BB Chapter 23 The Milky Way Galaxy(1).ppt - Chapter 23 The...

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Chapter 23 The Milky Way Galaxy
23.1Our Parent Galaxy23.2Measuring the Milky Way23.3Galactic Structure23.4The Formation of the Milky Way23.5Galactic Spiral Arms23.6The Mass of the Milky Way Galaxy23.7The Galactic CenterUnits of Chapter 23
From Earth, we seefew stars whenlooking out of ourgalaxy (red arrows)and many stars whenlooking in (white andsomewhat the bluearrows).Milky Way is what ourgalaxy appears as inthe night sky.23.1 Our Parent GalaxyThe Milky Way: Crash Course Astronomy #37
Our galaxy is a spiral galaxy. The AndromedaGalaxy, our closest spiral neighbor, probablyresembles the Milky Way fairly closely.Milky Way and Andromeda
What is a Galaxy?A galaxy is a large-scale system of stars, interstellar gas,dust, and plasma isolated in space and held together byits own gravity. Astronomers are aware of millions ofgalaxies beyond our own.The average galaxy contains more than 100 billion solarmasses and ranges in diameter from 1,500 to 300,000light-years, 90% of which is actually composed of largelyunknown substance called dark matter.Individual galaxies are separated by distances in excessof millions of light years.Lots of galaxy pictures and information:
Here are two other spiral galaxies similar tothe Milky Way, one viewed from the side andthe other from the top:M101NGC 4565
Explanation:
Our Sun lies in a part of theGalaxy known as the Galacticdisk—an immense, circular,flattened region containingmost of our Galaxy’s luminousstars and interstellar matter.Our Galaxy consists of acircular galactic disk of matterthat fattens to a Galactic bulgeat the center.The disk and bulge areembedded in a roughlyspherical ball of faint old starsknown as the Galactic halo.
Some Size AnalogiesIf the Sun were the size of abaseball, the density of the stars inour galaxy would be comparableto scattering fifty baseballs acrossthe United States, so that therewould be one star per state.If the distance from home plate tothe pitcher's mound were equal tothe distance from Earth to theSun, the next star would be 800miles away.
Classroom ModelFor a 15 m by 15 m classroom, use a scale of 1 m= 2000 pc.Center of Milky Way in center of room.A personabout 2 m high represents this.This central bulge is about 3 m in diameter.The Sun is about 4 m from the center.At thislocation the disk is about 15 cm thick.
At the Sun's location the disk is about 300 pcthick, about 1/100 of the galaxy diameter. (1 pc= 3.3 ly.)At the speed of light it would take 1000 years tocross this section of the galaxy.At this scale the Solar System (about 100 AU)would be about 0.2 microns in size.Youwould need a microscope to see it!Next two slides describe things from Sun’s pointof view.
From the position of the Sun, all the stars seenwith your naked eye are within 25 cm.

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Term
Fall
Professor
Fulton

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