1. Stars form from dense clouds of gas and dust in interstellar space. 2. Gravity causes the cloud to contract over many stages until fusion begins in the core and a new star is born on the main sequence. 3. The size and mass of a star determines its position on the HR diagram, with more massive stars being larger and hotter.

1. X. Star Formation
A. Interstellar Medium: The collective name for all
matter located between stars.
1. Made up of Gas & Dust
a. Gases produces absorption lines in the
star light that passes through it.
Absorption lines
b. Dust acts to block light at most
frequencies above infrared. Thus the
light from stars appear redder then they
would otherwise. This is called Reddening

2. B. Density & Composition
1) On average there is one atom of interstellar material per
square centimeter. VERY low density.
2) The gas is composed of 90% Hydrogen and 9% Helium.
3) The composition of interstellar dust, is mostly unknown,
but is believed to be heavier elements and “dirty” ices.

3. 4) Interstellar Clouds: are areas of higher then average
density of gas and dust.

4. 5) Types of Interstellar Clouds
a. Emission Nebula: HOT clouds of gas and dust that give off light.
- At or near the center
of an emission nebula
is located a hot O or B
type star.
- These are areas
where new stars are
forming.
- Can be several hundred
light years across.

5. 5) Types of Interstellar Clouds
b. Dark Dust Clouds: COOL clouds in space that do not radiate light.
- Tend to be denser
then emission nebulae
- Range from the size of
our solar system to a
few parsecs across.
- are not the site of new
star formation

6. B. Formation of Sun-Like Stars
- Over time the atoms & molecules in interstellar
clouds are affected by each others gravity and they
clump together which leads to star formation.
Stage 1) An Interstellar Cloud
a. Part of an interstellar cloud begins to
collapse.
b. This cloud is typically 1000’s of times the
mass of the sun and 10’s of parsecs
across.

7. Stage 1) An Interstellar Cloud
c. As it shrinks it begins to fracture into
smaller parts.
d. Each part of the could can give rise to a
new star.

8. Stage 2) Contracting Cloud Fragment
a. At stage 2, the piece of cloud is about 100x
the size of our solar system and has about
two solar masses worth of material.
b. Even though it is continuing to contract,
most of the heat is still lost to interstellar
space because the molecules of the
cloud are still very far apart.

9. Stage 3) Contracting Cloud Fragment
a. At stage 3 the cloud has contracted into a
sphere roughly the diameter of our solar
system.
b. The interior of the cloud has heated to over
10,000K and a protostar forms at the
center.

10. Stage 4) Protostar Evolution
a. The protostar has shrunk to roughly the
size of Mercury’s orbit.
b. The core of the protostar is about 1,000,000K
- Hot enough to strip electrons off of the
atoms but not hot enough to start fusion.

11. Stage 5) Protostar Evolution
a. The star heats to a point nearing that
needed for fusion.
b. The star is VERY bright but also very cool.
c. The brightness of the star comes from
energy released
as the star contracts
d. At this point we can
plot our protostar on
the HR diagram.

12. Stages 6 & 7) New formed Star
a. At stage 6 the core has heated enough to
begin fusing hydrogen atoms into helium,
but is still twice the size of the sun.
b. At stage 7 the star has completed
contracting and has reached the main
sequence.

13. X. Star Formation
C. Formation of Other Sized Stars
1. More Massive stars –
a. Form from more massive cloud fragments.
b. Are found “higher” on the main sequence
then the sun.
c. This means that classes
O-F are, generally, more
massive than the sun.

14. C. Formation of Other Sized Stars
2. Less Massive stars –
a. Form from less massive cloud fragments.
b. Are found “lower” on the main sequence
then the sun.
c. This means that classes
K & M are, generally, less
massive than the sun.

15. D. Formation Tracks
1. Remember that we can plot proto stars on the HR
diagram and follow them through the stages of
formation until they reach the main sequence
(stage 7)
2. The shape of star forming
tracks are the same but
their position depends on
their mass.
a. higher for high mass
b. lower for low mass

16. E) The End Result (dun… Dun… DUN!!!)
1) The end result of cloud collapse is a Star Cluster.
a. A star cluster is a group of stars that form
the same collapsing cloud of gas and dust.
b. These stars have similar characteristics.
- Very close ages
- Same initial composition
- Similar distance from the earth
c. This makes them ideal stellar laboratories
for testing different theories of stellar evolution.

