Lec 8
II-9 Normal Galaxies and Clusters of Galaxies
(Main Ref.: Lecture notes; FK 19-5, 23-1 to 3, 5, 6; Box
23-1; CD photos shown in class)
(i) Stellar Population (Main Ref.: Lecture notes; FK Sec. 19-5)
Two kinds of populations:
Population I: Stars in young clusters (e.g., associations, open clusters)
Young stars - blue stars,etc.
Metal rich; i.e., spectral lines of heavy elements strong
Population II: Stars in old clusters (e.g., globular clusters)
Old stars - red stars,etc.
Metal poor; i.e., spectral lines of heavy elements absent or weak
See, e.g., Fig.II-81
Population II stars created long time ago. Note: globular clusters are >
~ 1010 years old! All massive stars have finished life long ago, and
are gone. Only small mass stars with long life are still around. 1
Why population II stars are metal-poor? In the early Universe
right after the big bang, only light elements H and He are created.
All `metals’ (= elements heavier than He) are synthesized during
the pre-supernova stage of massive stars. Population II stars were
born early in the evolution of the Universe, soon after the big bang,
before metals are formed during the pre-supernova evolution of
massive stars. So, metal-poor!
Population I stars are born relatively recently, and so still young. So,
they are found in young clusters.
Why metal-rich? Pop. I stars were born recently from the dense
region of the interstellar medium (ISM), e.g. molecular clouds, dark
nebula,etc., which are contaminated by ashes of nucleosynthesis
which went on in earlier generation massive stars during the presupernova stages. Since the matter out of which Pop. I stars is born
comes out of the material ejected by supernova explosions of earlier
generation massive stars, they contain more metals, and hence,
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metal-rich!
See class notes for further details.
Now we also have Pop III stars (see class notes).
Will come back to more discussions of properties of Pop. I, II and III
stars in the galaxy section.
Fig. II-81: Spectra of Metal-Poor and Metal-Rich Stars
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(ii) Normal Galaxies (Main Ref.: Lecture notes; FK Sec. 23-2,3;
Reading FK Sec 23-1 optional – not covered in class)
Reading assignment – FK Ch 22 Our Galaxy (not covered in class.)
Galaxy Types: There are three types of galaxies – Spiral, Elliptical, and
Irregular. Properties - study Table II-7.
See class notes for further detailed explanation.
Enjoy CD photos shown in class and various pictures in FK.
Table II-7: Properties of different galaxies
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Examples: Elliptical – NGC 4365; Spiral – Andromeda (M31);
Barred Spiral – NGC 1365; Irregular - LMC
Hubble’s Classification and Hubble’s Tunig Fork Diagram:
Galaxies are classified according to the tuning fork diagram. The subclasses
are: Left to right (see Fig. II-82): Ellipticals – E0 to E7; Spirals – S0, Sa to
Sc; Barred Spirals – SB0, SBa to SBc
See class notes for changing Shapes from left to right, and what that means.
Note: among ellipticals, as we go from E0 to E7 the shape goes from circular
to more flattened (i.e., pancake) shape. Spirals are normal spirals and
barred spirals (spirals with a bar).
Among spirals and barred spirals, as
we go from a to c it goes from most
tightly coiled to most loosely winding
arms, and from the largest bulge to
the smallest.
Back to Pop I, II, and III stars:
Their age, location in different galaxies,
Velocities (Pop I low, Pop II high), etc.,
and why? See your class notes.
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Fig. II-82: Hubble’s Tuning Fork Diagram
(iii) Hubble Law
(Main Ref.: Lecture notes; FK Sec. 23-5)
What it says: Further away a galaxy is, faster it is receding from
us!
Discovery purely from observations of distance and redshift of
many galaxies. Galaxies are made up of stars, gas and dust. Star
light includes absorption lines. Gas in emission nebular emits
emission lines. So, we can measure redshift of these lines.
Distance is the hardest to measure, but could be done – come
back later. Each galaxy has its own local motion, but if
sufficiently far away, the receding velocity (hence redshift)
overtakes.
Importance: Proof of expanding Universe – one of the most
important discoveries in cosmology! – come back later.
Cosmological Redshift z = redshift due to cosmological
expansion of the Universe after big bang 
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Receding radial velocity vc
z = ( – 0) / 0 =   / 0 = vc / c (if non-rel) Eqn(II-28)
Hubble Law expressed mathematically is:
vc = H0 d
Eqn(II-29)
Where vc is receding velocity due to cosmological redshift; d is distance, and
H0 is Hubble’s constant. From Eqns (28) and (29), we get:
d = vc / H0 = z c / H0
Eqn(II-30)
Note: Eqn(30) is a powerful method to find distance d of galaxies far away
(when it cannot be known in other ways), if H0 is known. By simply measuring
redshift, can find d! So, it is important to find H0 correctly.
Hubble Constant: Find H0 by measuring distance d and redshift z of
galaxies whose distances can be found by other
methods (see later section on distance
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measurement).
Latest from HST (Hubble space
telescope):
H0 = 73 km/s-Mpc
Current range of unertainty:
60 < H0 < 90 km/s-Mpc
Here we use: H0 = 70 km/s-Mpc
EX 52: Singly ionized Ca K line
in NGC 4889.
Measured  = 401.8 nm;
0 = 393.3 nm. What is the
distance of NGC 4889?
Fig II-83: Hubble Law
Ans: d = 93 Mpc.
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See class notes for the details.
(iv) Clusters and Superclusters of Galaxies (Main Ref.:
Lecture notes; FK Sec. 23-6)
• FK gives good description of the subject. So, study your textbook
carefully. Here I give summary only.
• Galaxies come in clusters. The smallest is our local group,
consisting of our Milky Way Galaxy, Anromeda, LMC, SMC, etc.
See FK Fig. 23-19. The closest member of the Local Group – the
Canis Major Dwarf (see FK Fig. 23-20).
The largest – Andromeda = M31
.
• Large cluster of galaxies – e.g., The Coma Cluster, see FK Fig. 23-21
– many dense collection of galaxies, denser toward center. Have many
elliptical galaxies.
• Superclusters of galaxies: Distribution of clusters of galaxies – see
FK Fig. 23-23.They look like soap bubbles with big voids. Clusters of
galaxies lie along the walls of the bubble-like voids – filamentlooking in 2D(two dimensional) picture.
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See pictures in CD photos shown in class and those in FK.