telescopes

galileos observations

In 1610, Galileo looked at the night sky through the first telescope. This tool allowed him to make the following discoveries (among others): There are more stars in the night sky than the unaided eye can see. The band of light called the Milky Way consists of many stars. The Moon has craters (see Figure 23.10). Venus has phases like the Moon. Jupiter has moons orbiting around it. There are dark spots that move across the surface of the Sun. Galileos observations made people think differently about the universe. They made them think about the solar system and Earths place in it. Until that time, people believed that the Sun and planets revolved around Earth. One hundred years before Galileo, Copernicus had said that the Earth and the other planets revolved around the Sun. No one would believe him. But Galileos observations through his telescope proved that Copernicus was right.

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before telescopes

Humans have been studying the night sky for thousands of years. Knowing the motions of stars helped people keep track of seasons. With this information they could know when to plant crops. Stars were so important that the patterns they made in the sky were named. These patterns are called constellations. Even now, constellations help astronomers know where they are looking in the night sky. The ancient Greeks carefully observed the locations of stars in the sky. They noticed that some of the stars moved across the background of other stars. They called these bright spots in the sky planets. The word in Greek means wanderers. Today we know that the planets are not stars. They are objects in the solar system that orbit the Sun. Ancient astronomers made all of their observations without the aid of a telescope.

the electromagnetic spectrum

Visible light is the part of the electromagnetic spectrum (Figure 23.3) that humans can see. Visible light includes all the colors of the rainbow. Each color is determined by its wavelength. Visible light ranges from violet wavelengths of 400 nanometers (nm) through red at 700 nm. There are parts of the electromagnetic spectrum that humans cannot see. This radiation exists all around you. You just cant see it! Every star, including our Sun, emits radiation of many wavelengths. Astronomers can learn a lot from studying the details of the spectrum of radiation from a star. Many extremely interesting objects cant be seen with the unaided eye. Astronomers use telescopes to see objects at wavelengths all across the electromagnetic spectrum. Some very hot stars emit light primarily at ultraviolet wavelengths. There are extremely hot objects that emit X-rays and even gamma rays. Some very cool stars shine mostly in the infrared light wavelengths. Radio waves come from the faintest, most distant objects. To learn more about stars spectra, visit

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electromagnetic waves

Light is one type of electromagnetic radiation. Light is energy that travels in the form of an electromagnetic wave. Figure 23.2 shows a diagram of an electromagnetic wave. An electromagnetic (EM) wave has two parts: an electric field and a magnetic field. The electric and magnetic fields vibrate up and down, which makes the wave. The wavelength is the horizontal distance between two of the same points on the wave, like wave crest to wave crest. A waves frequency measures the number of wavelengths that pass a given point every second. As wavelength increases, frequency decreases. This means that as wavelengths get shorter, more waves move past a particular spot in the same amount of time.

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studying starlight with spectrometers

A spectrometer is a special tool that astronomers commonly use. Spectrometers allow them to study the light from a star or galaxy. A spectrometer produces a spectrum, like the one shown in Figure 23.11. A prism breaks light into all its colors. Gases from the outer atmosphere of a star absorb light. This forms dark lines in the spectrum. These dark lines reveal what elements the star contains. Astronomers use the spectrum to learn even more about the star. One thing they learn is how hot the star is. They also learn the direction the star is going and how fast. By carefully studying light from many stars, astronomers know how stars evolve. They have learned about the distribution and kinds of matter found throughout the universe. They even know something about how the universe might have formed. To find out what you can expect to see when looking through a telescope, check out

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observations with modern telescopes

Galileos telescope got people to think about the solar system in the right way. Modern tools have also transformed our way of thinking about the universe. Imagine this: Today you can see all of the things Galileo saw using a good pair of binoculars. You can see sunspots if you have special filters on the lenses. (Never look directly at the Sun without using the proper filters!) With the most basic telescope, you can see polar caps on Mars, the rings of Saturn, and bands in the atmosphere of Jupiter. You can see many times more stars with a telescope than without a telescope. Still, stars seen in a telescope look like single points of light. They are so far away. Only the red supergiant star Betelgeuse is large enough to appear as a disk. Except for our Sun, of course. Today, astronomers attach special instruments to telescopes. This allows them to collect a wide variety of data. The data is fed into computers so that it can be studied. An astronomer may take weeks to analyze all of the data collected from just a single night!

