Chapter 10 Measuring the Stars

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Chapter 10Measuring the Stars

• AST1002 Introduction to Astronomy
• Dr. Mike Reynolds

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The Distance to the Stars

• How far are the stars from the Earth?
• To measure the distance to nearby stars,
  astronomers can use Parallax
• Parallax The apparent shift of an object
  relative to some distant background as the
  observers point of view changes
• Parallax
• Allows you to determine close stellar distances
• How it works the finger and blinking demo
• Astronomers allow Earth to blink every six
  months on a specific star

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Stellar Parallax
Other Stars
Earth Sun
Star
Earth Position 1
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Stellar Parallax
Other Stars
Earth Sun
Star
Earth Position 1
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Stellar Parallax
Other Stars
Earth Sun
Star
Earth Position 2
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Stellar Parallax
Other Stars
Earth Sun
Star
Earth Position 2
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Stellar Parallax
Other Stars
Earth Sun
Star
Earth Position 1
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Stellar Parallax
Other Stars
Earth Sun
Star
Earth Position 2
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Stellar Parallax
Other Stars
Earth Sun
Star
Earth Position 1
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Stellar Parallax
Other Stars
Earth Sun
Star
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Stellar Parallax
Other Stars
Earth Sun
Star
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Stellar Parallax
Other Stars
Earth Sun
Star
Parallax Shift ½ the stars shift back and
forth, in degrees
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Stellar Parallax
Other Stars
Earth Sun
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Star
Parallax Shift ½ the stars shift back and
forth, in degrees
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The Issues with Measuring Parallax

• Everything is moving!
• The Earth and Sun
• The star you are measuring
• The background stars
• How Do You Overcome this?
• Make more than two measurements
• Take into account all the motions
• Now calculate the parallax, then distance
• Parallax only works with fairly-close stars
• ?WHY?

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How Close are the Close-By Stars?

• The closest star? The Sun!
• After the Sun
• Proxima Centauri, part of the three-star Alpha
  Centauri (a Cen) system
• About 4.3 light-years away
• A 50-mile high stack of toilet paper stretched
  out on the 1 sheet 1 million miles scale
• Alpha Centauri is often listed as the second
  closest star to Earth wrong!
• Next star Barnards Star is about 6 light-years
  away
• About 30 stars lie within 16.5 light-years of
  Earth

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Stellar Terminology

• Light Year the distance light travels in one
  year approximately 10 trillion kilometers
• Parsec the distance to an object with a parallax
  angle of 1 arcsecond unit is the pc
• One pc 3.26 light years (ly) 3.09 X 1013 km
• Proxima Centauri
• Parallax angle of 0.77 arcsecond
• Largest Parallax angle we see WHY?
• From the Sun, Proxima Centauri is
• 1.3 pc or 4.3 ly away
• 3.98 X 1013 km (2.43 X 1013 miles)

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Starlight, Starbright

• Luminosity a stars total amount of power
  radiated into space measured in watts
• Apparent Brightness the amount of light reaching
  us (per unit area) measured in flux
• Brightness varies by the Inverse Square Law
• If we see a star at twice the distance, we see
  1/4 the amount of light
• If the star is at 10 times the distance, we would
  see 1/100 the amount of light
• Can you think of examples that demonstrate this
  concept?

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The Magnitude System

• Solar Luminosity, Lsun comparing a stars
  luminosity to our Suns luminosity
• Lsun 3.8 X 1026 watts
• Magnitude System originated by Hipparchus
• A scale to show how bright the stars appear to
  our eyes (or binoculars and telescopes)
• Apparent Magnitude what we see in the sky
• Star charts use different size dots to
  represent
• Absolute Magnitude brightness if we moved all
  the stars to the same distance 10 pc
• Comparing apples to apples!

