Star Formation and HR diagrams
Star-Forming Clouds
• Stars form in dark
clouds of dusty gas
in interstellar space.
• The gas between the
stars is called the
________________
__________.
Gravity Versus Pressure
• Gravity can create stars only if it can overcome
the force of thermal pressure in a cloud.
• Gravity within a contracting gas cloud becomes
stronger as the gas becomes denser.
Mass of a Star-Forming Cloud
• A typical molecular cloud (T~ 30 K, n ~ 300
particles/cm3) must contain at least a few hundred
solar masses for gravity to overcome pressure.
• The cloud can prevent a pressure buildup by
converting thermal energy into infrared and radio
photons that escape the cloud.
Fragmentation of a Cloud
This simulation begins
with a turbulent cloud
containing 50 solar
masses of gas.
Fragmentation of a Cloud
The random motions
of different sections of
the cloud cause it to
become lumpy.
Fragmentation of a Cloud
Each lump of the cloud
in which gravity can
overcome pressure can
go on to become a star.
A large cloud can
make a whole cluster
of stars.
A _______of many stars can form out of a single
cloud.
____ cluster: A few thousand loosely packed stars
_________ cluster: Up to a million or more stars in a dense ball
bound together by gravity
Glowing Dust Grains
As stars begin to form,
dust grains that absorb
visible light heat up
and emit infrared light
of even longer
wavelength.
Multiwavelength Cloud
Glowing Dust Grains
Long-wavelength
infrared light is
brightest from
regions where many
stars are currently
forming.
Formation of Jets
Rotation also causes
jets of matter to shoot
out along the rotation
axis.
Jets are
observed
coming from
the centers of
disks around
protostars.
Lower Limit on a Star’s Mass
• Fusion will not begin in a contracting cloud if some
sort of force stops contraction before the core
temperature rises above 107 K.
• Thermal pressure cannot stop contraction because the
star is constantly losing thermal energy from its
surface through radiation.
• Is there another form of pressure that can stop
contraction?
___________ Pressure:
Laws of quantum mechanics prohibit two electrons
from occupying the same state in the same place.
Thermal Pressure:
Depends on heat content
The main form of pressure
in most stars
Degeneracy Pressure:
Particles can’t be in same
state in same place
Doesn’t depend on heat
content
_____ Dwarfs
• Degeneracy pressure
halts the contraction
of objects with
<0.08MSun before
the core temperature
becomes hot enough
for fusion.
• Starlike objects not
massive enough to
start fusion are
brown dwarfs.
Brown Dwarfs
• A brown dwarf
emits infrared light
because of heat left
over from
contraction.
• Its luminosity
gradually declines
with time as it loses
thermal energy.
Brown Dwarfs in Orion
• Infrared
observations can
reveal recently
formed brown
dwarfs because they
are still relatively
warm and luminous.
Weight of upper layers
compresses lower layers
_______
_______
Big nucleus splits into
smaller pieces
Small nuclei stick
together to make a
bigger one
(Nuclear power plants)
(Sun, stars)
High temperatures
enable nuclear
fusion to happen in
the core.
Sun releases energy by fusing four hydrogen nuclei into one
helium nucleus.
____________________ is how hydrogen fuses into helium in Sun
IN
4 protons
OUT
4He nucleus
2 gamma rays
2 positrons
2 neutrinos
Total mass is
0.7% lower.
Thought Question
What would happen inside the Sun if a slight rise in
core temperature led to a rapid rise in fusion energy?
A. The core would expand and heat up slightly.
B. The core would expand and cool.
C. The Sun would blow up like a hydrogen bomb.
Thought Question
What would happen inside the Sun if a slight rise in
core temperature led to a rapid rise in fusion energy?
A. The core would expand and heat up slightly.
B. The core would expand and cool.
C. The Sun would blow up like a hydrogen bomb.
________________ keeps burning rate steady
Solar Thermostat
Decline in core temperature
causes fusion rate to drop, so
core contracts and heats up
Rise in core temperature
causes fusion rate to rise, so
core expands and cools down
______________
equilibrium:
Energy provided
by fusion
maintains the
pressure.
Gravitational
contraction…
provided energy
that heated the
core as the Sun
was forming.
Contraction
stopped when
fusion began
replacing the
energy radiated
into space.
Sun’s Core
• Energy Transport
– _________ zone
• Energy move outward – photon radiation
• Photons take 100,000 years to reach surface
– __________ zone
• energy is transported by the rising and sinking of gas
– Convection manifests itself in the photosphere as
____________, numerous bright regions surrounded by
narrow dark zones
Core:
Energy generated
by nuclear fusion
~ 15 million K
Radiation zone:
Energy transported
upward by photons
Energy gradually leaks out of the radiation zone in the
form of randomly bouncing photons.
Convection zone:
Energy transported
upward by rising
hot gas
Convection (rising hot gas) takes energy to the surface.
