The crackling light of a star does not always mean its death – Skywatching

In 2019, the astronomy world was keeping a close eye on Betelgeuse, a red supergiant star marking the left shoulder of Orion, the hunter, the most spectacular winter constellation.

The star was dimming, which was believed to mark the star’s impending death in a supernova explosion. At a cosmically close distance of 642 light-years, the supernova would have been the brightest thing in our sky after the Sun and Moon. It would have been visible in broad daylight. However, the dimming stopped and the explosion never happened.

The dimming was due to the star ejecting a large amount of material. This cloud of material blocked starlight, making the star appear dimmer.

The Sun also throws up clouds of material, which we call coronal mass ejections or solar storms. However, the mass of material ejected from Betelgeuse was about 400 billion times the largest ejection we have ever seen produced by the Sun. Now that the material dissipates, the dimming has stopped. However the event changed the star.

For centuries we have kept an eye on this star. It cycled in brightness over a period of about 400 days. It stopped. Its surface is now bubbling, like boiling water in a saucepan. Despite this apparent recovery from the mass ejection event, Betelgeuse is still a very old star and nearing its end in a massive explosion. Our observations suggest that giant stars have a few crises before the last, which destroys the star.

Stars are held together by the balance of two forces: gravity pulling in and pressure pushing out. The high pressure is due to high core temperatures, which in turn are due to energy production in the central regions of the star.

If energy production stops, the core cools, the pressure drops and the star shrinks. As a star ages, the core region fills with waste products from energy production, with nuclear fusion occurring in a thick skin surrounding the core. This means that the area of ​​energy production is actually larger and closer to the surface.

The result is a stronger outward thrust that inflates the star into a giant. Since the gravitational pull decreases rapidly with increasing distance, the force that holds the outer layers in place becomes weaker and weaker. The star has little ability to cling to its outer layers, so any increase in pressure due to turbulence or sputtering in energy production can push matter out into space.

The end comes when a sputter lasts long enough for the temperature and pressure to drop, when the star collapses and explodes. Betelgeuse is going to do it at some point. Another candidate is the red giant star Antares, in the constellation Scorpius, which is low in the south on these evenings.

The birth of new stars is hidden in the depths of clouds of gas and dust. However, new instruments show us more and more what is going on there. These observations suggest that sputtering occurs when new stars are born.

The standard birth story is that a cloud of gas and dust collapses, and under the impact of falling matter, the temperature and pressure in the middle become high enough for nuclear fusion to start, and we have a new star.

Current research suggests that things are not as smooth as that. Under the impacts of falling materials, there may be occasional explosions and hiccups. When nuclear fusion begins, the increases in heat and pressure can push the material outward, lowering the pressure and stopping the fusion, and only after a few misfires does power generation set in and that we have a new star.

Although we have a good overall picture of how stars are born, live and die, we are finding more and more detail and variation in these processes.

Obviously, stars will never get boring.


Saturn and Jupiter are in the sky after sunset. Mars rises three hours later, followed, just as the sky begins to clear for dawn, by Venus. The Moon will reach its first quarter on September 3.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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