Our Sun is often described as being an average star. This is only true in a very narrow sense. Stars that, like the Sun, are maintaining their output of energy by converting hydrogen into helium deep in their interiors are said to lie on the ‘main sequence’ of a kind of graph, called the Hertzprung-Russell Diagram, which astronomers use to relate the temperature of a star to its mass. Because the Sun is on the Main Sequence of this diagram, it is regarded as an ordinary star. But ordinary does not mean average. Some 95 per cent of all stars are less massive than the Sun, and because the brightness of a star is related to its mass, this means that they are dimmer than the Sun. In that respect, the Sun is far from being average, and stars that are bigger and brighter than the Sun are even more rare than stars with the same mass as the Sun, even though massive stars are quite normal.
The Sun may not be ‘average’ in another way. There is some evidence that the brightness of the Sun varies by less than the variation in brightness of other stars with similar masses and chemical compositions. This is very hard to quantify, and we cannot be sure whether this has always been the case or is just a phase the Sun is going through today (or for the past few million years). But it does at least hint that the Sun may be an unusually stable star, with obvious benefits for the evolution of life on Earth.
On a larger scale, being either brighter or dimmer than the Sun has dramatic implications for the CHZ around a star. Most of the stars in the Galaxy – 95 per cent – are smaller and fainter than the Sun. Three quarters of all the stars in our neighbourhood are so-called red dwarfs, a category also known as M-type stars, which have only about a tenth as much mass as our Sun. Red dwarfs live for much longer than stars like the Sun (which is a yellow-orange G-type star; the initials are a historical accident and have no significance except as labels). This would be a good thing in terms of allowing time for intelligence to evolve. Unfortunately, though, the conditions on any planet orbiting a red dwarf are likely to be unsuitable for the emergence of a technological civilization.
The first problem is that the life zone around a red dwarf is very narrow, and very close to the parent star. In order to have liquid water on its surface, a planet would have to orbit within 5 million km of the star, at a distance only one thirtieth of the distance of the Earth from the Sun. Even at its closest, Mercury, the innermost planet in our Solar System, never gets within 46 million km of the Sun. It isn’t clear that planets could even form, or occupy stable orbits, within 5 million km of a star, but even if they could there would be complications. Just as tidal forces have locked the Moon into a rotation which keeps one face always turned towards the Earth, so planets in the life zone around a red dwarf would be locked into a rotation with one side always facing the star. So one side would be in eternal darkness, and the other in eternal light. Except, possibly, for a narrow twilight zone, the conditions would be either uncomfortably hot or uncomfortably cold. The most likely consequence of this is that convection would carry gases from the hot side of the planet to the cold side, where they would cool and freeze. Any atmosphere the planet originally possessed before the tidal locking was completed would freeze out on the dark side.
Another problem – as if that weren’t enough – is that red dwarf stars are much more active than the Sun. They produce frequent flares of activity which release large amounts of ultraviolet radiation, X-rays and particles. This would be particularly damaging because of the proximity of the planet to the star. Apart from the direct consequences for life, these outbursts would strip away any atmosphere that started to form around the planet. Overall, it seems we can rule out red dwarf systems as likely homes for other civilizations. We have already found that the Galactic Habitable Zone only includes 10 per cent of the stars in the Milky Way, and now we are ruling out 75 per cent of that 10 per cent. That leaves us with only 2.5 per cent of all the stars to consider, and we have barely started identifying all the reasons why we are here on Earth.
Bigger, brighter stars than the Sun form only a small part of that 2.5 per cent, and in terms of habitable zones alone are no better than red dwarf stars as possible places to find planets harbouring technological civilizations. A brighter star has a larger habitable zone, but it doesn’t live as long as the Sun, and the habitable zone moves out more rapidly than the Sun’s habitable zone as the star ages. A star with 30 times as much mass as our Sun would have to burn its nuclear fuel so fast that the rate at which it pours out energy is 10,000 times that of the Sun, and it will live for only a few tens of millions of years on the stable Main Sequence. Such stars also emit large amounts of ultraviolet radiation, damaging both to life and to the atmospheres of prospective Earth-like planets. The brightest stars on the Main Sequence, known as O and B stars, together make up less than one tenth of 1 per cent of all stars, though, so taking them out of the equation hardly makes much difference.
Slightly smaller, cooler A-type stars could provide any planets in their life zones with a stable environment for about a billion years, which is certainly long enough for life to get started, judging by the example of the rapid establishment of life on Earth, but may not be enough for a civilization like ours to develop. Even a star with just 1.5 times the mass of our Sun would leave the Main Sequence after only a couple of billion years. But there are some stars, the F-types, which are a little more massive than our Sun, have Main Sequence lifetimes of about 4 billion years, and which don’t seem to produce excessive amounts of ultraviolet radiation.
Putting everything together, reasonably large, reasonably long-lasting life zones may exist around stars which are in the Galactic Habitable Zone and are like the Sun (G-type), or stars a little more massive (the F-types) or a little less massive (known as K-types). A generous assessment would make that no more than 2 per cent of the stars in the Galaxy. In that sense, we can already see that the Sun is special. But even within that 2 per cent, the Sun is not an average star, because most stars have companions – they live in binary or even triple star systems.
