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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
The Milky Way Galaxy


Hubble’s Law

messier catalogue10
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
  • How many stars are in the Milky Way?
    • About 200 billion
  • How many galaxies are there?
    • billions and billions
  • 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.
are all stars members of the milky way
Are all stars members of the Milky Way?
  • No, most stars in the universe are in other galaxies.
where in the milky way is our solar system located
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.
sketch of the milky way galaxy
Sketch of the Milky Way Galaxy
  • Disk
  • Bulge
  • Halo
  • Spiral Arms
  • Sun
  • Globular Clusters
  • Open Clusters and Nebulae
how do we know that we are located in the spiral arm
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.
finding our place
Finding Our Place

Globular Clusters evenly distributed

Early view

Sun at center


Globular Clusters unevenly distributed

Harlow Shapley

the great debate

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!





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

M32 - E2

NGC 4125

M87 - E1

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

NGC 1097


NGC 4123

NGC 4477

barred spiral galaxies
Barred Spiral Galaxies
  • galaxies with a bar of stars running through the nuclear bulge
irregular galaxies
Irregular Galaxies
  • galaxies that are asymmetrical and are sometimes just two or more galaxies colliding
galaxy observations
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.
red shift and distance
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

galaxy observations46
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.
hubble s law
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

hubble s law50
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.
  • 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.

raisin cake model
Raisin Cake Model

Like raisins in rising raisin cake, galaxies move away

away from each other in our expanding universe.

why is the universe expanding
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.
  • What caused the Big Bang?
  • Where did the energy come from?
  • Why did it happen?