The Milky Way and Beyond (3 page)

Cygnus A

Great Attractor

Magellanic Clouds

M81 Group

Maffei I and II

Virgo A

Virgo Cluster

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PPENDIX:
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TARS AND
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TAR
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LUSTERS

G
LOSSARY

F
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or thousands of years, astronomers have worked to unlock the mysteries of the heavens. As they observed the stars and planets that parade majestically across the sky, one great source of wonder was a huge faint trail of light that was called the Milky Way Galaxy because it looked like milk, spilled across the darkness of night. When scientists started using telescopes in the 1600s, they began to understand more about the Milky Way. But it wasn't until the 20th century that new technology allowed scientists to take the full measure of this galaxy and others scattered across the universe. What they have uncovered is richly detailed in the pages of this book.

The primary elements of any galaxy are stars. A star is a massive body of gas that shines by radiation resulting from internal energy sources. There are so many stars in the universe it would be impossible to count them all. Only a very small fraction are actually visible to the unaided eye. In the Milky Way Galaxy alone there are hundreds of billions of stars. The easiest of these for scientists to study is the Sun, the star closest to Earth. It is about 150 million kilometres (93 million miles) away from us. It has a radius of about 700,000 km (430,000 miles.). Its mass is about the same as that of 330,000 Earth masses. It creates approximately 4 × 10
²³
kilowatts (4 × 10
³³
ergs) of luminosity per second, making it average in size, mass, and brightness. Scientists use these measurements as benchmarks when discussing other stars, which can be bigger or smaller, more or less bright, depending on their type and age.

While 150 million kilometres may sound like a very long distance, it is tiny in cosmic terms. The next closest star to Earth is Proxima Centauri, which is the smallest of three stars making up the triple star Alpha Centauri. Proxima Centauri is approximately 4.22 light years from Earth—or about 3.99 × 10
¹⁶
metres (2.48 × 10
¹³
miles). Even the fastest modern spacecraft would take countless lifetimes to reach this distant star.

Scientists have used several different methods to determine the distances from Earth to the stars. The earliest method, which is still used to determine the distances of closer stars, is a trigonometric parallax, which involves observing a star from two points on opposite sides of Earth's orbit. This technique depends on using the relatively unchanged backdrop of distant space for precise measurements. More-distant stars require other techniques, most of which rely on comparing luminosities.

Depending on its mass, a star may “live” 10 million years or 10 billion years.
During this time, it goes through many dramatic changes. Stars begin as clouds of interstellar gases, mostly hydrogen. Molecules in the clouds slowly begin to collapse and clump together, eventually forming areas of greater density. The clumps begin to rotate slowly as they grow more massive. In time, one or more of the clumps begins to collapse in on itself due to gravity. Often, several stars form from the same cloud, resulting in star groups.

Mass, gravity, and other forces cause the cloud to form a disk shape around the young star core. The temperature within the core continually rises, and when the temperature is high enough, hydrogen fusion begins, which allows stars to radiate tremendous amounts of heat and light. Soon after, the star has become fully formed. With a star the size of the Sun, this might take tens of millions of years. More massive stars, which burn hotter and more quickly, may take just a few hundred thousand years to reach this point.

For most of its life, a star continues to create helium through hydrogen fusion. This part of a star's life is called the main sequence. Fusion creates the light and heat that stars radiate and turns hydrogen into helium in the process. The power created by fusion is constantly pushing outward and fighting against the massive gravitational pull of the core. In time, as the hydrogen fuel in the core decreases, the core is converted to helium. Hydrogen is then burned on the surface of the helium core at a higher rate, which causes the outer layers of the star to expand. This is called the red giant stage. It can take anywhere from about a million years to hundreds of billions of years for a star to reach this stage.

Eventually, due to the lack of hydrogen, the fusion reaction begins to die down. Not all stars die in the same manner. They differ depending on how they formed, how dense they are, how big they are, and their age. In less-massive stars, the outer layers may drift away into space, leaving the slowly cooling core called a white dwarf. More-massive stars can explode as a supernova. The most-massive stars go supernova, then collapse in on themselves due to immense gravitational force. The result is a black hole—a force so powerful nothing, not even light, can escape its grasp.

