Chapter 4-2

The Scale of the Universe

three different images of a nebula
Three ways to see a nebula.
Observed in visible light (bottom), X-rays (top, left) and radio waves (top right), the eta Carinae region reveals wholly different aspects. Each is studied so that the physical processes at work in the nebula can be understood. The visible-light photo shows glowing, electrically excited gas, heated by radiation from adjacent hot stars, themselves not clearly distinguished. Dark lanes are clouds of interstellar dust that hide the bright nebula beyond. X-ray image from the HEAO-2 satellite reveals exact locations of the stars; X-rays come from their thin, hot outer atmospheres or coronae, while visible light comes from deeper layers. Faint blue glow around the stars in the X-ray picture (color supplied by a computer) reveals the presence of million-degree gas in a localized region of the nebula. Contours mapped by a radio telescope show that radio emission regions are highly structured (courtesy of D.S.Retallack, Kapteyn Institute). Infrared measurements (not shown) revealed that at some wavelengths, eta Carinae is the brightest object beyond the solar system, as seen from Earth.
To discuss the universe, we must abandon our ordinary frames of reference and think along almost unimaginable scales of distance and energy, and we must consider unusual, even fantastic, states of matter. True, our own horizons have expanded through space exploration. Humans have gone higher and faster than ever before. But our experience is still drastically limited by comparison to the immensities of structures in the universe.

Speed and distance

The scale for speed in the universe is set not by space flight but by light waves, which travel at 299,800 kilometers per second (186,000 miles per second). This velocity is a fundamental constant of the universe, and it provides a standard forjudging our progress into space. At the beginning of the Twentieth Century, the speed of light was more than 5 million times faster than any human had ever traveled. Now it is only 25,000 times faster than astronauts have flown on their way to the Moon. As space travel develops, it is no longer inconceivable that humans may someday travel at such high speeds that trips to the stars may become possible.

Dimensions in space are most conveniently expressed not in miles or kilometers, but in terms of the time it takes light to travel the distances involved. The distance from the Earth to the Moon (385,000 kilometers or 240,000 miles) is 1.3 lightseconds. The distance from the Earth to the Sun is 8 light minutes, or 150,000,000 kilometers (93,000,000 miles). The spacecraft Pioneer 10, launched in 1973 and now past the orbit of the planet Uranus, has gone more than 3 light hours from the Earth. Even the nearest stars are light years away. It takes 4.3 years for light from the nearest star beyond the Sun, Proxima Centauri, to reach the Earth, traveling 41 million million kilometers (25 trillion miles) in the process.

Galaxies, which are irregular, ellipsoidal, disk- or spiral-shaped systems of billions of stars, are aptly termed "island universes." Our own galaxy, the Milky Way, is a flat spiral that is 100,000 light years across and almost 1,000 light years thick, with a large central bulge. The distances between galaxies are greater still, often measured in millions of light years. And the universe itself extends at least 10 to 20 billion light years in every direction from the Earth.


The amounts of energy involved in celestial processes are equally difficult to appreciate from our own experience. A typical unit of human energy is the joule, about the amount of energy needed to lift a glass of water from the dinner table to your mouth. On earth, energy releases can reach a quadrillion (a thousand million million) joules, about equal to a megaton, the energy produced by the detonation of a million tons of TNT. This is also roughly the amount of energy contained in a tornado or in a small earthquake. A very different scale is needed for astronomical power. A useful unit is the energy released by one star -our Sun- which emits the equivalent of 100 billion megatons in the form of light every second. Even this immense quantity is tiny by cosmic standards. An average galaxy may contain 100 billion stars, many comparable to the Sun. The strange, distant objects called quasars are even more powerful, some individually releasing as much energy as a million galaxies.

Energy has many different forms in space. Energy is present in light, in the motions of particles, in magnetic fields, and in gravitational attraction. The temperature of atoms and mole cules moving randomly in space is proportional to the energy contained in each particle. As each particle moves, it radiates energy. By detecting and measuring this energy, we can measure the particle temperatures at dis tances of thousands of light years.

The scale of temperatures found by space astronomy runs from a few degrees above absolute zero, with particles moving at a slow 30 kilometers per hour (19 miles per hour), to almost 10 billion° C, where electrons move at close to the speed of light. Most of the universe is made of hydrogen, and for this reason the temperature of 10,000° C is a critical threshold. At this temperature, the energy of a moving particle is enough to knock the electron from a hydrogen atom when they collide. If energy is added to interstellar hydrogen, the gas will heat up until it reaches 10,000° C and then will stay at 10,000° C, even though energy is continuously added, until the hydrogen is completely ionized.


The density of matter in space is very different from conditions on the Earth. The best vacuums are found in space, especially between the stars, where there often is only one atom in more than a thousand cubic centimeters (60 cubic inches). An average star, with about a trillion trillion atoms in each cubic centimeter, is about as dense as water. At the extreme of astronomical density, however, are the neutron stars that form in super nova explosions. Their matter is so compressed that the individual atoms collapse into neutrons. A single cubic centimeter of a neutron star contains enough material to make a cubic kilometer of a normal star and may weigh as much as several billion tons.

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