Discovering the Universe Ninth Edition Discovering the Universe Ninth Edition Neil F. Comins William J. Kaufmann III CHAPTER 15 The Milky Way Galaxy CHAPTER 15 The Milky Way Galaxy

Understanding the Universe The Sombrero Galaxy (also designated M104, the 104th galaxy listed in the Messier catalog) is 29 million ly from us. By combining infrared, optical, and X- ray observations, we can gain insights into its disk of stars, gas, and dust, along with its central region, and the hot gas surrounding it.

WHAT DO YOU THINK? What is the shape of the Milky Way Galaxy? 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?

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

Schematic Diagrams of the Milky Way (a) This edge-on view shows the Milky Way’s disk, containing most of its stars, gas, and dust, and its halo, containing many old stars. Individual stars in the halo are too dim to be visible on this scale, so the bright regions in the halo represent clusters of stars. (b) Our Galaxy has two major arms and several shorter arm segments, all spiraling out from the ends of a bar of stars and gas that passes through the Galaxy’s center. The bar’s existence and the presence of two major arms were confirmed by the Spitzer Space Telescope.

Lord Rosse’s High-Tech 1.8-m Telescope of 1845 Built in 1845, this structure housed a 1.8-m-diameter telescope, the largest of its day. The improved resolution it provided over other telescopes was similar to the improvement that the Hubble Space Telescope provided over Earthbound optical instruments.

Results of 1845 High-Tech Telescope Using his telescope, Lord Rosse made this sketch of the spiral structure of the galaxy M51 and its companion galaxy NGC Below is a modern photograph of M51 (also called NGC 5194) and NGC The spiral galaxy M51 in the constellation of Canes Venatici is known as the Whirlpool Galaxy because of its distinctive appearance. The two galaxies are about 20 million ly from Earth.

A Cepheid Variable Star in Galaxy M100 One of the most reliable ways to determine the distance to moderately remote galaxies is to locate Cepheid variable stars in them, as discussed in the text. The distance of 50 million ly to the galaxy M100 in the constellation Coma Berenices was determined using Cepheids. (Insets) The Cepheid in this view, one of 20 located to date in M100, is shown at different stages in its brightness cycle, which recurs over several weeks.

The Period-Luminosity Relation This graph shows the relationship between the periods and average luminosities of classical (Type I) Cepheid variables and the closely related RR Lyrae stars (discussed in Chapter 12). Each dot represents a Cepheid or RR Lyrae whose luminosity and period have been measured.

Our Galaxy This wide-angle photograph spans half the Milky Way. 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.

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 4° 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, very little obscuring matter appears in this tiny section of the sky.

Electron Spin and the Hydrogen Atom Due to their spin, electrons and protons both act as tiny magnets. When an electron and the proton it orbits are spinning in the same direction, their energy is higher than when they are spinning in opposite directions. When the electron flips from the higher-energy to the lower-energy configuration, the atom loses a tiny amount of energy that is radiated as a radio photon with a wavelength of 21 cm.

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. The component of their motion away from us varies with their distance from the solar system. 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.

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 between two arms of the Milky Way Galaxy.

A Map of the Galaxy The spiral arms in the Milky Way.

Two Views of a Barred Spiral Galaxy The galaxy M83 is in the southern constellation of Centaurus, about 12 million ly from Earth. (a) At visible wavelengths, spiral arms are clearly illuminated by young stars and glowing H II regions. (b) A radio view at 21-cm wavelength shows the emission from neutral hydrogen gas. Note that the spiral arms are more clearly demarcated by visible stars and H II regions than by 21-cm radio emission.

Our Galaxy As seen from the side, three major visible components of our Galaxy are a thin disk, a central bulge, and a two-part halo system. As noted earlier, there is also a central bar. 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. (Inset) The visible matter in our Galaxy fills only a small volume compared to the distribution of dark matter, whose composition is presently unknown. Dark matter’s presence is felt by its gravitational effect on visible matter.

Infrared shows the expected concentration toward the galactic center Visible light shows no such concentration. Impression of being in center from visible light.

The Galactic Center Wide-angle and zooming in on the galactic center. Infrared is used to distinguish individual stars at the center of our galaxy.

Two Views of the Galactic Nucleus (a) A radio image taken 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 Sgr features are radio-bright objects. (b) The colored dots show the motion of seven stars in the vicinity of the unseen massive object (  ) at the position of the radio source Sagittarius A*, part of Sgr A. This plot indicates that the stars are held in orbit by a 4 x solar-mass black hole.

Orbits of Stars in Our Galaxy This disk galaxy, M58, 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 central bulge (blue), disk (red), and halo (yellow). Interstellar clouds, clusters, and other objects in the various components have similar orbits.

Differential Rotation of the Galaxy (a) If all 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. Stars and clouds farther from the center take longer to go around the Galaxy than do stars closer to the center. As a result, stars closer to the Galaxy’s center than the Sun are overtaking the solar system, whereas stars farther from the center are lagging behind us.

