Star Formation
(Chapter 9)
Student Learning Objectives
• Describe the interstellar medium and its effects on light
• Compare and contrast types of nebulae
• Describe the process of star formation
• Recall the balance that maintains stellar structure
What is the interstellar medium and how is it detected?
 The interstellar medium
(ISM) is gas and dust
between the stars.
 Giant clouds
 Stellar nurseries
 The ISM can be detected
because it alters light
from stars.
 Interstellar extinction
 Interstellar reddening
 Absorption lines
 Strong emission line
(21 cm)
ISM in Vicinity of Sun
APOD 2013 September 24
Practice
1) The ISM produces a strong emission line at 21 cm
(Radio). What does this tell us about the energy in
an ISM cloud?
2) How many stars can a single ISM cloud produce?
What are the three classifications of ISM?
Nebulae are regions of
ISM that absorb, reradiate, emit, and/or
scatter light.
Emission nebulae (HII
regions) contain
virtually no dust grains,
and are associated with
young stars.
The new star excites
surrounding gasses.
Emission Nebula IC 1396
APOD 2007 December 24
Reflection nebulae contain a few dust grains
which scatter light. (dust reflects starlight)
Dark nebulae contain many dust grains which block light.
APOD 2009 June 15
Practice
What is the key aspect that determines the
type of nebula?
What are the characteristics of interstellar dust grains?
 Solid dust grains account for
about 1% of the ISM.
 Grains align with magnetic
field.
Magnetic maps of nebulae
indicate where stars are
beginning to form.
“A TWO MICRON ALL SKY SURVEY ANALYSIS OF THE
STABILITY OF SOUTHERN BOK GLOBULES”
The Astrophyiscal Journal, Published 9 September 2009
How do stars form?
The Cloud
Inward Pull
• Gravity pulls toward
centers of mass
• Low temperature (10K)
= low outward pressure
Outward Push
• Magnetic fields
maintain shape
• Rotation and turbulence
= resistance to collapse
Shock Wave
Cloud collapse begins
Shock wave (change in pressure) disrupts balance
Gravity “wins”
Bok Globules form
 Dense gas regions heated
by contraction
 Pre-fusion masses
Bok globules become
either a star or a Brown
Dwarf.
Brown Dwarf Limit = 0.08 Msun
Proto-star forms
Bok globule grows
Warm dense core with associated envelope of material
A protostar will become a star
Proto-planetary disk forms (planets)
Contraction + Accretion  Heat + Rotation
Star
Star turns on
 As a protostar’s temperature
and luminosity change, it
will “move” on the HR
diagram
 Star enters main-sequence
Nuclear Fusion Begins
Birth Tracks to
Main Sequence
Practice
1) Why does the protostar get fainter and then
grow in brightness?
2) When does a protostar become a star?
APOD
 A star spends 90% of lifetime in the main-sequence stage.
Mass
(Solar Units)
1
5
10
Formation
(years)
1 x 108
5 x 106
6 x 105
Main-Sequence
(years)
9 x 109
6 x 107
1 x 107
Nuclear Fusion
Temperature determines the type of nuclear fusion.
Low Mass
T = 7 Million K
PPC
High Mass
T = 16 Million K
CNO
4H  He
C + N + O + H  He
Practice
1) Which type of nuclear fusion occurs in our Sun?
2) Which type of nuclear fusion occurs in a Blue Star?
3) What type of nuclear fusion occurs in an M-type Star?
What maintains stellar structure?
The outward radiation
pressure balances the
inward force of
gravity, at every layer
of the star. (maintains
shape of star)
Hydrostatic Equilibrium
Inward Gravity
Outward Pressure
Energy production  photons move outward  pressure
Deeper layers  more gravity  more pressure needed