observations with telescopes

radio telescopes

Radio telescopes collect radio waves. These telescopes are even larger telescopes than reflectors. Radio telescopes look a lot like satellite dishes. In fact, both are designed to collect and focus radio waves or microwaves from space. The largest single radio telescope in the world is at the Arecibo Observatory in Puerto Rico (see Figure 23.6). This telescope is located in a natural sinkhole. The sinkhole formed when water flowing underground dissolved the limestone. This telescope would collapse under its own weight if it were not supported by the ground. There is a big disadvantage to this design. The telescope can only observe the part of the sky that happens to be overhead at a given time. A group of radio telescopes can be linked together with a computer. The telescopes observe the same object. The computer then combines the data from each telescope. This makes the group function like one single telescope. An example is shown in Figure 23.7. To learn more about radio telescopes and radio astronomy in general, go to

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space telescopes

Telescopes on Earth all have one big problem: Incoming light must pass through the atmosphere. This blocks some wavelengths of radiation. Also, motion in the atmosphere distorts light. You see this when you see stars twinkling in the night sky. Many observatories are built on high mountains. There is less air above the telescope, so there is less interference from the atmosphere. Space telescopes avoid such problems completely since they orbit outside the atmosphere. The Hubble Space Telescope is the best known space telescope. Hubble is shown in Figure 23.8. Hubble began operations in 1994. Since then it has provided huge amounts of data. The telescope has helped astronomers answer many of the biggest questions in astronomy. The National Aeronautics and Space Administration (NASA) has placed three other major space telescopes in orbit. Each uses a different part of the electromagnetic spectrum. The James Webb Space Telescope will launch in 2014. The telescope will replace the aging Hubble. To learn more about NASAs great observatories, check out

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types of telescopes

optical telescopes

Humans have been making and using magnifying lenses for thousands of years. The first telescope was built by Galileo in 1608. His telescope used two lenses to make distant objects appear both nearer and larger. Telescopes that use lenses to bend light are called refracting telescopes, or refractors (Figure 23.4). The earliest telescopes were all refractors. Many amateur astronomers still use refractors today. Refractors are good for viewing details within our solar system. Craters on the surface of Earths Moon or the rings around Saturn are two such details. Around 1670, Sir Isaac Newton built a different kind of telescope. Newtons telescope used curved mirrors instead of lenses to focus light. This type of telescope is called a reflecting telescope, or reflector (see Figure 23.5). The mirrors in a reflecting telescope are much lighter than the heavy glass lenses in a refractor. This is important because a refracting telescope must be much stronger to support the heavy glass. Its 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. This means that they can study dimmer or more distant objects. The largest optical telescopes in the world today are reflectors. Telescopes can also be made to use both lenses and mirrors. For more on how telescopes were developed, visit http://galileo.rice.edu/sci/instruments/telescope.html .

the speed of light

In space, light travels at about 300,000,000 meters per second (670,000,000 miles per hour). How fast is that? A beam of light could travel from New York to Los Angeles and back again nearly 40 times in just one second. Even at that amazing rate, objects in space are so far away that it takes a lot of time for their light to reach us. Even light from the nearest star, our Sun, takes about 8 minutes to reach Earth.

electromagnetic spectrum

Earth is just a tiny speck in the universe. Our planet is surrounded by lots of space. Light travels across empty space. Astronomers can study light from stars to learn about the universe. Light is the visible part of the electromagnetic spectrum. Astronomers use the light that comes to us to gather information about the universe.

looking back in time

When we look at stars and galaxies, we are seeing over great distances. More importantly, we are also seeing back in time. When we see a distant galaxy, we are actually seeing 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. When you see an image of the galaxy what are you seeing? You are seeing the galaxy as it was 2.5 million years ago! Since scientists can look back in time they can better understand the Universes history. Check out http://science.n

lightyears

We need a really big unit to measure distances out in space because distances between stars are so great. A light- year, 9.5 trillion kilometers (5.9 trillion miles), is the distance that light travels in one year. Thats a long way! Out in space, its actually a pretty short distance. Proxima Centauri is the closest star to us after the Sun. This near neighbor is 4.22 light-years away. That means the light from Proxima Centauri takes 4.22 years to reach us. Our galaxy, the Milky Way Galaxy, is about 100,000 light-years across. So it takes light 100,000 years to travel from one side of the galaxy to the other! It turns out that even 100,000 light years is a short distance. The most distant galaxies we have detected are more than 13 billion light-years away. Thats over a hundred-billion-trillion kilometers!

instructional diagrams

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questions

Light travels as vibrating electric and magnetic fields.

