1. Introduction to modern astronomy 22
島袋隼⼠(Hayato Shimabukuro)（云南⼤学、
SWIFAR）
©GETTYIMAGES

2. Galactic spiral arms
•The milky way galaxy has “spiral arms”.
~30kpc

3. Galactic spiral arms
What do spiral arms contain?
•When we observe the galactic arm by radio wavelength,
we clearly see its existence of molecular gas.
The galactic spiral arm contains cold gas.
•The spiral arms contain HII regions and dark nebulae, which
are the main regions of star formation in spiral galaxies.

4. Galactic spiral arms
How does galactic spiral arms form?
•At first, let’s first see how traffic jam happens!

5. Galactic spiral arms
How does galactic spiral arms form?
•At first, let’s first see how traffic jam happens!

6. Galactic spiral arms
How does galactic spiral arms form?
•At first, let’s first see how traffic jam happens!

7. How does galactic spiral arms form?
Galactic spiral arms
•If the speed of cars is decreased on a sloping road, the velocity fluctuations are propagated
backward and the fluctuations are enhanced.
•Then, the density of cars become large and
cause traffic jam.
Speed down

8. Galactic spiral arms
How does galactic spiral arms form?

9. Galactic spiral arms
How does galactic spiral arms form?

10. Galactic spiral arms
How does galactic spiral arms form?

11. Galactic spiral arms
How does galactic spiral arms form?

12. The mass of milky way galaxy
m
v2
r
= G
Mm
r2
M =
rv2
G
The milky way galaxy and the sun
Equation of motion for the sun
: the mass of milky way galaxy (inside sun orbit).
M
: the mass of the sun.
m
: the distance between the galactic center and the sun.
r
M
m
r
We can estimate the mass of the
milky way galaxy by this formula

13. M =
rv2
G
r ∼ 8kpc ∼ 2.37 × 1017
km
G = 6.67 × 10−20
km3
kg−1
s−2
v = 240km/s
M = 2.05 × 1041
kg ∼ 1011
M⊙
The mass of milky way galaxy
The mass of the milky way galaxy is roughly 1011
M⊙

14. Galactic rotation
•The milky way galaxy is rotating. We can measure galactic rotation speed by
observing the Doppler shift of a 21cm emission line.
m
v2
r
= G
Mm
r2
Equation of motion for each star
r
v =
GM
r
∝ r−1/2
r
v

15. Galactic rotation
v =
GM
r
∝ r−1/2
•The observational galactic rotation curve data suggest that the rotation curve cannot be
explained by Kepler motion!!
Observational data
Theoretical prediction
Why??

16. Galactic rotation
Equation of motion for each star
v =
GM
r
∝ r−1/2
m
v2
r
= G
Mm
r2
•Except for stars, we may miss some missing mass. There is something that has gravity, but it
is not visible in the Milky Way galaxy.
Dark matter（暗物质）
•But, we have not known what dark matter is. Unknown particle? Blackhole?… It still remains
a mystery in the universe.

17. The galactic center
What exists in the center of the Milky Way galaxy?
Supermassive black hole (SMBH) called Sgr A*
But, how did we find a supermassive black hole in the center of the Milky Way galaxy?
Astronomers found a regular orbit of stars in the center of the Milky Way galaxy

18. The galactic center
It may be a supermassive black hole (SMBH)!
We need a massive object in the center of the Milky Way to explain this regular orbit
•The Nobel Prize was awarded in 2020 for this discovery.

19. The galactic center
It may be a supermassive black hole (SMBH)!
We need a massive object in the center of the Milky Way to explain this regular orbit
•The Nobel Prize was awarded in 2020 for this discovery.

20. The galactic center
•In 2022, EHT directly observed the Sgr A* located in the center of the Milky Way galaxy.

21. Summary
• The Milky Way Galaxy is a huge collection of stellar and
interstellar matters.
• In order to make a map of the Milky Way Galaxy, we need
to measure the distance to the stars. (e.g. stellar parallax,
variable stars) .
• The galactic spiral arms formed by density wave theory
• The rotation curve of stars suggests the existence of dark
matter.

22. 14. Galaxies
（星系）

23. Is the Milky Way galaxy the only galaxy??

24. No! Even children know that there are
many galaxies in the universe.
Is the Milky Way galaxy the only galaxy??

25. No! Even children know that there are
many galaxies in the universe.
Is the Milky Way galaxy the only galaxy??

26. No! Even children know that there are
many galaxies in the universe.
But, we did not know whether the Milky Way galaxy
was the only galaxy or whether other galaxies existed
in the universe.
Is the Milky Way galaxy the only galaxy??

27. Is the Milky Way galaxy the only galaxy??
•In 1920, Shapley and Curtis debated the following topics.
1. How large is the Milky Way galaxy?
2. Where is the spiral nebulae?
Shapley Curtis
These topics are related to the question
“Is the Milky Way galaxy the only
galaxy?”

28. Is the Milky Way galaxy the only galaxy??
Milky Way galaxy
Spiral nebula
Sun
Globular cluster
Shapley’s model Curtis’s model
Milky Way
galaxy
Spiral nebula
•Shapley: The size of the Milky Way Galaxy is ~100 kpc and spiral nebulae exist in the
Milky Way Galaxy.
•Curtis: The size of the Milky Way Galaxy is ~10 kpc and spiral nebulae exist outside the
Milky Way Galaxy.

29. Hubble’s answer
•Hubble observed variable stars in the Andromeda nebula
and measured its distance.
•He found that the Andromeda nebula is located outside the
Milky Way Galaxy.
Supporting Curtis’s model
•Currently Andromeda nebula is known as “Andromeda
galaxy”
Hubble’s discovery revealed there are galaxies other than
the Milky Way Galaxy!!
It opens “the galaxy universe view” !!