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Stars as They Appear
Apparent Brightness / Apparent Magnitude
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Stars as They Actually Are
Absolute Brightness / Absolute Magnitude
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Stars as They Actually Are
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Stars as They Actually Are
Stars at the same distance 10 pc
Apparent Brightness / Apparent Magnitude
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Star Temperatures and Colors

• Astronomers measure a stars surface temperature
  at its photosphere
• A stars surface temperature dictates its color
• Hotter stars are whiter
• Cooler stars are redder
• Our Sun is a yellow star
• We classify stars by their color / surface
  temperature E this is called a stars Spectral
  Type

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Spectral Types Examples
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Spectral Types Examples
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Spectral Types to Remember

• O
• B
• A
• F
• G
• K
• M
• RNS

• Oh
• Be
• A
• Fine
• Girl / Guy
• Kiss
• Me
• Right Now Sweetheart

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Stellar Masses

• Stellar Mass the most important stellar property
• Buthow do you measure this?
• Use an adaptation of Keplers Third Law as
  derived by our old friend, Sir Isaac Newton
• For Keplers 3rd Law you measure the period and
  average distance of the objects orbit
• However you need two objects to do this
• Guess what? Over 50 of stars have a companion
  star!

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Types of Star Systems

• Optical Double Stars not physically related to
  each other but happen to line up so that we see
  what appears to be a double star system
• More rare than one would think!
• Binary Stars systems in which physically-associat
  ed star systems are made up of two stars
• Visual Binary pair of stars that we can see
  orbiting each other

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Types of Star Systems, continued

• Spectroscopic Binary two stars that are so close
  that the only way we detect the binary nature is
  through a spectral (red) shift
• Eclipsing Binary pair of stars that orbit in the
  plane of our line of sight
• Often list as EBS (Eclipsing Binary Star)
• ...the Gorgon's head, a ghastly sight, deformed
  and dreadful, and a sight of woe
• Homer, writing about Algol in the Iliad

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EBS Position vs. Brightness over Time
A very bright white star orbiting a dimmer red
giant star
Brightness ?
Graph showing EBSs brightness over time
Time ?
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EBS Position vs. Brightness over Time
Brightness ?
Time ?
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EBS Position vs. Brightness over Time
Brightness ?
Time ?
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EBS Position vs. Brightness over Time
Brightness ?
Time ?
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EBS Position vs. Brightness over Time
Brightness ?
Time ?
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EBS Position vs. Brightness over Time
Brightness ?
Time ?
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EBS Position vs. Brightness over Time
Brightness ?
Time ?
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EBS Position vs. Brightness over Time
Brightness ?
Time ?
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EBS Position vs. Brightness over Time
Brightness ?
Time ?
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EBS Position vs. Brightness over Time
Brightness ?
Time ?
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The Hertzsprung-Russell Diagram

• Referred to as the H-R Diagram
• Plots two stellar properties
• The stars Temperature
• Temperature also refers to Spectral Class
• The stars Luminosity
• Luminosity can be measured in a couple of
  different ways
• Solar units (against our Sun)
• Absolute Magnitude
• ?Do these two stellar properties make sense? Why?

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A Simple H-R Diagram
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Stars break into four basic Groups or Types
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The Four Stellar Groups
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The Four H-R Stellar Groups

• Main Sequence most stars, like our Sun
• Supergiants cool stars which are very large and
  very bright
• Giants cool stars which are a little smaller and
  dimmer than the supergiants
• White Dwarfs very, very hot stars which are
  small in size
• The H-R Diagram can also show a stars life

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• The Suns Life Path
• Also referred to as Stellar Evolution

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Other Types of Stars

• Variable Stars stars that appear to change in
  brightness
• Two Major Classes of Variables
• Pulsating Variables and Cataclysmic Variables
• Five classes of Pulsating Variables one
  important example
• Cepheid Variables massive, high luminosity stars
  with short periods of one to 70 days and light
  variations of 0.1 to 2 magnitudes
• Astronomers use Cepheids as a standard for
  distances

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Star Clusters

• Two Major Groups of Star Clusters
• Open Clusters groups of young, hot stars that
  are physically related by being held together by
  gravity
• lt50 100gt stars
• Globular Clusters gravitationally-bound grouping
  of very old stars
• 10,000 to one million stars
• Look at the following pictures and determine
  which is Open and which is Globular

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Messier 13 Globular Cluster
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The Pleiades M45 Open Cluster
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Double Cluster in Perseus Open Cluster
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Omega Centauri NGC5139 Globular Cluster
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So How can you visually tell the difference
between Globular and Open Clusters?

• Think about the photos you just observed
• How globulars look overall
• Versus
• How open clusters look overall
• Anything similar? Astronomically-speaking
• Globulars old, many, many stars, organized
• Open young, fewer stars, not organized

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