Unit 05 - Light and Telescopes PDF

ASTR 1205 Unit 5 Dr. Bryan Rowsell Unit 5: Light and Telescopes [Alice Koning] 5.1 Basic Properties of Light and Matter How familiar are you with these three concepts: Light and Telescopes ·5−1· 5.1: Basic Properties of Light and Matte...

ASTR 1205 Unit 5 Dr. Bryan Rowsell Unit 5: Light and Telescopes [Alice Koning] 5.1 Basic Properties of Light and Matter How familiar are you with these three concepts: Light and Telescopes ·5−1· 5.1: Basic Properties of Light and Matter ASTR 1205 Unit 5 Dr. Bryan Rowsell Waves Physics definition: A wave is the propagation of a disturbance in a regular and organized way. Example: A drop of water falls on the surface of a pond. The disturbance propagates away from the place where the drop landed as ripples in the surface of the water. Two types of waves: 1. transverse waves 2. longitudinal waves Light is a self−propagating transverse wave that can travel through a vacuum or a medium, an electromagnetic wave, i.e. there is a magnetic and electric component at right angles. Light and Telescopes ·5−2· 5.1: Basic Properties of Light and Matter ASTR 1205 Unit 5 Dr. Bryan Rowsell A changing electric field creates a magnetic field. A changing magnetic field creates an electric field. Waves have two important properties: 1. wavelength or λ: distance between adjacent spots on a wave, i.e. peak−to−peak, trough−to−trough, etc. Usually in units of nm but sometimes m. 2. frequency, f or ν: number of wave peaks passing a given point 1 per second, in units of , or s−1 or Hz. 𝑠 In the wave shown on page 5−2, what is the λ and ν? Speed of a wave is given by distance over time, λ has distance units, and ν has units of inverse time, so: 1 𝑚 𝑐 = 𝜆𝜈 (𝑚 ) ( ) = 𝑠 𝑠 The speed of light, c, in a vacuum is constant at 2.98×108 m/s (can shortcut to 3×108). λ and ν are inversely related…if λ increases, what must happen to ν to keep c constant? Light can also be viewed as a particle as opposed to a wave…huh? Particles of light are called photons. Single photons interact with single atoms. Each photon has a wavelength (λ), frequency (f or ν),and energy (E). ℎ𝑐 𝑐 𝐸𝑝ℎ𝑜𝑡𝑜𝑛 = ℎ𝜈 = 𝑟𝑒𝑐𝑎𝑙𝑙 𝑐 = 𝜆𝜈, 𝑡ℎ𝑢𝑠 𝜈 = 𝜆 𝜆 In a vacuum, photons travel at the speed of light (c). Light and Telescopes ·5−3· 5.1: Basic Properties of Light and Matter ASTR 1205 Unit 5 Dr. Bryan Rowsell The Electromagnetic Spectrum The light our eyes can see (visible light) is a tiny part of the complete spectrum of light. The complete spectrum of all forms of light is called the electromagnetic spectrum. Radio waves are: (a) a form of light (b) a form of sound (c) a type of spectrum (d) all of the above In a vacuum, what is the speed of a radio wave? (a) about 300 m/s (b) about 300,000,000 m/s (c) depends on volume (d) depends on temperature In astronomy, we use the light coming from distant objects to decode information about those distant objects. To decode this information, we must understand the properties of matter and how it interacts with light. Light and Telescopes ·5−4· 5.1: Basic Properties of Light and Matter ASTR 1205 Unit 5 Dr. Bryan Rowsell Atomic Structure Matter is made of atoms. Atoms are teeny tiny! The number of atoms in a typical drop of water may exceed the number of stars in the observable universe. Atoms are made of neutrons, protons, and electrons. Atoms are actually mostly empty space…if the nucleus of a hydrogen atom (i.e. just a single proton) was a basketball, where would its electron orbit on average? about 1.5 km away, as far as Pluto is from the Sun on the same scale (Sun = baseketball, electron = Pluto) Light and Telescopes ·5−5· 5.1: Basic Properties of Light and Matter ASTR 1205 Unit 5 Dr. Bryan Rowsell Different types of atoms give rise to the different elements; the number of protons in an atom’s nucleus defines the element. Ordinary atoms have no net charge because they have the same number of protons and electrons. Atoms that have gained or lost an electron have a net charge and are called ions. Versions of an element with different numbers of neutrons arecalled isotopes. Within an atom or ion, there are energy levels of the electrons, not unlike the planets orbiting the sun (but in reality, they are VERY different). Electrons in atoms can only have a few specific energies. Energies between these specific values are not allowed. The allowed energies are called the energy levels (or energy states) of the atom. The energy level closest to the nucleus is called energy level 1 (also called the ground state). The energy levels in an atom/ion are like stairs on a staircase, electrons cannot exist between energy levels, the electron is either on a step or not on a step. These staircases (i.e. energy levels) are unique for every atom/ion: The