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Stars - PowerPoint PPT Presentation

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Stars. Composed of ~98% H and He Fusion in the core supports the star Full spectrum of masses. Key Properties. Apparent Brightness Luminosity Temperature / Color Mass Evolutionary State. Brightness. Absolute brightness Luminosity Power emitted by star into space Only depends on star

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Composed of ~98% H and He

  • Fusion in the core supports the star
  • Full spectrum of masses
key properties
Key Properties
  • Apparent Brightness
  • Luminosity
  • Temperature / Color
  • Mass
  • Evolutionary State
  • Absolute brightness
    • Luminosity
    • Power emitted by star into space
    • Only depends on star
    • Lsun = 4 X 1026 Watts
  • Apparent brightness
    • How bright star appears in the night sky
    • Power per unit area
    • Depends on star’s brightness and distance
inverse square law for light
Inverse square law for light
  • Apparent brightness measured in watts per square meter
  • Drops off as square of distance
measuring distance
Measuring Distance
  • Stellar Parallax
    • Caused by motion of Earth in its yearly orbit
    • d = 1/p

where p is in arcsecs and d is in parsecs

    • 1 parsec = 3.26 lyrs
  • Logarithmic
  • Large values are dim objects
  • Small values are bright objects

Absolute Magnitudes

Apparent Magnitudes

How objects appear from here on Earth

Depends on distance

We can only see objects with m≤6

  • A bright a star would appear if it were 10 pc away
  • Does not depend on distance
color and temperature
Color and Temperature
  • Color is the difference between intensity in two filters
  • B-V color is a good proxy for temperature
  • Color is independent of distance
spectral type
Spectral Type
  • Spectral types are subdivided for intermediate temperatures
  • Values run from 0-9
  • Smaller numbers are hotter
  • Larger numbers are cooler
  • Eg. B1 is hotter than B7
spectral types
Spectral Types
  • Order was alphabetical depending on strength of Hydrogen line
    • Williamina Flemming
  • Revised to follow a more natural order
    • Annie Cannon
measuring stellar masses

Measuring Stellar Masses

Using Binary Systems

hr diagram
HR Diagram
  • Main Sequence
  • Giants
  • Supergiants
  • White Dwarfs
hr diagram1
HR Diagram
  • Luminosity class gives size and luminosity information
main sequence
Main Sequence
  • Mass is the most important property for a star on the MS
  • Stars spend 90% of their lives here, burning H in their cores
  • MS lifetime depends on mass
main sequence1
Main Sequence
  • More massive stars live much shorter lives
    • Burn fuel very quickly to support such a large star
  • Less massive stars live longer
    • Less fuel, but burn it more slowly
life after the main sequence
Life After the Main Sequence
  • When stars run out of H in their cores, they evolve off the MS
  • Giants and Supergiants expand to extremely large sizes
    • Temperatures are very low
    • Luminosity is very high
  • White dwarfs are small and hot
    • Have no nuclear fusion
    • Heated by collapse of gas
star clusters
Star Clusters
  • All stars in the cluster formed about the same distance from Earth
  • All stars in the cluster formed at about the same time
  • Very useful in understanding stellar formation and evolution
    • Can use them as clocks
  • Most of what we know about stars comes from studying clusters
open clusters
Open Clusters
  • Only a few million years old
  • Contain lots of luminous blue stars
  • Contain several thousand stars
  • ~30 lyrs across
globular clusters
Globular Clusters
  • Often several billion years old
    • Some of the oldest objects in the galaxy
    • Contains mostly smaller stars
  • Around 105-106 stars concentrated in a relatively small volume
  • 50-150 lyrs across
age of cluster
Age of Cluster
  • Main Sequence Turnoff (MSTO) – more massive stars have evolved off of the Main Sequence
  • MSTO gives age of cluster
    • Lifetime of cluster same as MS lifetime of stars at the MSTO



Young clusters still have their massive stars on the MS

  • Old clusters are missing the massive blue stars on the MS