Chapter 13 Exploring Our Galaxy

Our Location in the Milky Way The Milky Way Galaxy is a disk-shaped collection of stars.

Views of the Milky Way When we look out at the night sky in the plane of the disk, the stars appear as a band of light that stretches all the way around the sky. When we look perpendicular to the plane of the Galaxy, we see only those relatively few stars that lie between us and the “top” or “bottom” of the disk.

Obscuring Dust Clouds Interstellar dust can absorb or scatter light from distant stars, causing them to appear dimmer than they otherwise would Therefore they seem farther or even fewer than they actually are. This interstellar dust is more concentrated in the disk of our Galaxy, obscuring our view, making objects in the disk appear dimmer than they really are. This makes it seem that the objects are farther away than they really are.

We determine our location in the Galaxy by observing globular clusters. The globular clusters form a spherical halo centered on the center of the Galaxy.

Cepheid Variables Henrietta Leavitt discovered the period-luminosity relation for Cepheid variables. The longer a Cepheid’s period, the greater its luminosity. The period-luminosity law can be used to determine distances. Knowing the star’s periodocity, you can find out how far away the star must be in order to give the observed brightness.

Cepheid and RR Lyrae Variables The more luminous the Cepheid, the longer its pulsation period. RR Lyrae variables are horizontal-branch stars that all have roughly the same average luminosity of about 100 L.

RR Lyrae Variables in Globular Clusters RR Lyrae variables are commonly found in globular clusters. By using the period-luminosity relationship for these stars, we can determine the distances to globular clusters.

The Infrared Milky Way In infrared wavelengths interstellar dust radiates more strongly than stars. A far-infrared view of the sky is principally a view of where the dust is.

The Structure of Our Galaxy There are three major components of our Galaxy: a disk, a central bulge, and a halo. The disk contains gas and dust along with metal-rich (Population I) stars. The halo is composed almost exclusively of old, metal-poor (Population II) stars. The central bulge is a mixture of Population I and Population II stars.

Measurements of our galaxy match images of other spiral galaxies.

Star Orbits in the Milky Way The different populations of stars in our Galaxy travel along different sorts of orbits. The galaxy in this visible-light image is the Milky Way’s near-twin NGC 7331.

Stellar Populations: Disk Versus Central Bulge The disk and central bulge of the Milky Way contain different populations of stars. The same is true for this galaxy, NGC 1309, which has a similar structure to the Milky Way Galaxy and happens to be oriented face-on to us.

Magnetic Interactions in the Hydrogen Atom Electrons and protons are both tiny magnets. When the electron flips from the higher-energy to the lower-energy configuration, the atom loses a tiny amount of energy and emits a radio photon with a wavelength of 21 cm.

The Sky at 21 Centimeters This image was made by mapping the sky with radio telescopes tuned to the 21-cm wavelength emitted by neutral interstellar hydrogen (H I). Black and blue represent the weakest emission, and red and white the strongest.

Detecting Our Galaxy’s Spiral Arms If we look within the plane of our Galaxy from our position at S, hydrogen clouds at different locations are moving at slightly different speeds relative to us. Radio waves from these various gas clouds are subjected to slightly different Doppler shifts. Radio astronomers sort out the gas clouds and map the Galaxy.

Neutral Hydrogen in Our Galaxy Surveys of 21-cm radiation show the distribution of hydrogen gas in a face-on view of our Galaxy. The distribution suggests a spiral structure. Details in the blank, wedge-shaped region at the bottom of the map are unknown. Gas in this part of the Galaxy is moving perpendicular to our line of sight and thus does not exhibit a detectable Doppler shift.

A Spiral Galaxy in Multiple Wavelengths Visible-light clearly shows the spiral arms. The presence of young stars and H II regions indicate that star formation takes place in spiral arms. Radio shows the emission from neutral interstellar hydrogen gas. The same pattern of spiral arms is traced out in this image as in the visible-light photograph. There is a much smoother appearance in this near-infrared view. This shows that cooler stars, which emit strongly in the infrared, are spread more uniformly across the galaxy’s disk.

The Spiral Arms Visible light allows us to see enough OB associations and HII regions to plot the spiral arms in our vicinity. Carbon monoxide in molecular clouds emits radio waves that are relatively unaffected by interstellar dust. They have been observed even in remote corners of the Galaxy. SO, we believe that the Milky Way has at least four major arms. We are located on a minor arm segment called the Orion Arm.

The Rotation of the Milky Way All stars and gas orbit in the same direction. All matter seems to orbit at about the same speed. Therefore, the Milky Way is not rotating as a solid disk, and The objects orbiting the center of the Milky Way do not appear to obey Kepler’s third law!

The Sun’s Orbit and the Mass of the Galaxy In our Solar System all objects orbit one large mass, the Sun, and obey Kepler’s third law. In the Milky Way, a star’s orbit is determined by all of the mass inside of its orbit (stars, gas, and dust) Our Sun’s orbit gives us clues as to the mass of the entire Milky Way.

Rotation Curves and the Mystery of Dark Matter The dashed red curve indicates how this orbital speed should decline beyond the confines of most of the Galaxy’s visible mass. Because there is no such decline, there must be an abundance of invisible dark matter that extends to great distances from the galactic center.

The Galaxy and Its Dark Matter Halo The dark matter in our Galaxy forms a spherical halo whose center is at the center of the visible Galaxy. The extent of the dark matter halo is unknown, but its diameter is at least 100 kiloparsecs. The total mass of the dark matter halo is at least 10 times the combined mass of all of the stars, dust, gas, and planets in the Milky Way.

Dark Matter Speculations MACHOS: Massive Compact Halo Objects Brown dwarfs, white dwarfs, black holes Can only account for about half of the dark matter halo Searched for via gravitational lensing WIMPS: Weakly Interacting Massive Particles Theoretical, and not-yet-detected

The Density-Wave Model Perhaps, the spiral arms of the galaxy are a pattern that moves through the Galaxy, like ripples in water.

Star Formation in the Density-Wave Model A spiral arm is a region where the density of material is higher than in the surrounding parts of a galaxy. Interstellar matter moves around the galactic center rapidly and is compressed as it passes through the slow-moving spiral arms. This compression triggers star formation in the interstellar matter, so that new stars appear on the “downstream” side of the densest part of the spiral arms.

Infrared and Radio Observations of the Galactic Nucleus In an infrared image, the reddish band is dust in the plane of the Galaxy and the fainter bluish blobs are interstellar clouds heated by young O and B stars. Adaptive optics reveals stars densely packed around the galactic center.

X rays from around a Supermassive Black Hole Looking toward the center of the Galaxy, toward Sagittarius A, we see one of the brightest radio sources in the sky. Magnetic fields shape nearby interstellar gas into immense, graceful arches. X-ray wavelengths show lobes of gas on either side of Sagittarius A.

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