energy levels in a hydrogen atom are different than the energy levels in a helium atom. Light and Telescopes ·5−6· 5.1: Basic Properties of Light and Matter ASTR 1205 Unit 5 Dr. Bryan Rowsell Matter emits, absorbs, transmits, and scatters/reflects light. These interactions between light and matter determine the appearance of everything around us. When an electron absorbs a photon, it “jumps” up to a higher energy level. When an electron emits a photon, it “jumps” down to a lower energy level. The energy of the absorbed/emitted photon is equal to the energy difference, i.e. Δ𝐸, between the two energy levels it jumped between. Light and Telescopes ·5−7· 5.1: Basic Properties of Light and Matter ASTR 1205 Unit 5 Dr. Bryan Rowsell Of the labelled energy level transitions in the diagram on the right, which one corresponds to the electron jump shown in the picture on the bottom of the previous page? Of the labelled energy level transitions in the diagram above, which one is not allowed? Of the labelled energy level transitions in the diagram above, which one represents the removal of an electron (thus leaving behind a positively charged ion)? → 5.2 Learning from Light The absorption/emission of photons with energies equal to the differences between energy levels results in a unique line spectrum for each atom (and molecule). There are three types of spectra (plural of spectrum) found in astronomy: Light and Telescopes ·5−8· 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell 1. Continuous Spectrum The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption. 2. Emission Line Spectrum A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines. 3. Absorption Line Spectrum A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum. If the cloud of gas wasn’t there, we’d see either the emission or continuous spectrum of the hot light source. Light and Telescopes ·5−9· 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell Emission vs. Absorption Of the labelled energy level transitions in the diagram on page 5−8, choose a transition that represents the emission of a photon. What elements are present in the cloud of gas that produced the observed absorption line spectrum in the top row of the image? (a) hydrogen (H) and helium (He) (b) H and nitrogen (N) (c) oxygen (O) and carbon (C) (d) He and C Light and Telescopes · 5 − 10 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell The following is all real science: In December 2020, a team of scientists published results of a study where they observed the spectra of an exoplanet’s atmosphere as it passed in front of its parent star. They discovered evidence for the presence of sodium and water vapor. What type of spectrum did scientists observe to make this discovery? (a) continuous spectrum (b) emission line spectrum (c) absorption line spectrum Doppler Shift Spectral lines shift to shorter (bluer) wavelengths when an object is moving towards us. The spectrum is said to be blueshifted. Spectral lines shift to longer (redder) wavelengths when an object is moving away from us. The spectrum is said to be redshifted. Think of car speeding past you. How does that sound? Sounds higher−pitched while car is coming toward you (high frequency or ν) and sounds lower−pitched when moving away from you (lower ν). Light and Telescopes · 5 − 11 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell Put these three objects in order from smallest to biggest observed Doppler shift. (a) 1, 2, 3 (b) 3, 2, 1 (c) 2, 3, 1 (d) 1, 3, 2 (e) all Doppler shifts are equal In addition to Doppler shifts, which can tell us if an object is moving closer to us or away from us, the thermal spectrum can tell us what the temperature of that object is. Nearly all large, dense objects (including stars, planets, and humans) emit a particular type of continuous spectrum called a thermal spectrum. Light and Telescopes · 5 − 12 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell Thermal spectra have a characteristic shape. The spectrum depends only on the object’s temperature. What patterns can you discern from the diagram to the right? 1. Hotter objects emit more light (per square meter of surface) at all wavelengths than cooler objects. 2. Hotter objects emit photons with a higher average energy. Why don’t humans glow in the dark? (a) people do not emit any kind of light (b) people only emit light that is invisible to our eyes (c) people are too small to emit enough light to see (d) people do not contain enough radioactive material In what region of the e/m spectrum was this photo taken? Light and Telescopes · 5 − 13 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell Wien’s Law Point #2 from the previous page is called Wien’s Law. Wilhelm Wien recognized there is a simple relationship between the temperature (T, measured in Kelvin) of the object and the wavelength at the peak of the thermal spectrum (λmax, measured in nm). 