Announcements. Topics To Be Covered in this Lecture

Transcription

1 Announcements! Tonight s observing session is cancelled (due to clouds)! the next one will be one week from now, weather permitting! The 3rd LearningCurve activity was due earlier today! Assignment 3 and Quiz 3 are due on Tuesday at 2 p.m., and will cover Chapter 3 from the textbook 1 Topics To Be Covered in this Lecture! I am assuming that you have read Chapter 3 in advance of this lecture! I will focus on the following in this lecture:! the electromagnetic spectrum! reflecting and refracting optical telescopes! light gathering ability and resolving power! multi-wavelength astronomy 2 1

2 LIGHT AND TELESCOPES 3 The Use of Light in Astronomy! All objects emit light (radiation)! Light can travel through the emptiness of space, traversing enormous distances at 300,000 km/s! This light can provide detailed information about distant stars and galaxies! e.g., size, distance, speed, chemical makeup! We will study the properties of light, and its use in astronomy 4 2

3 The Wavelength of a Light Wave! All light waves travel at the same speed in a vacuum! However, there are different types of light waves! One defining property of a light wave is its wavelength! Wavelength is the length of an individual wave cycle! for example, the distance between any two successive peaks 5 Other Properties of Light Waves! A related property is the frequency of a light wave! this gives the number of waves that pass a given location every second! high frequency light has short wavelengths! low frequency light has long wavelengths! The amount of energy carried by a light wave depends on its frequency! higher frequency light carries more energy 6 3

4 7 The Visible Spectrum of Light! Radiation that we can see with our eyes is referred to as visible light! Visible light comes in a range of colours! These colours correspond to different wavelengths of visible light! the longest wavelengths are red! the shortest wavelengths are violet 8 4

5 Wavelengths of Visible Light! The wavelengths of visible light are very small! We will use units called nanometres (nm) to describe these tiny wavelengths! a nanometre is 1 billion times smaller than a metre! 1 m = 10 9 nm! The visible spectrum ranges from 400 nm (violet) to 700 nm (red)! our eyes cannot detect light outside this range 9 The Electromagnetic Spectrum! Our eyes are sensitive only to the narrow wavelength range of visible light! this range is well suited to the available light here on the Earth! the Sun gives off most of its light in the visible part of the spectrum! our eyes have evolved to make use of this light! We now know that most radiation lies at either longer or shorter wavelengths! This radiation is invisible to the human eye, but can be detected in other ways 10 5

6 11 Long Wavelength Radiation! Radio waves have the longest wavelengths! radio wavelengths are larger than 10 cm! this part of the spectrum includes radar, AM/FM, and TV! radio waves are not sound waves (a radio converts electromagnetic radiation into sound)! Microwaves have slightly shorter wavelengths! 1 mm to 10 cm! Infrared light lies between microwaves and the red end of the visible spectrum! we cannot detect infrared light with our eyes! it is possible to feel some infrared light as heat 12 6

7 Short Wavelength Radiation! Ultraviolet light has shorter wavelengths than violet light! wavelengths 10 nm nm! this invisible radiation can damage our eyes and skin! X-rays have even shorter wavelengths! 0.01 nm - 10 nm! this high energy radiation can penetrate our bodies! Gamma-rays have wavelengths smaller than 0.01 nm! this very high energy radiation is often associated with radioactivity, and can be damaging to living cells 13 Atmospheric Transparency! Radiation from space must pass through Earth s atmosphere before reaching us! On a clear day or night, the Earth s atmosphere is transparent to visible light! this light can pass through the atmosphere! The same is not true for most other wavelengths of light! 14 7

8 Atmospheric Opacity! The Earth s atmosphere is opaque at most wavelengths! opacity is the extent to which radiation is blocked by the material it is travelling through! Many types of radiation from space are unable to reach the surface of the Earth 15 What Causes Opacity? Atmospheric particles and gases absorb radiation at certain wavelengths, making the atmosphere opaque! Clouds absorb visible light! optical telescopes can t be used when it is cloudy! The ozone layer absorbs ultraviolet, X-ray, and gammaray radiation! if it didn t, life could not survive on the Earth s surface! Water vapour and carbon dioxide absorb infrared light! this produces the greenhouse effect 16 8