-->  a. true

b. false

To view the stars in the night sky you should use

a. an electron microscope

-->  b. a refracting telescope

c. a magnifying glass

d. a radio telescope

All telescopes work by gathering and focusing visible light.

a. true

-->  b. false

If you see a star that is 30,000 light years away, you are viewing it as it

a. appears right now

b. appeared 1 light year ago

-->  c. appeared 30,000 years ago

d. appeared when it formed

Visible light includes all the colors of the rainbow.

-->  a. true

b. false

An electromagnetic wave

-->  a. has an electric field and a magnetic field

b. oscillates between high and low energy values

c. is visible to the human eye

d. all of these

Visible light is

-->  a. a small part of the electromagnetic spectrum

b. the only wavelengths that come from most stars

c. best for observing astronomical objects

d. all of these

The only radiation emitted by the sun is visible light.

a. true

-->  b. false

The light from distant stars is very old by the time it reaches Earth.

-->  a. true

b. false

The Greeks knew that planets were different from stars because they

a. are larger and brighter

b. move in the opposite direction

c. are not included in any constellations

-->  d. wander across the background of the other stars

The largest optical telescopes today are refracting telescopes.

a. true

-->  b. false

Radio telescopes look like satellite dishes.

-->  a. true

b. false

The Hubble telescope is the only space telescope ever placed in orbit.

a. true

-->  b. false

Galileo observed that Venus has phases like the moon.

-->  a. true

b. false

Galileos observations supported the theory that planets revolve around the sun.

-->  a. true

b. false

A light year is a measure of time.

a. true

-->  b. false

type of electromagnetic wave with the highest frequency

-->  a. gamma ray

b. light-year

c. optical telescope

d. radio wave

e. wavelength

f. wave frequency

g. radio telescope

The most distant objects are viewed with radio waves.

a. true

-->  b. false

number of waves that pass a given point per second

a. gamma ray

b. light-year

c. optical telescope

d. radio wave

e. wavelength

-->  f. wave frequency

g. radio telescope

type of electromagnetic wave with the longest wavelength

a. gamma ray

b. light-year

c. optical telescope

-->  d. radio wave

e. wavelength

f. wave frequency

g. radio telescope

Radio telescopes can be linked together to gather more data on a space object.

-->  a. true

b. false

horizontal distance between the same points on adjacent waves

a. gamma ray

b. light-year

c. optical telescope

d. radio wave

-->  e. wavelength

f. wave frequency

g. radio telescope

The longer the wavelength, the higher the frequency.

-->  a. true

b. false

Space telescopes are able to gather more types of waves than land-based telescopes.

a. true

-->  b. false

device that collects and focuses radio waves from space

a. gamma ray

b. light-year

c. optical telescope

d. radio wave

e. wavelength

f. wave frequency

-->  g. radio telescope

device that gathers and magnifies visible light from space

a. gamma ray

b. light-year

-->  c. optical telescope

d. radio wave

e. wavelength

f. wave frequency

g. radio telescope

unit for measuring the vast distances of space

a. gamma ray

-->  b. light-year

c. optical telescope

d. radio wave

e. wavelength

f. wave frequency

g. radio telescope

The speed of light through space is

a. 300 thousand m/s.

b. 3 million m/s.

c. 30 million m/s.

-->  d. 300 million m/s.

The ancient Greeks observed that some stars moved across the background of other stars. They named these stars wanderers. Today, we call them

a. moons.

-->  b. planets.

c. galaxies.

d. constellations.

The hottest stars emit primarily

a. radio waves.

b. microwaves.

c. visible light.

-->  d. X rays and gamma rays.

The earliest telescopes were

a. reflecting telescopes.

-->  b. refracting telescopes.

c. radio telescopes.

d. none of the above

The main reason that space telescopes can gather more information than telescopes on Earths surface is that space telescopes are

a. closer to objects in space.

-->  b. above Earths atmosphere.

c. optical telescopes.

d. two of the above

Which of the following discoveries as made by Galileo with a telescope?

a. Earth has a moon.

-->  b. The moon has craters.

c. Venus has moons.

d. all of the above

The spectrum of light from a star can be used to learn the stars

a. speed.

b. direction.

c. temperature.

-->  d. all of the above

diagram questions

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