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1. 1. Introduction to modern astronomy 島袋隼⼠(Hayato Shimabukuro)（云南⼤学、 SWIFAR） ©GETTYIMAGES
2. 2. 3.Radiation（辐射） : Information from cosmos
3. 3. Information from skies This is a picture of Andromeda galaxy. For human, galaxies on the sky is seen with naked eye a as faint. Of course, it is hard to visit the galaxy (With speed of light, it takes 2.5 million year for Andromeda.) So, how do we obtain the information on astrophysical objects such as galaxies from the sky??
4. 4. Electromagnetic wave
5. 5. Light and electromagnetic wave Radiation（辐射） or electromagnetic wave(电磁波) is a way in which energy is transmitted through space from to another. We observe the universe by radiation (electromagnetic wave) and obtain information. In the lecture today, one of the purposes is to understand “electromagnetic wave”
6. 6. Wave motion Wave is a way in which energy is transferred from place to place without the physical movement of material from one location to another. For example, look at point 1. The location of point 1 is not changed. But, the wave transfers information for other points.
7. 7. The property of wave Period（周期）: the number of seconds needed for the wave repeat itself at any give point in spa Wavelength（波长）: the number of meters needed for the wave to repeat itself at a given moment in time. It can be measured as the distance between two wave crests. Amplitude（振幅）: the maximum departure of the wave from the undisturbed state.
8. 8. The property of wave Frequency（频率）: the number of wave crest passing any given point per unit. (For example, how many the wave oscillate per second.) Frequency = 1 Period (ex.) A wave with a period of 5 seconds has a frequency of a 1/5=0.2 Hz, meaning one wave crest passes a given point in space every 5 seconds.
9. 9. The property of wave Wave velocity（波速） velocity = wavelength Period or velocity = wavelength × frequency
10. 10. Components of visible light When the white light passed through a prism, it splits into its component colors, spanning red to violet in the visible part of the electromagnetic spectrum. The divided rainbow of these basic color is called a spectrum（光谱）.
11. 11. Electromagnetic waves So far, we understand that electromagnetic wave is important to get information in the universe. What generates electromagnetic wave? We consider charged particle such as proton（质⼦） and electron(电⼦). (a) particles which have same charge are pushed, whereas particles with unlike charges attract. (b) Charged particle generates electric field. (c) If charged particle begins to vibrate, its electric field changes.
12. 12. Electromagnetic waves •The laws of physics tell us that a magnetic field（磁场） must accompany every changing electric field. •As we saw previous slide, moving charged particle changes electronic field. This generates magnetic field and also changing magnetic field generates electronic field. •Electric and magnetic fields vibrate perpendicularly teach other. Together they form an electromagnetic waves that moves through space at the speed of light c. c = 3.00 × 105 km/s •The speed of light is finite. So, light does not travel instantaneously from place to place (e.g Sun).
13. 13. Magnetism • Earth’s magnetic field interacts with a magnetic compass needle, causing the needle to become aligned with the field. •Aurora is generated because the earth has magnetic field.
14. 14. Electromagnetic spectrum
15. 15. The universe seen by different wavelength
17. 17. Thermal radiation •All macroscopic objects, such as fire, ice, people, stars, emit radiation at all times. •Thermal radiation（热辐射） is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material is converted to electromagnetic radiation Thermal energy is converted to electromagnetic wave. What is the relation between thermal energy and electromagnetic wave? Let’s consider it !
18. 18. Temperature •We first begins to consider “Temperature(温度)” •We usually use degree(℃) as measure of temperature. •In physics or astronomy, we often use Kelvin Temperature instead of degrees K(Kelvin) = degree Celsius (℃)＋273 For example, 0℃ is 273K and water freezes. •All thermal motion stopps at 0K (-273℃) •Water boils at 373K(100℃)
19. 19. The blackbody spectrum •Intensity is a term often used to specify the amount or strength of radiation •All objects emit all its radiation which is spread out over a range of frequencies. Blackbody(⿊体谱) : an object that absorbs all radiation falling on it and reemit the same amount of energy it absorbs.
20. 20. The blackbody spectrum •Blackbody curve only depends on temperature. •Black body spectrum depends on frequency. •As temperature increases, the peak wavelength becomes longer. λmax = 2.9mm T Wien’s law Wien’s law tells us that the hotter the object, the bluer is its radiation. Redder Bluer hotter colder Does it match your sense??
21. 21. Radiation law The total energy per unit area is given by Stefan- Boltzmann equation Total amount of energy only depends on its temperature. •You may know that total amount of energy increases as the temperature of a object increases yes, it is correct
22. 22. The blackbody spectrum What is the relation between thermal energy and electromagnetic wave? Remember our first question ! Answer The electromagnetic wave is related to temperature via blackbody radiation. 1. Given temperature, blackbody curve is determined. 2. Once blackbody curve is determined, we can know how much the intensity it is at given frequency.
23. 23. Astronomical applications (a) A cool dark galactic gas, the temperature is around 60K. The peak wave length is located at radio wavelength. Thus, it emits mostly radio radiation. (b) A young star with T=600K, the radiation is mainly infrared. (c)The sun with T=6000K is brightest in the visible region. (d) The Andromeda galaxy, seen at far-ultraviolet wavelengths, showing the galaxy’s hottest stars and active core.
24. 24. The Doppler Effect •If one is moving toward a source of radiation, the wavelengths seem shorter (blue shift); if moving away, they seem longer (redshift). •You know the sound of ambulance is changed when it is approaching you or it goes away from you. •This Doppler effect is case of sound, but electromagnetic wave can also experience the Doppler effect
25. 25. The Doppler Effect •In the universe, astrophysical objects move, the Doppler effect will occur. •For example, we will learn later, the universe is expanding. The expansion of the universe causes the Doppler effect. •However, it is difficult to measure Doppler effect in the case of blackbody curve, simply because it is spread over many wavelengths. •In the context of astronomy, we measure the Dopper effect for line emission as we learn next class.
26. 26. Summary • In the astronomy, we use electromagnetic wave to observe astrophysical objects and obtain information. • The electromagnetic wave consists of radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma-rays. • Blackbody radiation is related to temperature. Given temperature, blackbody curve is determined and we measure intensity at given frequency. • The doppler e ff ect occurs due to the motion of radiation source.