Cosmic Rays

Cosmic Rays

Imagine carrying a Geiger counter and a piece of uranium ore to some place deep beneath the Earth – a gold mine, say, or a Java tube, a cave carved through the Earth by a river of molten rock. The sensitive counter clicks when exposed to gamma rays or to such high-energy charged particles as protons and helium nuclei. If we bring it close to the uranium ore, which is emitting helium nuclei in a spontaneous nuclear decay, the count rate, the number of clicks per minute, increases dramatically.

If we drop the uranium ore into a heavy lead canister, the count rate declines substantially; the lead has absorbed the uranium radiation. But some clicks can still be heard. Of the remaining counts, a fraction come from natural radioactivity in the walls of the cave. But there are more clicks than can be accounted for by radioactivity. Some of them are caused by high-energy particles penetrating the roof. We are listening to cosmic rays, produced in another age in the depths of space. Cosmic rays, mainly electrons and protons, have bombarded the Earth for the entire history of life on our planet. A star destroys itself thousands of light-years away and produces cosmic rays that spiral through the Milky Way Galaxy for millions of years until, quite by accident, some of them strike the Earth, and our hereditary material. Perhaps some key steps in the development of the genetic code, or the Cambrian explosion, or bipedal stature among our ancestors were initiated by cosmic rays.

Cosmic Rays. Supernovae are routinely observed in other galaxies. Image : © Elena

On July 4, in the year 1054, Chinese astronomers recorded what they called a “guest star” in the constellation of Taurus, the Bull. A star never before seen became brighter than any star in the sky. Halfway around the world, in the American Southwest, there was then a high culture, rich in astronomical tradition, that also witnessed this brilliant new star (Moslem observers noted it as well. But there is not a word about it in all the chronicles of Europe.

The remarkable star, 5,000 light-years distant, is now called the Crab Supernova, because an astronomer, centuries later was unaccountably reminded of a crab when looking at the explosion remnant through his telescope. The Crab Nebula is the remains of a massive star that blew itself up. The explosion was seen on Earth with the naked eye for three months. Easily visibly in broad daylight, you could read by it at night. On the average, a supernova occurs in a given galaxy about once every century. During the lifetime of a typical galaxy, about ten billion years, a hundred million stars will have exploded – a great many, but still only about one star in a thousand.

In the Milky Way, after the event of 1054, there was a supernova observed in 1572, and described by Tycho Brahe, and another, just after, in 1604, described by Johannes Kepler.

On the New Stars

On the New Stars

Unhappily, no supernova explosions have been observed in our Galaxy since the invention of the telescope, and astronomers have been chafing at the bit for some centuries.

However, supernovae are now routinely observed in other galaxies. Among candidates for the sentence that would most thoroughly astonish an astronomer of the early 1900’ is the following, from a paper by David Helfand and Knox Long in the December 6, 1979, issue of the British journal Nature: “On 5 March, 1979, an extremely intense burst of hard X-rays and gamma rays was recorded by the nine interplanetary spacecraft of the burst sensor network, and localized by time-of-flight determinations to a position coincident with the supernova remnant N49 in the Large Magellanic Cloud” (the Large Magellanic Cloud, so called because the first inhabitant of the Northern Hemisphere to notice it was Magellan, is a small satellite galaxy of the Milky Way, 180,000 light-years distant. There is also, as you might expect, a Small Magellanic Cloud).

Supernovae may be exhausts of alien star-ships on its long voyage home. Image: © Elena

However, in the same issue of Nature, E. P. Mazets and colleagues of the Ioffe Institute, Leningrad – who observed this source with the gammary burst detector aboard the Venera 11 and 12 spacecraft on their way to land on Venus – argue that what is being seen is a flaring pulsar only a few hundred light-years away. Despite the close agreement in position Helfand and Long do not insist that the gamma-ray outburst is associated with the supernova remnant. They charitably consider many alternatives, including the surprising possibility that the source lies within the solar system. Perhaps it is the exhaust of an alien star-ship on its long voyage home. But a rousing of the stellar fires in N49 is a simpler hypothesis: we are sure there are such things as supernovae.

