Death and the hunter

Written by      

At last the apparently endless clouds are drawing off the country and we are again being permitted to see the stars. Orion, lord of the summer nights, is already visible in the east as the sky darkens, and he will dominate our heavens until lost in the afterglow of autumn sunsets. Uniquely amongst the constellations, Orion shows—even in small telescopes—examples of stars both newly born and undergoing the stellar equivalent of terminal Cheyne-Stokes respiration.

Next to Crux, the Southern Cross, the group of stars forming Orion’s belt and sword are probably the pattern most commonly recognised by people living south of the equator. Being a northern constellation, Orion is seen upside down by us. It requires an act of will to recognise the sword hanging from his belt, for in our hemisphere these stars suggest the shape of a humble saucepan.

In the middle of the handle of the Pot there is visible to the naked eye a hazy patch which is no star but the Great Nebula, M42, the brightest of all the luminous nebulae as seen from Earth. Although it appears impressively large with even a small tel­e-scope, most of this nebula is invisible, being a huge cloud of rather cool gas and dust which radiates in the infrared—wavelengths too long for our eyes to detect. In the centre of this cloud, gravitational contraction has produced pressures and temperatures so great that new stars are forming. As their nuclear processes switch on, so they begin to shine, and it is this intense radiation that has begun to blow away the parent gas cloud and riven a hole in our side of the cloud. At the bottom of this hole even a quite modest telescope will show the four stars of the Trape­zium—youngsters perhaps as little as 100,000 years old. These are amongst the youngest stars visible in ordinary telescopes.

Marking the right armpit of Orion is the distinctive bright, orange-tinted star Betelgeuse, a Orionis, known to the Arabs as Ibt al Jauzah, the Armpit of the Central One. Orion is facing Taurus, and the stars forming his club must be in his right hand while his left arm carries the shield to ward off the Bull, and hinges on Bellatrix, y Orionis. Diirer’s chart of the southern hemisphere shows Orion with his back to the viewer, because the celestial sphere was drawn from the outside looking in. Although later cartogra­phers showed a geocentric view, they slavishly copied Diirer’s figure, thus inad­vertently making Orion left-handed.

In visible light (wave­lengths from 400run to 700nm) Betelgeuse is the tenth brightest star in the night sky. Typically, although not invariably, the brightest star in a constella­tion is labelled a, and then so on through the Greek alphabet. Although today Rigel, 13 Orionis, is obvi­ously the brighter of the two, this has not always been so, for Betelgeuse is unique amongst the first magnitude stars in being of variable brightness. As recently as 1852, Sir John Herschel, who first noticed this variability, estimated it to be then the brightest star in the northern hemisphere, leaving only Sirius, Canopus and a Centauri as rivals.

However, if one in­cludes infrared radiation in the estimation of brightness, then Betelgeuse moves up to the top of the list, as it is then the brightest star in the sky. This great outpouring of invisible infrared is because Betelgeuse is a supermassive star ap­proaching the end of its life; a red supergiant with a hugely bloated low temperature atmosphere. Were Betelgeuse centred on the Sun, then, even at its smallest, its pulsating envelope would reach out somewhat beyond the orbit of Mars, and Earth would be orbiting through a tenuous gas at 3700K. In response to events in the core, this envelope ex­pands every five or six years out to about the distance of Jupiter from the Sun. This expansion cools the gas, but the reduction of light at visible wavelengths is more than outweighed by the in­crease in the surface area of the envelope, and so we see it brightening.

Red supergiants have consumed most of the hydrogen in their cores, which are now substan­tially helium—the “ash” of hydrogen fusion. The remaining hydrogen is now burning in a shell around this spherical ashpit. But so massive is this core that as hydrogen fusion peters out, the core contracts under its own weight, with the result that the core temperature rises to 100 million degrees Kelvin and ignites the “triple alpha” process, in  which three helium nuclei fuse to form a carbon nucleus. Initially, this process is very rapid, leading to the “helium flash”, which results in the star brightening and heating up, and is detect­able as a change from red to blue light emission.

