High School Earth Science/Telescopes

Many scientists can interact directly with what they are studying. Biologists can collect cells, seeds, or sea urchins and put them in a controlled laboratory environment. Physicists can subject metals to stress or smash atoms into each other. Geologists can chip away at rocks to see what is inside. But astronomers, scientists who study the universe beyond Earth, rarely have a chance for direct contact with their subject. Instead, astronomers have to observe their subjects at a distance, usually a very large distance!

Lesson Objectives

• Explain how astronomers use the whole electromagnetic spectrum to study the universe beyond Earth.
• Identify different types of telescopes.
• Describe historical and modern observations made with telescopes.

Electromagnetic Radiation

Earth is separated from the rest of the universe by very large expanses of space. Occasionally, matter from the outside reaches Earth, such as when a meteorite makes it through the atmosphere. But for the most part, astronomers have one main source for their data—light. Light can travel across empty space, and as it does so, it carries both energy and information. Light is one type of electromagnetic (EM) radiation, or energy transmitted through space as a wave.

The Speed of Light

Light travels faster than anything else in the universe. In the almost completely empty vacuum of space, light travels at a speed of approximately 300,000,000 meters per second (670,000,000 miles per hour). To give you an idea of how fast that is, a beam of light could travel from New York to Los Angeles and back again nearly 40 times in just one second. Even though light travels extremely fast, objects in space are so far away that it takes a significant amount of time for light from those objects to reach us. For example, light from the Sun takes about 8 minutes to reach Earth.

Light-Years

Because astronomical distances are so large, it helps to have a unit of measurement that is good for expressing those large distances. A light-year is a unit of distance that is defined as the distance that light travels in one year. One light-year is approximately equal to 9,500,000,000,000 (9.5 trillion) kilometers, or 5,900,000,000,000 (5.9 trillion) miles. That's a long way! By astronomical standards, it's actually a pretty short distance.

Proxima Centauri, the closest star to us after the Sun, is 4.22 light-years away. That means the light from Proxima Centauri takes 4.22 years to reach us. The galaxy we live in, the Milky Way Galaxy, is about 100,000 light-years across. So, how long does it take light to travel from one side of the galaxy to the other? 100,000 years! Even 100,000 light years is a short distance on the scale of the whole universe. The most distant galaxies we have detected so far are more than 13 billion light-years away. That's over a hundred-billion-trillion (100,000,000,000,000,000,000,000) kilometers!

Looking Back in Time

When we look at astronomical objects such as stars and galaxies, we are not just seeing over great distances—we are also seeing back in time. Because light takes time to travel, the image we see of a distant galaxy is an image of how the galaxy used to look. For example, the Andromeda Galaxy, shown in Figure 23.1, is about 2.5 million light years from Earth. If you look at the Andromeda Galaxy in a telescope, you will see the galaxy as it was 2.5 million years ago. If you want to see the galaxy as it is now, you will have to wait and look again 2.5 million years into the future!

Figure 23.1: This recent picture of the Andromeda Galaxy actually shows the galaxy as it was about 2.5 million years ago.

Electromagnetic Waves

Earlier, we said that light is one type of electromagnetic (EM) radiation. That means light is energy that travels in the form of an electromagnetic wave. Figure 23.2 shows a diagram of an electromagnetic wave. An EM wave has two components: an electric field and a magnetic field. Each of these components oscillates between positive and negative values, which is what makes the "wavy" shape in the diagram.

Figure 23.2: An electromagnetic wave consists of oscillating electric and magnetic fields. The distance between two adjacent oscillations is called wavelength.

Notice the horizontal arrow at the top left of the diagram. This measurement corresponds to the wavelength, or the distance between two adjacent points on the wave. A related value is frequency, which measures the number of wavelengths that pass a given point every second. Wavelength and frequency are reciprocal, which means that as one increases, the other decreases.

The Electromagnetic Spectrum

Visible light—the light that human eyes can see—comes in a variety of colors. The color of visible light is determined by its wavelength. Visible light ranges from wavelengths of 400 nm to 700 nm, corresponding to the colors violet through red. But what about EM radiation with wavelengths shorter than 400 nm or longer than 700 nm? Such radiation exists all around you—you just can't see it! Visible light is part of a larger electromagnetic spectrum, as Figure 23.3 illustrates.

Figure 23.3: Visible light is part of a larger electromagnetic spectrum. The EM spectrum ranges from gamma rays with very short wavelengths, to radio waves with very long wavelengths.

What does the electromagnetic spectrum have to do with astronomy? Every star, including our Sun, emits light at a wide range of wavelengths, all across the visible spectrum, and even outside the visible spectrum. Astronomers can learn a lot from studying the details of the spectrum of light from a star.

