Science 30 Unit C Electromagnetic Energy

Transcription

1 Science 30 Unit C Electromagnetic Energy Outcome 2: Students will describe the properties of the electromagnetic spectrum and their applications in medical technologies, communication systems and remote-sensing technologies used to study the universe. Specific Outcome 2.7: Explain, in general terms, the design of telescopes that are used to gather information about the universe through the collection of as much EMR as possible; i.e. reflecting and refracting optical and radio telescopes. Specific Outcome 2.9: Describe, in general terms, how a spectroscope can be used to determine the composition of incandescent objects or substances, and the conditions necessary to produce emission (bright line) and absorption (dark line) spectra, in terms of light source and temperature. Specific Outcome 2.10: Describe technologies used to study stars o Spectroscopes used to analyze the distribution of energy in a star s continuous emission spectrum can be used to estimate the surface o temperature of the star Doppler-shift technology used to measure the speed of distant stars provides evidence that the universe is expanding Specific Outcome 2.11: Describe, in general terms, the evolution of stars and the existence of black holes, white dwarves and neutron stars. Textbook reference pages: p in Science 30 Most of what we know about our universe comes from studying electromagnetic waves (EMR) from space. Using telescopes, astronomers can detect distant stars, planets and galaxies. o The nearest galaxy to our own is 2.9 million light years away. That means we are seeing it as it was 2.9 million years ago! TELESCOPES REFRACTING TELESCOPES Refracting telescopes consist of 2 lenses to focus the light Alberta Education This type of telescope has it drawbacks due to its use of lenses. o The lenses need to be made of high quality glass. o The size of the lenses needed to be relatively small due to mass of the lenses. 1

2 REFLECTING TELESCOPES Reflecting telescopes have two mirrors to focus the light and reflect the light from the stars Alberta Education Reflecting telescopes have a few advantages over refracting telescopes o Mirrors do not break light into its component colours o Reflecting telescopes can gather infrared and ultraviolet light as well o The telescopes can have very large openings since the light reflects off the mirror o Curved mirrors can be larger than glass lenses as they can support their weight better than a large lens. Modern reflecting telescopes use much larger mirrors to collect and reflect more light. o The Canada-France-Hawaii telescope (CFHT) uses a mirror that is 3.6 m in diameter. o The Giant Magellan Telescope is under construction in the Atacama Desert, Chile and will be completed in It will consist of seven 8.4 m diameter mirrors creating a reflecting telescope with a collecting area equivalent to a 22 metre diameter mirror. RADIO TELESCOPES Radio waves have the longest wavelengths of the EMR so a very large dish is required to collect this radiation The radio waves are collected and transformed into an electrical signal for interpretation of composition and distribution of interstellar matter Radio observatories are often set up in valleys to shield them from other electromagnetic interference A 305 m dish located in Puerto Rico is the world s largest full dish radio telescope fixed in the ground. To improve resolution of the long radio waves, radio telescopes are grouped into arrays long lines of radio telescopes. The Very Large Array in New Mexico has twenty-seven 25-m radio telescopes arranged in a Y-shape that would represent a telescope with a diameter of 27 km

3 SPACE TELESCOPES Reflecting and refracting telescopes on Earth are affected by the atmosphere and weather. The Hubble Space Telescope gathers, infrared, ultraviolet and visible light without the interference of air and light pollution on Earth. Most of radiation from interstellar matter, planets, comets and asteroids are in the infrared region of the EMS. Three galaxies colliding seen by Hubble, about 400 million light-years away (~ km). The Chandra X-ray telescope is a space-based telescope used to gather X-ray radiation. o X-rays are at the high energy end of the EMS and behaves more like a particle than a wave o The telescope was designed to detect X-ray radiation from exploded stars, galaxy clusters and matter around black holes very hot regions of the Universe Interesting Facts about Chandra Taken from Alberta Education Chandra s resolving power is equivalent to the ability to read a stop sign at a distance of 19 kilometres. The light from some of the quasars observed by Chandra will have been traveling through space for ten billion years. Chandra can observe X-rays from particles up to the last second before they fall into a black hole. 3

