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Stars
Star Field as seen through the Hubble Space Telescope
2
Stars –
1. Definition- a large gaseous body that generates
energy through nuclear fusion in its core
( Although the term is often also applied to objects that are in the
process of becoming stars or to the remains of stars that have died.)
2. Spectra (light) of Stars- Allows astronomers to determine the star’s
a. Composition
b. Temperature
c. Luminosity
d. Velocity and Rotation rate in Space
e. Mass
There are three different types of spectra produced when light is
passed through a prism depending on the source of the light:
Stars (cont.)
2. Spectra (light) of Stars(cont.)
A. Continuous Spectraproduced by a glowing solid, liquid, or very high
density gas under certain conditions. (A normal light bulb
produces a continuous spectra.)
B. Absorption Spectra (Dark Line)produced when a cooler gas lies between the observer
and the object emitting a continuous spectra.
- The gas absorbs some of the wavelengths of light leaving behind dark lines.
The wavelengths absorbed depends on the composition of the gas and the
temperature of the light source.
-This is the spectra used to classify stars
Stars (cont.)
2. Spectra (light) of Stars(cont.)
C. Emissions Spectra (Bright Line) –
-produced when a glowing gas emits energy at specific
wavelengths, characteristic of the element composing
the gas.
- used to study nebulae (Clouds of gas)
Stars (cont.)
3. Classifications of Stars- Stars are essentially all made of the same material!!!
- So WHY don’t they all have the same color or absorption line spectra?
***The spectral difference is due to the difference in
temperature of the star.
The different temperatures also leads to the difference in colors that we
see:
- Hotter stars appear Blue
- Cooler Stars appear Red
A. Classification system
The classification scheme used today divides the
star up into seven major spectral or temperature
classes
O, B, A , F, G , K, M (Oh Be A Fine Girl (Guy) Kiss Me
O – Hottest Stars
Stellar Spectra Absorption Lines
Stellar Spectra
Absorption Lines
and
Classifications
The Spectral Sequence
Spectral
Class
O
Temperature
Color
Spectral Lines
Example
30,000 to
50,000 K
BlueViolet
Ionized Helium
Minataka
B
11,000 to
30,000 K
7,500 to
11,000 K
5,900 to
7,500 K
5,200 to
5,900 K
BlueWhite
White
Neutral HELIUM,
Hydrogen
Hydrogen (Strong)
Rigel, Spica
YellowWhite
Yellow
Ionized Metals
Procyon
Ionized CALCIUM,
Ionized and Neutral
Metals
The Sun,
Capella
Orange
Neutral Metals
RedOrange
Neutral Metals,
Molecular Bands
Arcturus,
Aldebaran
Betelgeuse,
Antares
A
F
G
K
M
3,900 to
5,200 K
2,500 to
3,900 K
Sirius, Vega
Stars (cont.)
3. Classifications of Stars (cont)A. Classification system (cont.)
Since 1995 Astronomers have found new stars with surface temps
even lower than spectral class M. These bodies which are not truly stars
are called Brown Dwarfs- Heat is generated by
contraction of gases not Nuclear Fusion.
(Give off a lot of light in the infrared range.)
B. H-R Diagram (Hertzsprung + Russel)
- In 1912 classification scheme for stars invented
- Stars are plotted according to:
1. Luminosity (Absolute Magnitude)
Brightest Stars at the Top
2. Temperature (Spectral Class)
Hotter Stars on the Left – Temperature
Decreases as you move to the right
13
H-R
Diagram of
Some of the
Most
Prominent
Stars in the
Night Sky
Stars (cont.)
3. Classifications of Stars (cont)B. H-R Diagram (cont.)
3. Super-giants:
- Very few rare stars that are bigger and brighter
than typical giants
- 1000 times larger than the Sun
EX- Betelguese in Orion and Antares in Scorpius
4. White dwarfs- Remaining 9% of stars located in the lower left of
the H-R Diagram
- Although Very Hot, they have low luminosities due
to their small size. (About the size of Earth)
- (So dim can only be seen with a telescope)
**- NO nuclear Fusion in core, only shines due to stored
heat remaining from contraction of core.
