An Image of Our Universe

An Image of Our Universe

What Is the Structure of the Universe? 

Contrasting views of the universe drawn by artist hundreds of years ago.

Think about the mysterious spectacle of a clear night sky. What objects are up there? How big are they? How far away are they? How do they move? How are they arranged? In addressing such questions, ancient philosophers first distinguished between stars (points of light whose locations relative to each other are fixed) and planets (tiny spots of light that move relative to the backdrop of stars). Over the centuries, two schools of thought developed concerning how to explain the configuration of stars and planets, and their relationships to the Earth, Sun, and Moon. The first school advocated a geocentric model (figure above a), in which the Earth sat without moving at the centre of the Universe, while the Moon and the planets whirled around it within a revolving globe of stars. The second school advocated a heliocentric model (figure above b), in which the Sun lay at the centre of the Universe, with the Earth and other planets orbiting around it.

The geocentric image eventually gained the most followers, due to the influence of an Egyptian mathematician,  Ptolemy (100–170 C.E.), for he developed equations that appeared to predict the wanderings of the planets in the context of the geocentric model. During the Middle Ages (ca. 476–1400 C.E.), church leaders in Europe adopted Ptolemy’s geocentric model as dogma, because it justified the comforting thought that humanity’s home occupies the most important place in the Universe. Anyone who disagreed with this view risked charges of heresy. 

Then came the Renaissance. In 15th-century Europe, bold thinkers spawned a new age of exploration and scientific discovery. Thanks to the efforts of Nicolaus Copernicus (1473–1543) and Galileo Galilei (1564–1642), people gradually came to realize that the Earth and planets did indeed orbit the Sun and could not be at the centre of the Universe. And when Isaac Newton (1643–1727) explained gravity, the attractive force that one object exerts on another, it finally became possible to understand why these objects follow the orbits that they do. 

In the centuries following Newton, scientists gradually adopted modern terminology for discussing the Universe. In this language, the Universe contains two related entities: matter and energy. Matter is the substance of the Universe it takes up space and you can feel it. We refer to the amount of matter in an object as its mass, so an object with greater mass contains more matter. Density refers to the amount of mass occupying a given volume of space. The mass of an object determines its weight, the force that acts on an object due to gravity. 

An object always has the same mass, but its weight varies depending on where it is. For example, on the Moon, you weigh much less than on the Earth. The matter in the Universe does not sit still. Components vibrate and spin, they move  from one place to another, they pull on or push against each other, and they break apart or combine. In a general sense, we consider such changes to be kinds of “work.” Physicists refer to the ability to do work as energy. One piece of matter can do work directly on another by striking it. Heat, light, magnetism, and gravity all provide energy that can cause change at a distance. 

A galaxy may contain about 300 billion stars.

As the understanding of matter and energy improved, and telescopes became refined so that astronomers could see and measure features progressively farther into space, the interpretation of stars evolved. Though it looks like a point of light, a star is actually an immense ball of incandescent gas that emits intense heat and light. Stars are not randomly scattered through the Universe; gravity holds them together in immense groups called galaxies. The Sun and over 300 billion stars together form the Milky Way galaxy. More than 100 billion galaxies constitute the visible Universe (figure above a). 

From our vantage point on Earth, the Milky Way looks like a hazy band (figure above b), but if we could view the Milky Way from a great distance, it would look like a flattened spiral with great curving arms slowly swirling around a glowing, disk-like centre (figure above c). Presently, our Sun lies near the outer edge of one of these arms and rotates around the centre of the galaxy about once every 250 million years. So, we hurtle through space, relative to an observer standing outside the galaxy, at about 200 km per second. 

Clearly, human understanding of Earth’s place in the Universe has evolved radically over the past few centuries. Neither the Earth, nor the Sun, nor even the Milky Way occupy the centre of the Universe and everything is in motion. 

The Nature of Our Solar System 

Eventually, astronomical study demonstrated that our Sun is a rather ordinary, medium-sized star. It looks like a sphere, instead of a point of light, because it is much closer to the Earth than are the stars. The Sun is “only” 150 million km (93  million miles) from the Earth. Stars are so far away that we measure their distance in light years, where 1 light year is the distance travelled by light in one year, about 10 trillion km, or 6 trillion miles the nearest star beyond the Sun is over 4 light years away. How can we picture distances? If we imagined that the Sun were the size of a golf ball (about 4.3 cm), then the Earth would be a grain of sand about one meter away, and the nearest star would be 270 km (168 miles) away. (Note that the distance between stars is tiny by galactic standards, the Milky Way galaxy is 120,000 light years across!)

 The relative sizes and positions of planets in the Solar System.

Our Sun is not alone as it journeys through the heavens. Its gravitational pull holds on to many other objects which, together with the Sun, comprise the Solar System (figure above a, b). The Sun accounts for 99.8% of the mass in the Solar System. The remaining 0.2% includes a great variety of objects, the largest of which are planets. Astronomers define a planet as an object that orbits a star, is roughly spherical, and has “cleared its neighbourhood of other objects.” The last phrase in this definition sounds a bit strange at first, but merely implies that a planet’s gravity has pulled in all particles of matter in its orbit.

According to this definition, which was formalized in 2005, our Solar System includes eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. In 1930, astronomers discovered Pluto, a 2,390-km-diameter sphere of ice, whose orbit generally lies outside that of Neptune’s. Until 2005, astronomers considered Pluto to be a planet. But since it does not fit the modern definition, it has been dropped from the roster. Our Solar System is not alone in hosting planets; in recent years, astronomers have found planets orbiting stars in many other systems. As of 2012, over 760 of these “exoplanets” have been found. 

Planets in our Solar System differ radically from one another both in size and composition. The inner planets (Mercury, Venus, Earth, and Mars), the ones closer to the Sun, are relatively small. Astronomers commonly refer to these as terrestrial planets because, like Earth, they consist of a shell of rock surrounding a ball of metallic iron alloy. The outer planets (Jupiter, Saturn, Uranus, and Neptune) are known as the giant planets, or Jovian planets. The adjective giant certainly seems appropriate, for these planets are huge Jupiter, for example, has a mass 318 times larger than that of Earth and accounts for about 71% of the non-solar mass in the Solar System. The overall composition of the giant planets is very different from that of the terrestrial planets. Specifically, most of the mass of Neptune and Uranus contain solid forms of water, ammonia, and methane, so these planets are known as the ice giants. Most of the mass of Jupiter and Saturn consists of hydrogen and helium gas or liquefied gas, so these planets are known as the gas giants.

In addition to the planets, the Solar System contains a great many smaller objects. Of these, the largest are moons. A moon is a sizeable body locked in orbit around a planet. All but two planets (Mercury and Venus) have moons in varying numbers Earth has one, Mars has two, and Jupiter has at least 63. Some moons, such as Earth’s Moon, are large and spherical, but most are small and have irregular shapes. In addition to moons, millions of asteroids (chunks of rock and/or metal) comprise a belt between the orbits of Mars and Jupiter. Asteroids range in size from less than a centimetre to about 930 km in diameter. And about a trillion bodies of ice lie in belts or clouds beyond the orbit of Neptune. Most of these icy objects are tiny, but a few (including Pluto) have diameters of over 2,000 km and may be thought of as “dwarf planets.” The gravitational pull of the main planets has sent some of the icy objects on paths that take them into the inner part of the Solar System, where they begin to evaporate and form long tails of gas we call such objects comets.
Credits: Stephen Marshak

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