2900000 𝜆𝑚𝑎𝑥 = 𝑇 The Sun’s surface temperature is 5777 K. At about what wavelength does its thermal spectrum peak? 500 nm, which is green. See previous page, the 𝜆𝑚𝑎𝑥 is in the visible region, and the tiny bit of extra green light won’t make much difference. What colour is the Sun from space? Look south on a February evening and you’ll find the constellation Orion. Follow the stars of Orion’s belt down, and you’ll find the star Sirius. Sirius is (seriously) the brightest star in the entire night sky and its thermal spectrum peaks at a wavelength of 290 nm. [Stellarium] What is Sirius’ surface temperature, in K? (a) 1000 K (b) 8400 K (c) 10,000 K (d) 100,000 K (e) 841,000,000 K Light and Telescopes · 5 − 14 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell Interpreting a Sample Astrophysical Spectrum Reflected Sunlight Thermal Radiation Continuous spectrum of visible light is Infrared spectrum peaks at wavelength like the Sun’s, except some blue light has corresponding to a T of 225 K. been absorbed – the object must look red. UV Emission IR and Visible Absorption Presence of hot gas in the upper atmosphere of the object. Absorption lines are the fingerprint of CO2. By carefully studying the features in a spectrum, we can learn a great deal about the object that created it. Spectra of astrophysical objects are usually combinations of the three basic types. Notice how the emission and absorption lines are “added” and “subtracted” against the continuous spectrum. 5.3 Collecting Light with Telescopes Myth: Telescopes increase the magnification of an image. Truth: Magnification does not add any light or new detail. High magnification is not the purpose of a telescope! Light and Telescopes · 5 − 15 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell [Joe Roberts, www.rocketroberts.com] The true purpose of telescopes is to be giant pupils, i.e. huge light−gathering devices! Thus, bigger is always better! Light-collecting area = Area of whatever is collecting light For a human, this is the area of the pupil. For a telescope, this is the area of the primary mirror (or lens). A larger light-collecting area can detect fainter objects and resolve objects that are closer together. Light and Telescopes · 5 − 16 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell Angular Resolution Angular resolution = Minimum angle between two objects where you (or a telescope) is still able to resolve the two objects as distinct. Two stars closer together than this minimum angle will look like one blobby star (they are no longer distinct). A telescope with “high angular resolution” can see objects that are very close together as distinct—its angular resolution is a small angle. Light and Telescopes · 5 − 17 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell Which statement is correct about the images below? (a) Image 1 has the lowest angular resolution and was therefore taken by the telescope with the largest light collecting area. (b) Image 1 has the lowest angular resolution and was therefore taken by the telescope with the smallest light collecting area. (c) Image 1 has the highest angular resolution and was therefore taken by the telescope with the largest light collecting area. (d) Image 1 has the highest angular resolution and was therefore taken by the telescope with the smallest light collecting area. The angular resolution of large telescopes on the ground doesn’t get as good as is theoretically possible because of atmospheric blurring. Light and Telescopes · 5 − 18 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell [Nick Strobel at www.astronomynotes.com] The Hubble Space Telescope obtains higher-resolution images than most ground-based telescopes because it is: (a) larger (b) closer to the stars (c) above the Earth’s atmosphere (d) all of the above (e) none of the above [HST, NASA] 5 Weapons to Battle Atmospheric Blurring 1. Observe objects when they’re near your zenith (as opposed to when they’re near your horizon). 2. Put your telescope where there’s less atmosphere (for example, on a mountain). 3. Put your telescope where the atmosphere is calmer (for example, on an island). 4. Give your telescope adaptive optics (AO). 5. Put your telescope in space. Light and Telescopes · 5 − 19 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell Adaptive Optics Rapid but minor changes in mirror shape compensate for atmospheric turbulence. See images of Neptune below: [NASA] Adaptive optics improves a telescope’s: (a) light−collecting area (d) all of the above (b) magnification (e) none of the above (c) angular resolution Another enemy of angular resolution is light pollution, i.e. the unwanted brightening of the night sky due to artificial light sources. Light and Telescopes · 5 − 20 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell 2 Weapons to Battle Atmospheric Blurring 1. Put your telescope far from civilization (on a mountain or remote island). 