Astronomy: The Solar System and Beyond 5th edition

1. Astronomy:The Solar System and Beyond 5th edition Michael Seeds

2. The strongest thing that’s given us to see with’s A telescope. Someone in every town Seems to me owes it to the town to keep one. ROBERT FROST The Star-Splitter Chapter 5

3. Starlight is going to waste. • Every night, light from the stars falls on trees, oceans, roofs, and empty parking lots, and it is all wasted. • To an astronomer, nothing is so precious as starlight. • It is our only link to the sky, and the astronomer’s quest is to gather as much starlight as possible and extract from it the secrets of the stars.

4. The telescope is the emblematic tool of the astronomer, because its purpose is to gather and concentrate light for analysis. • Nearly all the interesting objects in the sky are faint sources of light. • So, modern astronomers are driven to build the largest possible telescopes to gather the maximum amount of light.

5. Thus, any discussion of astronomical instruments is concentrated on large telescopes and the specialized tools used to analyze light.

6. If you wish to gather visible light, a normal telescope will do. • However, visible light is only one kind of radiation.

7. Astronomers can also extract information from other forms of radiation—by using specialized telescopes. • Radio telescopes provide an entirely different view of the sky. • Some specialized telescopes can be used from Earth’s surface. • However, some must go into orbit above Earth’s atmosphere. For instance, telescopes that observe X rays must be placed in orbit.

8. As you study the sophisticated telescopes and instruments that modern astronomers use, keep in mind Frost’s suggestion: In every town, someone should keep a telescope. • Astronomy is more than technology and scientific analysis. • It tells us what we are, and every town should have a telescope to keep us looking upward.

9. Radiation: Information from Space • Just as a book on baking bread might begin with a discussion of flour, this chapter on telescopes begins with a discussion of light. • This is not just visible light, but the entire range of radiation from the sky.

10. Light as a Wave and a Particle • When you admire the colors of a rainbow, you are seeing light behave as a wave. • However, when you use a camera to take a photo of the same rainbow, the light entering the camera’s light meter acts as a particle. • Light is very strange, and there is nothing else like it in the universe.

11. Light as a Wave and a Particle • Light is both wave and particle. • How it acts at a given time depends on how you observe it. • Astronomers observe both wave and particle properties of light as they gather information about the stars.

12. Light as a Wave and a Particle • Light is a form of electromagnetic radiation. • We use the word light to refer to electromagnetic radiation that we can see. • However, visible light is only one part of a range that also includes X rays and radio waves.

13. Light as a Wave and a Particle • Electromagnetic radiation travels through space at 300,000 km/s (180,000 mi/s). • This is commonly referred to as the speed of light, c. • However, this is the speed of all electromagnetic radiation. • Electromagnetic radiation travels through space as electric and magnetic waves.

14. Light as a Wave and a Particle • You are familiar with waves in water: if you disturb a pool of water, waves spread across its surface. • Imagine that you use a meterstick to measure the distance between successive peaks of a wave. • This distance is the wavelength, usually represented by the Greek letter lambda (λ).

15. Light as a Wave and a Particle • The colors you see in a rainbow or on the surface of a soap bubble are caused by differing wavelengths of the light that reaches your eye. • You sense different wavelengths of light as different colors.

16. Light as a Wave and a Particle • Sound also travels in waves. • You hear different wavelengths of sound as different pitches. • Unlike sound, however, electromagnetic waves—including light—do not require a medium and can travel through space where there is no sound.

17. Light as a Wave and a Particle • Although light does behave as a wave, it also behaves as a particle. • A particle of light is called a photon. • You can think of a photon as a bundle of waves.

18. Light as a Wave and a Particle • The amount of energy a photon carries depends inversely on its wavelength. • That is, shorter-wavelength photons carry more energy and longer-wavelength photons carry less. • You can express this relationship in a simple formula: E = (hc) / λ • Here, h is Planck’s constant (6.6262 x 10-34 joule second) and c is the speed of light (3 x 108 m/s). • This book will not use this formula for a calculation.

19. Light as a Wave and a Particle • The important point is the inverse relationship between the energy E and the wavelength λ. • As λ gets smaller, E gets larger. • Thus, a photon of visible light carries a very small amount of energy, but a photon with a wavelength much shorter than that of visible light can carry much more energy.

