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 ofthe Milky Way Edge-on view showing the Milky Way’s disk
Schematic Diagrams ofthe 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)
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.
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.
Our Galaxy • Diameter is about 100,000 ly • Sun about 26,000 ly from the galactic center • Disk contains gas, dust and Population I stars • Halo is composed mostly of Population II stars.
Infrared View of the Milky Way Taken by the COBE satellite in 1997
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.
Microlensing by Dark Matterin the Galactic Halo Gravitational fields cause light to change direction.
Microlensing by Dark Matterin 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