Some very hot stars emit light primarily at ultraviolet wavelengths, while some very cool stars emit mostly in the infrared. There are extremely hot objects that emit X-rays and even gamma rays. Light from some of the faintest, most distant objects is in the form of radio waves. In fact, a lot of the objects most interesting to astronomers today can't even be seen with the naked eye. Astronomers use telescopes to detect the faint light from distant objects and to see objects at wavelengths all across the electromagnetic spectrum.

Types of Telescopes

Figure 23.4: The largest refracting telescope in the world is at the University of Chicago's Yerkes Observatory in Wisconsin. This telescope was built in 1897. Its largest lens has a diameter of 102 cm.

Optical Telescopes

Humans have been making and using lenses for magnification for thousands and thousands of years. However, the first true telescopes were made in Europe in the late 16^th century. These telescopes used a combination of two lenses to make distant objects appear both nearer and larger. The term telescope was coined by the Italian scientist and mathematician Galileo Galilei (1564–1642). Galileo built his first telescope in 1608 and subsequently made many improvements to telescope design.

Telescopes that rely on the refraction, or bending, of light by lenses are called refracting telescopes, or simply refractors. The earliest telescopes, including Galileo's, were all refractors. Many of the small telescopes used by amateur astronomers today are refractors with a design similar to Galileo's. Refractors are particularly good for viewing details within our solar system, such as the surface of Earth's moon or the rings around Saturn. Figure 23.4 shows the biggest refracting telescope in the world.

Figure 23.5: The telescope still looks much the same today.

Around 1670, another famous scientist and mathematician—Sir Isaac Newton (1643–1727)—built a different kind of telescope. Figure 23.5 shows a telescope similar in design to Newton's.

Figure 23.6: Reflecting telescopes used by amateur astronomers today are similar to the one designed by Isaac Newton in the 17^th century.

Figure 23.8: Many amateur astronomers today use catadioptric telescopes. These telescopes have large mirrors to collect a lot of light, but short tubes for portability.

Newton's telescope used curved mirrors instead of lenses to focus light. Telescopes that use mirrors are called reflecting telescopes, or reflectors (Figure 23.6). The mirrors in a reflecting telescope are much lighter than the heavy glass lenses in a refractor. This is significant, because thick glass lenses in a telescope mean that the whole telescope must be much stronger to support the heavy glass. In addition, it's much easier to precisely make mirrors than to precisely make glass lenses. For that reason, reflectors can be made larger than refractors. Larger telescopes can collect more light, which means they can study dimmer or more distant objects. The largest optical telescopes in the world today are reflectors, like the one in Figure 23.7.

Many consumer telescopes today use a combination of mirrors and lenses to focus light. These telescopes are called catadioptric telescopes. By using both kinds of elements, catadioptric telescopes can be made with large diameters but shorter lengths so they are less awkward to move around. Figure 23.8 shows a typical catadioptric telescope.

Figure 23.7: The South African Large Telescope (SALT) is one of the largest reflecting telescopes on Earth. SALT's primary mirror consists of 91 smaller hexagonal mirrors, each with sides 1 m long.

Radio Telescopes

Notice it says above that the largest optical telescopes in the world are reflectors. Optical telescopes are designed to collect visible light. There are even larger telescopes that collect light at longer wavelengths—radio waves. These telescopes are called—can you guess?—radio telescopes. Radio telescopes look a lot like satellite dishes. In fact, both are designed to do the same thing—to collect and focus radio waves or microwaves from space.

The largest single telescope in the world is at the Arecibo Observatory in Puerto Rico (see Figure 23.9). This telescope is located in a naturally-occurring sinkhole that formed when water flowing underground dissolved the limestone rock. If this telescope were not supported by the ground, it would collapse under its own weight. The downside of this design is that the telescope cannot be aimed to different parts of the sky—it can only observe the part of the sky that happens to be overhead at a given time.

A group of radio telescopes, such as those shown in Figure 23.10, can be linked together with a computer so that they are all observing the same object. The computer can combine the data from each telescope, making the group function like one single telescope.

Figure 23.10: The Very Large Array in New Mexico has 27 radio dishes, each 25 meters in diameter. When all the dishes are spread out and pointed at the same object, they act like a single telescope with a diameter of 22.3 mi.