EX- Sirius B a companion star to Sirius A.
Stars (cont.)
3. Classifications of Stars (cont)B. H-R Diagram (cont.)
- Data points (Stars) on the diagram are NOT
scattered randomly, but rather appear grouped in
a few distinct regions:
1. Main Sequence Stars:
- About 90% of stars fall in this band stretching diagonally across the
diagram.
-Extends from the hot, luminous blue stars to the
cool, dim red stars
Ex- Sun is a Main sequence star
2. Giants:
- Upper right hand side of diagram
- Stars are both luminous and cool.
In order to be as luminous as they are they must be large or giants
- Approximately 10 to 100 times larger than our Sun
Ex- Aldebaran in Taurus
Relative Size of
some Well Known
Stars
H-R Diagram of
some Nearby stars
H-R Diagram of the
Brightest Stars in
the Night Sky
Stars (cont.)
4. Stellar Evolution- Stars DO NOT Live forever
- Eventually the fuel which powers the nuclear reactions will run out and the
star will cease to shine.
- Changes that a star undergoes is referred to as its
LIFE CYCLE
A. Pre-Main Sequence Stage Star
- Stars form in a dense cold, cloud of dust and gas
(Mostly Hydrogen and Helium) called a Cocoon Nebula
that begins to condense and form a Proto-star
Possible Reasons for Condensationa. Nearby Supernova Outburst
b. Stellar Winds from hot nearby stars
1. Proto-Star- Forms as the cloud condenses by the
gravitational accretion of gas and dust. As it grows the
contraction of the particles causes it to heat and begin
to glow.
Stars (cont.)
4. Stellar Evolution (cont.)
A. Pre-Main Sequence Stage (cont.)
2. Protostar(cont.)
- As protostar begins to heat and glow, it spins
faster. Which starts Bipolar Outflow
- NO FUSION YET – Heat only generated by contraction
- Evidence of star formation:
a. T-Tauri Stars
b. Herbig-Haro Objects- Bipolar outflow collides with
surrounding interstellar medium
c. EGG’s (Evaporating Gaseous Globules) smalll dense
clouds in the act of contracting
d. Protoplanetary disks (PROPLYDS)
- If you see any of these there would most likely be a star forming there,
but no planets and no fusion yet!!!!
Star Formation Process
Collapse of an Interstellar Cloud and Formation of
many Stars
Protostar showing Bipolar Outflow
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Hubble Space
Telescope
Picture
showing
Bipolar Jets
Artist’s
Conception of
Bipolar Jets
Herbig Haro Object- Shows Bipolar flow colliding
with interstellar medium
27
Orion Nebula
showing HerbigHaro Objects
The Eagle
Nebula –
Possible
formation
of Many
stars.
Example of
an EGG
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Protoplanetary Disk- Photo taken by Hubble Space
Telescope
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Time Frame for Interstellar
Evolution and Star Formation
Stars (cont.)
4. Stellar Evolution (cont.)
A. Pre-Main Sequence Stage (cont.)
2. Protostar(cont.)
-Eventually contraction of gasses produces a high enough
temperature at the core so that Nuclear Fusion Starts.
***-Once Hydrogen fusion begins  A MAIN
SEQUENCE STAR IS BORN
-Time frame for formation:
A. The more mass there is, the more heat
generated by contraction, the faster the Star
forms
(15- solar masses takes about 60,000 years to form)
B. The less mass there is, the less heat generated
by contraction, the slower the star forms
( .5 solar masses takes 150 million years to form)
C. Our sun probably took about 50 million years to
form
15MSun
9MSun
3MSun
1MSun
0.5MSun
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Stellar Evolution of
Pre-Main Sequence
Stars
Stars (cont.)
