ASTR 1020

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ASTR 1020 Introductory
Astronomy II Stars Galaxies
Week 6 Lecture 1 Stars, magnitudes, spectra, HR
diagram
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Announcements Thursday
Midterm on Thursday Observing
on Thrusday at 700 PM Summary of Week 5
The Sun gravity ltgt
pressure 4H gt 4He energy
Stars 0.1 to 100
Mo H-R diagram Should
have read Chapter 15 (Surveying the Stars)
Stellar properties distances,
luminosities, masses How do we
determine these? The H-R diagram
Evolution of stars towards the
stellar graveyard
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Measuring the Stars

• Measuring stellar luminosities
• Measuring distances
• Classifying stars

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Inverse Square Law of Brightness
Apparent Brightness Lo / (distance)2
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Stellar Luminosity

• What we measure APPARENT BRIGHTNESS
• how bright it appears to us here on Earth
• What we want to know (absolute) LUMINOSITY
• how much energy is emitted (Joules/sec or watts)
• Need to know DISTANCE to the star

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How Do We Measure the Distances to Astronomical
Objects?

• Well keep asking this question again and again
  over the semester
• Several techniques, each valid for different
  objects at different distances
• Technique 1 Parallax

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Parallax
Determining Distance Using

• Parallactic angle (p) 1/2 of the change in
  angular position over 6 months
• Larger for closer objects
• Smaller for farther objects

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Parallax formula

• New Distance Unit invented for just this method
  of distance measurement!!
• Parsec (parallaxarcsecond)
• An object at a distance of one parsec has a
  parallax of 1 arcsecond
• Distance (parsecs) 1/p (arcsec)
• 1 parsec 1 pc 3.26 light years
• Remember 1 arcsecond 1/3600 degree!

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The biggest ground-based telescopes with adaptive
optics can measure stars positions to accuracies
of about 0.01 arcseconds. How far away can they
map the positions of stars via parallax?
Clicker Question

• 1 pc
• 10 pc
• 100 pc
• 1000 pc

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Parallax

• B. maximum distance is set by the accuracy with
  which you can measure positions in the sky (space
  does better than ground)
• Distance (pc)
• 1 / 0.01 arcsec
• 100 pc 320 ly
  

d (in parsecs) 1 / p (in arcsec)
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Brad and Angelina are two stars that have the
same apparent brightness. Brad has a larger
parallax than Angelina. Which star is more
luminous?
Clicker Question

• Brad
• Angelina
• Not enough information to know

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ANSWER

• Brad has a larger PARALLAX. Thus, he is closer to
  us.
• They both have the same APPARENT BRIGHTNESS, but
  Brad is closer
• B. Angelina must be more luminous.

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Best parallax measurer Hipparcos satellite
(1989-1993)

• Space measurements not affected by atmosphere
• Measurement made many times until accurate to
  0.001 arcsec (?3300 light years)
• 100,000 stars mapped
• (2.5 million to slightly lesser accuracy)

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Apparent Magnitudes

• Dates back to the original Hipparchus (the
  person! 190-120 B.C.)
• Brightest stars were of first magnitude
• Dimmest stars were of sixth magnitude
• Everything else sorted in between.
• Later calculated more precisely and found our eye
  sees on a logarithmic scale.
• Linear difference of 5 in magnitude is a factor
  of 100 in apparent brightness
• Tied to the brightness of Vega gt 0-th magnitude
• 
  5-th is 100 times dimmer
• 
  10-th is 104 times dimmer
• 
  15-th is 106 timed dimmer
• M -2.5 log (Flux / Flux of Vega)
• NOTE THE BACKWARDS SCALE!
• Bigger number is fainter!

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Tom is a magnitude 5 star and Katie is a
magnitude 10 star. Who is brighter and by how
much?
Clicker Question

• Tom appears 5 times brighter than Katie.
• Katie appears 5 times brighter than Tom.
• Tom appears 100 times brighter than Katie.
• Katie appears 100 times brighter than Tom.
• They are both the same brightness, just different
  colors.

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Two ways to measure temperature

• 1) Thermal spectrum (i.e. Wiens Law, Chapter 5)
• Hotter bluer cooler redder

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2.) Spectral class even better!

