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The New Zealand Railways Magazine, Volume 13, Issue 5 (August 1, 1938)

Partial Impact

Partial Impact.

Stated very briefly the theory is that a Nova is caused by the partial, or grazing, impact of two stars, drawn together by their mutual gravitation. Each having some original velocity, they do not meet directly, but whirl in hyperbolic orbits around their common centre of gravity. If they come so close as to graze one another, the parts that meet are struck off and coalesce to form a fiery whirling unstable mass with extraordinary characteristics. This “Third Body,” or “Cosmic Spark,” as Bickerton called it, is found to furnish the key to the enigma. The wounded stars pass on, and make little show in the spectacular display. The “Third Body” is a twisted spindle shaped mass, with the lightest elements at the centre and the heaviest at its ends. It is intensely heated, but in a most unusual way. Since all have had the same onward motion transformed into atomic agitation or heat, the different elements are at widely different temperatures. Initially the helium is four times and the lead 207 times as hot as the hydrogen. But the average temperature being hundreds or thousands of million degrees, the body, with its compartively small mass, is thermodynamically unstable. Its atoms have more than the critical velocity of escape. Its disappearance is not page 32
(Photo., G. W. Ritchey, Yerkes Observatory.) The Great Nebula in Andromeda as seen through the 24 inch Yerkes reflector, Sept. 18th, 1901. Exposure, 4 hours.

(Photo., G. W. Ritchey, Yerkes Observatory.)
The Great Nebula in Andromeda as seen through the 24 inch Yerkes reflector, Sept. 18th, 1901. Exposure, 4 hours.

due to cooling. The mass is dissipated into space because it is too hot to hold together. It depends on the fraction struck off whether the wounded stars, each with a long lake of fire on its surface, escape from one another, or whether they are wedded into a binary system by the attraction of their brilliant but short-lived son.

The elements in the turbulent “Third Body” try to adjust the distribution of energy so that each atom has an equal share. To do this the heavy atoms must give heat to the lighter ones. Thus hydrogen soon leads the outward rush, with helium following at about half the speed. The brilliant nucleus becomes surrounded by expanding luminous shells of gas, whose light, when analysed by the spectroscope, tells what they are made of, and how fast they are flying. The atoms coming directly towards us absorb some of their appropriate radiation, so each bright line becomes fringed by a dark border on the violet side.

The peculiarities of each particular Nova depend on the characteristics of the colliding stars and on the depth of the impact.