4. Stellar Evolution (cont.)
B. Main Sequence Stars- Once Hydrogen fusion begins the temperature and
pressure in the core become strong enough to resist
further contraction
***- Hydrostatic Equilibrium is reached and the star
becomes a stable Main sequence Star
Hydrostatic
Equilibrium –
The outward
pressure of
Nuclear Fusion
is EQUAL to the
inward Pull of
Gravity
Our Sun- A Main Sequence Star
Hydrogen Vs. Helium
Concentrations over the
Life of the SUN
Stars (cont.)
4. Stellar Evolution (cont.)
B. Main Sequence Stars (cont.)- Time frame for Main sequence Star:
1. More Massive Stars have to burn hotter and faster to
resist gravitational contraction and therefore use up
their fuel quicker.
( A 15 solar mass star will burn for about 10 million years)
** Higher internal temps makes these stars more
luminous
2. Less massive stars burn cooler and therefore can last
longer
( A .5 solar mass star will live for 100 billion years)
** Low temps mean they are NOT as luminous
3. Our Sun will fuse hydrogen (burn) for about 10
billion years
Stars (cont.)
4. Stellar Evolution (cont.)
B. Main Sequence Stars (cont.)- The short life span of massive stars implies that
observed ones MUST be YOUNG!!! -> Would you expect to find
Life around planets that orbit these massive stars???
C. Post Main Sequence Stage- Core’s Hydrogen supply runs out Fusion stops and
core begins to contract under gravity.
- The release of heat from contraction causes outer layers of hydrogen to
fuse at an incredible rate and outer layer expands to become a RED GIANT
STAR
1. Red Giant or Super-giant:
Very luminous due to its size but heat is spread out
over a larger area so cooler than main sequence
star….That’s why it turns red!!!
Ex- Betelguese in Orion is a Star that has left the Main sequence stage and
become a Red Supergiant.
Formation
of a RED
Giant or
Supergiant
Star
Red Giant
phase on the
H-R diagram
Size of Supergiant,
Betelguese, compared
to orbit of Earth and
Jupiter
44
Artist’s view of Earth and the Sun as a Red Giant Star
45
Stars (cont.)
4. Stellar Evolution (cont.)
C. Post main Sequence Stage (cont.)
what happens to a star after Fusion stops depends on the original mass of
the star.
a. Low mass stars such as our sun will become Red
giants
b. Higher Mass stars will expand much further to
become Red Super-giants. (ex- Betelguese)
Stars (cont.)
4. Stellar Evolution (cont.)
D. Death of a Star – 4 Solar Masses or less
- Core of Red Giant will heat up due to contraction and start fusing helium
to carbon at a very high rate.
- When Helium runs out Fusion stops and Carbon Core begins to contract
which again causes outer layers to heat up and expand.
- Outer layers of gas will be ejected into space to form a
Planetary Nebula:
a huge shell of brightly glowing gas and dust lighted
by the very hot exposed core of a star. (Will become
White Dwarf Star)
Final Phase of a
Red Giant Star like
our SUN
Instability of the
envelope of gases
that surround a Red
Giant Star
Stellar Evolution of a
Star like our Sun
Represented on a H-R
Diagram
Stellar Evolution
of a Star like our
SUN
Formation of a Planetary Nebula
Ring Nebula in Lyra (Relatively young nebula because core is
not yet visible)
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Cat’s Eye Nebula in Draco
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Eskimo Nebula in Draco
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Hourglass Nebula in Musca
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Butterfly Nebula in Ophiucus
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Stars (cont.)
4. Stellar Evolution (cont.)
D. Death of a Star - 4 Solar Masses or less (cont.)
- Due to lack of mass carbon will not be able to condense enough to fuse
into oxygen.
- After Planetary Nebula Gases Spread out all that remains is a
White Dwarf “Star”:
- Stellar Core Remnant that has about 1.4 Solar
Masses or less
(About the mass of the SUN in what will shrink down to the size of the
Earth – 1 teaspoon of matter would weigh 5 tons on earth)
- Generates light and heat from contracting of matter
under gravity (NOT FUSION)
- Very hot but not luminous because of small size
- Eventually will stop shrinking (electrons prevent further collapse) and
will slowly cool off over 10’s of billions year and become a black dwarf.