• Different atoms and molecules can be
  characterized as tough or fragile
• The more complex an atom or molecule (more
  electrons, more atoms), the more fragile it is
• Fragile types are more easily ionized or knocked
  apart by collisions in high temperature regions
• ? If there are signs of fragile atoms and
  molecules, the temperature must be low

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Spectral Classification O B A F G K M

• Hottest stars O B mostly helium, little hydrogen
• Hot stars A F helium, hydrogen
• Cooler stars G hydrogen, heavier atoms
• Coolest stars M molecules, (complex absorption
  bands)

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Spectra help classify stars
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A bit of history Classifying Stars

• 1900 , Harvard College observatory
• Women were hired by the observatory director as
  computers to help with a new survey of the
  Milky Way
• Most had studied astronomy, but were not allowed
  to work as scientists

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Devising the strange temperature code

• Original classification of spectra (1890) was
• A strongest hydrogen feature
• B less strong hydrogen C, D, etc.
• Annie Jump Cannon realized that a different
  sequence made more sense (1910)

? O B A F G K M !!
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• Important the different spectral lines seen are
  NOT primarily because stars are made of different
  elements
• Most stars are made mostly of hydrogen
• The variety in spectra is due to temperature via
  the survival of electrons attached to atoms and
  molecules in at the stars surface

Cecelia Payne-Gaposchkin figured this out
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Astronomers ToolboxWhat do we know how to do
now?

• Measure Distance
• parallaxgood to nearby stars but not beyond
• Measure Luminosity
• measure apparent brightness and distance, infer
  luminosity
• Measure Temperature
• Wiens law, or, better yet, take spectra and use
  spectral classification.
• Next Mass

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Masses are much harder than distance, luminosity,
or temperature

• Since we are only ever seeing a point source, it
  is hard to determine how much mass is contained.
• If we could see another nearby object (another
  star maybe?) we could use the gravity between the
  objects as a measure of the mass.

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Binary Stars to the Rescue!!

• Types of binary star systems
• Visual Binary
• Eclipsing Binary
• Spectroscopic Binary
• About half of all stars are in binary systems

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Visual Binary
We can directly observe the orbital motions of
these stars
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Eclipsing Binary
We can measure periodic eclipses
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Spectroscopic Binary
We determine the orbit by measuring Doppler shifts
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Newtons Laws of gravity provide the mass
Direct mass measurements are possible only for
stars in binary star systems Once we know
p period a average separation We can
solve Newtons equations for mass (M)
Isaac Newton
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ASTR 1020 Introductory
Astronomy II Stars Galaxies
Week 7 Lecture 1 HR diagram Stellar Evolution
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Announcements Thursday
Midterm on Thursday Observing
on Thrusday at 700 PM Summary of Week 5
The Sun gravity ltgt
pressure 4H gt 4He energy
Stars 0.1 to 100
Mo H-R diagram Should
have read Chapter 15 (Surveying the Stars)
Stellar properties distances,
luminosities, masses How do we
determine these? The H-R diagram
Evolution of stars towards the
stellar graveyard
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Most massive stars 100 MSun Least
massive stars 0.08 MSun (MSun is the
mass of the Sun)
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Main sequence stars

• Burning hydrogen in their cores
• Temperatures are hotter for more massive stars
  (more gravitational pressure ? higher T, remember
  Equation of State)
• More luminous (higher T ? much higher fusion
  rates)

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Lifetimes on Main Sequence (MS)

• Stars spend 90 of their lives on MS
• Lifetime on MS amount of time star fuses
  hydrogen (gradually) in its core
• For Sun (G), this is about 10 billion years
• For more massive stars (OBAF), lifetime is (much)
  shorter
• For less massive stars (KM), lifetime is longer

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George and Abe are two main sequence stars
George is an M star and Abe is a B star. Which is
more massive? Which is redder in color?
Clicker Question

• George is more massive and redder
• Abe is more massive and redder
• George is more massive Abe is redder
• Abe is more massive George is redder
• They are both main sequence, theyre the same
  mass and same color.

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Main-Sequence Star Summary
High Mass High Luminosity Short-Lived
Large Radius Hot Blue Low Mass Low
Luminosity Long-Lived Small Radius Cool
Red
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Astronomers ToolboxWhat do we know how to do
now?

• Measure Distance
• parallaxgood to nearby stars but not beyond
• Measure Luminosity
• measure apparent brightness and distance, infer
  luminosity
• Measure Temperature
• Wiens law, or, better yet, take spectra and use
  spectral classification.
• Measure Mass
• For stars in binary orbits, if we can get their
  orbital parameters, we can figure out their mass

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H-R diagram depicts Temperature Color
Spectral Type Luminosity Radius
Luminosity
Temperature
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Luminosity Class I. Supergiants II. Bright
Giants III.Giants IV. Subgiants V. Main
Sequence
Luminosity
The Sun is a G2 V star
Temperature
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• As stars run out of hydrogen fuel their
  properties change (generally they turn into red
  giants-more on why next week)
• Top end of main sequence starts to peel off
• Pleiades star cluster shown ? no more O and B
  stars

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Main-sequence turnoff point of a cluster tells us
its age
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Where we see this best Star Clusters

• Groups of 100s to millions of stars
• All about the same distance (apparent brightness
  tracks luminosity well)
• All formed about the same time (i.e. all are same
  age)
• Range of different mass stars!