Photosphere:
Visible surface of
Sun
~ 6,000 K
Bright blobs on photosphere where hot gas reaches the surface
Chromosphere:
Middle layer of
solar atmosphere
~ 104–105 K
Corona:
Outermost
layer of solar
atmosphere
~1 million K
Solar wind:
A flow of
charged
particles from
the surface of
the Sun
Radius:
6.9  108 m
(109 times Earth)
Mass:
2  1030 kg
(300,000 Earths)
Luminosity:
3.8  1026 watts
We learn about the inside of the Sun by …
• making mathematical models.
• observing solar vibrations.
• observing solar neutrinos.
Patterns of
vibration on the
surface tell us
about what the
Sun is like inside.
Data on solar
vibrations agree
with mathematical
models of solar
interior.
_________ created
during fusion fly
directly through the
Sun.
Observations of
these solar neutrinos
can tell us what’s
happening in the
core.
Solar neutrino problem:
Early searches for solar
neutrinos failed to find
the predicted number.
Solar neutrino problem:
Early searches for solar
neutrinos failed to find
the predicted number.
More recent observations
find the right number of
neutrinos, but some have
changed form.
Solar activity is like “weather”
• _________
• Solar ______
• Solar ___________
All are related to ________ fields.
Solar Magnetic Activity
• Sunspots are the most common type of solar
magnetic activity:
– Dark-appearing regions ranging in size from a few
hundred to a few thousand kilometers across
– Last a few days to over a month
– Darker because they are cooler than their surroundings
(4500 K vs 6000 K)
– Cooler due to stronger magnetic fields within them
Sunspots…
Are ______
than other
parts of the
Sun’s
surface
(4,000 K)
Are regions
with strong
magnetic
fields
Zeeman
Effect
We can
measure
magnetic
fields in
sunspots by
observing
the splitting
of spectral
lines
Charged particles spiral along magnetic field lines.
Loops of bright gas often connect sunspot pairs.
Magnetic activity
causes solar flares
that send bursts of
X-rays and
charged particles
into space.
Magnetic activity
also causes solar
prominences that
erupt high above
the Sun’s surface.
The corona
appears bright in
X-ray photos in
places where
magnetic fields
trap hot gas.
Coronal mass
ejections send
bursts of energetic
charged particles
out through the
solar system.
Charged particles streaming from the Sun can disrupt electrical power
grids and disable communications satellites.
The number of sunspots rises and falls in 11-year cycles.
The sunspot cycle has something to do with the winding and
twisting of the Sun’s magnetic field.
The ___________
• Introduction
– Sunspot, flare, and prominence activity change yearly
in a pattern called the solar cycle
• Cause of the Solar Cycle
– Differential rotation
•
•
•
•
Equator 25 days
Poles 30 days
Magnetic field wind’s up
The cycle ends when the field twists too “tightly” and
collapses – the process then repeats
• Changes in the Solar Cycle
– Varies from 6 to 16 years
– Cycle is 22 years if the polarity of sunspots is considered
• Links Between the Solar Cycle and Terrestrial
Climate
– Midwestern United States and Canada experience a 22-year
drought cycle
– Few sunspots existed from 1645-1715, the Maunder
Minimum, the same time of the “little ice age in Europe and
North America
– Number of sunspots correlates with change in ocean
temperatures
Plot illustrating that the number of sunspots changes with time, showing the Maunder
minimum and the solar cycle. (Courtesy John A. Eddy.)
Curves showing the change in ocean temperatures on Earth and the change in
sunspot numbers over several decades. Notice that the curves change approximately
in step. Astronomers deduce from these curves that solar magnetic activity alters our
climate. (The spot numbers are averaged over 11 year intervals).
Brightness of a star depends on both ________ and _________
__________:
Amount of power a star
radiates
(energy per second =
watts)
Apparent brightness:
Amount of starlight that
reaches Earth
(energy per second per
square meter)
Luminosity passing
through each sphere
is the same
Area of sphere:
4π (radius)2
Divide luminosity by
area to get brightness.
The relationship between apparent brightness
and luminosity depends on distance:
Brightness =
Luminosity
4π (distance)2
We can determine a star’s luminosity if we can
measure its distance and apparent brightness:
Luminosity = 4π (distance)2  (Brightness)
________
is the
apparent shift
in position of
a nearby
object against
a background
of more
distant
objects.
Introduction to Parallax
Apparent
positions of
the nearest
stars shift
by about an
arcsecond
as Earth
orbits the
Sun.
Parallax of a Nearby Star
The parallax
angle
depends on
distance.
Parallax Angle as a Function of Distance
Parallax is
measured by
comparing
snapshots
taken at
different times
and measuring
the shift in
angle to star.
Measuring Parallax Angle
Parallax and Distance
p = parallax angle
1
d (in parsecs) =
p (in arcseconds)
1
d (in light-years) = 3.26 
p (in arcseconds)
Most luminous
stars:
106 LSun
Least luminous
stars:
10−4 LSun
(LSun is luminosity
of Sun)
The Magnitude Scale
m  apparent magnitude
M  absolute magnitude
apparent brightness of Star 1
m m
 (1001/5 ) 1 2
apparent brightness of Star 2
luminosity of Star 1
1/5 M1  M 2
 (100 )
luminosity of Star 2
How do we measure stellar
temperatures?