Space Telescopes

Telescopes on Earth all have one significant limitation: the electromagnetic radiation they gather must pass through Earth's atmosphere. The atmosphere blocks some radiation in the infrared part of the spectrum and almost all radiation in the ultraviolet and higher frequency ranges. Furthermore, motion in the atmosphere distorts light. You see evidence of this distortion when you see stars twinkling in the night sky. To minimize these problems, many observatories are built on high mountains, where there is less atmosphere above the telescope. Space telescopes avoid such problems completely because they are outside Earth's atmosphere altogether—in space.

The Hubble Space Telescope (HST), shown in Figure 23.11, is perhaps the best known space telescope. The Hubble was put into orbit by the Space Shuttle Atlantis in 1990. Once it was in orbit, scientists discovered that there was a flaw in the shape of the mirror. A servicing mission to the Hubble by the Space Shuttle Endeavor in 1994 corrected the problem. Since that time, the Hubble has provided huge amounts of data that have helped to answer many of the biggest questions in astronomy.

Figure 23.11: The Hubble Space Telescope orbits Earth at an altitude of 589 km (366 mi). It collects data in visible, infrared, and ultraviolet wavelengths.

In addition to the Hubble, the National Aeronautics and Space Administration (NASA) has placed three other major space telescopes in orbit: the Compton Gamma-Ray Observatory (CGRO), the Chandra X-Ray Observatory (CXO), and the Spitzer Space Telescope (SST). Together, these four telescopes comprise what NASA calls the "Great Observatories". Figure 23.12 shows how each of these telescopes specializes in a different part of the electromagnetic spectrum. Of these, all but the Compton are still in orbit and active. NASA is planning for another telescope, the James Webb Space Telescope, to serve as a replacement for the aging Hubble. The James Webb is scheduled to launch no earlier than 2013.

Figure 23.12: NASA's four space-based Great Observatories were designed to view the universe in different ranges of the electromagnetic spectrum. Clockwise from top left: Hubble Space Telescope (visible light), Compton Gamma Ray Observatory (gamma ray), Spitzer Space Telescope (infrared), Chandra X-ray Observatory (X-ray).

Observations with Telescopes

Ancient Astronomers

Humans have been studying the night sky for thousands of years. Observing the patterns and motions in the sky helped ancient peoples keep track of time. This was important to them because it helped them know when to plant crops. They also timed many of their religious ceremonies to coincide with events in the heavens.

The ancient Greeks made careful observations of the locations of stars in the sky. They noticed that some of what they thought were "stars" moved against the background of other stars. They called these bright spots in the sky planets, which in Greek means "wanderers". Today we know that the planets are not stars, but members of our solar system that orbit the Sun. The Greeks also identified constellations, patterns of stars in the sky. They associated the constellations with stories and myths from their culture. Constellations still help astronomers today; they are used to identify different regions of the night sky.

Galileo's Observations

Ancient astronomers knew a lot about the patterns of stars and the movement of objects in the sky, but they did not know much about what these objects actually were. All of that changed in the year 1610, when Galileo turned a telescope toward the heavens. Using a telescope, Galileo made the following discoveries (among others):

• There are more stars in the night sky than the naked eye can see.
• The band of stars called the Milky Way consists of many stars.
• The Moon has craters (See Figure 23.13).
• Venus has phases like the Moon.
• Jupiter has moons orbiting around it.
• There are dark spots that move across the surface of the Sun.

Figure 23.13: Galileo was the first person known to look at the Moon through a telescope. Galileo made the drawing on the left in 1610. The image on the right is a modern photograph of the Moon.

Galileo's observations challenged people to think in new ways about the universe and Earth's place in it. About 100 years before Galileo, Nicolaus Copernicus had proposed a controversial new model of the universe. According to Copernicus's model, Earth and the other planets revolve around the Sun. In Galileo's time, most people believed that the Sun and planets revolved around Earth. Galileo's observations provided direct evidence to support Copernicus' model.

Observations with Modern Telescopes

Today, equipped with no more than a good pair of binoculars, you can see all of the things Galileo saw, and more. You can even see sunspots, but you need special filters on the lenses to protect your eyes. Never look directly at the Sun without using the proper filters! With a basic telescope like those used by many amateur astronomers, you can also see polar caps on Mars, the rings of Saturn, and bands in the atmosphere of Jupiter.

We now know that all of these objects are within our solar system. You can also see many times more stars with a telescope than without a telescope. However, stars seen in a telescope still look like single points of light. Because they are so far away, stars continue to appear as points of light in even the most powerful professional telescopes. Figure 23.14 shows one rare exception.

Today, very few professional astronomers look directly through the eyepiece of a telescope. Instead, they attach sophisticated instruments to telescopes. These instruments capture and process the light from a telescope, and astronomers then look at the images or data shown on these instruments. Most of the time, the instruments then pass the data on to a computer where the data can be stored for later use. It can take an astronomer weeks or months to analyze all the data collected from just a single night!

