7. Our parent galaxy
•Our solar system is one of the stars in the Milky Way galaxy.
•The milky way galaxy is also called “The Galaxy”
8. Structure of the milky way
Galactic disk: a large, circular, flattened region containing most of our Galaxy’s luminous
stars and the interstellar medium.
9. Structure of the milky way
Galactic disk: a large, circular, flattened region containing most of our Galaxy’s luminous
stars and the interstellar medium.
10. Structure of the milky way
Galactic bulge: a tightly packed group of stars.
11. Structure of the milky way
Galactic bulge: a tightly packed group of stars.
12. Structure of the milky way
Globular clusters: a group of stars that has a spherical shape
13. Structure of the milky way
Globular clusters: a group of stars that has a spherical shape
14. Structure of the milky way
Globular halo: the region which consists of a spherical ball of faint old stars.
15. Structure of the milky way
Globular halo: the region which consists of a spherical ball of faint old stars.
17. Measuring the milky way
•We already know the structure of the milky way. But, until the beginning of the 20th century,
we had not only known the structure of the milky way but also the idea of a “galaxy” !!
So, how could we understand the milky way galaxy??
18. Measuring the milky way
•We already know the structure of the milky way. But, until the beginning of the 20th century,
we had not only known the structure of the milky way but also the idea of a “galaxy” !!
So, how could we understand the milky way galaxy??
19. Measuring the milky way
•If you would like to know where we live, it is a good idea to make a map.
•The situation is the same in the case of the universe. If we would like to know where the earth
is located, we should make a map!
20. Measuring the milky way
(Question) If you need to make a map, what is important to make the map?
21. Measuring the milky way
(Question) If you need to make a map, what is important to make the map?
•Direction •Distance
22. Measuring the milky way
(Question) If you need to make a map, what is important to make the map?
•Direction •Distance
23. Measuring the milky way
(Question) If you need to make a map, what is important to make the map?
•Direction •Distance
In particular, it is very important to measure the distance to stars for making the map
of the milky way galaxy.
24. Measuring the milky way
•As the first step to measure the distance to stars is “Annual parallax”
m − M = 5 log10 (
d[pc]
10pc )
•Another method to measure the distance to stars is using the relation
between “brightness” and “distance”
•If a star is distance [pc] far away from us, there is a relation between
distance, apparent magnitude , and absolute magnitude .
d
m M
•I introduce a new method to measure the distance to stars using
“variable stars”
27. Variable stars
•There are some stars whose luminosity changes with time.
Cepheid variable star
RR Lyrae variable star
•Cepheid variable stars are more massive and
luminous than RR Lyrae variable star.
Cepheid variable
RR Lyrae variable
28. Variable stars
•It is known that there is a relation between period and luminosity in Cepheid stars.
•Thus, once we measure the period of the Cepheid star, we can estimate the luminosity
of the Cepheid star.
m − M = 5 log10 (
d[pc]
10pc )
We can calculate the distance to the Cepheid star!
30. The distribution of stars in the Milky Way
•At the beginning of the 20th century, astronomers measured
distances to stars and globular clusters.
We understand where our solar system is
located.
31. The distribution of stars in the Milky Way
•At the beginning of the 20th century, astronomers measured
distances to stars and globular clusters.
We understand where our solar system is
located.
•500 years ago, humans believed the earth is located in the center of the universe. But,
we are not at the center of the universe!
33. The milky way by MeerKAT
•Radio image of the milky way by MeerKAT.
34. The milky way by MeerKAT
•Same as previous image, but different colors.
35. When did the milky way galaxy form? How old is the milky way galaxy?
Formation of the milky way
1·~50亿岁 2·~75亿岁 3·~100亿岁 4·~130亿岁 5·~150亿岁
How can we estimate the age of the milky way galaxy?
36. When did the milky way galaxy form? How old is the milky way galaxy?
Formation of the milky way
We expect the milky way galaxy formed ~13 Gyr ago(130亿年前)
1·~50亿岁 2·~75亿岁 3·~100亿岁 4·~130亿岁 5·~150亿岁
How can we estimate the age of the milky way galaxy?
37. When did the milky way galaxy form? How old is the milky way galaxy?
Formation of the milky way
We expect the milky way galaxy formed ~13 Gyr ago(130亿年前)
1·~50亿岁 2·~75亿岁 3·~100亿岁 4·~130亿岁 5·~150亿岁
How can we estimate the age of the milky way galaxy?
•We need to find the oldest star in the milky
way galaxy.
38. When did the milky way galaxy form? How old is the milky way galaxy?
Formation of the milky way
We expect the milky way galaxy formed ~13 Gyr ago(130亿年前)
1·~50亿岁 2·~75亿岁 3·~100亿岁 4·~130亿岁 5·~150亿岁
How can we estimate the age of the milky way galaxy?
•We found very old stars in the milky way
galaxy and their ages are roughly 13 Gyr old.
SMSS J031300.36-670839.3
•We need to find the oldest star in the milky
way galaxy.
39. Formation of the milky way
How did the milky way galaxy form??
We have not well understood the whole of galaxy formation.
40. Formation of the milky way
How did the milky way galaxy form??
We have not well understood the whole of galaxy formation.
(a) At first, several young stars are born
in gas and dust.
41. Formation of the milky way
How did the milky way galaxy form??
We have not well understood the whole of galaxy formation.
(a) At first, several young stars are born
in gas and dust.
(a)The stars and gas form dwarf galaxies
and they merged to form the milky way
galaxy.
42. Formation of the milky way
How did the milky way galaxy form??
We have not well understood the whole of galaxy formation.
(a) At first, several young stars are born
in gas and dust.
(a)The stars and gas form dwarf galaxies
and they merged to form the milky way
galaxy.
(b) Merged galaxy is rotating. The
stars are randomly distributed.
43. Formation of the milky way
How did the milky way galaxy form??
We have not well understood the whole of galaxy formation.
44. Formation of the milky way
How did the milky way galaxy form??
We have not well understood the whole of galaxy formation.
(c) Because the stars and gas are rotating,
the rotation makes stars and gas flattened
on the plane. Thus, they are distributed
on the galactic disk.
45. Formation of the milky way
How did the milky way galaxy form??
We have not well understood the whole of galaxy formation.
(c) Because the stars and gas are rotating,
the rotation makes stars and gas flattened
on the plane. Thus, they are distributed
on the galactic disk.
(d) As a result, although it is very
simplified, the milky way galaxy formed.
After galactic disk formed, it still keeps
rotating because it has angular momentum.
