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The Milky Way Galaxy and Hubble’s Law Messier Catalogue Messier Catalogue Charles Messier made the first catalog of non-stellar “fuzzy” objects during the 1770's. He made a list of 109 such objects that he did not want to mistake for comets .

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The milky way galaxy and hubble s law l.jpg

The Milky Way Galaxy


Hubble’s Law

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Messier Catalogue

  • Charles Messier made the first catalog of non-stellar “fuzzy” objects during the 1770's.

  • He made a list of 109 such objects that he did not want to mistake for comets.

  • These objects turned out to be the following:

    • galaxies

    • star clusters

    • nebulae

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  • How many stars are in the Milky Way?

    • About 200 billion

  • How many galaxies are there?

    • billions and billions

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  • How old is the Milky Way Galaxy?

    • It is 15 billion years old and will remain active for a another 10 billion years.

    • Recall: The Sun is 4.5 billion years old.

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Are all stars members of the Milky Way?

  • No, most stars in the universe are in other galaxies.

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Where in the Milky Way is our solar system located?

  • The solar system is located in a spiral arm about 28,000 light years from the center of the Milky Way.

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Sketch of the Milky Way Galaxy

  • Disk

  • Bulge

  • Halo

  • Spiral Arms

  • Sun

  • Globular Clusters

  • Open Clusters and Nebulae

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How do we know that we are located in the spiral arm?

  • In 1917 Harlow Shapley discovered that the globular clusters form a huge spherical system that is not centered on the Earth.

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Finding Our Place

Globular Clusters evenly distributed

Early view

Sun at center


Globular Clusters unevenly distributed

Harlow Shapley

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Harlow Shapley

Hebert Curtis

The Great Debate

April 26, 1920

A galaxy is a nebula

within the Milky Way!

A galaxy is an

island universes!

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Spiral Galaxies

  • galaxies like the Milky Way with arcing structures lying in a plane and emanating from the nuclear bulge

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M32 - E2

NGC 4125

M87 - E1

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Elliptical Galaxies

  • galaxies with an elliptical shape, no spiral arms, and little interstellar matter

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NGC 1097


NGC 4123

NGC 4477

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Barred Spiral Galaxies

  • galaxies with a bar of stars running through the nuclear bulge

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Irregular Galaxies

  • galaxies that are asymmetrical and are sometimes just two or more galaxies colliding

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Galaxy Observations

  • During the 1920's Edwin Hubble and Milton Humason photographed the spectra of many galaxies with the 100 inch telescope at Mount Wilson.

  • They found that most of the spectra contained absorption lines with a large redshift.

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Red Shift and Distance

24 Mpc

1200 km/s

300 Mpc

15,000 km/s

780 Mpc

39,000 km/s

1220 Mpc

61,000 km/s

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Galaxy Observations

  • Using the Doppler effect, Hubble calculated the velocity at which each galaxy is receding from us.

  • Using the period and brightness of Cepheid variables in distant galaxies, Hubble estimated to distances to each of the galaxies.

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Hubble’s Law

  • Hubble noticed that there was a linear relationship between the recessional velocity and the distance to the galaxies.

  • This relationship is know as Hubble’s Law:

    V = H D

    recessional velocity = Hubble’s Constant  Distance

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Hubble’s Law

  • H is known as the Hubble constant and is about 75km/s/Mpc.

  • This means that a galaxy that is 1 megaparsec from Earth will be moving away from us at a speed of 75km/s.

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  • The study of the origin, structure and evolution of the universe.

  • What does the Hubble law tells us about our universe?

    Our universe is expanding.

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Raisin Cake Model

Like raisins in rising raisin cake, galaxies move away

away from each other in our expanding universe.

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Why is the universe expanding?

  • The expanding universe probably originated in an explosion called the Big Bang between 12 and 18 billion years ago.

  • Will the universe end?

    • Present observations suggest that is will expand forever.

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  • What caused the Big Bang?

  • Where did the energy come from?

  • Why did it happen?