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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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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



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