Stars are not the only bright points of light in the night sky. A few of those twinkling specks are nebulae. A nebula is an interstellar cloud of gas and dust. The matter that makes up nebulae is called the interstellar medium, which can be found just about everywhere in the universe, although it is more dense in nebulae. The composition of nebulae is approximately 90 percent hydrogen and nearly 10 percent helium, with a very small amount of other elements mixed in, namely oxygen, carbon, neon, and nitrogen. Many of the characteristics of nebulae are determined by the physical state of the hydrogen in them.

Based on their components and behaviour, there are two main
categories of nebulae, dark and bright. Dark nebulae, which are also called molecular clouds, are dense and cold. While molecular clouds can contain approximately 60 different kinds of molecules, the majority of their composition is molecular hydrogen (H
₂
). They are opaque because of the relatively high concentration of solid grains in them. Densities generally are millions of hydrogen molecules per cubic centimetre. These are the clouds from which most new stars form through gravitational collapse.

Although a very small percentage of the interstellar medium, like that found in dark nebulae, is in solid grains, that percentage is very important to the creation of stars and solar systems. Unlike the gases in dark nebulae, solid grains absorb starlight. In turn, they are able to heat and cool the gases.

Bright nebulae are usually not as dense as the dark variety. However, as the name suggests, they are visible. There are several different kinds of bright nebulae. Reflection nebulae are molecular clouds just like dark nebulae. However, they are visible because light from nearby stars reflect off of their solid grains.

H II regions are cosmic clouds that glow because they have been ionized by the radiation produced by a neighbouring hot star. Ionization occurs when the hydrogen atoms in the cloud separate into positive hydrogen ions (H
⁺
) and free electrons, causing the cloud to glow. Another kind of nebula, called the diffuse nebula, is visible due to the ionization of hydrogen, nitrogen, and sulfur. Diffuse nebulae require the most energy of all the kinds of nebulae and are found in the vicinity of the hottest and most-massive stars.

When a star goes supernova, the resulting explosion can last for several weeks. After the supernova dies down, a bright, colourful nebula, sometimes called a supernova remnant, is left behind. Stars that don't go supernova can create planetary nebulae. This occurs when the envelope of gases around the dying star begins to expand and spread out. Planetary nebulae often have a round, compact shape. Scientists believe the Milky Way Galaxy contains about 20,000 planetary nebulae.

Despite their many differences, nebulae have basic traits in common. For example, all nebulae exhibit chaotic motions scientists call turbulence. This is similar to the ripples and whirlpools we see when we add a coloured liquid to a clear liquid. The disorganized flow of gases creates energy and heat. Scientists know that turbulence has a great effect on the behaviour of nebulae, but they do not fully understand why or how. They hope to learn more about turbulence and nebulae by continuing to study known nebulae and by discovering new ones.

Galaxies are massive, self-contained collections of stars. Scientists believe that most galaxies formed shortly after the birth of the universe, about 13 billion years ago. They can look very different, based on how they formed and evolved. Some are very small, while others, like the Milky Way, have huge spiral arms reaching deep into space.

Scientists have had difficulty studying the Milky Way because of a thick layer of interstellar medium that obscures their view of it, even with powerful telescopes. Many think the galaxy's diameter is about 100,000 light-years, and that our sun resides in a spiral arm about 30,000 light-years from the galaxy's center.

The Milky Way is one galaxy in a cluster of galaxies called the Local Group. Galaxy clusters are groups of galaxies that can be hundreds of millions of light-years across. The word “cluster,” however, might be a bit misleading. The Magellanic Clouds are two satellite galaxies orbiting the Milky Way. They are probably between 160,000 and 190,000 light-years away. Scientists have learned much about nebulae and stars by observing these neighbouring galaxies.