Kepler published in 1606 a book called De stele Nova, “On the New Star”, in which he wonders if a supernova is the result of some random concatenation of atoms in the heavens. He presents what he says is “… not my own opinion, but my wife’s: Yesterday, when weary with writing, I was called to supper, and a salad I had asked for was set before me. “It seems then, “I said, “if pewter dishes, leaves of lettuce, grains of salt, drops of water, vinegar, oil and slices of eggs had been flying about in the air for all eternity, it might at last happen by chance that there would come a salad”. “Yes”, responded my lovely, “but not so nice as this one of mine”.

The Fate of the Solar System

The Fate of the Solar System

The fate of the inner solar system as the Sun becomes a red giant is grim enough. But at least the planets will never be melted and frizzled by an erupting supernova. This is a fate reserved for planets near stars more massive than the Sun. Since these stars with higher temperatures and pressure run rapidly through their store of nuclear fuel, their lifetimes are much shorter than the Sun’s. A star tens of times more massive than the Sun can stably convert hydrogen to helium for only a few millions years before moving briefly on to more exotic nuclear reactions. Thus there is almost certainly not enough time for the evolution of advanced forms of life on any accompanying planets; and it will be rare that beings elsewhere can ever know that their star will become a supernova: if they live long enough to understand supernovae, their star is unlikely to become one.

The essential preliminary to a supernova explosion is the generation by silicon fusion of a massive iron core. Under enormous pressure, the free electrons in the stellar interior are forceably melted with protons of the iron nuclei, the equal and opposite electrical charges canceling each other out; the inside of the star is turned into a single giant atomic nucleus, occupying a much smaller volume than the precursor electrons and iron nuclei.

The supernova explosion ejects into space most of the matter of the precursor star. Image: © Elena

The core implodes violently, the exterior rebounds and a supernova explosion results. A supernova can be brighter than the combined radiance of all the other stars in the galaxy within which it is embedded. All those recently hatched massive blue-white supergiant stars in Orion are destined in the next fem million years to become supernovae, a continuing cosmic fireworks in the constellation of the hunter.

The awesome supernovae explosion ejects into space most of the matter of the precursor star – a little residual hydrogen and helium and significant amounts of other atoms, carbon and silicon, iron and uranium. Remaining is a core of hot neutrons, bound together by nuclear forces, a single, massive atomic nucleus with an atomic weight about 10 (56), a sun thirty kilometers across; a tiny, shrunken, dense, withered stellar fragment, a rapidly rotating neutron star. As the core of a massive red giant star collapses to form such a neutron star, it spins faster. The neutron star at the center of the Crab Nebula is an immense atomic nucleus, about the size of Manhattan, spinning thirty times a second. Its powerful magnetic field, amplified during the collapse, traps charges particles rather as the much tinier magnetic field emit beamed radiation not only at radio frequencies but in visible light as well.

Our Vision of Mars

Our Vision of Mars

Percival Lowell’s notebooks are full of what he thought he saw: bright and dark areas, a hint of polar caps, and canals, a planet festooned with canals. Lowell believed he was seeing a globe-girdling network of great irrigation ditches, carrying water from the melting polar caps to the thirsty inhabitants of the equatorial cities. He believed the planet to be inhabited by an older and wiser race, perhaps very different from us. He believed that the seasonal changes in the dark areas were due to the growth and decay of vegetation. He believed that Mars was, very closely, Earth-like. All in all, he believed too much.

Lowell conjured up a Mars that was ancient, arid, withered, a desert world. Still, it was an Earth-like desert. Lowell’s Mars had many features in common with the American Southwest, where the Lowell Observatory was located. He imagined the Martian temperatures a little on the chilly side but still as comfortable as the South of England. The air was thin, but the elegant network of canals carried the life-giving fluid all over the planet.

Our vision of Mars has a mythic quality as old as Genesis. Image: © Elena

What was in retrospect the most serious contemporary challenge to Lowell’s ideas came from an unlikely source. In 1907, Alfred Russel Wallace, co-discoverer of evolution by natural selection, was asked to review one of Lowell’s books. He had been an engineer in his youth and, while somewhat credulous on such issues as extrasensory perception, was admirable sceptical on the habitability of Mars.

