15 Million Degrees by Lucie Green

Author:Lucie Green [Green, Lucie]
Language: eng
Format: azw3, epub
ISBN: 9780241963562
Publisher: Penguin Books Ltd
Published: 2016-03-30T16:00:00+00:00

TAILS

Up until now we have been looking at the Sun as a sphere of plasma, which can be broadly split into different shells. Then, during the time of a total solar eclipse, we get a glimpse that the plasma stretches up from the photosphere to form an atmosphere. The impression that we get from the total-eclipse observations is that the atmosphere gets thinner and thinner with height until it eventually fades and runs out of material.

But over the centuries there have been some icy visitors to the inner Solar System which hinted that the tenuous corona we saw in the previous chapter doesn’t end where we see it end with our eyes during an eclipse. They gave the first clue that the atmosphere of the Sun extends beyond what our eyes perceive. These icy visitors are the comets.

Comets are frozen lumpy bodies of ice and dust several kilometres across that mostly reside in a region called the ‘Oort Cloud’, named after the Dutch astronomer who proposed that a vast shell of comets surrounds the Solar System at one fifth of the distance to the nearest star. That’s over 10,000 times further out than Jupiter. At that distance, there is no way that we can see any comets directly, not even with the most powerful telescope. But, still, the Oort Cloud is thought to exist. At that vast distance, comets are only loosely gravitationally bound to the Sun. So they can be easily knocked out of their icy ghetto by the gravitational influence of a star passing nearby. This interaction can change the orbit of a comet so that it comes in closer to the Sun. Again, this may seem far-fetched, but during the lifetime of the Solar System, as we fly through the Galaxy, such incredible events can happen. This gravitational disruption is thought to explain why we have comets that repeatedly come and visit us in the inner Solar System, like Halley’s comet (named after Edmond Halley, whom we met before), which comes back to us every seventy-six years.

At their distant outpost comets can take millions of years to orbit the Sun, but when one gets dislodged from the Oort Cloud and comes into the inner Solar System, its journey close to the Sun causes it to heat up. Basking in the increasing amount of sunlight falling on it, ice that has been frozen for millennia turns into gas and particles of dust trapped amongst the ice grains are released. As the comet reaches roughly the orbit of Jupiter, a diffuse cloud of gas and dust called a ‘coma’ starts to form around the giant iceberg. And then something spectacular happens. A beautiful tail forms behind the comet that stretches out for millions of kilometres. And it is this tail that shows something intriguing.

Comet tails all have something in common: they always point away from the Sun – within a few degrees – no matter whether the comet is approaching the Sun or whether it has passed around the Sun and is heading back out into the depths of space.

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