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- 1. Introduction to modern astronomy 6 島袋隼⼠(Hayato Shimabukuro)（云南⼤学、 SWIFAR） ©GETTYIMAGES
- 2. 3.Radiation（辐射） : Information from cosmos
- 3. Information from sky This is a picture of Andromeda galaxy. For humans, galaxies in the sky are seen with the naked eye as faint. Of course, it is hard to visit the galaxy (With the speed of light, it takes 2.5 million years for Andromeda.) So, how do we obtain the information on astrophysical objects such as galaxies from the sky??
- 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. 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）.
- 7. Components of sun light •Rainbows are the result of the spectroscopy of sunlight •When sunlight enters into prizm, the sunlight is divided into spectrum. •Newton studied this phenomenon, called spectroscopy(谱系)
- 8. Electromagnetic spectrum •We can use the different wavelengths of the electromagnetic wave for different purposes in daily life.
- 9. The sun seen by different wavelength •We can use the different wavelengths of the electromagnetic wave for different purposes in daily life. Not only in daily life, but also in astronomy, we can use different wavelengths of the electromagnetic wave. •The picture below is the sun seen by different wavelengths of electromagnetic wave.
- 10. Galaxy seen by different wavelength •Why do we use different wavelengths to observe universe? We can obtain different information by different wavelengths of electromagnetic wave
- 11. Galaxy seen by different wavelength •Why do we use different wavelengths to observe universe? We can obtain different information by different wavelengths of electromagnetic wave
- 12. Galaxy seen by different wavelength •Why do we use different wavelengths to observe universe? We can obtain different information by different wavelengths of electromagnetic wave
- 13. 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.
- 14. Magnetism • Earth’s magnetic field(磁场) interacts with a magnetic compass needle, causing the needle to become aligned with the field. •Then, Aurora appears. •Charged particles are coming from the sun and they interact with the magnetic field at the earth.
- 15. Electromagnetic waves •The laws of physics tell us that a magnetic field 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
- 18. •The speed of light is finite. So, light does not travel instantaneously from place to place (e.g Sun). We see the sun 8 mins ago c = 3.00 × 105 km/s km 1.5 × 108 From the sun to the earth, the light travels for…. t = 1.5 × 108 km 3.0 × 105km/s = 5.0 × 102 s ∼ 8mins We see the sun 8minutes ago!
- 19. Distant galaxies If the distance from us is much farther….
- 20. Distant galaxies If the distance from us is much farther…. More distant galaxies take a longer time to reach us!
- 21. Distant galaxies If the distance from us is much farther…. More distant galaxies take a longer time to reach us! •This is most distant galaxy from us.
- 22. Distant galaxies If the distance from us is much farther…. More distant galaxies take a longer time to reach us! •This is most distant galaxy from us. •We are observing a galaxy 13.4 billion years ago(134亿年前) !
- 25. The Doppler Effect •If one is moving toward a source of radiation, the wavelengths become shorter (blue shift), if moving away, the wavelengths become 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
- 26. •We often use “redshift(红移)” to express distant or past（过去） universe. Redshift Redshift Due to the expansion of the universe, the wavelength of the distant or past universe extends. z = 0 0 Original wavelength Extended wavelength (Ex) •21cm radio wave in the earth extends to 63cm in the past galaxy. Then what is the redshift of the galaxy?
- 27. •We often use “redshift(红移)” to express distant or past（过去） universe. Redshift Redshift Due to the expansion of the universe, the wavelength of the distant or past universe extends. z = 0 0 Original wavelength Extended wavelength (Ex) •21cm radio wave in the earth extends to 63cm in the past galaxy. Then what is the redshift of the galaxy? z = (63 − 21) 21 = 2
- 28. Redshift •The redshift of a most distant galaxy is z ∼ 11 z=0: present universe z=0.1: 1.4 Gyr ago z=1: 7.8 Gyr ago z=10: 13.2 Gyr ago z=20: 13.7 Gyr ago •The candidate of galaxy z = 16 G:Giga, 10亿年
- 29. Summary • We observe the universe by electromagnetic wave • When we use di ff erent wavelength, we can observe di ff erent universe. • The redshift of electromagnetic wave is very useful to evaluate how the universe is old.