1. Electromagnetic waves, including radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays, are used to observe astrophysical objects and obtain information. 2. Blackbody radiation depends on an object's temperature - the hotter the object, the shorter the peak wavelength of its blackbody spectrum. 3. The Doppler effect causes the wavelengths of radiation to appear shorter if the source is moving towards the observer, or longer if moving away, due to the motion of the radiation source.

1. Introduction to modern astronomy
島袋隼⼠(Hayato Shimabukuro)（云南⼤学、
SWIFAR）
©GETTYIMAGES

2. 3.Radiation（辐射） : Information
from cosmos

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. Electromagnetic wave

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. 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. 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. 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. The property of wave
Wave velocity（波速）
velocity =
wavelength
Period
or velocity = wavelength × frequency

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. 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. 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. 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. Electromagnetic spectrum

15. The universe seen by different wavelength

16. Thermal radiation（热辐射）

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. 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. 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. 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. 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. 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. 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. 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. 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. 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.