9 Limitations of the Human Eye Most objects studied by astronomers cannot be seen with the naked eye. There are several reasons for this:! the human eye cannot see very faint objects! e.g., distant galaxies or even 2 solar system planets!! the human eye is unable to resolve detailed features in anything but very nearby objects! e.g, the Moon! the human eye is sensitive only to a small part of the electromagnetic spectrum! we cannot see radio waves, X-rays, etc. 17 Enhancing Our Vision There are a number of ways that we can improve our eyesight! Telescopes are the primary tool used by astronomers in this regard! Various instruments can be used to collect and analyze light from a telescope! Sophisticated detection methods help us to improve things even more 18 9

10 Optical Telescopes! Galileo first used an optical telescope in 1609! Optical telescopes collect large amounts of visible light and bring it to a focus! It can then be seen by the human eye or other detectors (such as cameras)! There are 2 main types of optical telescopes:! refractors, which use lenses! reflectors, which use mirrors Galileo s telescope (1609) Hevelius s telescope (1673) 19 Refraction! Light travels at different speeds in different media! e.g., vacuum, air, water, glass! As a result, light changes direction when it passes from one medium into another! This is called refraction 20 10

11 How Does a Lens Work?! When light passes through a glass window pane, its medium changes from air to glass and back to air! this causes the light to be refracted (bent) twice! If the glass is curved, it can be used to bring all of the light to the same place! this location is called the focal point! This type of curved glass is called a lens! A lens can be used to collect light over a large area and bring it to a focus in one place 21 Taking Advantage of Refraction 22 11

12 Light Gathered by a Telescope! Photographic film (or a screen) can be placed at the focal point in order to capture this light! However, if you placed your eye at the focal point, the object would be all out of focus!! this is because your eye is a lens too! An extra lens, called an eyepiece, can be used to bring the light back out of focus! the light will still be more concentrated than before! your eye can now bring the light back into focus! your retina is located at your eye s focal point 23 Light Passing Through a Refracting Telescope See LaunchPad Animation

13 Reflecting Telescopes! Reflecting telescopes use mirrors (instead of lenses) to gather light and bring it to a focus! Mirrors are generally made of smooth glass coated with a reflective material (e.g., aluminum)! Light striking a flat surface is always reflected at a predictable angle (the principle of reflection)! A mirror can be curved so that the light from a distant object is reflected to the same place! this location is called the mirror s focal point 25 Reflection 26 13

14 Different Types of Reflecting Telescopes! Prime focus: there is a primary mirror only! light comes to a focus at the top of the telescope! it can be awkward to place a detector at this location! Newtonian focus: light is reflected from a secondary mirror to the side of the telescope! Cassegrain focus: light is reflected from a secondary mirror down through a hole in the primary mirror! Coudé focus: light is reflected from two or more extra mirrors and brought to a focus elsewhere 27 Reflecting Telescopes See LaunchPad Animation

15 Reflectors vs. Refractors! Reflecting and refracting telescopes perform the same function. However, all large telescopes are reflectors, not refractors. Why is this?! The amount of refraction that light undergoes depends on its colour! this is called chromatic aberration! this phenomenon blurs an object s colour image! A lens absorbs some light that passes through it! It is difficult to evenly support the weight of a large lens from its edges! this causes large lenses to sag and deform 29 The 1-metre Yerkes Refractor 30 15

16 The Keck 10-metre Reflecting Telescopes 31 Light Gathering Ability! The most important characteristic of a telescope is its ability to gather light! This is determined by the size of its primary mirror or lens! In particular, the amount of light that is gathered depends on the area of the primary mirror or lens 32 16

17 The Thirty-Metre Telescope TMT has been planned but not yet built 33 Telescope Size! Telescope sizes are usually given in terms of the diameter of the primary mirror or lens! Area is proportional to the square of the diameter (D)! area = πd 2 /4! for example, a 3-metre diameter mirror collects 9 times as much light as a 1-metre diameter mirror! Larger telescopes can more easily detect very faint objects 34 17

