Our Galaxy: Measuring The Milky Way and its Structure

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Our GalaxyMeasuring The Milky Way and its
Structure

• Group 30
• Faryn Agiert Grace Siu
• Teby Philip Lauren Miles

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Galaxy
Definition a humungous collection of stellar
and interstellar matter Isolated in space and
held together by its own gravity Milky Way
Galaxy our home galaxy aka the Galaxy (with a
capital G) Galactic Disk the part of the
Galaxy where the Sun lies Spiral Galaxies
Andromeda Galaxy nearest major galaxy to Milky
Way Galaxy - has an elongated shape -
consists of a circular galactic disk of matter
that fattens to a galactic bulge at the
center - Galactic Halo a spherical ball of
faint old stars in which the galactic disk and
galactic bulge are embedded
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Star Counts

• English astronomer William Herschel estimated
  size and shape of our galaxy.
• the Milky Way is several tens of kiloparsecs
  across and the Sun lies far from the center.

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Observations of Variable Stars
Variable Stars Two types of Pulsating Variable
Stars RR Lyrae Variables Cepheid Variables Both
Variable Stars are easily recognizable Pulsating
variable ? normal stars experiencing
instability As a star brightens, its surface ?
hotter radius ? shrinks Astronomers use
pulsating variables as means of determining
distances.
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A New Yardstick
How to infer a variable stars luminosity? Also
known as period-luminosity relationship For RR
Lyrae- same luminosity For Cepheids- close
correlation between average luminosity and
pulsation period The distance-measurement
technique works well provided the variable star
can be clearly identified and its pulsation
period measured.
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The Size and Shape of Our Galaxy
Many RR Lyrae variables found in globular
clusters American Astronomer Harlow Shapley,
using observations of RR Lyrae stars, made two
discoveries. 1) globular clusters reside at great
distances 2) Measuring direction distance of
cluster, able to determine their three
dimensional distribution is space. Shapley
demonstrated the mapping of globular
clusters. The center of distribution lies nowhere
near the sun. The location is nearly 8 kpc away
from us in the direction of constellation
sagittarius, known as the galactic center.
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Mapping Our Galaxy

• Our Galaxy Disk, Bulge and Halo
• Much of our knowledge is based on radio
  observations, particularly of the 21-cm radio
  emission line
• The center of the gas distribution coincides
  roughly with the center of the globular cluster
  system
• Galactic disk 300 pc thick
• Galactic bulge 6 kpc by 4 kpc

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Overall Properties
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Orbital Motion

• The motion of stars and clouds on small scales
  seem random but on larger scales its much more
  orderly
• The entire disk is rotating about the Galactic
  center
• The rotation period of material is shorter closer
  to the center and longer further away from the
  center
• Old globular clusters in the halo and faint,
  reddish stars in the halo and the bulge have
  largely random orbits

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Formation of the Milky Way

• Like the star formation, it starts from a
  contracting cloud of pregalactic dust
• Young stars form and spread throughout the
  contracting cloud of pregalactic gas
• Rotation flattens gas into a thin disk stars
  that formed are left behind, forming the halo
• New stars forming in the disk inherit its overall
  rotation and orbit the Galactic center on
  ordered, circular orbits

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THE END Thanks everyone and have an awesome day!