Stars

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Stars
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Star Field as seen through the Hubble Space
Telescope
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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 stars
• 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

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Stars (cont.)

• 2. Spectra (light) of Stars(cont.)
• A. Continuous Spectra-
• produced 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

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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)

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Stars (cont.)

• 3. Classifications of Stars-
• - Stars are essentially all made of the same
  material!!!
• - So WHY dont 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
• M Coolest Stars

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Stellar Spectra Absorption Lines
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Stellar Spectra Absorption Lines and
Classifications
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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

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H-R Diagram of Some of the Most Prominent Stars
in the Night Sky
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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.

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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

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Relative Size of some Well Known Stars
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H-R Diagram of some Nearby stars
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H-R Diagram of the Brightest Stars in the Night
Sky
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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 Condensation-
• a. 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.

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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. EGGs (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!!!!

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Star Formation Process
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Collapse of an Interstellar Cloud and Formation
of many Stars
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Protostar showing Bipolar Outflow
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Hubble Space Telescope Picture showing Bipolar
Jets
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Artists Conception of Bipolar Jets
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Herbig Haro Object- Shows Bipolar flow colliding
with interstellar medium
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Orion Nebula showing Herbig-Haro Objects
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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
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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

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15MSun
9MSun
3MSun
1MSun
0.5MSun
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Stellar Evolution of Pre-Main Sequence Stars
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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

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Hydrostatic Equilibrium The outward pressure of
Nuclear Fusion is EQUAL to the inward Pull of
Gravity
Our Sun- A Main Sequence Star
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Hydrogen Vs. Helium Concentrations over the Life
of the SUN
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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

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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!!! -gt Would you
  expect to find Life around planets that orbit
  these massive stars???
• C. Post Main Sequence Stage-
• - Cores 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.Thats why it turns red!!!
• Ex- Betelguese in Orion is a Star that has left
  the Main sequence stage and become a Red
  Supergiant.

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Formation of a RED Giant or Supergiant Star
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Red Giant phase on the H-R diagram
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Size of Supergiant, Betelguese, compared to
orbit of Earth and Jupiter
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Artists view of Earth and the Sun as a Red
Giant Star
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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)

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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)

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Final Phase of a Red Giant Star like our SUN
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Instability of the envelope of gases that
surround a Red Giant Star
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Stellar Evolution of a Star like our Sun
Represented on a H-R Diagram
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Stellar Evolution of a Star like our SUN
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Formation of a Planetary Nebula
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Ring Nebula in Lyra (Relatively young nebula
because core is not yet visible)
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Cats 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 10s of billions year and become a black
  dwarf.

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Sirius B is a white dwarf star shown next to
companion star, much brighter Sirius A.
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White Dwarf Star and Companion Star which
wandered to close to white Dwarf will probably
lead to a Type I Supernova
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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)

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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)

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Layers of a Super-Giant Red Star right prior to
Supernova Explosion
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Fusion up to Iron Releases energy but Fusion past
Iron requires Energy
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Process of a Type II Supernova Explosion
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Supernova 1987 A Same star field seen before
supernova and after Supernova explosion
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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)

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Size of a Neutron Star
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Formation of Pulsars by Neutron Stars
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Pulsars
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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. -gt 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.)

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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

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Black Holes Effect on the Warping of Space-Time
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Formation of a Black Hole
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Artists View of a Black Holes Effect on
a Planet