Milky Way Galaxy

1
Milky Way Galaxy

• Unit Five Exam
• Chapter 12
• Mr. Saks
• Astronomy

2
Introduction

• The Milky Way Galaxy is a spiral galaxy our sun
  and solar system are a small part of it
• Most of the stars that we can see are in the
  Milky Way Galaxy
• The main plane of the Milky Way looks like a
  faint band of white in the night sky
• This spiral galaxy formed about 14 billion years
  ago
• It takes the sun roughly 250 million years to
  orbit once around the Milky Way
• The Earth is about 26,000 light-years from the
  center of the Milky Way Galaxy
• The major arms of the Milky Way galaxy are the
  Perseus Arm, Sagittarius Arm, Centaurus Arm, and
  Cygnus Arm our Solar System is in a minor arm
  called the Orion Spur

3
The Galaxy we call Home

• The Milky Way
• Our galaxy is about 100,000 ly in diameter
• 1,000 ly thick
• It contains over a 100 billion stars.
• IF Earth is the only inhabited place then
• It makes the galaxy ours
• Each person owns about 50 stars to themselves
  plus some comets, moons, and maybe a planet.

4
The Galaxy

• There are spiral arms and a bright central part
• The Sun is far from the center of the Galaxy,
  halfway to the edge of the Galaxy along the
  Orion spiral arm
• The Sun is revolving around the center of the
  Galaxy at a speed of half a million miles per
  hour, yet it will still take 200 million years
  for it to go around once.

5
What we see

• Almost everything that we see in the night sky is
  strictly in the Milky Way galaxy
• Exception The Andromeda galaxy found as a faint
  patch of light in the constellation Andromeda.
• The faint cloud we see in the nighttime sky are
  the glow of a cloud of distant stars.
• The Greeks named that band galaxies kuklos (milky
  circle).
• Romans changed the name to via lacteal (milky
  road or milky way).
• It would not be until the 20th century that we
  discovered we live inside a great wheel of stars
  and the universe has other such places.

6
Our Discovery

• Two hundred years ago astronomers thought all
  stars were scattered through space uniformly.
• One hundred years ago astronomers understood the
  stars we see lie in a great wheel shaped star
  system but they thought the sun was near the
  center.
• A life time ago, a woman studying variable stars
  and a man studying a star cluster unlocked a
  secret we live in a galaxy.

7
Variable Stars

• Although we think of stars as unchanging, we have
  known for centuries that stars change in
  brightness
• Novae and Supernovae appear and fade
• Variable stars are stars that get brighter, then
  fainter, then brighter, then fainter, and back
• Some are eclipsing binaries but others are not
• Some stars just constantly pulse like beating
  heart

8
Cepheid Variable stars

• Cepheid variable stars are named after the first
  such star discovered
• ? Cephei
• Many variable stars have periods from 1 to 60
  days
• There are on the instability strip of the HR
  diagram
• Cepheids are clearly giant stars
• RR Lyrae variable stars have a period of about
  half a day (located in Lyra)

9
Why do they pulse

• A star may have a layer in their atmosphere of
  partially ionized helium which absorbs and
  releases energy
• Stars can pass through this strip as they evolve
• Massive stars pulsate slow and smaller mass stars
  pulsate faster
• Long pulsing then we can determine star is
  massive and so on

10
Period-Luminosity Relation

• 1912 Henrietta Leavitt studied stars in the Small
  Magellanic Cloud
• Noticed variable stars
• Noticed brightest stars were longer periods of
  pulsating
• Found relationship between period of pulse and
  luminosity now known as period-luminosity relation

11
Size of Galaxy

• Early 20th century Astronomers believed that we
  lived near the center of a disk shaped cloud of
  stars that was not very large
• Many believed that the star system was isolated
  in an otherwise empty universe
• Howard Shapley began to discover the true nature
  of galaxy when he noticed different kinds of star
  cluster in different parts of the sky

12
Star Clusters

• Open star cluster contain 100-100 stars in a
  diameter of 10 to 60 ly
• Concentrated along the band of the Milky Way
• Pleiades is a well known open star cluster
• Globular star clusters contain 100,000
  1,000,000 stars in a diameter of about 75 ly
• Noticed these are concentrated in one part of sky
• Almost half of all known globular clusters are
  found in Constellation Sagittarius

Pleiades star cluster
13
Shapley Needed Henrietta to find Distance

• To find distance you need a parallax, which the
  known Cepheid and RR Lyrae stars were too far to
  have one or be able to measure their proper
  motion
• Proper motion is the movement of stars through
  space
• Proper motion allows distance to be figured
• Shapley was able to find 11 Cephied and RR Lyrae
  stars with proper motion and therefore he could
  find distance
• Knowing distance he could then shift Henriettas
  scale of apparent magnitude with absolute
  magnitude
• When looking at star clusters and finding these
  variable stars he could compare their apparent
  magnitude with the scale for absolute magnitude
  and thus find the distance to the star clusters

14
Calibrated Cepheids

• Astronomers give great credit to Shapley for
  calibrating all Cepheids
• With his work, variable stars could be used as a
  measuring tool for distance
• What he did
• Found distance to variable stars
• Found their absolute magnitude
• Change apparent magnitude for absolute magnitude
  on Henriettas period-luminosity relation chary
• Comparing apparent magnitude with absolute
  magnitude for variable stars too far to be
  measure allowed distances to be found to that
  star and star cluster (magnitude-distance
  relationship)