IN STARRY SKIES

Evening Post, Volume CXV, Issue 71, 25 March 1933, Page 23

IN STARRY SKIES

CAN THE WOUNDED STARS

ESCAPE ?

(By "Omega Centauri.")

The spectacular part in the outburst of a Nova, as viewed from stellar distances, is always played by 'the exploding "Third Body." But an important question remains: "What becomes of .the wounded stars?" Have they energy to continue their travels through ihe immensity of space? Two' comparatively small stars like, our sun, while being drawn together by gavitation, set up a relative velocity of nearly 400 miles per second. If they pass by one another, this speed is exactly what is needed to carry them again to an infinite ,distance .apart. But two stars attracted towards one another cannot pass unless they have initially some relative velocity. Starting from rest, and uninfluenced by other bodies, they would of course meet directly centre towards' centre and coalesce into a single bod}-. This conception formed a part of the dismal doctrine of the dissipation of energy that held the field so long. Every col-lision-was pictured as resulting in an aggregation of mass. The marvellous beauty of the universe was believed to be evanescent, and astronomers, peering intd the remotest future, seemed to dimly see the glorious cosmic systems reduced to one vast . dead and absolutely cold mass of inert matter. One of the earliest triumphs of the impact theory was to point the way to escape from such an unworthy view of creation. Bickerton saw agencies at work counteracting the tendency, towards aggregation. New Worlds are continually being born as old ones die. As far as we can see, the universo as a whole is very young. It carries the impress of immortality. From this point of view the details of stellar encounters are of outstanding importance. All stars appear to be in motion. .Of course, we have no point of rest to measure from. Our sun is moving like the others.' Campbell and Moore at the Lick Observatory in 1925, from a study of.the radial motions of 2034 stars, estimated the sun's speed to be 19 kilometres, or about 11.8 miles, per second. It appears to be moving almost directly towards the star Mv Herculis. This speed appears to be slightly greater than the a-verage; the

velocities differ strangely with stars of different types. A star's speed appears to increase with. age. It looks ■as if a stellar encounter, which revivifies the pair, sometimes blending them into one, destroys' in doing so much of their velocity, which then is gradually built up again by gravitation in the course of their long lives. But whatever the cause may be, stars of type B appear to have radial, speeds of barely four miles per second, whilst those of type M have ones averaging more than 10^ miles per second. Individual motions are far more startling; about a dozen stars are known whose speeds range from ■ 75. to over 400 miles per second. Stars which, ultimately collide probably have velocities directed nearly towards one another, whilst still at great distances apart. This motion, of course, will hardly ever be absolutely centra to centre... In. nearly, every case, the collision is brought about by gravitation curving their paths, and the .resulting encounters are partial, that is, of a grazing character. In. such a case, as we have already seen, the parts which actually collide are fused into the third body. The wounded stars pass on. The work done in tearing ofi the impacting parts accounts for only a very minute fraction of the energy available"; In the case- of small stars

like our sun this is about 45 millions of calories per .-■ gramme, which is enough to raise oxygen to a temperature o£ over 200 million and lead to. 1500 million degrees centigrade. When more massive stars collide the sudden transformation of energy ib often very much greater. But though, for the moment, the wounded stars pass on, they niay not ultimately escape. The gravitation of the third body is generally the determining factor. As the two /stars separate the third body is whirling and exploding in between-them. The wounded stars get a slight start. The velocity of each is directly from the other. In the third body the''motion of- translation is transformed into atomic agitation or heat. The atoms are moving in- all directions and incessantly colliding one; with another. ' All having at first tho sama. average atomic speed, different elements will lie at widely different temperatures. Oxygen for instance will be 16 times, and lead. 207 times, as hot as hydrogen. Every atomic collision will do something tow.ards equalising these temperatures. A hydrogen atom striking one of lead, will bound from it with greatly accelerated speed.. For a time the. pressure will be so intense that the outer layers of the third body will be pushed outwards-with increasing velocity. But gradually-atfinv sorting will take place. The heavy elements, i losing atomic speed, will hang back, whilst hydrbgeh and helium will form a vast exploding atmosphere: The hydrogen will sometimes attain a velocity of thousands of mijea per seoond. It therefore catches up and passes beyond the wounded stars. Until this takes place, the whole mass of the third body, exerts its full gravitational effect. But while the : wounded . stars are flying apart we may consider it as practically standing still midway between tliem. Being only half as far from each . its gravitational pull on either is four times as effective. Let us take some numerical illustrations. In order to be able to take simple fractions, let us call the mass of each star- 60. Then when they are at unit distance apart we may' call' the force on unit mass of either 60. Now suppose one-twelfth has been struck from each. Then the wounded stars have each mass 55 and the third body has mass 10. Being only half the distance its pull on each unit of mass is 40, making the total 95. If a tenth is struck from each' the third body has a niass 12 and each wounded star 54, which makes the force 102.' If a sixth is struck from each the masses are 56, 20, and 50, and the force 130.- If a fifth is struck off the force .becomes 144,

and if the fraction is increased to a fourth a third or a half the effective force will become 165, 200, or 270 instead of the original-60. We may put this generally and say 'that if X per cent, is struck from' each star the restraining force will be 100 plus,7X times as great as if the stars had just escaped collision: This shows that the two stars will be unable to escape un)ess the graze is a slight one, or unless they had .a'high, initial rela-, tivo velocity. But it is only just at first that the full retarding force is in action. As the third body expands it envelops the retreating stars and" the portion that has' passed beyond them ceases to act as a restraint, whilst all that remains between the two continues to produce its full effect.' If the portioM struck off amount to as much'as a sixth- of each it will b© impossible for the ■stars, to escape unless they had an unusually high= initial relative velocity. Very often then a partial impact must wed two. stars into an orbitally connected binary. In such a. case each member of the pair will be variable, until its' blazing sear' has cooled, or has raised the rest of the body to its temperature. For a time the dissipated substance of the:.third body will form-a nebulous.atmosphere.around the variable pair.. : ■-'.•