If you live in the Northern hemisphere, in winter there’s a very conspicuous part of the night sky which rises high above the horizon.
It is not difficult to find the constellation named after Orion, the Hunter, which looks like this: and is worth viewing with binoculars.
With a VERY powerful telescope, Orion looks brilliant:
The bright reddish star that marks one of Orion’s shoulders is called BETELGEUSE: there’s another bright star called Aldebaran, then if you look about the same amount further on, you’ll see a group or “cluster” of stars called the PLEIADES, which are well worth finding. Maybe sometimes where you live, the sky is dark enough to see them, if not you can probably see them if you go on holiday to a “dark sky” area. They can be seen in winter but in summer they are hidden below the horizon.
. We’ll come back to them later.
That very bright and massive star, Betelgeuse, is an example of what is called a “supergiant”. There are several other supergiants that are not hard to find: these stars are Deneb, Antares, Rigel and Betelgeuse, and are some of the most prominent stars in our sky.
We are interested in Betelgeuse, because the study of supergiants like that help cosmologists to work out how the various heavy elements in the universe (iron, calcium, and so on) are created.
Betelgeuse is so huge that if our sun were as big, we’d be inside it - its inside would enclose the whole of our earth’s orbit. Another such supergiant is Deneb. What was perhaps a bit worrying in 2020 was that Betelgeuse began to vary in brightness, which, like many other “variable stars”, it had been doing for a long time. These variations in brightness were getting more and more noticeable, so astronomers were wondering whether Betelgeuse has become a “pulsating eruptive” variable? If so, this means that it could become one of a type of exploding star, maybe as bright as the full moon and even visible in daylight: the first such star in our galaxy for more than 300 years. But when might it explode?
Answer: some time in the next 100,000 years, so probably not to-night, but you never know! c stage towards the end of a star's life is characterised by a sudden and dramatic rise in brightness – in other words, it becomes a type of exploding star.
These high-mass stars are rare and have very short lifespans. However, the nuclear physicists were able to work out what happens in the most massive stars of all. Such an event is extremely rare, but still very important. The experts were able to confirm how one would expect a cataclysmic explosion towards the end of the star’s life, characterised by a sudden and dramatic brightening. The story of “Zeus’s thunderbolts” describes how an event like this did happen in historical times, with the brightening lasting for several days or maybe a few weeks, outshining all the other stars in our galaxy!
Here is another supergiant:
Is it possible that a nearby explosion event could be dangerous for us? The greatest risk is to the Earth's protective ozone layer, producing effects on life and climate. The shape of our galaxy is a central bulge, and radiating from it are a few spiral-shaped concentrations of stars, gas and dust. In a few billion years as the Sun orbits the centre of our Galaxy, it will at times enter regions of greater stellar density. This may put the Earth at greater risk of being near an exploding supergiant - if such a one occurred within a few light years of the Earth, then the resulting release of radiation and stellar material would be sufficient to destroy all life on earth. A recent study has found that mass extinctions in the past, correlate closely with passages of the Sun through the dense spiral arms of our galaxy. While the explosion seen in 1006 was not near enough to affect the earth significantly, a sign of its outburst can be found in nitrate deposits in Antarctic ice.
Luckily, neither Betelgeuse nor any other massive stars are close enough to the Earth, to cause a mass extinction in the foreseeable future.
Let’s look again at the bunch of stars called the Pleiades, all squashed up together to make what’s called a cluster. When the sky is nice and dark you can see six of these Pleiades. People with very good eyes can sometimes see seven. Of course with a telescope you can see more of them:
The ancient Greeks said that the Pleiades were the seven daughters of Atlas who was a Titan. They were terribly sad because Atlas was condemned by Zeus, the king of the gods, to carry the heavens on his shoulders for ever. So they committed suicide but to cheer them up and make them immortal, Zeus changed them first into doves, and then into those stars, which are often called the Seven Sisters. The trouble with that was that Orion, the Hunter, started chasing them across the night sky, as we can still see (if you believe all that stuff).
Occasionally there’s a special night when the moon slowly moves across the Pleiades, and blocks them off! The moon’s path doesn’t often do that. Maybe you have a telescope which you use to look at the moon. On such a night you will see the Pleiades looking like lights being suddenly switched off one by one, and switched on again after the moon has moved on. It all takes about three minutes.
In the days before mirrors could be placed on the moon to reflect a light signal beamed from earth, groups of people sometimes used a stopwatch to measure the time each star disappeared, just as exactly as when a runner’s lap time is measured in a race. Knowing the precise longitude and latitude of each observer, and with careful synchronisation of all the stopwatches to the broadcast time signal, a simple maths calculation gave a figure for how distant the moon is, from the earth, to the nearest kilometre or so
Now with modern methods it is possible to measure the distance of the moon to within two millimetres! although it varies by as much as 12% over the course of a month, since that moon’s orbit is not a perfect circle. The very precise measurements now possible confirm that the moon is spiralling away from the Earth at an average rate of 3.8 cm per year. Anyone studying mechanics to an advanced level at school will learn about something known as “angular momentum”: because of the friction resulting from the tides caused by the moon’s gravitational pull, some angular momentum of the earth's rotation is slowly being transferred to the Moon's orbit. The result is that the Earth's rate of spin is gradually decreasing, so our days are getting longer by about two millionths of a second every month…
Fossil studies of mollusk shells from the Campanian era (80 million years ago) show that there were 372 days (of 23 h 33 min) per year.
We said that we are interested in supergiants like Betelgeuse, because they help cosmologists to work out how the various heavy elements in the universe, such as iron and copper, are created. So how do the nuclear physics experts think this happens?
For all stars (not just the ones that will become supergiants), the creation of elements begins when the star first forms from a collapsing cloud of hydrogen, the simplest and lightest atom of all and the first to form once the universe had cooled enough after the “big bang”. The gravitational pressure at the star's core generates heat, which ignites a “thermonuclear fusion” reaction that converts the core's hydrogen into helium (this reaction is what made possible that devastating potential weapon known as the hydrogen bomb). The process, called ”nuclear fusion” or "nucleosynthesis," continues until the core's hydrogen is exhausted:
Observations indicate that most stars are massive enough to enter a second round of nucleosynthesis. The depleted core - now rich in helium - contracts further, generating enough heat to undergo a second wave of thermonuclear fusion, turning its helium into carbon and oxygen:
But the more massive the star, the more likely it is to experience even more rounds of nucleosynthesis! The most massive stars can have several layers of fusion going on at the same time:
each shell of material is fusing to make the element in the shell inside it, and so on, down to a core in which, for the most massive stars, iron is produced.
In these rounds of nucleosynthesis, the nuclear fusion reactions produce energy, creating an outward pressure that counterbalances the inward pressure of gravity. You might think that the formation of more and more of those shells could continue indefinitely, until ALL the elements listed in the periodic table had been created? What prevents this happening is that when iron is formed, iron fusion uses up energy instead of producing it. So the outward pressure stops and even reverses, gravity takes over, and if the star is massive enough, it rapidly implodes until suddenly a vast number of mysterious particles called neutrinos blast out of the core, blowing the rest of the star to bits in an explosion that may be as bright as an entire galaxy.
Such a star is called a SUPERNOVA.
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