Chapter 15: The Milky Way Galaxy For great photos and other data our galaxy consults the SEDS web site:  www.seds.org/messier/more/mw.html.

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 is made of billions of stars, and gas and dust, all bound together by mutual gravitational attraction. The properties of our Milky Way Galaxy. Earth’s location in the Milky Way. That there is a huge Black Hole at the center of our Galaxy. That observations reveal the presence of significant hidden mass in the Milky Way – but we don’t know what it is.

The Milky Way – Side View FIGURE 15-1 Schematic Diagrams of the Milky Way (a) This edge-on view shows the Milky Way’s disk, containing most of the stars, gas, and dust, and its halo, containing many old stars. Individual stars in the halo are too dim to show, so the bright regions in the halo are clusters of stars. Edge-on view showing the Milky Way’s disk, nuclear bulge, and halo.

The Milky Way – Top View FIGURE 15-1 Schematic Diagrams of the Milky Way (b, c) Two possible distributions of the spiral arms of our Galaxy. Our Galaxy has at least four major spiral arms and several shorter arm segments. Because we are in the disk, surrounded by gas and dust, we cannot see much of the spiral structure directly. Therefore, we are not yet sure of its exact shape. (b, c: Illustration by Dennis Davidson, courtesy of AMNH/Hayden Planetarium) Top view showing spiral arms. Latest research shows that the Milky Way also has a bar in the center.

How We Map the Galaxy FIGURE 15-8 A Technique for Mapping the Galaxy Hydrogen clouds at different locations along our line of sight are moving around the center of the Galaxy at different speeds. Radio waves from the various gas clouds therefore exhibit slightly different Doppler shifts, permitting astronomers to sort out the gas clouds and map the Galaxy. Radio waves from different gas clouds show slightly different Doppler shifts, permitting astronomers to sort out the gas clouds and map the Galaxy.

Map of the Galaxy FIGURE 15-9 A Map of the Galaxy This map, based on radio telescope surveys of 21-cm radiation, shows the distribution of hydrogen gas in a face-on view of the Galaxy. This view just hints at spiral structure. The galactic nucleus is marked with a dot surrounded by a circle. Details in the large, blank, wedge-shaped region toward the upper left of the map are unknown, because gas in this part of the sky is moving perpendicular to our line of sight and thus does not exhibit a detectable Doppler shift. Inset: This drawing, based on visible-light data, shows that our solar system lies just inside the Orion arm of the Milky Way Galaxy. (Courtesy of G. Westerhout; inset: National Geographic) This map (left), based on radio telescope surveys of 21-cm radiation, shows the distribution of hydrogen gas in the Milky Way.

How Big? How Many Stars? Diameter 100,000 light years (only the visible stuff – dark matter area is much larger). 2,000 light years thick where we are. 200 billion stars. Sun is 26,000 light years from the galactic center. FIGURE 15-11 Our Galaxy Inset: As seen from the side, the three major visible components of our Galaxy are a thin disk, a nuclear bulge, and a halo. The visible Galaxy’s diameter is about 100,000 ly, and the Sun is about 26,000 ly from the galactic center. The disk contains gas and dust along with Population I (young, metal-rich) stars. The halo is composed almost exclusively of Population II (old, metal-poor) stars. The visible matter in our Galaxy fills only a small volume compared to the distribution of dark matter, whose composition is presently unknown. Its presence is felt by its gravitational effect on visible matter.

What Kinds of Stars? Nuclear bulge about 20,000 light years diameter. Disk contains gas, dust, and Population I stars (young, metal-rich stars). Halo is composed mostly of Population II stars (old, metal-poor stars). FIGURE 15-11 Our Galaxy Inset: As seen from the side, the three major visible components of our Galaxy are a thin disk, a nuclear bulge, and a halo. The visible Galaxy’s diameter is about 100,000 ly, and the Sun is about 26,000 ly from the galactic center. The disk contains gas and dust along with Population I (young, metal-rich) stars. The halo is composed almost exclusively of Population II (old, metal-poor) stars. The visible matter in our Galaxy fills only a small volume compared to the distribution of dark matter, whose composition is presently unknown. Its presence is felt by its gravitational effect on visible matter.

View Toward the Galactic Center FIGURE 15-6 A 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. Beyond the disk stars you can see two prominent globular clusters. Although most regions of the sky toward Sagittarius are thick with dust, there is very little obscuring matter in this tiny section of the sky. (Harvard Observatory) More than a million stars in the disk of our Galaxy fill this view, looking toward the galactic nucleus in Sagittarius.

