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)
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!
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.
