The Sizes of the Stars

Chapter 8 The Sizes of the Stars The “Plough” in Ursa Major, photographed by Nik Szymanek Look up into the sky, and you will see the stars as tiny, twinkling points. The twinkling is due entirely to the Earth’s atmosphere; from space (or on the Moon) stars do not twinkle (scintillate) at all, and if you have the chance of seeing stars while you are travelling in a high-flying jet you will find that the twinkling is much less then it is at sea level. But with the naked eye, no star appears as anything but a dot. If you use a star as an obvious disk, you may be assured that there is something wrong. Almost certainly the telescope is out of focus. This being so, it takes an effort of the imagina- tion to appreciate that some of the stars are huge enough to contain the whole orbit of the Earth round the Sun – while admittedly others are so tiny that they could fit com- fortably into the ring road of a small city. For the last the programme of 2005 I was joined by Professors Richard Harrison and Lucy Green to say something about the Sun, the only star close enough to be examined in a great deal, and then by Drs John Mason and Barrie Jones, to discuss the sizes of the various types of stars. P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_8, 29 © Springer Science+Business Media, LLC 2010 30 8 The Sizes of the Stars If no stars show obvious disks, then how do we measure their diameters? There are various methods. Of course, there is no problem at all with the Sun, which is a normal Main Sequence star of Type G; it is 865,000 miles across. We may not have discovered all its secrets, but we do know a great deal about it, and it gives us a guide to other stars – though the stars are amazingly diverse. Some are much larger than the Sun, others much smaller; some are far more luminous, others remarkably feeble. Let us deal first with exceptionally luminous stars – which are not necessarily the largest. According to one set of measurements, the record holder is a remote celestial search light catalogued as LBV 1906-20, said to be the equal of 40 million Suns, which is about as powerful as a star could be without disrupting itself by the pressure of radiation. (LBV, by the way, stands for Luminous Blue Variable.) This does seem rather dubious. Then we have the Pistol Star in Sagittarius, so nicknamed by the shape of the nebula, which it illuminates. It is approximately 25,000 light years away, in the direction of the galactic centre, and it is certainly very powerful and massive. Were it is not so masked by interstellar dust, it would be an easy naked-eye object; in fact, it remained undetected until the Hubble Space Telescope imaged it in infra-red. Its mass seems to be about 150 times that of the Sun, and its diameter has been given as around 300 times that of the Sun, i.e. roughly 250,000,000 miles, so that it could contain the whole of the Earth’s orbit. However, the data for Eta Carinae, the erratic variable in the southern hemisphere of the sky, are more reliable. The luminosity is at least 5,000,000 times that of the Sun, and it is one of the most massive stars known. It is also wildly unstable; for a while around 1840 it shone as the most brilliant star in the sky apart from Sirius, though for well over a century now it has hovered on the brink of naked-eye visibility. In the foreseeable future – perhaps tomorrow, perhaps not for a million years – it will explode as a supernova, ending up as either a neutron star or a black hole. The largest of all stars are red supergiants. A star begins its career by condensing out of the material inside nebula; it shrinks, under the influence of gravitation, and the inside heats up. When the core temperature reaches about ten million degrees, nuclear reactions are triggered off. The main “fuel” is hydrogen, the most abundant element in the universe; the hydrogen atoms combine to form helium, and the star begins to shine. (Yes, I know this is horribly oversimplified, but it will suffice for the moment!) When the supply of the available hydrogen runs low, different reactions begin, and elements heavier than helium are built up. With a modest star such as the Sun, the process is halted before it can go too far. The star will briefly become a red giant (not a supergiant) and will puff off its outer layers and become a beautiful “planetary nebula”. When the outer layers are finally lost, what is left of the star collapses into what is known as the white dwarf stage. It will then go on shining feebly until all its light and heat have gone, leaving it as cold, dead globe – a black dwarf; – it is quite possible that the universe is not yet old enough for any black dwarfs to have formed. But with a much more massive star, equal to (say) over ten Suns, the story is different. The star evolves much more quickly, and the element-building process is not halted so soon. The star heats up until its core is at a temperature of millions of degrees, and the globe is blown out to produce a supergiant. The surface has 8 The Sizes of the Stars 31 cooled-hence the red colour – but the luminosity is immense, though without matching Eta Carinae. The best known red supergiant is Betelgeux in Orion (the star marks the great hunter’s shoulder, the name can be spelt in various ways and some people pronounce it “Beetle Juice”). Its apparent magnitude varies slowly between 0.2 and 1; sometimes it equals Rigel, the other brilliant star in Orion, while at others it is comparable with Aldebaran, in Taurus, which looks like the same colour but is a giant rather than a supergiant. Betelgeux is just over 500 light-years away, it must have a diameter of around 550 million miles, and shine 15,000 times more powerful as the Sun. In luminosity it cannot match Rigel, well over 40,000 Sun power, but Rigel is hot, bluish white star, and it is not nearly as large as a red supergiant. But Betelgeux, vast though it is, is by no means the record-holder. Even larger is Mu Cephei, in the far north of the sky not far from the W of Cassiopeia; over Britain if it never sets. It is variable between magnitudes 3.6 and 6, but as its seldom drops between the fifth magnitude it is almost always within naked-eye range. It is so red that William Herschel christened it the “Garnet Star”, and the nickname has stuck; through binoculars it looks rather like a glowing coal. It is further away than Betelgeux (perhaps 5,000 light-years) and much larger, more massive and more luminous, since it could equal 350,000 Suns. For a long time it was said to be the largest star known but we have now found that it is outmatched by four others – VV Cephei, V354 Cephei, KW Sagittari and KY Cygni – and possibly also a fifth, VY Canis Majoris, though the various measurements used here do not agree really well. Consider KY Cygni around 5,200 light years away in the constellation of the Swan. The diameter is thought to be around 1,000,000,000 miles. Imagine that you could stand upon the surface and go for a walk, how long would it take you to go right around, walking at a steady 3 mph and never stopping? The answer – 150,000 years. Yet, although KY is 300,000 times as luminous as the Sun, it has only 25 times the solar mass. Large stars are always less dense than smaller ones; it is almost like balancing a lead pellet against a meringue. Go and look for KY Cygni by all means; its position is RA 20h 26m 52s2, dec. +38° 21¢11″, but I warn you that it will not be easy. It lies in a rich area, but its apparent magnitude is a modest 13.3. Rather surprisingly, the star with the largest known apparent diameter is none of these supergiants, but R Doradus in the far southern constellation of the Swordfish. The distance is 200 light-years, the luminosity 6,500 times that of the Sun and the diameter 150 million miles. It is red, and a variable star of the pulsating type. From incredibly large stars to very small ones, we have noticed that a modest star like the Sun will become a white dwarf when its supply of hydrogen fuel is exhausted. We know a great many white dwarfs, the most famous is the faint companion of Sirius which was also the first to be identified. All the atoms are crushed and broken, and the component parts packed together with almost no wasted space; matter of this sort is termed “degenerate”, and a cupful of it would balance the weight of an ocean liner. Atoms in a normal state are mostly empty space. The best analogy I can give – not a good one, I know – is to picture a snooker table upon which the balls are set out ready for a game. They take up a good deal of room – but pack all the balls together, and 32 8 The Sizes of the Stars you can cram them into a suitcase.

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