Our place In The Universe

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Chapter 1

• Our place In The Universe

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Student information Sheets
Semester ______________________ Astronomy 105
Section ___________ Dr. Robert Friedfeld
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How To Access on-line Materials
http//www.physics.sfasu.edu/
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http//www.astronomyplace.com/
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What is this lecture section?

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A Modern View of the Universe

• The sun, moon, planets and stars appear to circle
  around the earth.
• We cannot feel the earth move, although it does.
• It seems natural to place the earth at the center
  of everything (Geocentric model).
• Today, we know that the earth is one of nine
  planets which orbits the Sun.

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Our Cosmic Address
40 Galaxies
More than 100 billion stars
The Local Supercluster
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Our Cosmic Origins

• How did we come to be?

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• First lets look at what we know.

Telescopic evidence points to an expanding
universe.
From the observed rate of expansion, Astronomers
estimate that the expansion began sometime
between (12-16) billion years ago.
The universe as a whole continues to expand, but
on smaller scales, gravity brings matter
together to form galaxies and clusters of
galaxies.
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Our Cosmic Origins
Galaxies form a few billion years after the Big
Bang
Star forms
A Region of Space Expands with time
Star explodes
Earth was made of elements
produced in stars
long ago.
Star shines
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• Within galaxies, gravity causes the collapse of
  clouds of gas and dust, forming stars and
  planets.
• When stars die, they release much of their
  content back into interstellar space.
• Galaxies function as cosmic recycling plants.
• All elements heavier than H, He, and Li were
  manufactured by stars through nuclear fusion.

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Images of Time

• We study the universe by studying light from
  distant stars and galaxies.
• The speed of light is 300,000 km/s.
• This is fast enough to circle the earth 8 times
  in 1 second.
• Light from stars can take many years to reach us.
• We measure distances to the stars in
  Light-Years. (ly)
• 1 ly is the distance light can travel in one
  year.

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• 1 ly 9.46 x 1012km (9.46 trillion km)
• Because it takes time for light to travel through
  space, the farther away we look in distance, the
  further back we look in time. (Look back time)
• The speed of light limits the portion of the
  universe that we can see.

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• Any picture of a distant galaxy is a picture of
  both space and time.
• The Great Spiral Galaxy in Andromeda (aka
  M31)
• lies about 2.5 million light-years from the
  earth.
• A picture of the Galaxy taken now is a picture of
  how M31 looked about 2.5 million years ago.
• The diameter of M31 is 100,000 ly.
• light from the far side of the galaxy took
  100,000 years longer to reach us than the light
  from the near side.

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• The Great Spiral Galaxy in Andromeda (aka M31)
• lies about 2.5 million light-years from the
  earth.
• A picture of the Galaxy taken now is a picture of
  how M31 looked about 2.5 million years ago.
• The diameter of M31 is 100,000 ly.
• light from the far side of the galaxy took
  100,000 years longer to reach us than the light
  from the near side.

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When we study the universe, it is impossible to
separate space and time.
The Great Spiral Galaxy in Andromeda
100,000 ly
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The Scale of the Universe- The Solar System
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The Sizes of the Sun and the planets (11010)
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Onward To The Stars

• The nearest star system to our own is called
  Alpha Centauri.
• It is approximately 4.4 ly from us.

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The Milky Way Galaxy and Beyond

• How many stars are there in the visible universe?

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-more than all the grains of sand on all the
beaches of the world.
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Spaceship Earth
The Earth rotates on its axis once each day.
As the Earth rotates, your speed around the
Earths axis depends on your latitude.
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The Earth revolves around the Sun once each year.
The average distance of the earth from the Sun is
called an astronomical unit or AU.
1AU 150 million km 93 million miles.
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The plane of the Earths orbit around the Sun
is called the ecliptic plane.
Axial Tilt of 23.5o
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Seasons

• The combination of Earths axial tilt, axial
  rotation and its revolution around the Sun
  explains why we have seasons.

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Spring (Vernal) Equinox

• On about March 21 each year.
• Both hemispheres receive equal amounts of
  sunlight.
• Beginning of Spring in the Northern Hemisphere.
• Beginning of Fall in the Southern Hemisphere.

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Summer Solstice

• On around June 21, the northern hemisphere
  receives its most direct sunlight.
• The northern hemisphere has the longest period of
  daylight of any day of the year.
• This is usually considered the first day of
  summer in the northern hemisphere.
• The Southern Hemisphere receives its least direct
  sunlight.
• and has its shortest period of daylight of
  any day of the year.
• This is the first day of winter in the Southern
  Hemisphere.

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Fall (Autumnal) Equinox

• Occurs around September 21.
• Both hemispheres receive the same amount of
  sunlight. but now the seasons are the reverse of
  the spring (Vernal) equinox.
• In the Northern Hemisphere we have the beginning
  of Fall.
• In the Southern Hemisphere we have the beginning
  of Spring.

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Winter Solstice

• Around December 21.
• We have the reverse situation to that of the
  summer solstice.
• It is usually considered the first day of winter
  for the northern hemisphere and the first day of
  summer for the southern hemisphere.

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Overview of Seasons
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Precession

• The axis of the Earth will remain pointed toward
  Polaris throughout our lifetimes.
• However, this has not always been the case and
  the direction will change again in the distant
  future.
• The reason is a that the rotation axis of the
  Earth precesses - like a spinning top.

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Like a Spinning top, the axis of the Earth
precesses.
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Traveling in the Milky Way Galaxy

• The local solar neighborhood is only a tiny
  portion of the Milky Way Galaxy.
• The stars in the local solar neighborhood move
  quite fast relative to our solar system. (
  70,000 km/h)

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The Entire Milky Way Galaxy Rotates

• Our Sun and solar system are located about 28,000
  ly from the galactic center.
• At this distance, each orbit around the galactic
  center takes about 230 million years.

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Edge-On View of the Milky Way Galaxy.

• Most visible stars reside within the galaxys
  thin disk.
• Careful study of galactic rotation shows that
  most of the mass lies in the galactic halo.
• Because this mass emits no light that we have
  detected, we call it dark matter.

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The Expanding Universe

• When we look outside the local group of galaxies
  (remember, this is the group to which the Milky
  Way belongs.) we find two astonishing facts.
• Virtually every galaxy outside the Local Group
  is moving away from us.
• The more distant the galaxy, the faster it
  appears to be moving away from us.
• ? The entire universe is expanding

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An expanding raisin cake illustrates basic
principles of the expansion of the universe
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• End of Section