To help us imagine cosmic distances, physicist Robert Jastrow suggests the following analogy. Imagine the sun scaled down to the size of an orange. Then the earth would be a mere grain of sand circling in orbit around the sun at a distance of 30 feet [9 m]. Jupiter would be like a cherry pit revolving around the orange a city block away, and Pluto would be still another sand grain at a distance of ten city blocks from our imaginary orange, the sun. On that same scale, the sun’s nearest neighbor, the star Alpha Centauri, would be 1,300 miles [2,100 km] away, and the entire Milky Way a loose cluster of oranges separated from one another by about 2,000 miles [3,200 km], with an overall diameter of 20 million miles [30 million km]. Even when everything is scaled down, the figures soon get out of hand.

It is not just the distances that are astounding. As scientists have unveiled the secrets of the universe, peculiar phenomena have come to light. There are neutron stars consisting of matter so dense that a mere teaspoonful weighs as much as 200 million elephants. There are tiny stars called pulsars, one of which winks on and off some 600 times a second.

The Universe​—Some Secrets Unlocked

ON THE 4th of July, in the year 1054, Yang Wei Te gazed up at the early morning sky. As official astronomer of China’s Imperial Court, he was meticulously observing the movement of the stars when suddenly a bright light near the constellation of Orion attracted his attention.

A “guest star”​—the name the ancient Chinese gave to such a rare occurrence—​had made its appearance. After dutifully reporting to his emperor, Yang noted that the “guest star” had become so bright that it even outshone Venus and could be seen in broad daylight for several weeks.

Nine hundred years were to pass before this spectacle could be adequately explained. It is now believed that the Chinese astronomer was witnessing a supernova, the cataclysmic death throes of a massive star. The whys and wherefores of such an extraordinary phenomenon are just some of the secrets that astronomy is trying to unlock. The following is one explanation that astronomers have painstakingly pieced together.

Although stars like our sun may have immensely long and stable lives, their formation and demise give rise to the most spectacular sights in the skies. Scientists believe that the life story of a star begins inside a nebula.

Nebula. This is the name given to an interstellar cloud of gas and dust. Nebulas are among the most beautiful objects in the night sky. The one seen on the cover of this magazine is called the Trifid Nebula (or nebula with three clefts). Inside this nebula new stars have been born, which cause the nebula to give off a reddish glow.

Apparently, stars form inside a nebula when the diffuse matter condenses under the force of gravity into contracting regions of gas. These huge balls of gas stabilize when they reach the temperature at which nuclear reactions begin in the core of the cloud, preventing further contraction. Thus a star is born, often in conjunction with others, with which it makes up a star cluster.

Star clusters. In the photograph on page 8, we see a small cluster called the Jewel Box, thought to have been formed just a few million years ago. Its name was coined from the graphic description by 19th-​century astronomer John Herschel: “a casket of variously coloured precious stones.” Our galaxy alone is known to have over a thousand similar clusters.

The star’s energy. A nascent, or developing, star stabilizes as a nuclear furnace is fired in its interior. It starts converting hydrogen to helium by a fusion process somewhat like that which occurs in a hydrogen bomb. Such is the mass of a typical star, like the sun, that it can burn its nuclear fuel for billions of years without exhausting the supply.

But what happens when such a star eventually uses up its hydrogen fuel? The core contracts, and the temperature rises as the star exhausts the hydrogen in the central regions. Meanwhile, the outer layers expand enormously, increasing the star’s radius 50 or more times, and it becomes a red giant.

Red giants. A red giant is a star with a surface temperature that is relatively cool; its color therefore appears red, rather than white or yellow. This phase in a star’s life is relatively short, and it ends​—when most of the helium supply runs out—​with a celestial fireworks display. The star, still burning helium, ejects its outer layers, which form a planetary nebula, glowing because of energy received from its mother star. Eventually, the star contracts dramatically to become a faintly shining white dwarf.

If the original star is massive enough, however, the final outcome is that the star itself explodes. That is a supernova.

Supernovas. A supernova is the explosion that ends the life of a star that was originally much more massive than the sun. Huge amounts of dust and gas are spewed into space by violent shock waves at speeds of over 6,000 miles a second [10,000 km/​sec]. The intense light of the explosion is so bright that it outshines a billion suns, appearing as a sparkling diamond in the sky. The energy liberated in a single supernova explosion corresponds to the total energy the sun would radiate in nine billion years.

Nine hundred years after Yang observed his supernova, astronomers can still see the scattered debris of that explosion, a structure called the Crab Nebula. But something more than the nebula was left behind. At its center they discovered something else​—a tiny object, rotating 33 times a second, called a pulsar.

Pulsars and neutron stars. A pulsar is understood to be a superdense, spinning core of matter left over after a supernova explosion of a star no more than three times as massive as the sun. Having diameters of less than 20 miles [30 km], they are rarely detected by optical telescopes. But they can be identified by radio telescopes, which detect the radio signals that are produced by their rapid rotation. A beam of radio waves rotates with the star, like the beam of a lighthouse, appearing as a pulse to an observer, giving rise to the name pulsar. Pulsars are also called neutron stars because they are principally composed of tightly packed neutrons. This accounts for their incredible density​—over a billion tons per cubic inch [over a hundred million tons per cubic centimeter].

But what would happen if a really massive star went supernova? According to astronomers’ calculations, the core could continue its collapse beyond the neutron-​star stage. Theoretically, the force of gravity compressing the core would be so great that a so-​called black hole would result.