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Massive Star’s Dying Blast Caught By Pure Chance!

By Dave Reneke

A massive star’s dying blast has been caught by pure chance, and how early moons collided to form today’s Moon.

On 25 June 2016, an international team of 31 astronomers caught a massive star as it died in a titanic explosion deep in space. The blast released in about 40 seconds as much energy as the Sun releases over its entire lifetime, all focused into a tight beam of gamma rays and fortuitously aimed directly toward Earth. The team’s findings provide strong evidence for one of two competing models for how gamma-ray bursters produce their energy.

The gamma-ray blast was detected by two NASA satellites that monitor the sky for such events: the Fermi Gamma-ray Space Telescope and the Swift Gamma-Ray Burst Mission. These detected the burst of gamma rays, identified from where in the sky it came, and sent its celestial position within seconds to automated telescopes on the ground.

The MASTER-IRC telescope at the Teide Observatory in the Canary Islands observed it first, within a minute of the satellite notification. It made optical light observations while the initial phase was still active, gathering data on the amount of polarised optical light relative to the total light produced.

Gamma-ray bursts are detected approximately once per day, and are brief but intense flashes of gamma radiation. They come from all different directions in the sky, and they last from tens of milliseconds to about a minute, making it hard to observe them in detail.

“While gamma-ray bursters have been known for about 50 years, astronomers are still mostly in the dark about how they erupt,” said A/Prof Nathaniel Butler of Arizona State University. Astronomers, though, believe that most of these explosions are associated with supernovae.

Continued instrument observations over weeks following the outburst showed that the gamma rays were shot out in a beam about 2° wide, or roughly four times the apparent size of the Moon. It was sheer chance that Earth happened to lie within the beam.

“We think the gamma-ray emission is due to highly energetic electrons propelled outward like a fireball,” Butler said. “This is the first strong evidence that the early shocks generated by these bursts are magnetically driven.”

How Early Moons Collided To Form Today’s Moon

The Moon, and how it was formed, has long been a source of fascination and wonder. Now a team of Israeli researchers from the Technion-Israel Institute of Technology and the Weizmann Institute of Science suggests that the Moon we see every night is not Earth’s first moon, but rather the last in a series of moons that orbited the Earth in the past.

The newly proposed theory runs counter to the commonly held “giant impact” paradigm that the Moon is a single object that was formed following a single giant collision between a small Mars-like planet and the ancient Earth.

“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” said co-author Prof Hagai Perets. “It’s likely that such moonlets were later ejected or collided with the Earth or with each other to form bigger moons.”

To check the conditions for the formation of such mini-moons or moonlets, the researchers ran 800 simulations of impacts with the Earth. The new model is consistent with science’s current understanding of the formation of the Earth. In its last growth stages, the Earth experienced many giant impacts with other bodies. Each of these impacts contributed more material to the proto-Earth until it reached its current size.

“We believe the Earth had many previous moons,” said Perets, who added that “a previously formed moon could therefore already exist when another moon-forming giant impact occurs”.

The tidal forces from the Earth could cause moons to migrate slowly outwards (the current Moon is slowly doing that at a pace of about 1 cm /year). A pre-existing moon would slowly move out by the time another moon forms. However, their mutual gravitational attraction would eventually cause the moons to affect each other and change their orbits.

“It’s likely that small moons formed through the process could cross orbits, collide and merge,” Perets said. “A long series of such moon–moon collisions could gradually build up a bigger moon – the Moon we see today.” It’s a colourful theory, backed up by sound research, and will no doubt command much scientific discussion.


David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com