2. Put your telescope in space. A third enemy of angular resolution is atmospheric absorption. What two types of light can pass through Earth’s atmosphere to reach the ground? visible and radio 1 Weapon to Battle Atmospheric Absorption 1. Put your telescope in space Light and Telescopes · 5 − 21 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell Astronomers want to learn more about a distant object with a surface temperature of 1.7 million K. What type of telescope should they build to observe this object at its peak wavelength? (a) radio telescope on the ground (b) optical telescope on the ground (c) x−ray telescope in space (d) gamma−ray telescope in space (e) none of the above Hubble vs. Webb Hubble is a 2.4 m diameter telescope which detects mostly visible/UV telescope, James Webb is a 6.5 m diameter lens which detects primarily in the infra−red region. [NASA] Want to buy your own telescope? Advice from an astrophysicist: Buy binoculars first (e.g., 7×35) − you get much more for the same amount of money. Ignore magnification (sales pitch!). Notice: aperture size, optical quality, portability. Consumer research: Astronomy, Sky & Telescope, Mercury magazines; astronomy clubs. Light and Telescopes · 5 − 22 · 5.3: Collecting Light with Telescopes ASTR 1205 Unit 5 Dr. Bryan Rowsell Chapter 5: The Essential Cosmic Perspective End−of−Chapter Questions: 1−34, 38−45. Solutions found in Bb. Extra Resources: Watch the following Crash Course Astronomy videos: Light [11 min]: https://www.youtube.com/watch?v=jjy-eqWM38g Telescopes [12 min]: https://www.youtube.com/watch?v=mYhy7eaazIk Watch this BBC video that explains and recreates Newton's famous prism experiment [3 min]: https://www.youtube.com/watch?v=--b1F6jUx44 Watch this TedEd video that explores the history of our understanding of light [4 min]: https://www.youtube.com/watch?v=J1yIApZtLos Watch Neil deGrasse Tyson explain how infrared and ultraviolet light were discovered [4 min]: https://www.youtube.com/watch?v=Pr4qEhcGPq8 Explore this NASA website all about light—there are videos to watch, explanations to read, and informative diagrams and pictures to look at: https://science.nasa.gov/ems/ Interact with the animals at NASA's Infrared Zoo: https://coolcosmos.ipac.caltech.edu/infrared_gallery/1 Watch this Veritasium video to see the world in UV [11 min]: https://www.youtube.com/watch?v=V9K6gjR07Po Watch this video for another explanation of how absorption and emission spectra are created [5 min]: https://www.youtube.com/watch?v=fPt0jXHwqUw Read the info on this website and check out the interactive illustration of an absorption spectrum with a few different elements at the bottom of the page: https://www.khanacademy.org/science/class-11-chemistry-india/xfbb6cb8fc2bd00c8:in- instructure-of-atom/xfbb6cb8fc2bd00c8:in-in-bohr-s-model-of- hydrogenatom/a/absorptionemission-lines Watch this video for a straightforward explanation of the Doppler Effect [3 min]: https://www.youtube.com/watch?v=h4OnBYrbCjY Watch this video for another explanation of the Doppler Effect [6 min]: https://www.youtube.com/watch?v=3mJTRXCMU6o Interact with this website to blueshift and redshift a sample spectrum: http://physics.bu.edu/~duffy/HTML5/EM_Doppler.html Interact with the thermal spectra on this website: https://phet.colorado.edu/sims/html/blackbody- spectrum/latest/blackbody-spectrum_en.html Watch this Physics Demos video on emission and continuous spectra of different elements [6 min]: https://www.youtube.com/watch?v=oae5fa-f0S0 Watch this MinutePhysics video on why we put telescopes in space [2 min]: https://www.youtube.com/watch?v=Ij-u2bHo_fw Watch this TedEd video that gives a good review of the properties of light and explains the importance of exploring the universe at multiple wavelengths [6 min]: https://www.youtube.com/watch?v=O0PawPSdk28 Read about light pollution—what it is, why it matters, and what you can do about it: https://www.darksky.org/light-pollution/ Listen or Read this NASA podcast about how the raw data from telescopes gets transformed into the beautiful colour images released to the public: https://www.nasa.gov/mediacast/gravity-assist- how-we-make-webb-and-hubble-images Watch this video that quickly shows the process of how Hubble colour images are made [2 min]: https://www.youtube.com/watch?v=p5c1XoL1KFs Light and Telescopes · 5 − 23 · Questions and Resources