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Ch. 30 Sec. 1. The Milky Way Galaxy. Stars with varying light output allowed astronomers to map the Milky Way, which has a halo, spiral arms, and a massive black hole at its center. Review Vocabulary.
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Ch. 30 Sec. 1 The Milky Way Galaxy
Stars with varying light output allowed astronomers to map the Milky Way, which has a halo, spiral arms, and a massive black hole at its center. Review Vocabulary galaxy: any of the very large groups of stars and associated matter found throughout the universe
I. Discovering the Milky Way It is difficult to tell how big the Milky Way galaxy is, where its center is, or what Earth’s location is within this vast expanse of stars. Though astronomers have answers to these questions, they are still refining their measurements.
A. Variable stars • pulsate in brightness because of the expansion and contraction of their outer layers • brightest at their largest diameters and dimmest at their smallest diameters
B. Types of variables For certain types of variable stars, there is a relationship between a star’s luminosity and its pulsation period, which is the time between its brightest pulses. The longer the period of pulsation takes, the greater the luminosity of the star. Mira type variable star Cygnus
RR Lyrae variables • a. Pulsation period between 1.5 – 24 hrs. • b. Luminosity remains relatively constant
2. Cepheid a. Pulsation period is 1 - 100 days b. Luminosity doubles By measuring a star’s period of pulsation, astronomers can determine the star’s absolute luminosity, and thus how far away the star is.
B. The galactic center • 28,000 light-years (ly) away • High star density
II. The Shape of the Milky Way • Discovered using radio waves & infrared radiation • 1. Nuclear bulge
2. Halo a. Spherical region b. Surrounds nuclear bulge and disk c. Contains globular clusters
B. Spiral arms Using hydrogen emission spectra as a guide, and knowing that the Milky Way galaxy has a disklike shape with a central bulge, astronomers speculated that it might also have spiral arms, as do many other galaxies.
Four major spiral arms and numerous minor arms • Sun • a. Located in the minor Orion spiral arm • b. 220 km/s • c. Orbital path takes 240 mill.yrs. • d. Has orbited the bulge approx. 20 times
C. Nuclear bulge or bar? Many spiral galaxies have a barlike shape rather than having a round disk. The nuclear bulge of a galaxy is typically made up of older, red stars. The bar in a galaxy center, however, is associated with younger stars. Most stars are about 1000 AU apart compared to 207,000 AU separation in the locale of the Sun. Recent measurements of 30 million stars in the Milky Way indicate a bar about 27,000 ly in length.
III. Mass of the Milky Way A. Mass of the halo • 90% of the galaxy’s mass • Older, dimmer stars • a. White dwarfs • b. Neutron stars • c. Black holes
B. A galactic black hole 1. Supermassive black holes occupy the centers of most galaxies. 2. Center of the Milky Way a. Using infrared and radio wavelengths b. Sagittarius A (Sgr A), with sub-source called Sgr* (Sagittarius star) c. Actual point around which the whole galaxy rotates
3. Sagittarius A* (pronounced A star) a. Supermassive black hole b. Glows brightly because of hot gas spiraling in
4. Formation of supermassive black hole a. Collapse of a dense gas cloud b. Accumulation of mass c. Releases photons d. Jet of matter Fig. 30.6 Page 865
C. Stellar populations in the Milky Way The differences among stars include differences in location, motion, and age, leading to the notion of stellar populations. The population of a star provides information about its galactic history.
1. Population I stars a. Approximately 2% heavy elements b. Found in the disk and arms c. Young stars (ex: Sun) d. Circular orbits with low (flat) eccentricity e. Orbits lie close to the plane of the disk
2. Population II stars Smaller traces of heavy elements Globular Clusters in halo and bulge 12 – 14 billion yrs. old Little star formation or interstellar material
Section 30.1 The Milky Way Galaxy
IV. Formation and Evolution of the Milky Way The fact that the halo and nuclear bulge are made exclusively of old stars suggests that these parts of the galaxy formed first, before the disk that contains only younger stars.
Astronomers hypothesize that the galaxy began as a spherical cloud in space. The first stars formed while this cloud was round. The nuclear bulge, which is also round, represents the inner portion of the original cloud.
The original cloud eventually collapsed under the force of its own gravity, and rotation forced it into a disklike shape. Stars that formed after this time have orbits lying in the plane of the disk. They also contain greater quantities of heavy elements.
V. Spiral Arms Shape retention hypothesis 1. Spiral density wave 2. Moves through gas and dust 3. Temporary buildup of material
Fig. 30.9 Page 868 A slow truck on a highway causing a buildup of cars around it illustrates one theory as to how spiral density waves maintain spiral arms in a galaxy.
B. Second hypothesis 1. Not permanent structures 2. Continually forming a. disturbances (supernova explosions) 3. Broken spiral-arm pattern a. fits this second model best
C. Third possibility 1. Faraway galaxies 2. Arms visible a. Contain hot blue stars b. Stand out more than red stars
Stars with varying light output allowed astronomers to map the Milky Way, which has a halo, spiral arms, and a massive galactic black hole at its center. • The discovery of variable stars aided in determining the shape of the Milky Way. • RR Lyrae and Cepheid are two types of variable stars used to measure distances.
The nuclear bulge and halo of the Milky Way is a globular cluster of old stars. • The spiral arms of the Milky Way are made of younger stars and gaseous nebulae. • Population I stars are found in the spiral arms, while Population II stars are in the central bulge and halo.