It is actually very difficult to make stars. The large clouds of gas and dust in the thin disc of the Milky Way (known as giant molecular clouds, because they are big and contain molecules) rotate, which tends to stop them collapsing, and are threaded by magnetic fields which also help to hold them up against the inward tug of gravity. If a star with the same mass as the Sun formed from a cloud spread out to the density of a slowly rotating interstellar cloud, by the time this had shrunk to the size of the Sun it would be spinning so fast that its surface would be moving at 80 per cent of the speed of light. This is because a property known as angular momentum is conserved when a cloud shrinks – or, indeed, when it expands. In order to have the same angular momentum, provided it has the same mass a small object has to spin faster than a large object. This is exactly why a spinning ice skater can spin faster or slower by pulling their arms in or out. In order to shrink, a collapsing cloud of gas has to get rid of angular momentum. If two or more stars form from the same collapsing cloud, a lot of the angular momentum goes in to the orbital motion of the stars around each other, rather than into the spin of the stars themselves.
An average giant molecular cloud is about 65 light years across and contains about a third of a million solar masses of material. When a cloud passes through the density jump of a spiral arm, it gets squeezed, and if a supernova explodes nearby shock waves from the blast will go rolling through it. Under these conditions, turbulence stirring up the cloud can produce regions of greater density where gravity can take over and cause some of those local regions to collapse to form stars. Stellar ‘nurseries’ where this process is going on have been photographed in the infrared part of the spectrum, where radiation penetrates the dust in the clouds, from unmanned space observatories such as Herschel, confirming astronomers’ understanding of what goes on based on their knowledge of the laws of physics.
Turbulence seems to produce ‘pre-stellar cores’ on which stars grow as gravity tugs more matter towards them. A typical core would be about a fifth of a light year across, and contain about 70 per cent as much mass as the Sun. Only the very centre of such a core collapses and heats up to the point where it begins to generate energy by nuclear fusion, becoming initially a tiny proto-star with less than a hundredth (perhaps as little as a thousandth) of the mass of our Sun; the nuclear reactions begin when it has grown to about a fifth of the mass of the Sun. The final size of the star that grows onto this core doesn’t depend on the size of the core – all such cores start out with roughly the same mass. What matters is the amount of matter close enough to be captured by the gravity of the young star, before the radiation from the star and any companions forming nearby disperses the clump in the giant molecular cloud from which they have formed. For a star like the Sun, 99 per cent of its mass is gathered in this way by accretion. But this is a very inefficient process. Although roughly half of the mass in a clump gets turned into stars, only a few per cent of the material in the whole cloud is converted into stars as it makes the passage through a spiral arm.
Because of the angular momentum problem, it is hard to see how a star could form in isolation, and observations of our stellar neighbourhood show that at least 70 per cent of Sun-like stars have at least one companion, although systems with more than three stars bound together by gravity are extremely rare. Computer simulations of the way stars in multiple systems interact with one another and with nearby systems explain how this proportion has arisen, and why there are at least some stars which, like our Sun, do not have a stellar companion.
When three stars are orbiting around one another, they follow a complicated dance in which it is quite easy for one of the stars to gain a lot of energy and be ejected from the system, carrying angular momentum off with it, while the other two move closer together in a tighter embrace. Binary pairs are more stable, unless they pass close by another star (or a binary or a triple), in which case gravitational interactions can break up the pair and leave at least one isolated star, although its companion can sometimes be captured by the other system. Computer simulations suggest that if out of every 100 new star systems 40 are triple and 60 are binaries (making a total of 240 stars) then, allowing for how close these systems are in the star-forming regions of the Milky Way, by the time the star systems have moved apart into the Galaxy at large and things have settled down there will be 25 triples, 65 binaries and just 35 single stars. The same 240 stars are now shared out in such a way that just under 20 per cent are unaccompanied, roughly matching our observations of the stars in our neighbourhood.
Binary and triple star systems are bad news for life – certainly for the prospects of a technological civilization arising on any planet in such a system. Stable orbits can exist, either if the two stars in a binary are very close together (within about a fifth of the distance from the Earth to the Sun) and the planets orbit around both stars, or if the two stars are far apart (at least 50 times the distance from the Earth to the Sun) and the planets orbit one of the stars. But although the orbits may be stable, they will not be as beautifully circular as the Earth’s orbit around the Sun, and the planets will be affected by the heat and light from two stars, making it difficult to establish a long-lasting habitable zone. Judging by the evidence of the geological record of the evolution of life on Earth, even a change in the amount of heat reaching a planet from its star or stars of 10 per cent could cause severe problems. A rough rule of thumb is that a 1 per cent change in the output of the Sun causes a 1 °C change in the average temperature at the surface of the Earth, and there is serious concern today that a global warming of 4–5 °C could cause the collapse of civilization.
~~Alone in The Universe: Why Our Planet is Unique -by- John Gribbin
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