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1.) Open Clusters

• Loose groups of 1000s of stars
• This is where most stars in the Galaxy are born

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• Pleiades an open cluster of stars about 100
  million years old
• Compare with Suns age of about 4.6 BILLION years
  old

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2.) Globular Clusters

• Generally much older- up to 13 BILLION years
• millions of stars, densely packed
• Intense gravitational interactions

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Variable Stars

• Any star that varies significantly in brightness
  with time is called a variable star
• Some stars vary in brightness because they cannot
  achieve proper balance between power welling up
  from the core and power radiated from the surface
• Such a star alternately expands and contracts,
  varying in brightness as it tries to find a
  balance

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Pulsating Variable Stars

• The light curve of this pulsating variable star
  shows that its brightness alternately rises and
  falls over a 50-day period

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Cepheid Variable Stars

• Most pulsating variable stars inhabit an
  instability strip on the H-R diagram
• The most luminous ones are known as Cepheid
  variables

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Evolution of Low Mass Stars (less than 2x Suns
mass)

• Protostars ? Main sequence
• Most of its life on Main sequence (billions of
  years)
• What happens when it runs out of hydrogen?

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When The Core Runs Out Of Hydrogen, All That is
Left in the Center is Helium

• But the temperature is not hot enough to fuse
  helium.
• With fusion no longer occurring in the core,
  gravity causes core collapse ? key theme
• Core temperature starts to heat up
• Now Hydrogen fusion has moved to shells
  surrounding the core
• Pushes outer layers of the star out.

RED GIANT
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Red Giants

• As core collapses, hydrogen SHELL burns faster
  and faster more energy created
• Luminosity increases, lifts outer parts of star
• Star becomes brighter, larger and cooler!!
• All the while, the core is continuing to shrink
  and is heating up.

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What happens to the Earth?

• Red giants have sizes up to 100 x the Suns
  radius, 1000 times the luminosity
• Sun will swallow Mercury, Venus EARTH!!!
• In 5-7 billion years, we will be toast.

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Putting Doom into Perspective

• 65 million years ago, the dinosaurs died
• Present-day mammals (like us) evolved from small
  rodents alive at that time
• In the next 5 billion years, we have about 80
  equal sized time intervals - enough time to
  re-evolve over and over again if necessary

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Eventually, The Core Is Hot Enough To Burn Helium

• At Temp gt108 K helium flash occurs and helium
  ignites.
• Hydrostatic equilibrium has been restored and the
  core is now balanced again happily burning
  helium to carbon as a Horizontal Branch Star

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Electron Degeneracy Pressure Causes The Helium
Flash

• Typical Conditions
• Temperature rise of gas corresponds to pressure
  rise (SOLAR THERMOSTAT)
• As pressure goes up, gas expands and temperature
  drops back down.
• Nuclear fusion is kept at a constant rate
• Degenerate conditions (extremely high pressures)
• As temperature rises pressure does not change.
• No expansion, no cooling, no stabilization(no
  ther
• RUNAWAY FUSION
• (lasts for only a few hours)

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After the Helium Flash

• Helium burning into Carbon in the core- stable!
• Hydrogen still burning in a shell outside the
  core

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On the HR Diagram

• Helium burning expands the core Hydrogen shell
  also lifted and slightly cooled
• Energy output DECREASES slightly after helium
  flash (still brighter than main sequence)
• Outer layers fall and heat

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A star moves upwards and to the right on the HR
diagram. What is probably happening in the core?
Clicker Question

• The core has just started to burn a new element
• All nuclear burning is slowing down
• The inner core temperature is cooling
• The inner core is collapsing and heating up
  shell burning is increasing

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HR Diagram for an old globular cluster
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Time scales for Evolution of Sun-like Star

• H core burning Main Sequence 1010 yr
• 10 billion years
• Inactive He core, H shell burning Red Giant 108
  yr
• 100 million years
• He core burning (unstable), Helium Flash Hours
  
• He core burning (stable), Horizontal
  Branch 107 yr
• 10 million years

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Variable Stars

• Any star that varies significantly in brightness
  with time is called a variable star
• Some stars vary in brightness because they cannot
  achieve proper balance between power welling up
  from the core and power radiated from the surface
• Such a star alternately expands and contracts,
  varying in brightness as it tries to find a
  balance

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Pulsating Variable Stars

• The light curve of this pulsating variable star
  shows that its brightness alternately rises and
  falls over a 50-day period

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Cepheid Variable Stars

• Most pulsating variable stars inhabit an
  instability strip on the H-R diagram
• The most luminous ones are known as Cepheid
  variables

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