The Milky Way GalaxyPg. 131

The Milky Way Galaxy A disk made of stars orbiting a

central point Perhaps 100 billion stars in the milky

way

Discovering the Milky Way Difficult to see because we are inside

of it Can’t tell how big it is, where the

center is or where Earth is in it Scientists began by mapping globular

clusters Saw they were located above and

below the plane of the galactic disk

Galactic Center The center of a galaxy is a region of

very high star density The direction of the galactic center is

toward the constellation Sagittarius.

The shape of the Milky Way

Use radio waves to map it Can penetrate interstellar gas and dust Center is called the nucleus Has a bulge that sticks out of the disk Halo (spherical region where clusters are

found) exists outside the nucleus

Spiral arms of the milky way

Scientists use Hydrogen to discover the spiral arms

Our Sun is located in the minor arm Orion (28000 ly away from the center)

Approximated the sun has orbited the center about 20 times

Mass of the Milky Way About 100 billion times the mass of

the sun 90% of mass contained in the halo Some matter can’t be seen (neutron

stars, black holes, white dwarfs) Called black matter

Stars in the Milky Way Halo contains the oldest known objects in the

galaxy, globular clusters About 99.9% of the stars are hydrogen and

helium Young stars are located in the spiral arms Can be broken into 2 regions: stars in the halo

and bulge; stars in the disk (esp. spiral arms) Population I stars are in the disk and arms and

contain some heavy elements (our Sun) Population II stars in the halo and bulge that

contain hardly any heavy elements

Formation and evolution of the Milky Way

Galaxy began as a spherical cloud in space Cloud eventually collapsed under the force

of its own gravity Rotation forced it into a disk Spiral arms are maintained by a spiral

density wave (dense and less dense regions that are frozen and rotate in a rigid pattern)

May also be continually forming because of supernova explosions

Draw a rough sketch of the Milky Way galaxy and include the following (you may look up a picture of the galaxy on your phone or computer):• the spiral arms• Put a circle where the halo would be. • Draw stars to represent the globular clusters• Put a filled in circle where the nucleus and the

nuclear bulge would be. • Where would the younger stars be located?• Where would the Population II stars be?

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