Twinkle, twinkle, little stars

Boyle: I ofen looked up at the sky an' assed meself the question - what is the stars, what is the stars?

Joxer: Ah, that's the question, that's the question.

Sean O'Casey, Juno and the Paycock (1924) The sun seen in a foggy sky (or through smoky glass) looks circular, no matter what day of the year we see it. Since we on earth move around the sun during the course of the year, this means it looks circular no matter from what orientation we view it. Because of this we know it is spherical. Moreover, the earth orbits the sun because of the effect of the strong gravitational pull of the sun's mass; it is this symmetric pull, radial to the sun's centre, which keeps the sun very nearly exactly spherical.

The sun shines. We see light coming from it and we feel its heat; it is hot. Its radiation sustains life on earth and has done so for three billion years, neither boiling, nor letting freeze the oceans nor the water solvents in which biochemistry works so well on your and my behalf. From this we can deduce that the sun has not changed substantially in all this time.

However, in the sky at night we see a variety of stars. They differ in temperature. Some stars are merely red hot-"Aldebaran like a ruby aflame'' (William Roscoe, 1823-59); others are white hot-"azure Lyra, like a woman's eye,/Burning with soft blue lustre," (Nathaniel Parker Willis, 1807-67). Stars differ too in brightness, and astronomers have invented an imaginative range of names to classify them-supergiants, giants, dwarfs.

The study of stellar evolution seeks to explain stars as spherical, self-gravitating, equilibrium structures, slowly changing from one sort of star to another; for instance, the sun is a yellow dwarf and will change to a red giant and then to a white dwarf.

The two books under review illustrate the success of this physics in action, by explaining what the sun and stars are and how they change. Both books are meant for the use of undergraduates, starting by outlining observations of stars and interpreting the data.

O star . . .

Say something to us we can learn By heart and when alone repeat.

Say something! And it says "I burn."

But say with what degree of heat.

Talk Fahrenheit, talk Centigrade.

Use language we can comprehend.

(Robert Frost, Fireflies in the Garden , 1945.) The reason that stars support themselves against their own force of gravity is that they are hot; it is demonstrable that a typical star is tens of millions of degrees inside, and at these temperatures a star ignites nuclear fusion, at first fusing hydrogen to helium, then, its core shrinking stage by stage, fusing helium to carbon, carbon to oxygen, and continuing to iron: A star, shrinking under gravity's long pull, seethes under pressure to a new ignition point, flings its temper to the frontiers of space, pulses, bellows but can never swell again.

(John Latham, The Valedictory Lecture, 1990.) In this way the chemical elements are built up from the hydrogen formed in the Big Bang.

And though spectral analysis reveals stars are largely H, Interstellar gas similarly composed, nevertheless, let us also refrain from worshipping Hydrogen Or from writing a pastiche of the Old Testament: Hydrogen begat Helium Helium begat Carbon, Carbon begat Oxygen, Oxygen begat Sodium and Calcium, Sodium begat Iron.

(Ronald Duncan, Man: the Cantos, 1970.) Eventually all stars run out of nuclear fuel. Some become cooling white dwarfs, some slow to a standstill from rapidly spinning neutron stars, some collapse immediately to black holes: all become constituents of dark matter.

The sky is strewn with horrible dead suns, Dense sediments of mangled atoms: Only desperate heaviness emanates from them, Not energy, not messages, not particles, not light.

Light itself falls back down, broken by its own weight.

(Primo Levi, The black stars.) R. J. Tayler's book ranges slightly more widely than A. C. Phillips's, taking into account the variations in the life of a star that loss of material can bring. For instance, if a star is a member of a close binary system it can lose or gain mass to or from the other star. This can alter its future life.

The unravelling of stellar evolution is a success story of physics. And by the time a subject has reached the status of an undergraduate text, it is pretty well tied up. But the sun and the stars are natural phenomena, not clean laboratory experiments. It is good to know that there are still problems to solve, and in the case of stellar structure the big issue is the solar neutrino problem.

The nuclear reactions in the sun generate neutrinos.

Neutrinos, they are very small.

They have no charge and have no mass And do not interact at all.

The Earth is just a silly ball To them, through which they simply pass.

(John Updike, Cosmic Gall c1965).

Although the earth is practically transparent to neutrinos, some particularly sensitive apparatus can catch enough of them to see how many arrive here from the sun. The problem is that astronomers see too few neutrinos-about a third as many as theory predicts. What part of the theory is wrong? Are the conditions in the sun different from what astronomers predict through the application of the theories in these two books? Both Phillips and Tayler suggest not: it may be that the properties of neutrinos are not well enough known.

For writers and poets as well as for the aspiring scientists to whom these books are directed, the life of stars is an inspiration. It will be another triumph of the human imagination if a major property of fundamental particles, neutrinos, turns out to be deduced from the failure of such an unworldly construct as the physics of stellar structure. To quote Alexander Hutchinson (Switching channels, 1990):

Being plausible about what might give mass to this miasma the man from Jodrell Bank jumps to pluck one word ("neutrino") from his hoard of words and perkily confesses physics as we know it now is dodo-like to guarantee what's what.

So let me tell you, so you know: it is a mantis taking prey, it is the disposition to believe, it is a bumbast circumstance, it is a beauty and a bel esprit.

Paul Murdin is head of astronomy, Particle Physics and Astronomy Research Council.