Every object emits _______ radiation with
a spectrum that depends on its temperature.
An object of
fixed size
grows more
luminous as its
temperature
rises.
Relationship Between Temperature and Luminosity
Hottest stars:
50,000 K
Coolest stars:
3,000 K
(Sun’s surface
is 5,800 K)
106 K
105 K
104 K
Ionized
Gas
(Plasma)
103 K
Neutral Gas
102 K
Molecules
10 K
Solid
Level of ionization
also reveals a star’s
temperature.
Absorption lines in a star’s spectrum tell us its ionization level.
Lines in a star’s spectrum correspond to a ________ type
that reveals its temperature:
(Hottest)
O B A F G K M
(Coolest)
Remembering Spectral Types
(Hottest)
O B A F G K M
(Coolest)
• Oh, Be A Fine Girl/Guy, Kiss Me
• Only Boys Accepting Feminism Get Kissed
Meaningfully
We measure mass using gravity.
Direct mass measurements are
possible only for stars in binary
star systems.
p2 =
4π2
a3
G (M1 + M2)
p = period
a = average separation
Isaac Newton
Orbit of a binary star system depends on strength of gravity
Types of Binary Star Systems
• Visual binary
• Eclipsing binary
• Spectroscopic binary
About half of all stars are in binary systems.
Visual Binary
We can directly observe the orbital motions of these stars.
Eclipsing Binary
We can measure periodic eclipses.
Spectroscopic Binary
We determine the orbit by measuring Doppler shifts.
Need two out of three
observables to measure mass:
1. Orbital period (p)
2. Orbital separation (a or r = radius)
3. Orbital velocity (v)
v
r
For circular orbits, v = 2pr / p
M
Most massive
stars:
100 MSun
Least massive
stars:
0.08 MSun
(MSun is the
mass of the
Sun.)
Luminosity
An H-R
diagram plots
the
___________
and
___________
of stars.
Temperature
Most stars fall
somewhere on
the
_____________
of the H-R
diagram.
Large radius
Stars with lower
T and higher L
than mainsequence stars
must have larger
radii:
______ and
supergiants
Stars with
higher T and
lower L than
main-sequence
stars must have
smaller radii:
_____ dwarfs
Small radius
H-R diagram
depicts:
Temperature
Luminosity
Color
Spectral type
Luminosity
Radius
Temperature
Protostar to Main Sequence
• A protostar contracts and heats until the core
temperature is sufficient for hydrogen fusion.
• Contraction ends when energy released by
hydrogen fusion balances energy radiated from the
surface.
• It takes 50 million years for a star like the Sun
(less time for more massive stars).
Main-sequence
stars are ______
hydrogen into
helium in their
cores, like the
Sun.
Luminous mainsequence stars are
hot (blue).
Less luminous
ones are cooler
(yellow or red).
High-mass stars
Low-mass stars
Mass
measurements of
main-sequence
stars show that the
hot, ____ stars are
much more
massive than the
cool, ___ ones.
High-mass stars
Low-mass stars
The ____ of a
normal,
hydrogen-burning
star determines its
luminosity and
spectral type!
The core pressure
and temperature
of a higher-mass
star need to be
higher in order to
balance gravity.
A higher core
temperature
boosts the fusion
rate, leading to
greater
luminosity.
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen
(10% of total) is
used up
Life expectancy of a 10 MSun star:
10 times as much fuel, uses it 104 times as fast
10 million years ~ 10 billion years  10/104
Life expectancy of a 0.1 MSun star:
0.1 times as much fuel, uses it 0.01 times as fast
100 billion years ~ 10 billion years  0.1/0.01
Main-Sequence Star Summary
____-mass:
High luminosity
Short-lived
Large radius
Blue
___-mass:
Low luminosity
Long-lived
Small radius
Red
Upper Limit on a Star’s Mass
• Photons exert a
slight amount of
pressure when they
strike matter.
• Very massive stars
are so luminous that
the collective
pressure of photons
drives their matter
into space.
Upper Limit on a Star’s Mass
• Models of stars
suggest that
radiation pressure
limits how massive
a star can be without
blowing itself apart.
• Observations have
not found stars more
massive than about
150MSun.
Luminosity
Stars more
massive
than
150MSun
would blow
apart.
Temperature
Stars less
massive
than
0.08MSun
can’t
sustain
fusion.
Luminosity
Very
massive
stars are
rare.
Low-mass
stars are
common.
Temperature
Life Track After Main Sequence
• Observations of star
clusters show that a
star becomes larger,
redder, and more
luminous after its
time on the main
sequence is over.
A star
remains on
the main
sequence as
long as it can
fuse hydrogen
into helium in
its core.
Main-Sequence Lifetimes and Stellar Masses
Off the Main Sequence
• Stellar properties depend on both mass and age:
those that have finished fusing H to He in their
cores are no longer on the main sequence.
• All stars become larger and redder after
exhausting their core hydrogen: giants and
supergiants.
• Most stars end up small and white after fusion has
ceased: white dwarfs.