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Chapter 1 Our place In The Universe Student information Sheets Semester: ______________________ Astronomy 105 Section ___________ Dr. Robert Friedfeld How To Access on-line Materials http://www.physics.sfasu.edu/ http://www.astronomyplace.com/ What is this lecture section? 001 002 003

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Chapter 1 l.jpg

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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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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  • 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 gas and dust, forming stars and planets.

  • 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 10 gas and dust, forming stars and planets.12km (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) and time.

    • 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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Onward To The Stars space and time.

  • 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 space and time.

  • How many stars are there in the visible universe?


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- space and time.more than all the grains of sand on all the beaches of the world.


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Spaceship Earth space and time.

The Earth rotates on its axis once each day.

As the Earth rotates, your speed around the Earth’s axis depends on your latitude.


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The Earth revolves around the Sun once each year. space and time.

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 Earth’s orbit around the Sun is called the ecliptic plane.

Axial Tilt of 23.5o


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Seasons is called the

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


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Spring (Vernal) Equinox is called the

  • 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 is called the

  • 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 is called the

  • 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 is called the

  • 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 is called the


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Precession is called the

  • 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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Traveling in the Milky Way Galaxy is called the

  • 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 is called the

  • 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. is called the

  • Most visible stars reside within the galaxy’s 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 is called the

  • 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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End of Section the expansion of the universe


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