17. 2) Open Clusters: Small star clusters a few pc
across with 100’s to 10,000’s of stars.
a. Generally found in the plane of the Milky Way.
b. These have a high abundance of upper main
sequence stars which indicates that these clusters
are very young.
c. Example:
The Pleiades

18. 3) Globular Clusters: Larger star clusters
containing 100,000’s or millions of stars.
a. Generally found outside the plane of the milky
way.
b. These clusters lack most upper main
sequence stars indicating that they are much
older then open clusters.
c. Example
M92

19. 4) HR Diagrams and Cluster age
a. Zero-Age Main Sequence
- When a star cluster first forms the stars
somewhat evenly distributed across the
main sequence.

20. b. Older-Main Sequence
- Over time the higher mass stars burn through
their fuel faster and die off.
- So as time goes on, the stars located higher
on the chart begin to leave the main sequence.

21. F. Estimating the Size of Stars
1. Stefan-Boltzmann law
a. Most stars cannot have their radius directly
measured using geometry, and it must be measured
indirectly.
b. We know that as the temperature of a star goes up
so does the luminosity
L T4
c. We also know that the larger a star is the more
energy it gives off. Thus…
L r2

22. d. By combining these proportionalities…
Luminosity Radius2 X Temperature4
e. Remove the proportionality and rearrange…
R‫√( = סּ‬L ‫/)סּ‬T2‫סּ‬
Example: Betelgeuse is 0.517x the temperature of the sun
and is 80x as luminous. How many times the size of the
sun is the star? If the suns radius is 696,000km then how
many km is this star?

23. 2) Applying Radius to the HR
Diagram
a. Because the HR diagram is
a chart of
temperatures vs
luminosities we can estimate
the relative size of stars based
on their position on the diagram.
- Up to the right is larger
- Down towards the left is smaller

24. H. Forces in a Forming Star
1. There are two forces acting on Stars
a. The force of gravity
- Acts to cause the star to contract
- Caused by the large amount of
Hydrogen Gas
b. Internal Pressure Forces
- Resists the force of gravity
- Caused by:
* Heat generated within the star
* Repulsion between particles of
like charge.

25. 1. Two forces (a balancing act)
c. When the cloud of gas and dust first starts
forming, the force of gravity GREATLY
overpowers the internal pressure force.
- This causes the cloud to start
contracting.

26. 1. Two forces (a balancing act)
d. As the cloud gets smaller the force of
gravity remains the same. But as the
molecules get closer together pressure
builds up inside the cloud to resist gravity.
- This causes contraction to greatly
slow.

27. 1. Two forces (a balancing act)
e. Eventually the when the protostar forms
the pressure of the gas almost balances
out the force of gravity.
- This is the slowest part of contraction.

28. 1. Two forces (a balancing act)
f. When the star reaches the main sequence,
and begins fusing hydrogen, the interior
pressure force perfectly balances gravity
and contraction stops
- This is called Hydrostatic Equilibrium.
- Stars on the main sequence DO NOT
CHANGE RADIUS

29. I. Powering Stars (The Proton-Proton Chain)
1. Main Sequence Stars
a. Use Hydrogen as a source of fuel
+ b. The hydrogen is fused together to form
- Helium.
c. The process of combining Hydrogen
together into Helium is called The Proton-
Proton Chain.
d. Remember that at the center of the sun it
is hot enough to strip the electrons off
of+ any atoms. Thus creating a whole
bunch of floating protons.

30. Proton-Proton Chain: Step 1
Two Protons (1H) combine to form a Hydrogen
& a Neutron (2H), a positron, and a neutrino.
-
+ + +
+
H + 1H  2H + e+ + v
1

31. Proton-Proton Chain: Step 2
Two Heavy-Hydrogen's (2H) combine to form
Light Helium (3He) and a Gamma Ray.
+
+ + γ
+
2
H + 2H  3He + γ

32. Proton-Proton Chain: Step 3
Two Light-Helium’s (3He) combine to make a
stable Helium (4He) and two Hydrogen Ions (1H).
+ +
+
+ + +
+ +
He + 3He 4He + 1H + 1H
3

33. 2) Energy releasing steps
a. In step 2 we release a gamma ray.
γ
- This is just high energy light!
- It gets absorbed and remitted on its trip to
the surface of the sun, so when it emerges it
has less energy.
b. In step 1 we created a positron.
- This is Antimatter. Essentially an electron with a
+ positive charge.
- Eventually it will contact a normal-matter
electron and they will annihilate each other
- releasing massive amounts of energy.