Chapter 15:Chapter 15:The Milky

Way Galaxy

Lesson PlanLesson Plan Idea of GallaxiesIdea of Gallaxies

Parson’s observationsParson’s observations Hubble’s observationsHubble’s observations

Cerpheid VariablesCerpheid Variables Milky WayMilky Way

CenterCenter Hershel’s observationsHershel’s observations Interstellar gasInterstellar gas Shapley’s observations (pr #7, pg 400)Shapley’s observations (pr #7, pg 400)

Disk mappingDisk mapping 21sm – radiation (pr #9, pg 405)21sm – radiation (pr #9, pg 405)

NucleusNucleus Infrarred viewInfrarred view Synchrotron radiation (pr #11, pg 409)Synchrotron radiation (pr #11, pg 409) X-ray radiationX-ray radiation Suppermassive black hole (pr #13, pg 410)Suppermassive black hole (pr #13, pg 410)

RotationRotation 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…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 ofSchematic Diagrams ofthe Milky Waythe Milky Way

Edge-on view showing the Milky Way’s disk

Schematic Diagrams ofSchematic Diagrams ofthe Milky Waythe 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-Telescope of the Mid-Nineteenth CenturyNineteenth Century

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

Telescope of the Mid-Telescope of the Mid-Nineteenth CenturyNineteenth 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 CepheidsMessier 100 and Cepheids

Period-Luminosity RelationPeriod-Luminosity Relation

Our GalaxyOur Galaxy

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

View Toward the Galactic CenterView 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 AtomElectron Spin & the Hydrogen Atom

Mapping the GalaxyMapping 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 GalaxyMap 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 M83Two Views of Spiral Galaxy M83

Our GalaxyOur Galaxy

Diameter is about 100,000 Diameter is about 100,000 lyly

Sun about 26,000 ly from Sun about 26,000 ly from the galactic centerthe galactic center

Disk contains gas, dust Disk contains gas, dust and Population I starsand Population I stars

Halo is composed mostly Halo is composed mostly of Population II stars.of Population II stars.

Infrared View of the Milky WayInfrared View of the Milky Way

Taken by the COBE satellite in 1997

The Galactic CenterThe Galactic Center

Two Views of the Galactic NucleusTwo Views of the Galactic Nucleus

Radio image taken at the VLA

Two Views of the Galactic NucleusTwo 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 GalaxyOrbits of Stars in Our Galaxy

NGC 4144, very similar to the Milky Way

The Nearest GalaxyThe Nearest Galaxy

Canis Major dwarf elliptical galaxy

Differential Rotation of the GalaxyDifferential 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 CurveThe Galaxy’s Rotation Curve

Microlensing by Dark MatterMicrolensing by Dark Matterin the Galactic Haloin the Galactic Halo

Gravitational fields cause light to change direction.

Microlensing by Dark MatterMicrolensing by Dark Matterin the Galactic Haloin 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 TermsKey Terms

dark matter (missing mass)disk (of a galaxy)distance modulusgalactic cannibalismgalactic nucleusgalaxyhalo (of a galaxy)microlensingMilky Way Galaxymissing mass

nebula (plural nebulae)nuclear bulgerotation curve (of a galaxy)Sagittarius AShapley–Curtis debatespin (of an electron or proton)spiral armsynchrotron radiation21-cm radio radiation