Chapter 15: Chapter 15: The Milky Way Galaxy

Lesson Plan Idea of Gallaxies Idea of Gallaxies Parson’s observations Parson’s observations Hubble’s observations Hubble’s observations Cerpheid Variables Cerpheid Variables Milky Way Milky Way Center Center Hershel’s observations Hershel’s observations Interstellar gas Interstellar gas Shapley’s observations (pr #7, pg 400) Shapley’s observations (pr #7, pg 400) Disk mapping Disk mapping 21sm – radiation (pr #9, pg 405) 21sm – radiation (pr #9, pg 405) Nucleus Nucleus Infrarred view Infrarred view Synchrotron radiation (pr #11, pg 409) Synchrotron radiation (pr #11, pg 409) X-ray radiation X-ray radiation Suppermassive black hole (pr #13, pg 410) Suppermassive black hole (pr #13, pg 410) Rotation Rotation Dark matter (pr #10, pg 412) Dark matter (pr #10, pg 412)

WHAT DO YOU THINK? How many stars does the Milky Way Galaxy contain? Where is our Solar System located in the Milky Way Galaxy? Is the Sun moving through the Milky Way Galaxy and, if so, about how fast?

You will discover… the Milky Way Galaxy—billions of stars along with gas and dust bound together by mutual gravitational attraction the properties of our Milky Way Galaxy Earth’s location in the Milky Way how interstellar gas and dust enable star formation to continue in our Galaxy that observations reveal the presence of significant mass in the Milky Way that astronomers have yet to identify that there is a black hole at the center of our Galaxy

Schematic Diagrams of the Milky Way Edge-on view showing the Milky Way’s disk

Schematic Diagrams of the Milky Way Two possible distributions of the spiral arms of our Galaxy. Our Galaxy has at least four major spiral arms and several shorter arm segments.

Telescope of the Mid- Nineteenth Century Built by the Earl of Rosse in 1845, a 1.8-m-diameter telescope.

Telescope of the Mid- Nineteenth Century Lord Rosse’s sketch of the spiral structure of the galaxy M51 A modern photograph of M51 (also called NGC 5194)

Messier 100 and Cepheids

Period-Luminosity Relation

Our Galaxy Wide-angle photograph spanning half the Milky Way, as seen from the equatorial latitudes

View Toward the Galactic Center More than a million stars in the disk of our Galaxy fill this view, which covers a relatively clear window just 4º south of the galactic nucleus in Sagittarius.

Electron Spin & the Hydrogen Atom

Mapping the Galaxy Radio waves from various gas clouds exhibit slightly different Doppler shifts, permitting astronomers to sort out the gas clouds and map the Galaxy.

Map of the Galaxy This map (left), based on radio telescope surveys of 21-cm radiation, shows the distribution of hydrogen gas in the Milky Way.

Two Views of Spiral Galaxy M83

Our Galaxy Diameter is about 100,000 ly Diameter is about 100,000 ly Sun about 26,000 ly from the galactic center Sun about 26,000 ly from the galactic center Disk contains gas, dust and Population I stars Disk contains gas, dust and Population I stars Halo is composed mostly of Population II stars. Halo is composed mostly of Population II stars.

Infrared View of the Milky Way Taken by the COBE satellite in 1997

The Galactic Center

Two Views of the Galactic Nucleus Radio image taken at the VLA

Two Views of the Galactic Nucleus Infrared image showing the motion of six stars in the vicinity of the unseen massive object at the position of the radio source Sagittarius A* (  ).

Orbits of Stars in Our Galaxy NGC 4144, very similar to the Milky Way

The Nearest Galaxy Canis Major dwarf elliptical galaxy

Differential Rotation of the Galaxy Stars closer to the Galaxy’s center than the Sun are overtaking the solar system, while stars farther from the center are lagging behind us.

The Galaxy’s Rotation Curve

Microlensing by Dark Matter in the Galactic Halo Gravitational fields cause light to change direction.

Microlensing by Dark Matter in the Galactic Halo The light curve of the gravitational microlensing of light from a star in the Galaxy’s nuclear bulge by an intervening object.

WHAT DID YOU THINK? How many stars does the Milky Way Galaxy contain? The Milky Way has about 200 billion stars. Where is our solar system located in the Milky Way Galaxy? The solar system is between the Sagittarius and Perseus spiral arms about 26,000 ly from the center of the Galaxy. Is the Sun moving through the Milky Way Galaxy and, if so, how fast? The Sun orbits the center of the Milky Way Galaxy at a speed of 828,000 km per hour.

Key Terms dark matter (missing mass) disk (of a galaxy) distance modulus galactic cannibalism galactic nucleus galaxy halo (of a galaxy) microlensing Milky Way Galaxy missing mass nebula (plural nebulae) nuclear bulge rotation curve (of a galaxy) Sagittarius A Shapley–Curtis debate spin (of an electron or proton) spiral arm synchrotron radiation 21-cm radio radiation