Looking toward the Galaxy Center in Sagittarius – Visible Light FIGURE 15-5 Our Galaxy This wide-angle photograph spans half the Milky Way as seen from the equatorial latitudes. The Northern Cross is at the left, and the Southern Cross is at the right. The center of the Galaxy is in the constellation Sagittarius, in the middle of this photograph. The dark lines and blotches are caused by hundreds of interstellar clouds of gas and dust that obscure the light from background stars, rather than by a lack of stars. (Dirk Hoppe)

Looking toward the Galaxy Center in Sagittarius – Infrared Light FIGURE 15-5 Our Galaxy This wide-angle photograph spans half the Milky Way as seen from the equatorial latitudes. The Northern Cross is at the left, and the Southern Cross is at the right. The center of the Galaxy is in the constellation Sagittarius, in the middle of this photograph. The dark lines and blotches are caused by hundreds of interstellar clouds of gas and dust that obscure the light from background stars, rather than by a lack of stars. (Dirk Hoppe)

The Galactic Center – Zooming in, in Infrared Light FIGURE 15-13 The Galactic Center (a) This wideangle view at infrared wavelengths shows a 50o segment of the Milky Way centered on the nucleus of the Galaxy. Black represents the dimmest regions of infrared emission, with blue the next strongest, followed by yellow and red; white represents the strongest emission. The prominent band diagonally across this photograph is a layer of dust in the plane of the Galaxy. Numerous knots and blobs along the plane of the Galaxy are interstellar clouds of gas and dust heated by nearby stars. (b) This close-up infrared view of the galactic center covers the area outlined by the white rectangle in (a). (c) This infrared image shows about 300 of the brightest stars less than 1 ly from Sagittarius A*, which is at the center of the picture. The distribution of stars and their observed motions around the galactic center imply a very high density (about a million solar masses per cubic light-year) of less luminous stars. (a, b: NASA; c: A. Eckart, R. Genzel, R. Hofman, B.J. Sams, and C.E. Tocconi- Garman, ESO) Shows very high density at the Center.

Galactic Nucleus – in Radio Waves FIGURE 15-14 Two Views of the Galactic Nucleus (a) A radio image taken at the VLA of the galactic nucleus and environs. This image covers an area of the sky 8 times wider than the Moon. SNR means supernova remnant. The numbers following each SNR are its right ascension and declination. The Sgr (Sagittarius) features are radio-bright objects. Sagittarius A (Sgr A) is at the Galaxy Center, and is very bright in Radio Waves. SNR = Supernova Remnant.

Galactic Nucleus – in Infrared FIGURE 15-14 Two Views of the Galactic Nucleus (a) A radio image taken at the VLA of the galactic nucleus and environs. This image covers an area of the sky 8 times wider than the Moon. SNR means supernova remnant. The numbers following each SNR are its right ascension and declination. The Sgr (Sagittarius) features are radio-bright objects. Stars orbiting a MASSIVE object that cannot be seen – a 4 million solar mass BLACK HOLE – where the Sgr A radio source is located.

The Black Hole Area at the Center of our Galaxy might look something like this.

Orbits of Stars in Different Parts of Our Galaxy FIGURE 15-15 Orbits of Stars in Our Galaxy This disk galaxy, NGC 4144, looks very similar to what the Milky Way Galaxy would look like from far away. The colored arrows show typical orbits of stars in the nuclear bulge (blue), disk (red), and halo (yellow). Interstellar clouds, clusters, and other objects in the various components have similar orbits. (Hubble Heritage Team, AURA/STScI/NASA) NGC 4144, very similar to the Milky Way.

Our Galaxy rotates, and the center rotates faster than the outer parts do – Kepler’s 3rd Law ! FIGURE 15-17 Differential Rotation of the Galaxy (a) If all the stars in the Galaxy had the same angular speed, they would orbit in lockstep. (b) However, stars at different distances from the galactic center have different angular speeds. It takes stars and clouds farther from the center longer to go around the Galaxy than it does stars closer to the center. As a result, 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. By convention, galactic rotation is illustrated as proceeding clockwise. NOT like this. Like THIS.

The Galaxy’s Rotation Curve – shows there’s Dark Matter beyond the visible edge of the Galaxy. FIGURE 15-18 The Galaxy’s Rotation Curve The blue curve shows the orbital speeds of stars and gas in the Galaxy, while the dashed red curve shows Keplerian orbits due to the gravitational force from known objects. Because the data (blue curve) do not show any such decline, there apparently is an abundance of dark matter that extends to great distances from the galactic center. This additional mass gives the outer stars higher speeds than they would have otherwise.

How Much Dark Matter Does Our Galaxy Have? The stars and other objects that give off radiation account for only 10% of the Galaxy’s total mass. The other 90% of our Galaxy is made of stuff that we can’t see at any wavelength of radiation – so we call it “Dark Matter.” Dark Matter forms a sphere around the Galaxy, at least 12 times farther from the center than we are. We don’t know what this Dark Matter is, but it does have gravity.

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 in the Orion Arm, about 26,000 light years 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. How many miles per hour is that?