18 Light Gathering Power 35 Telescope Sizes! Backyard telescopes typically have diameters of no more than 20 cm! The smallest professional optical telescopes have diameters of about 1-2 metres! At present, the largest optical telescopes have diameters of 8-10 metres! e.g., the Keck and Gemini telescopes! The next generation of optical telescopes will be substantially larger! The 25-metre Giant Magellan Telescope (GMT)! The Thirty Metre Telescope (TMT)! The 39-metre European Extremely Large Telescope (E-ELT) 36 18

19 The European Extremely Large Telescope 37 Resolving Power! Larger telescopes are also better able to resolve small details in objects they observe! Every telescope has a fundamental limit to its resolving power (or resolving ability)! this is called the diffraction limit! this refers to the size of the smallest (angular) feature that can be seen with the telescope! this limit results from the wave-like nature of light! A small diffraction limit makes it possible to see very fine details! A large diffraction limit can make some things look blurry 38 19

20 Low Resolution vs. High Resolution Small backyard telescope (0.25m) Hubble Space Telescope (2.4m) 39 What Determines Resolving Power?! A telescope s diffraction limit is inversely proportional to its diameter! larger telescopes have smaller diffraction limits! this means that they can resolve smaller features! In addition, the diffraction limit is directly proportional to the wavelength of the light being observed! it is easier to resolve details at shorter wavelengths 40 20

21 Atmospheric Blurring Effects! In practice, most ground-based telescopes are unable to achieve resolutions as small as their diffraction limits! This is due to the blurring effects of the atmosphere 41 Atmospheric Seeing! Air turbulence and refraction cause nearby beams of light to take slightly different paths through the air! This leads to a blurring effect called seeing 42 21

22 What Contributes to Seeing?! Seeing varies with location, and also varies at a given location over time. Why is this?! Seeing depends primarily on the amount of atmosphere that light passes through! seeing is less of a problem at higher altitudes! seeing is worse when looking near the horizon than directly overhead! Seeing is worsened by temperature differences between the telescope, the telescope s dome, and the surrounding air 43 High Resolution Astronomy! Astronomers have devised clever techniques to minimize or remove the detrimental effects of seeing at ground-based telescopes:! Active optics make rapid changes to the tilt of the mirror to compensate for degraded image quality! Adaptive optics goes a step further, by actually deforming the mirror to correct for the effects of seeing! In both cases, very rapid corrections are needed 44 22

23 Resolution of the TMT 45 Telescopes in Space! An even better method of avoiding the Earth s atmosphere is to put a telescope in space!! There are now many telescopes in orbit around the Earth (and further away, in some cases)! These telescopes do not suffer from the effects of seeing! The most famous is the Hubble Space Telescope! This telescope has revolutionized astronomy with its high resolution images of planets, stars, galaxies, etc

24 The Hubble Space Telescope 47 The Hubble extreme Deep Field (XDF) 48 24

25 The XDF 49 The Depth of the XDF! The XDF is the deepest image of the sky ever taken! roughly 23 days worth of exposures were combined to create this image! The faintest galaxies seen are about 10 billion times too faint to see with the naked eye! The most distant galaxy found on this image is seen as it was only 600 million years after the Big Bang! it is about 13.2 billion light years away from us 50 25

26 Collecting and Storing Light! After light has travelled through a telescope, it can be collected and stored in various ways! Attaching an eyepiece lens allows one to see all of the visible light arriving at the telescope! this is convenient for backyard astronomy, but does not allow one to store the light that is seen! moreover, the eye is unable to accumulate light over long periods of time! also, the eye is only able to detect visible light 51 Photography and CCD s! Photographic film (or photographic plates) can be placed in a telescope s focal plane! this allows the astronomer to keep a permanent record of the observations! it also permits long exposures, which can reveal faint light not seen otherwise! Charge-coupled devices (CCD s) are even better! CCD s are basically sophisticated digital cameras! they too allow for long exposures 52 26

27 Advantages of CCDs! CCDs are much more sensitive to faint light than photographic film or plates! They are also capable of better resolution! CCDs are sensitive to a wide range of wavelengths, from ultraviolet to infrared! Digital images are much easier to store and transport! Digital images can be analyzed with sophisticated computer software programs! Most modern day astronomy is done using CCDs 53 CCD Images of the Moon 54 27