Wallace showed that Lowell had erred in calculation of the average temperatures on Mars; instead of being as temperate as the South of England, they were, with few exceptions, everywhere below the freezing point of water. There should be permafrost, a perpetually frozen subsurface. The air was much thinner than Lowell had calculated. Craters should be as abundant as on the Moon. And as for the water in the canals:

Any attempt to make that scanty surplus of water, by means of overflowing canals, travel across the equator into the opposite hemisphere, through such terrible desert regions and exposed to such a cloudless sky as Mr. Lowell describes, would be the work of a body of madmen rather than of intelligent beings. It may be safely asserted that not one drop of water would escape evaporation or insoak at even a hundred miles from its source.

The devastating and largely correct physical analysis was written in Wallac’s eight-fourth year. His conclusion was that life on Mars – by this he meant civil engineers with an interest in hydraulics – was impossible. He offered no opinion on microorganisms.

The Great Mystery of the Martian Canals

When Paul Fox of Cornell and Carl Sagan compared Lowell’s maps of Mars with the Mariner 9 orbital imagery – sometimes with a resolution a thousand times superior to that of Lowell’s Earthbound twenty-four-inch refracting telescope – they found virtually no correlation at all. It was not that Lowell’s eye had stung up disconnected fine detail on the Martian surface into illusionary straight lines. There was no dark mottling of crater chains in the position of most of his canals. There were no features there at all. Then how could he have drawn the same canals years after years? How could other astronomers – some of who said they had not examined Lowell’s maps closely until after their own observations – had drawn the same canals?

One of the great findings of Mariner 9 mission to Mars was that there are time-variable streaks and splotches on the Martian surface – many connected with the ramparts of impact craters – which change with the seasons. They are due to windblown dust, the patterns varying with the seasonal winds. But the streaks do not have the character of the canals, they are not in the position of the canals, and none of them is large enough individually to be seen from the Earth in the first place. It is unlikely that there were real features on Mars even slightly resembling Lowell’s canals in the first few decades of this century that have disappeared without a trace as soon as close-up spacecraft investigations became possible.

Which side of the telescope the intelligence is on? Image: Colors © Megan Jorgensen (Elena)

The canals on Mars seem to be the same malfunction, under difficult seeing conditions, of the human hand/eye/brain combination (or at least for some humans; many other astronomers, observing with equally god instruments in Lowell’s time and after, claimed there were no canals whatever). But this is hardly a comprehensive explanation, and I have the nagging suspicion that some essential feature of the Martian canal problem still remains undiscovered.

Lowell always said that the regularity of the canals was an unmistakable sign that they were of intelligent origin. This is certainly true. The only unresolved question was which side of the telescope the intelligence was on.

Lowell’s Martians were benign and hopeful even a little godlike, very different from the malevolent menace posed by Wells and Welles in the War of the Worlds. Both sets of ideas passed into the public imagination through Sunday supplements and science fiction.

Galaxy is Cultivated

Galaxy is Cultivated

Even if think nobody has anything to learn from you because you’re technologically so backward, there are other merits to a civilization: music, lovingkindness, dreams… Humans are very good at dreaming. There may be cultures all over the Galaxy that would trade dreams.

We don’t care about rapacious, bloodthirsty civilizations which develop interstellar flight, because this never happens. In the long run, the aggressive civilizations destroy themselves, always. It’s their nature. They can’t help it. Just make sure that no one bothers them and let them work out their destiny.

Galaxy is cultivated. Who is all-powerfull should fear everything (Pierre Corneille, Cinna, 1640, Act. IV, Scene II). Image: © Elena

There certainly are cooperative projects between galaxies. We mustn’t think of the universe as a wilderness. It hasn’t been that for billions of years. We should think of it as cultivated.

The problem is that the universe is expanding, and there’s not enough matter in it to stop the expansion. After a while, no new galaxies, no new stars, no new planets, no newly arisen lifeforms – just the same old crowd. Everything’s getting run-down. It’ll be boring.

As flies to wanton boys are we to the gods – they kill us for their sport (William Shakespeare, King Lear, IV, I, 36).