2. Hubble’s discovery opened the “Galaxy universe” view.
Our universe contains many galaxies
other than the Milky Way Galaxy.
3. Various kind of galaxies
•In the universe, there exist many galaxies. Thus, we expect to see variety of galaxies.
4. Spiral galaxies
•Some galaxies have spiral arms. The Milky Way Galaxy and Andromeda galaxy are also
kind of spiral galaxies.
•Spiral galaxy is denoted by the letter “S” such as Sa, Sb, Sc. Type Sa spiral galaxies tend
to have tight arms although Type Sc spiral galaxies have poor spiral.
5. Spiral galaxies
•The bulges and halos of spiral galaxies contain large
numbers of red old stars and globular clusters.
•On the other hand, the spiral arms contain gas and dust.
•By radio observations, we find that young stars form
within spiral arms.
6. Barred Spiral galaxies
•Some spiral galaxies have a “Bar” passing through the center and extending beyond
the bulge into the disk. These galaxies are called “barred spiral galaxies”.
•Barred spiral galaxies are denoted by SBa, SBb, and SBc.
•The Milky Way galaxy is thought to be SBb galaxy.
7. Elliptical（椭圆） galaxies
•Elliptical galaxies do not have spiral arms and, in most cases, no obvious galactic disk.
•Elliptical galaxies are denoted by “E” (e.g. E1-E7). E0 galaxies are more circular and E7
galaxies are more elliptical.
8. Elliptical（椭圆） galaxies
•Elliptical galaxies do not contain gas and dust.
This indicates star formation is not active.
•Actually, there is no evidence of young stars or
ongoing star formation.
•Elliptical galaxies are made up mostly of old and
low-mass stars.
X-ray image of elliptical galaxy
•X-ray observations reveal that a large
amount of very hot interstellar gas(~ )
is distributed through their interiors.
106
K
9. S0 galaxies
•Intermediate between E7 elliptical galaxy and Sa spiral galaxy is called S0(or SB0) galaxy.
Not spiral galaxies, but not elliptical galaxies
10. Irregular galaxies
•An irregular galaxy is a galaxy that does not have a regular shape, unlike a spiral or an
elliptical galaxy.
•The irregular galaxy is denoted by “Irr”
11. Magellanic Clouds
•Magellanic Clouds are famous pair of irregular galaxies.
Small Magellanic cloud
Large Magellanic cloud
•Magellanic Clouds orbit the Milky Way Galaxy. It is located at 50kpc away from the
center of our Galaxy.
12. Can you classify galaxies?
(Question 1)
(1) Elliptical galaxy
(2) Spiral galaxy
(3) Irregular galaxy
13. Can you classify galaxies?
(Question 1)
(1) Elliptical galaxy
(2) Spiral galaxy
(3) Irregular galaxy
14. Can you classify galaxies?
(1) Elliptical galaxy
(2) Spiral galaxy
(3) Irregular galaxy
(Question 2)
15. Can you classify galaxies?
(1) Elliptical galaxy
(2) Spiral galaxy
(3) Irregular galaxy
(Question 2)
16. Can you classify galaxies?
(1) Elliptical galaxy
(2) Spiral galaxy
(3) Irregular galaxy
(Question 3)
17. Can you classify galaxies?
(1) Elliptical galaxy
(2) Spiral galaxy
(3) Irregular galaxy
(Question 3)
19. Hubble’s galaxy classification
•Note that galaxies DO NOT evolve along the Hubble sequence.
Elliptical galaxies
Barred-spiral galaxies
Spiral galaxies
20. Distance to galaxies
Do you remember how to measure the distance to stars?
m − M = 5 log10 (
d[pc]
10pc )
•If we can measure the absolute magnitude( ) or
Luminosity ( ), we can calculate the distance to stars.
M
L
•In the case of galaxies, we can also apply the same approach. If we know the luminosity of
galaxies, we can estimate the distance to galaxies.
•We can use useful relation called the “Tully-Fisher relation” for spiral galaxies.
Tully-Fisher relation
L ∝ Vn The luminosity of spiral galaxies
L :
The circular velocity of spiral galaxies
V :
If we measure the circular velocity of spiral galaxies, we can estimate the luminosity
of spiral galaxies.
22. Distance to galaxies
Finally, we can obtain the distance
to galaxies!
L ∝ Vn m − M = 5 log10 (
d[pc]
10pc )
If we can measure the circular velocity of spiral galaxies….
We can estimate the luminosity of the
spiral galaxies by Tully-Fisher’s
relation.
Q. So, how can we measure the circular velocity of spiral galaxies?
A. We can use the Doppler effect of emission lines of galaxies.
23. Clusters of galaxies
Andromeda galaxy
•The Milky Way galaxy is a
member of the local galactic
group.
The Milky Way galaxy
•The local group is made up of
more than 50 (small) galaxies.
24. Clusters of galaxies
•Galaxy cluster is a collection of galaxies bigger than a group of galaxies.
Virgo cluster
Abell 1689
~2500 galaxies
~160,000 globular clusters
~3Mpc
~100Mpc