28 Radio Astronomy! So far, we have discussed observations in the optical part of the spectrum only! Now, we switch to the study of radio waves, which can also penetrate Earth s atmosphere! Radio waves from space were first detected by Karl Jansky in 1931! He deduced that this radiation had originated in our Galaxy, the Milky Way! Thus began the discipline of radio astronomy 55 Radio Telescopes! The long wavelengths of radio waves require key differences in telescope design! Radio telescopes need to be much larger than optical telescopes, since resolution is proportional to wavelength! even so, they tend to have very poor resolution compared to their optical counterparts! They do not need to be as smooth as optical telescopes! They are very sensitive to faint radiation, due to their large sizes 56 28

29 The 305m Arecibo Radio Telescope (Puerto Rico) 57 The 500m FAST Radio Telescope (China) FAST is now the world s largest single-dish radio telescope

30 Advantages of Radio Telescopes! Observations can be made during the daytime and during poor weather! Some objects which emit very little optical light can be detected quite easily at radio wavelengths! Radio waves are least affected by absorption due to dust! The universe emits most of its light at long wavelengths 59 Interferometry! A technique called interferometry can greatly improve the resolution in some cases! Interferometry is the simultaneous observation of an object using two or more telescopes! the further apart the telescopes are, the better the effective resolution will be! The Very Large Array in New Mexico consists of 27 radio telescopes spread out over 30 km! The Atacama Large Millimetre Array (ALMA) in Chile will have 66 telescopes covering up to 16 km! The Very Long Baseline Interferometer (VLBI) uses radio telescopes on different continents! 60 30

31 The Very Large Array (VLA) 61 The Atacama Large Millimetre Array (ALMA) 62 31

32 A Protoplanetary Disk Imaged by ALMA 63 VLBA (Very Long Baseline Array) 64 32

33 VLBA Detection of Voyager 1! As an example of the VLBA s size and resolution, consider this detection of Voyager 1 s radio transmitter! Distance: 18.5 billion km (123 AU)! Power: 22 Watts (comparable to the light in your fridge) 0.5 arcseconds 65 Infrared Astronomy! Some infrared astronomy is possible from the ground, even though the atmosphere is at least partly opaque! to minimize atmospheric opacity, infrared telescopes are often built high on mountains! Prospects are much better at very high altitudes (balloons) or in orbit (satellites) 66 33

34 The Spitzer Space Telescope! This 0.85 metre IR telescope was launched in 2003! It trails behind the Earth in its orbit around the Sun! This telescope has greatly advanced infrared astronomy 67 The James Webb Space Telescope (JWST)! The 6.5 metre JWST will be the world s most powerful IR telescope! It will orbit the Sun 1.5 million km away from the Earth! Its launch is scheduled for October

35 Ultraviolet Astronomy! Earth s atmosphere is completely opaque to most ultraviolet (UV) radiation! Therefore, these observations can only be carried out from space! A number of satellites have been used to learn about the universe at UV wavelengths! The International Ultraviolet Explorer (IUE) collected data from 1978 to 1996, and revolutionized UV astronomy! More recently, the Galaxy Evolution Explorer (GALEX) has made tremendous advances in UV astronomy 69 GALEX GALEX, launched in 2003 Galaxy M81 in visible vs. UV light 70 35

36 High Energy Astronomy! Astronomy at shorter wavelengths is challenging! high energy radiation tends to penetrate mirrors instead of being reflected! the universe gives off much less light at these wavelengths compared to radio or even optical light! as with UV astronomy, these observations are only possible from space! The Chandra and XMM-Newton X-ray satellite telescopes have made great advances in recent years! Gamma-ray telescopes such as the Swift and Fermi satellites have also been very successful 71 X-ray and Gamma-ray Observations A gamma-ray burst as seen by SWIFT The Chandra Deep Field (blue objects are distant black holes) 72 36

37 Before Next Class! Complete Assignment 3 and Quiz 3! due on Tue. Oct. 4 at 2 p.m.! Read Chapter 4 (we will cover it in class next week)! Complete LearningCurve: Atomic Physics and Spectra! due on Thu. Oct. 6 at noon 73 37