20. The Electromagnetic Spectrum • A spectrum is an array of electromagnetic radiation in order of wavelength. • You are most familiar with the spectrum of visible light, which you see in rainbows. • The colors of the spectrum differ in wavelength, with red having the longest wavelength and violet the shortest.

21. The Electromagnetic Spectrum • The average wavelength of visible light is about 0.0005 mm. • You could put 50 light waves end to end across the thickness of a sheet of household plastic wrap. • It is too awkward to measure such short distances in millimeters. • So, scientists measure the wavelength of light using the nanometer (nm), one-billionth of a meter (10–9 m).

22. The Electromagnetic Spectrum • Another unit that astronomers commonly use is called the angstrom (Å), named after the Swedish astronomer Anders Jonas Ångström. • One angstrom is 10–10 m. • The wavelength of visible light ranges between 400 nm and 700 nm, or between 4000 Å and 7000 Å. • Radio astronomers often refer to long radio wavelengths using meters, centimeters, or millimeters.

23. The Electromagnetic Spectrum • The visible spectrum makes up only a small part of the electromagnetic spectrum. • Beyond the red end of the visible spectrum lies infrared radiation, where wavelengths range from 700 nm to about 1 mm.

24. The Electromagnetic Spectrum • Your eyes are not sensitive to this radiation, but your skin senses it as heat. • A heat lamp is nothing more than a bulb that gives off large amounts of infrared radiation. • Beyond the infrared part of the electromagnetic spectrum lie radio waves. • The radio radiation used for AM radio transmissions has wavelengths of a few kilometers down to a few hundred meters.

25. The Electromagnetic Spectrum • FM, television, and military, governmental, and ham radio transmissions have wavelengths that range down to a few tens of centimeters. • For instance, microwave transmissions—used for radar and long-distance telephone communications—have wavelengths from a few centimeters down to about 1 mm.

26. The Electromagnetic Spectrum • You may not think of radio waves in terms of wavelength, because radio dials are marked in units of frequency—the number of waves that pass a stationary point in 1 second. • To calculate the wavelength of a radio wave, divide the speed of light by the frequency. • Thus, when you tune in your favorite FM station at 89.5 MHz (million cycles per second), you are adjusting your radio to detect radio photons with a wavelength of 335 cm.

27. The Electromagnetic Spectrum • The distinction between the wavelength ranges is not sharp. • Long-wavelength infrared radiation and the shortest microwave radio waves are the same. • Similarly, there is no clear division between the short-wavelength infrared and the long wavelength part of the visible spectrum. • It is all electromagnetic radiation.

28. The Electromagnetic Spectrum • At the other end of the spectrum, you will notice that electromagnetic waves shorter than violet are called ultraviolet. • Even shorter electromagnetic waves are called X rays. • The shortest are gamma rays. • Again, the boundaries between these ranges are not clearly defined.

29. The Electromagnetic Spectrum • Remember the formula for the energy of a photon. • High-energy X rays and gamma rays can be dangerous, and even ultraviolet photons have enough energy to do you harm. • Small doses produce a suntan, larger doses can cause sunburn, and extreme doses might produce skin cancers.

30. The Electromagnetic Spectrum • Contrast this with the lower-energy infrared photons. • Individually, they have too little energy to affect skin pigment, a fact that explains why you can’t get a tan from a heat lamp. • Only by concentrating many low-energy photons in a small area, as in a microwave oven, can you transfer significant amounts of energy.

31. The Electromagnetic Spectrum • Astronomers are interested in electromagnetic radiation because it carries clues to the nature of stars, planets, and other celestial objects. • Earth’s atmosphere is opaque to most electromagnetic radiation, as displayed by the graph.

32. The Electromagnetic Spectrum • Gamma rays, X rays, and some radio waves are absorbed high in Earth’s atmosphere. • A layer of ozone (O3) at an altitude of about 30 km absorbs ultraviolet radiation. • Water vapor in the lower atmosphere absorbs the longer-wavelength infrared radiation.