4 ANALYZING STARLIGHT By studying the Universe across the spectrum we can get a more complete understanding of objects in space. The light from each part of the electromagnetic spectrum brings us valuable and unique information. o X-Rays bring us information about high energy phenomena such as black holes, supernova remnants, hot gas, and neutron stars. o Ultraviolet radiation reveals hot stars and quasars while visible light shows us warmer stars, planets, nebulae, and galaxies. o In the infrared spectrum we see cool stars, regions of star birth, cool dusty regions of space, and the core of our galaxy. o Radiation in the radio wave frequency shows us cold molecular clouds and the radiation left over from the Big Bang. X-ray image showing hot gas near the center of our Milky Way Galaxy (CXO). Ultraviolet view of hot white dwarf stars in a nearby galaxy (ASTRO-1). Visible light image showing stars of different colors. Infrared view of glowing dust near the center of our Galaxy (2MASS). Radio image of a supernova remnant (NRAO). We have the technology to be able to tell what stars are made of, if they are moving or not and in what direction they are moving. SPECTROSCOPES Instruments with a diffraction grating to study the spectrum of a star. A diffraction grating is a piece of glass or plastic with thousands of tightly spaced lines etched on the surface to produce spectra (different wavelengths of light). The spectrum of a star can determine the stars composition what elements it is made up of. 4

5 CONTINUOUS SPECTRUM A spectrum having no distinct lines that is distributed over an unbroken band of wavelengths. Most continuous spectra are from hot, dense objects like stars, planets, or moons ABSORPTION SPECTRUM (dark-line spectrum) A spectrum that has a pattern of dark lines due to the light passing through an absorbing medium; can be used to identify a material Each type of atom absorbs a different wavelength of light and therefore creates its own absorption spectrum EMISSION SPECTRUM (bright-line spectrum) A spectrum that has a pattern of separate bright lines that is emitted from an excited gas under low pressure; can be used to identify a material A gas will emit the same wavelength of light that is absorbed during its excitement THE ABSORPTION AND EMISSION SPECTRA TOGTHER FOR A GAS SHOULD MAKE A CONTINUOUS SPECTRUM 5

6 Examples: 1. Use the radiation spectrums to explain what star 1 and star 2 are composed of. Hydrogen Helium Sodium Star 1 Star 2 Star 1 is composed of helium and hydrogen Star 2 is composed of helium and sodium 2. Use the following spectral lines to identify the composition of the star. The star is composed of helium and hydrogen 6

7 DOPPLER SHIFT A change in pitch is called the DOPPLER EFFECT and is caused by the change in a sound s wavelength. How does this relate to a star??? If a star is approaching you, its wavelengths become compressed o So the dark lines in the stars spectrum shift towards the blue end BLUE SHIFTED. before after If a star is going away from you, the wavelengths will be longer o So The dark lines in the stars spectrum shift towards the red end. RED SHIFTED! before after The Doppler Effect Explained.. 7

8 EVOLUTION OF STARS Stars begin in dust clouds called nebulae. They condense and heat up under the influence of gravity. When it gets hot enough, the fusion reaction begins. When the star finally runs out of fuel, it dies. The brightness of stars depends on their mass and temperature Most stars are called mainsequence stars SUN-LIKE STARS NEBULA: an interstellar cloud of dust and gas MASSIVE STARS NEBULA: an interstellar cloud of dust and gas RED GIANT: a star of great size and age that has a relatively low surface temperature RED SUPERGIANT: a massive star that has increased in size and become very bright WHITE DWARF: found in the last stage of sun-like (low mass) stars - the star becomes compact as it collapses; very hot but very faint SUPERNOVA: a stellar explosion that produces a very bright cloud of ionized gas that remains a very bright object for weeks or months BLACK DWARF: hypothetical final stage of sun-like (low mass) stars very cool and invisible NEUTRON STAR: a super-dense star consisting mainly of neutrons formed as the last stage of an intermediate-mass star; remnants of a supernova PULSAR: a rotating neutron star that emits radiation in regular pulses 8

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10 BLACK HOLES An area in space with a gravitational field so strong that neither matter nor EMR can escape; formed as the last stage in the evolution of high-mass stars. Black holes are detected by the gravity effect on nearby stars Material ripped from nearby stars and falling into a black hole create collisions with atoms that heat the material to millions of degrees 10