A spectrometer is a tool that astronomers commonly use to study the light from a telescope. A spectrometer uses a prism or other device to break light down into its component colors. This produces a spectrum like the one shown in Figure 23.15. The dark lines in the spectrum of light from a star are caused by gases in the outer atmosphere of the star absorbing light. This spectrum can be observed directly, captured on film, or stored digitally on a computer.

Figure 23.15: This is a simplified example of what light from a star looks like after it passes through a spectrometer.

From a single spectrum of a star, an astronomer can tell:

• How hot the star is (by the relative brightness of different colors).
• What elements the star contains (by the pattern of dark lines).
• Whether and how fast the star is moving toward or away from Earth (by how far the dark lines are shifted from their normal positions).

Using telescopes, astronomers can also learn how stars evolve, what kind of matter is found throughout the universe, and how it is distributed, and even how the universe might have formed.

Lesson Summary

• Astronomers study light from distant objects.
• Light travels at 300,000,000 meters per second—faster than anything else in the universe.
• A light-year is a unit of distance equal to the distance light travels in one year, 9.5 trillion kilometers.
• When we see distant objects, we see them as they were in the past, because their light has been traveling to us for many years.
• Light is energy that travels as a wave.
• Visible light is part of the electromagnetic spectrum.
• Telescopes make distant objects appear both nearer and larger. You can see many more stars through a telescope than with the unaided eye.
• Optical telescopes are designed to collect visible light. The three main types of optical telescopes are reflecting telescopes, refracting telescopes, and catadioptric telescopes.
• Radio telescopes collect and focus radio waves from distant objects.
• Space telescopes are telescopes orbiting Earth. They can collect wavelengths of light that are normally blocked by the atmosphere.
• Galileo was the first person known to use a telescope to study the sky. His discoveries helped change the way humans think about the universe.
• Modern telescopes collect data that can be stored on a computer.
• A spectrometer produces a spectrum from starlight. Astronomers can learn a lot about a star by studying its spectrum.

Review Questions

1. Proxima Centauri is 4.22 light-years from Earth. Light travels 9.5 trillion kilometers in one year. How far away is Proxima Centauri in kilometers?
2. Identify four regions of the electromagnetic spectrum that astronomers use when observing objects in space.
3. List the three main types of optical telescopes, and describe their differences.
4. Explain the advantages of putting a telescope into orbit around Earth.
5. Describe two observations that Galileo was the first to make with his telescope.
6. List three things that an astronomer can learn about a star by studying its spectrum.

Vocabulary

catadioptric telescope

Telescopes that use a combination of mirrors and lenses to focus light.

constellations

Patterns of stars as observed from Earth.

electromagnetic radiation

Energy transmitted through space as a wave.

electromagnetic spectrum

The full range of electromagnetic radiation.

frequency

The number of wavelengths that pass a given point every second.

gamma rays

A penetrating form of electromagnetic radiation.

infrared

Electromagnetic waves with frequencies between radio waves and red light; about 1 mm to 750 nanometers.

light-year

The distance light can travel in one year; 9.5 trillion kilometers.

microwaves

The shortest wavelength radio waves.

planets

Around celestial object orbiting a star that has cleared its neighboring region of planetesimals.

radio telescope

A radio antenna that collects radio waves.

radio waves

The longest wavelengths of the electromagnetic spectrum; from 1 mm to more than thousands of kilometers.

reflecting telescope

Telescopes that use mirrors to collect and focus light.

refracting telescope

Telescopes that use convex lenses to collect and focus light.

space telescope

Telescopes in orbit above Earth's atmosphere.

spectrometer

A tool that uses a prism to break light into its component colors.

ultraviolet

Electromagnetic radiation having wavelengths shorter than the violet.

wavelength

Horizontal distance measured from wave crest to wave crest, or wave trough to wave trough.

visible light

The portion of light in the electromagnetic spectrum that is visible to humans.

X-rays

A band of electromagnetic radiation between gamma and ultraviolet.

Points to Consider

• Radio waves are used for communicating with spacecraft. A round-trip communication from Earth to Mars takes anywhere from 6 to 42 minutes. What challenges might this

present for sending unmanned spacecraft and probes to Mars?

• The Hubble Space Telescope is a very important source of data for astronomers. The fascinating and beautiful images from the Hubble also help to maintain public support for science. However, the Hubble is growing old. Missions to service and maintain the telescope are extremely expensive and put the lives of astronauts at risk.
• Do you think there should be another servicing mission to the Hubble?