IN STARRY SKIES

Evening Post, Issue 47, 23 August 1935, Page 16

IN STARRY SKIES

AN OBLIQUE ENCOUNTER:

TWO WOUNDED STARS AND A COSMIC SPARK

(By lOmega Centauri.")

Most stellar collisions are brought about by gravitation; and all such are oblique. IE stars moved in "straight Jines. encounters would be very rare phenomena.. Every star, however, is 'pulling every other, and these mutual attractions take control of stellar' destinies. A star's velocity is the only factor that can enable it to escape. But (J actual velocity, however great, is powerless in this respect if another star happens to be moving with equal speed in a parallel, direction. It is relative velocity alone that counts. Now, .■whilst all stars are in motion, and whilst many traverse a dozen miles or more per second, the movementss are very far from being indiscriminate. Many. group's, of stars have been" discovered whose motions'are nearly equal and are along approximately parallel lines. The Taurus cluster, for ex- . ample, contains 39 stars from 5 to 100 times' as luminous as our sun, which appear to.be,moving at about £8 miles .a second towards a point in the sky a little to the east of Alpha Orionis. Another group of a dozen stars in Ursa Major consists of stars from 7 to 400 times as brilliant.'as, the sun, moving 18 miles per" second towards a point near Altair. The Pleiades and Praesepe are similar groups, and others are f ound in Perseus, Orion, and Scorpio. Kapteyn discovered, moreover, that the ~motions of the stars are largely in two opposite directions, as if the galaxy

were composed of two inter-penetra-ting streams of stars. Concentrating attention' on. any single pair of stars, we find that, 'if their relative velocityis high, gravitation makes.little alteration in their paths. If on the other hand this relative velocity is low, gravitation .controls their orbits and increases enormously the chance of coir lision. r For example suppose a body is .at. the same distance as Neptune from the sun. If it is moving at 5 miles per second relatively to the sun, it will be able to escape altogether from its attraction. There.will be no collision unless the v motion is one of almost direct approach.' ' The critical velocity at that distance is i little more than>42 miles per second, /which implies an energy of nearly 7000 calories per gram. If the-body has only one-tenth of this energy, it will strike the sun if the initial direction, of ' its motion is within S degrees 3 minutes of the sun's centre.- This implies that the chance of collision, is increased 323,600 times by the sun's, gravitation. Even if the -relative motion were at right angles to the direction c the sun, the body could not escape collision unless the initial speed-was appreciably greater than 3 \ miles' per minute.

Now when a distant, or a near approach is turned into an actual collision., by .gravitation the impact must be oblique. The result depends greatly on the closeness with which the centres try to pass. If two stars similar to our;sun, but a shade more,massive; •are" drawn -together, jgravitation, will add to.any energy they may have had before, over twelve million calories per gram. They will meet with a reJtative speed of 400 miles per second. If the graze is a comparatively shallow one two things seem certain. Tha portions that come 'directly' in one another's'way will be arrested. The remainder, that is the main part of each star, will for the time -at least, pass on. The energy required to cut the stars is a minute fraction of that available. ' The gravitational restraint, exercised by the portion stopped, is a very much more' serious matter. The arrested parts coalesce into, a third body .with most extraordinary characteristics. Bickerton felicitously termed it a cosmic spark. The portion struck from either star is-of peculiar shape. The middle line of its back is the-arc of a circle, that of .its breast as nearly a parabola. The cross sec.tion at right angles to its length is composed of segments of a circle on opposite sides of the same chord. Two masses of this peculiar shape are forced together lengthwise with the pressure developed by the destruction of a relative velocity of if our hundred miles a second. In less than]

an: hour the other parts have torn themselves away and start flying from the scene of the encounter with initial j speeds yery slightly • less than two hundred miles per second. During the collision each has had a hew rotation impressed on. it, and combined with any that it had before. ] Each has a long scar on one side. From this gigantic scar the lighter and cooler matter has been removed. In its endeavour to regain sphericity, gravitation forces up heavier and hotter material from below. One side 61 each star thus becomes 'consider .bly brighter than the other. Therefore seen from a distance they should appear as a pair of variables. At first, however, their light is completely overwhelmed and hidden by the blaze of the cosmic spark. The energy liberated by a complete collision is only sufficient, as we have, seen, to double the brighti ness of the pair.. The increase due to the fiery scars on' the wounded • stars must be much less than this, if both [ Stars are originally gaseous. The variations due to the rotation of the retreating stars 'is unlikely to be more I than'a: fraction of a single magnitude. A very interesting question arises as to whether the' wounded stars can get away from one another or whether .they will be welded into a binary. But before considering this, we should focus attention on the third body, or cosmic spark. Half of it has been torn from each star.

In the diagram the two stars are represented by A and B. The motion of A alone is shown, A 2 and B2 being simultaneous 'positions. As A /moves along its curved path towards P and B along its path towards Q the shaded portions X and L are violently pressed into sone another about .where L >' is shown. The unshaded portions pass on. But each, experiencing the extra* .gravitational pull of X and L combined, is pulled more or less out of its course, towards. S or, T' respectively. The lightest portion of X meets

the densest part of L, and the lightest of L meets the. densest of K. The momentum is thus unbalanced and the third "body rotates. .A small fraction of the energy due to the speed of 200 miles per second is used in this rota--tion, but the greater part of it is transformed within the hour into the atomic motion of heat. Now all kinds of elements were travelling together in each star, so initially, after the collision, all kinds of'atoms have the same average speed. This means that they are at .widely different temperatures, the iron being nearly 56, and the gold 196' times as hot as the hydrogen. Now" the elements are communists.. In a mixture of gases they always endeavour .to share the available energy equally. In every- collision between different kinds of atoms in the cosmic spark the lighter one gains and the heavier one loses energy. They try to arrange that, on an average, the hydrogen atom moves twice as fast as one of helium, four times as fast as one of oxygen, and fourteen ..times as fast as one of gold. But this ideal state cannot be reached in an hour or' a day. The. pressure is tremendous arid all the outer parts of the third body are pressed outwards together, with accelerated speed. ' Much of the lighter material was imprisoned in the centre of the whirling mass. It has difficulty, in finding its way out, and this diffusion takes time. But gradually the light gases tend to take the lead with speeds that often exceed 1000 miles per second. There are moreover certain regions of weakness. Two of these are on the sides of the swirj niidway between the points where the heaviest material is located. The others are at the poles of the rotating mass. , The two former regions are likely to be turned towards us~ alternately. The hydrogen is coming from them with constantly increasing speed, so when such, a region points towards us, two or more jets, approaching" .towards1 us at different speeds may be between • the nucleus and the ' observer. .We should therefore expect the dark lines which border the bright bands in the spectrum.to be doubled or trebled at a certain stage. The wounded stars get the best of the start. They leave the scene of the encounter with a speed of nearly 200 miles per second, whilst the third body is still being compressed. : But they. lose speed- more quickly even than they'gained it, because of the extra attraction of the third body midway between them. The light gases leaving the latter, on the other hand, have a speed far greater than the y critical velocity, a speed which gravitation is powerless to materially reduce. ;The flying gases therefore overtake the retreating stars.