The Galaxy’s Rotation Curve The blue curve shows the orbital speeds of stars and gas in the Galaxy, and the dashed red curve shows Keplerian orbits that would be caused by the gravitational force from all the known objects in the Galaxy. Because the data (blue curve) do not show any such decline, there is, apparently, 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.

From Newton’s Laws v = √(G M enclosed / R) “Kepler” case: M enclosed is constant, v decreases But M enclosed increases with R – more stars But the mass of stars is insignificant outside 40,000 Ly Detailed calculations: not enough stars inside, either Conclusion: Dark Matter! - to supply the extra gravity

Microlensing by Dark Matter in the Galactic Halo This technique searches for Massive Compact Halo Objects (MACHOs). It finds them, but not NEARLY enough of them. The other hypothesis: WIMPS (Weakly Interacting Massive Particles, meaning subatomic particles). The mystery: it can’t be any KNOWN subatomic particle, so what IS it?

Summary of Key Ideas

Discovering the Milky Way A century ago, astronomers were divided on whether or not the Milky Way Galaxy and the universe were the same thing. A century ago, astronomers were divided on whether or not the Milky Way Galaxy and the universe were the same thing. The Shapley–Curtis debate was the first major public discussion between astronomers as to whether the Milky Way contains all the stars in the universe. The Shapley–Curtis debate was the first major public discussion between astronomers as to whether the Milky Way contains all the stars in the universe. Cepheid variable stars are important in determining the distance to other galaxies. Cepheid variable stars are important in determining the distance to other galaxies. Edwin Hubble proved that there are other galaxies far outside of the Milky Way. Edwin Hubble proved that there are other galaxies far outside of the Milky Way.

The Structure of Our Galaxy Our Galaxy has a disk about 100,000 ly in diameter and about 2000 ly thick, with a high concentration of interstellar dust and gas. It contains around 200 billion stars. Our Galaxy has a disk about 100,000 ly in diameter and about 2000 ly thick, with a high concentration of interstellar dust and gas. It contains around 200 billion stars. Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths. However, hydrogen clouds can be detected beyond this dust by the 21-cm radio waves emitted by changes in the relative spins of electrons and protons in the clouds, as well as by other nonvisible emissions. Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths. However, hydrogen clouds can be detected beyond this dust by the 21-cm radio waves emitted by changes in the relative spins of electrons and protons in the clouds, as well as by other nonvisible emissions.

The Structure of Our Galaxy The center, or galactic nucleus, has been studied at gamma-ray, X-ray, infrared, and radio wavelengths, which pass readily through intervening interstellar dust and H II regions that illuminate the spiral arms. These observations have revealed the dynamic nature of the galactic nucleus, but much about it remains unexplained. The center, or galactic nucleus, has been studied at gamma-ray, X-ray, infrared, and radio wavelengths, which pass readily through intervening interstellar dust and H II regions that illuminate the spiral arms. These observations have revealed the dynamic nature of the galactic nucleus, but much about it remains unexplained. A supermassive black hole of about 4.3 x 10 6 solar masses exists in the galactic nucleus. A supermassive black hole of about 4.3 x 10 6 solar masses exists in the galactic nucleus. The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called the central bulge, through which a bar of stars and gas extends. The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called the central bulge, through which a bar of stars and gas extends.

The Structure of Our Galaxy A disk with at least four bright arms of stars, gas, and dust spirals out from the ends of the bar in the galactic central bulge. A disk with at least four bright arms of stars, gas, and dust spirals out from the ends of the bar in the galactic central bulge. Young OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms where stars are forming. Young OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms where stars are forming. The Sun is located about 26,000 ly from the galactic nucleus, between the spiral arms. The Sun moves in its orbit at a speed of about 878,000 km/h and takes about 230 million years to complete one orbit around the center of the Galaxy. The Sun is located about 26,000 ly from the galactic nucleus, between the spiral arms. The Sun moves in its orbit at a speed of about 878,000 km/h and takes about 230 million years to complete one orbit around the center of the Galaxy.

The Structure of Our Galaxy The entire Galaxy is surrounded by two halos of matter. The inner halo includes a spherical distribution of globular clusters and field stars, as well as large amounts of dark matter. It orbits in the same general direction as the disk. The outer halo is composed of dark matter and very old stars, which have retrograde orbits. The entire Galaxy is surrounded by two halos of matter. The inner halo includes a spherical distribution of globular clusters and field stars, as well as large amounts of dark matter. It orbits in the same general direction as the disk. The outer halo is composed of dark matter and very old stars, which have retrograde orbits.

Key Terms central bulge 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

WHAT DID YOU THINK? What is the shape of the Milky Way Galaxy? The Milky Way is a barred spiral galaxy. A bar of stars, gas, and dust runs through its central region. It has two major spiral arms, several minor arms, and is surrounded by a complex spherical halo system of stars and dark matter.

WHAT DID YOU THINK? 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 (about halfway out to the visible edge of the galactic disk).

WHAT DID YOU THINK? Is the Sun moving through the Milky Way Galaxy and, if so, about how fast? Yes. The Sun orbits the center of the Milky Way Galaxy at a speed of 878,000 km/h.