After this outburst, the triple alpha process maintains the star in a steady state for a while, but as the helium is consumed the reduction in energy output leads to another phase of contraction, with a consequent further heating of the core and the ignition of the carbon. This cycle of consumption, contraction and ignition of the “ash” continues at an ever-increasing pace as the nuclei of the heavier elements—oxygen, neon, magnesium, silicon and so on, through to iron—are produced. Each step in this chain releases less energy than the one before, and the fusion of iron to produce heavier nuclides actually absorbs energy.

As soon as the fusion of iron commences, there is no longer any radiation pressure being generated, and no longer any force to resist the gravitational pull on the material of the star, with the result that there is an immediate and cata­strophic collapse of the entire system of concentric shells and the extended outer atmosphere. Gas though a star may be, it nevertheless has mass, and the outer layers falling in towards the core of the star are the equivalent of several suns falling tens of millions of miles in a

strong gravitational field. The kinetic energy of this material is enormous, and as it arrives at the core it can go no further, and must be converted into pressure and heat.

Towards the centre, this added energy allows the energy-consuming fusion reactions which create the elements heavier than iron to proceed. Further out, the fusion of the lighter nuclides is accelerated, but motion towards the centre is reversed as the material bounces off the now ultra-dense core, is accelerated outward by the sudden increase in radiation pressure, and is blown off into the surrounding space. This effect is called a supernova, an explosion akin to that created by the blue supergiant Sandulek ­69 202 in 1987, now known as S(uper)N(ova) 1987A.

Being only 300 light years away (as opposed to the 170,000 light years of SN1987A), when Betelgeuse does go super­nova it will blaze with unparalleled magnificence, with an apparent magni­tude of about -10—the brightness of a three-quarter moon. This blazing point will dominate the sky and be easily visible even at noon.

But this will be a transitory glory, for Betelgeuse will be dead. In the space of a few months its brilliance will fade as the expanding shell of gas blown off by the explosion cools and dims, and the core of the original star will pay the ultimate price of its great size: it will vanish.

Massive stars (Betelgeuse is estimated to be 17 times the mass of the Sun) not only live fast, consuming their substance at a profligate rate, but at their end are fated to disappear at the bottom of a gravitational well. Once a star has exhausted its nuclear fuels there is nothing to stop its contraction under the force of gravity except the intra-atomic forces. For low mass stars, such as the Sun, this process leads to a white dwarf, a sphere of degener­ate matter in which the nuclei have been stripped of their electrons and the density is of the order of 107– 1011kg ni3 (water has a density of 103 kg m-3), the material being prevented from further contraction by the degeneracy pressure of the electrons. For star remnants over the Chandrasekhar Limit, about 1.5 solar masses, the force of gravity is greater than the degeneracy pressure, and the free electrons are forced to combine with protons, so that all the material is closely packed neutrons—hence the name “neutron stars”. Such stars have densities of the order of 10″ kg m-3, of the order of ten thousand million million times the density of water.

Such a massive, yet compressed, object—its diameter but a few kilome­tres—produces an intense gravitational field in its vicinity. Indeed, for massive neutron stars this field can be so great that even light itself cannot meet the required escape velocity, and the object is then a black hole: that into which anything may fall but nothing can escape.

When Betelgeuse eventually goes supernova it will blow off several solar masses of its outer layers, but, nevertheless, the remaining core will be so massive that it will become a black hole. As surrounding material, including some of its own gaseous shrapnel, is drawn in towards the core, it will form an accretion disc about the equator. The violent acceleration of this material as it approaches the event horizon, that boundary from within which nothing can escape, will cause intense radia­tion and form a bright screen around the core, which will itself be invisible.

Just how far Betelgeuse is along this last part of its evolutionary path we do not know. The evidence is that it is in the last few thousand years of its life, possibly within decades of its catastrophic end. In fact, the odds in favour of something happening during a human lifetime are better than those of being a solo winner of a Lotto jackpot. Watch that space!