Sirius B is a white
dwarf star shown
next to companion
star, much brighter
Sirius A.
White Dwarf Star and Companion Star which wandered to close
to white Dwarf will probably lead to a Type I Supernova
60
Stars (cont.)
4. Stellar Evolution (cont.)
E. Death of a Star - 4 Solar Masses or more
- Eventually due to extremely high mass of the Star, the
core will eventually become hot enough to have fusion
all the way to Iron
- As it tries to fuse into heavier elements it actually loses energy that is
supporting the core against gravity.
- The core shrinks very rapidly and rebounds with a
tremendous shock wave that blows apart the entire
shell of the star in an explosion called a Supernova
(Type II)
Stars (cont.)
4. Stellar Evolution (cont.)
E. Death of a Star - 4 Solar Masses or more (cont.)
Supernova (Type II)- An explosion that causes a star to suddenly
increases dramatically in brightness
- Energy released is more than 100 times what the sun will radiate over
ts entire 10 billion year lifetime
- Very rare only about 1 every hundred years per
galaxy (But there are billions of galaxies in the universe)
- Star will outshine ALL the stars in its own galaxy
COMBINED!!
- May even be visible on earth during daylight hours
-Nucleosynthesis- creation of heavier elements from
lighter elements. (All elements heavier than Iron
could only be created in Supernova Explosions)
Layers of a
Super-Giant
Red Star right
prior to
Supernova
Explosion
Fusion up to Iron Releases energy but Fusion past Iron
requires Energy
Process of a Type II Supernova Explosion
Supernova 1987 A – Same star field seen before supernova
and after Supernova explosion
1987 Supernova in the Large Magellanic Cloud – Hubble
Space Telescope
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Veil Nebula – Remnant of a supernova that exploded about
15,000 years ago
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Crab Nebula- A Remnant of a Supernova Explosion
observed in 1054 AD which was bright enough to be seen
during the day for over three weeks and during the night
for 6 months
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Stars (cont.)
E. Death of a Star - 4 Solar Masses or more (cont).
-After Supernova explosion, stellar remnant is
dependant upon how much of core is left.
-1. Neutron “Star”-- Core remnant is between 1.4 and 3.0 solar masses
- Compression will be so great that protons and electrons of matter in core
will combine to form neutrons – Atoms will be able to become very close
together (Neutrons prevent further collapse)
-
- Only Massive stars 5-10 solar masses can become
Neutron stars
-
- More Massive than a white dwarf star BUT only the size of a large city!!!!!
(A paper clip made from a Neutron star would outweigh Mt. Everest )
-
- Emit strong radio waves
-- Pulsars (Pulsating Radio waves) are evidence for the
existence of Neutron Stars
-
**- Pulsars detected in at Center of Both Crab and Veil Nebula
(Remnants of a Supernova)
Size of a Neutron Star
Formation of Pulsars by Neutron Stars
Pulsars
Stars (cont.)
E. Death of a Star - 4 Solar Masses or more (cont).
2. Black Hole
- Core remnant is greater than three solar masses
- Compression of core is so great that even neutrons cannot hold the
core up against its own gravity.
- Gravity squeezes three solar masses into an
infinitesimally small point (Smaller than the size of a
pinhead) called a singularity
-The area that separates the black hole from the
surrounding space is called the Event Horizon. ->
Within the event horizon gravity is so strong that even
light does not travel fast enough to escape the gravity.
(At the singularity the infinite gravity causes space and time to be
jumbled and the laws of physics as we know them do not apply.)
Stars (cont.)
E. Death of a Star - 4 Solar Masses or more (cont).
2. Black Hole (cont.)
- Black holes are usually detected in binary star
systems where one of those stars has become a
black hole
- Only massive main sequence star (10 solar masses
or more) will become black holes
Black Hole’s
Effect on the
Warping of
Space-Time
Formation of a Black Hole
Artist’s View of a Black Hole’s Effect on a Planet