33. The Electromagnetic Spectrum • Only visible light, some shorter-wavelength infrared, and some radio waves reach Earth’s surface through two wavelength regions called atmospheric windows. • Obviously, if you wish to study the sky from Earth’s surface, you must look out through one of these windows.

34. Building Scientific Arguments • What could you see if your eyes were sensitive only to X rays? • As you build this scientific argument, you must imagine a totally new situation. • That is sometimes a powerful tool in the critical analysis of an idea.

35. Building Scientific Arguments • In this case, you might at first expect to be able to see through walls, but remember that your eyes detect only light that already exists. • There are almost no X rays bouncing around at Earth’s surface. • So, if you had X-ray eyes, you would be in the dark and would be unable to see anything. • Even when you looked up at the sky, you would see nothing, because Earth’s atmosphere is not transparent to X rays. • So, if Superman can see through walls, it is not because his eyes can detect X rays.

36. Building Scientific Arguments • Now, imagine a slightly different situation and modify your argument. • Would you be in the dark if your eyes were sensitive only to radio wavelengths? • Earth has two atmospheric windows. • So, there are two main types of ground-based telescopes.

37. Optical Telescopes • Astronomers build optical telescopes to gather light and focus it into sharp images. • This requires sophisticated optical and mechanical designs, and it leads astronomers to build gigantic telescopes on the tops of high mountains.

38. Optical Telescopes • To begin, you need to understand the terminology of telescopes. • However, it is more important to understand how different kinds of telescopes work and why some are better than others.

39. Two Kinds of Optical Telescopes • Optical telescopes focus light into an image in one of two ways, as displayed. • A lens bends (refracts) the light as it passes through the glass and brings it to a focus to form a small, inverted image.

40. Two Kinds of Optical Telescopes • Telescopes also use concave mirrors to focus an image by reflecting light. • The mirrors used in these telescopes are concave pieces of glass with a reflective coating on the front surface.

41. Two Kinds of Optical Telescopes • In either case, the focal length is the distance from the lens or mirror to the image formed of a distant light source, such as a star. • Short-focal-length lenses and mirrors must be strongly curved, and long-focal-length lenses and mirrors are less strongly curved.

42. Two Kinds of Optical Telescopes • Grinding the proper optical shapes is an expensive process. • The surfaces of lenses and mirrors must be shaped and polished to accuracies of less than the wavelength of light (less than 0.0005 mm). • Creating the optics for a large telescope can take months or years, involve huge, precision machinery, and employ expert optical engineers and scientists.

43. Two Kinds of Optical Telescopes • The main lens in a refracting telescope is called the primary lens. • The main mirror in a reflecting telescope is called the primary mirror. • These are also called the objective lens and objective mirror.

44. Two Kinds of Optical Telescopes • Both kinds of telescopes form a very small, inverted image that is difficult to observe directly. • So, astronomers use a small lens called the eyepiece to magnify the image and make it convenient to view.

45. Two Kinds of Optical Telescopes • The two types of telescopes make use of the two ways to focus light. • Refracting telescopes use a large lens to gather and focus the light. • Reflecting telescopes use a concave mirror. • The advantages of the reflecting telescope have made it the preferred design for modern observatories.

46. Two Kinds of Optical Telescopes • Refracting telescopes suffer from a serious optical distortion that limits their usefulness. • When light is refracted through glass, shorter wavelengths bend more than longer wavelengths, and blue light comes to a focus closer to the lens than does red light.

47. Two Kinds of Optical Telescopes • If you focus the eyepiece on the blue image, the red light is out of focus, and you see a red blur around the image. • If you focus on the red image, the blue light blurs. • The color separation is called chromatic aberration.

48. Two Kinds of Optical Telescopes • Telescope designers can grind a telescope lens of two components made of different kinds of glass, and so bring two different wavelengths to the same focus.

49. Two Kinds of Optical Telescopes • This does improve the image, but these achromatic lenses are not totally free of chromatic aberration, because other wavelengths still blur. • Telescopes made with such lenses were popular until the end of the 19th century.

50. Two Kinds of Optical Telescopes • The primary lens of a refracting telescope is very expensive to make. • This is because it must be a two-piece achromatic lens, and the glass must also be pure and flawless because the light passes through the lens. • Also, the four surfaces must be ground precisely, and the lens can be supported only along its edge.