Stars

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Stars
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What is a star?

• A huge collection of very hot gas
• Stars emit light and other types of EM radiation
  during the process of fusion

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How far away is that star?

• Because the distances are so big, special units
  are used
• Light Year
• The distance light travels through a vacuum in
  one year
• 9.5 trillion km (9500000000000 km)

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How far away is that star?

• Parallax
• The change in the apparent position of an
  object against a distant background
• Look a thumb with one eye open
• As the Earth moves in its orbit, scientists look
  at stars from different angles

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Parallax
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How far away is that star?

• Parallax
• The change in the apparent position of an
  object against a distant background
• Look a thumb with one eye open
• As the Earth moves in its orbit, scientists look
  at stars from different angles
• The closer the star is to Earth, the greater the
  parallax

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Characteristics of Stars

• Color Temperature
• The temperature of a flame can be estimated by
  the color of the flame
• Color of stars can be used to estimate their
  temperature
• Hottest stars appear blue (above 30,000 K)
• Medium stars appear yellow (5000-6000 K)
• Coolest stars appear red (3000 K)

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Characteristics of Stars

• Brightness
• Closer stars appear brighter
• Brightness can vary by huge amounts
• Ex the Sun vs. other stars
• Apparent Brightness brightness of the star as it
  appears on Earth
• Absolute Brightness how bright the star actually
  is, based on the amount of energy it gives off

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Apparent Brightness vs. Absolute Brightness
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Characteristics of Stars

• Size Mass
• Once the temperature and absolute brightness are
  measured, the diameter and volume can be
  estimated
• The mass of the star can be estimated by
  observing the gravitational effects it has on
  objects around it

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Characteristics of Stars

• Composition (What is it made of?)
• Light emitted from stars is analyzed using a
  spectrograph

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Absorption Spectrum from Spectrograph
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Characteristics of Stars

• Composition (What is it made of?)
• Light emitted from stars is analyzed using a
  spectrograph
• Each star has a unique pattern
• The pattern is determined by the type of elements
  the star contains
• Most stars have similar composition to the Sun
  (96-99.9 hydrogen and helium)

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Hertzprung Russell Diagram

• Graph of
• Surface Temperature (determined by color)
• Absolute Brightness
• Measuring these characteristics allows
  astronomers to
• Estimate Size
• Estimate Distances
• Predict what will happen to stars next

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H-R Diagram
Absolute Brightness on y-axis Brightest stars
towards top Faint stars towards bottom
Temperature on x-axis Hot Stars (blue) on
left Cool Stars (red) on right
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Interpreting the H-R Diagram

• Main Sequence Stars
• Majority of stars lie along a diagonal band from
  bottom right to top left

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H-R DiagramMain Sequence
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Interpreting the H-R Diagram

• Main Sequence Stars
• Supergiants Giants
• Located in top right of HR-Diagram
• Stars that are not very hot, but very bright
• They must be very large to give off huge amounts
  of energy
• 100-1000x the diameter of the Sun

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H-R DiagramSupergiants Giants
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Interpreting the H-R Diagram

• Main Sequence Stars
• Supergiants Giants
• Dwarfs
• Bottom left corner of H-R diagram
• Stars that are very hot, but not very bright
• They must be small in order for the amount of
  energy given off to be so small

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H-R DiagramDwarfs