we found that all these
optical flashes were produced by white dwarfs," says Degenaar. "This observation was crucial in interpreting our result and for the discovery
of micronovae," Scaringi adds.

The discovery of micronovae adds to the repertoire of known stellar
explosions.

The team now want to capture more of these elusive events, requiring
large scale surveys and quick follow-up measurements. "Rapid response
from telescopes such as the VLT or ESO's New Technology Telescope and
the suite of available instruments will allow us to unravel in more
detail what these mysterious micronovae are," Scaringi concludes.

Note: *We use trillion to mean a million million (1,000,000,000,000 or
1012) and billion to mean a thousand million (1,000,000,000 or 109). The
weight of the Great Pyramid of Giza in Cairo, Egypt (also known as the
Pyramid of Khufu or Pyramid of Cheops) is about 5,900,000,000 kg.


========================================================================== Story Source: Materials provided by ESO. Note: Content may be edited
for style and length.


========================================================================== Related Multimedia:
* Artist's_impression_and_animation_of_a_micronova ========================================================================== Journal Reference:
1. Scaringi, S., Groot, P.J., Knigge, C. et al. Localized thermonuclear
bursts from accreting magnetic white dwarfs. Nature, 2022 DOI:
10.1038/ s41586-022-04495-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220420122315.htm

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