11 Practice Questions: 1. Using the spectra to the right Which elements make up star A? (Remember there may be more than one and they may be slightly shifted) Hydrogen and mercury Star B? hydrogen Star C? Sodium and mercury 2. Which direction is star A moving? Away from you 3. Which direction is star B moving? Not moving 4. Which direction is star C moving? Toward you Practice Questions: Page & Astronomical observatories for infrared radiation are sometimes located in special aircraft that can fly at high altitudes because Earth s atmosphere absorbs most of the infrared radiation that arrives from sources in space. 34. A radio telescope is a device that can detect EMR from the radio-wave region of the electromagnetic spectrum. The energy in the radio waves is used to produce an electrical signal, which is then used to produce a visible representation of the information contained in the radio waves. Since radio waves have the longest wavelengths of all the types of radiation in the electromagnetic spectrum, the dishes 11

12 that collect these waves must be very large. In addition, radio waves are the EMR with the lowest energy content. A very large reflecting dish means that more radio-wave energy can be reflected to the detector, allowing for the study of weak signals. Page & A continuous spectrum is the full rainbow of colours with no dark lines or bands to interrupt the flow from one colour to the next. The word continuous refers to the fact that one colour continues into the next colour, forming an unbroken band of wavelengths. When observed through a spectroscope, an emission spectrum consists of a series of individual bright lines. Each discrete line corresponds to a particular wavelength of emitted light. Emission spectra are produced when gases under low pressure are energized by an external source, such as the electric current supplied to a gasdischarge tube. When observed through a spectroscope, an absorption spectrum consists of a pattern of dark lines superimposed on a continuous spectrum. Each dark line corresponds to a wavelength of light that is absorbed. Absorption spectra are produced when light passes through a gas at low pressure. The lines that a particular gas will absorb in its absorption spectrum correspond to the same lines that the gas emits in its emission spectrum. 36. The evidence supporting the idea that the universe is expanding comes from the spectral analysis of the light from remote galaxies. In every case, the pattern of spectral lines has been shifted to the red end of the spectrum. Since red shift indicates that the source of light is moving away from the observer, every remote galaxy in the universe must be moving away from Earth. If remote galaxies are increasing their distance from Earth, the universe must be expanding. Page , 38 & Referring to the graph The Continuous Spectra of Stars: Brightness of Emitted Light Versus Wavelength on page 451, you can see the overall trend is that as the surface temperature of a star rises, the brightness of the emitted light increases and the peak of the curve moves closer to the blue end of the spectrum. So, if a star produces light that is less yellow and more white than the light emitted by the Sun, the surface of this star must have a higher temperature than the surface of the Sun. Similarly, if a star produces light that is more orange than the light emitted by the Sun, the surface of this star must have a lower temperature than the surface of the Sun. 38. The feature that determines the endpoint of a star in stellar evolution is the initial mass of the gas and dust that forms the star. If the initial mass is between 0.1 and 1.4 solar masses, the end result of solar evolution will be a white dwarf. If the initial mass is between 1.4 and 8 solar masses, the end result of solar evolution will be a neutron star. Finally, if the initial mass is greater than 8 solar masses, the end result will be a black hole. 12

13 39. A black hole is an area in space with a gravitational field that is so strong that neither matter nor EMR can escape. A telescope is a device designed to detect the light emitted or reflected from distant objects. Since no light is emitted from a black hole, a telescope would be unable to detect it. Page 455 4, 5 & 6 4. Since the star that produced the Crab Nebula involved a supernova, it must have been an intermediate-mass star. An intermediate-mass star begins as a cloud of gas and dust, progresses to form an intermediate-mass star, and eventually forms a supergiant star before exploding as a supernova and forming a neutron star. 5. Each type of EMR yields unique information about the Crab Nebula. By using as many types of EMR as possible, scientists obtain a much richer collection of data than if they studied the Crab Nebula using only one type of radiation. 6. a. The success of an agricultural economy depends upon obtaining the optimal harvest of the crops that are planted. It is important to know the best times of the year to plant the seeds, when to expect the seasonal rains, and when to harvest. Astronomical observations could have provided the ancient Pueblo People (Anasazi) with an accurate way to keep track of these key periods in the growing season. b. Given that astronomical observations may have played an important role in their agricultural economy, it is likely that they were already routinely looking up to the night sky. It is also likely that the events in the sky were taken quite seriously as their survival was linked to their ability to record and track changes in the positions of constellations. However, unlike everyday occurrences, this astronomical event would have been a truly amazing sight, something completely out of the ordinary. Given these circumstances, it seems only natural to make a record of such an unusual event. 13