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Objectives

Determine the size and shape of our galaxy. Distinguish the different kinds of variable stars. Identify the different kinds of stars in a galaxy and their locations. Section 30.1. Objectives. The Milky Way Galaxy.

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Objectives

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  1. Determine the size and shape of our galaxy. Distinguish the different kinds of variable stars. Identify the different kinds of stars in a galaxy and their locations. Section 30.1 Objectives The Milky Way Galaxy

  2. 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. Section 30.1 The Milky Way Galaxy Review Vocabulary galaxy: any of the very large groups of stars and associated matter found throughout the universe

  3. Section 30.1 Discovering the Milky Way The Milky Way Galaxy 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.

  4. Section 30.1 The Milky Way Galaxy Variable stars In the 1920s, astronomers focused their attention on mapping out the locations of globular clusters of stars. Astronomers estimated the distances to the clusters by identifying variable stars in them.

  5. Section 30.1 The Milky Way Galaxy Variable stars Variable starspulsate in brightness because of the expansion and contraction of their outer layers. Variable stars are brightest at their largest diameters and dimmest at their smallest diameters.

  6. Section 30.1 The Milky Way Galaxy 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.

  7. Section 30.1 The Milky Way Galaxy Types of variables RR Lyrae variables are stars that have periods of pulsation between 1.5 hours and 1 day, and on average, they have the same luminosity.

  8. Section 30.1 The Milky Way Galaxy Types of variables Cepheid variables have pulsation periods between 1 and 100 days, and the luminosity as much as doubles from dimmest to brightest. By measuring a star’s period of pulsation, astronomers can determine the star’s absolute luminosity, and thus how far away the star is.

  9. Section 30.1 The Milky Way Galaxy The galactic center After reasoning there were globular clusters orbiting the center of the Milky Way, astronomers then used RR Lyrae variables to determine the distances to them.

  10. Section 30.1 The Milky Way Galaxy The galactic center Astronomers discovered that globular clusters are far from our solar system, and that their distribution in space is centered on a distance point 28,000 light-years (ly) away. The galactic center is a region of high star density, much of which is obscured by interstellar gas and dust.

  11. Section 30.1 The Milky Way Galaxy The Shape of the Milky Way By measuring radio waves as well as infrared radiation, astronomers have discovered that the galactic center is surrounded by a nuclear bulge, which sticks out of the galactic disk much like the yolk in a fried egg.

  12. Section 30.1 The Milky Way Galaxy The Shape of the Milky Way Around the nuclear bulge and disk is the halo, a spherical region where globular clusters are located.

  13. Section 30.1 The Milky Way Galaxy Spiral arms 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.

  14. Section 30.1 The Milky Way Galaxy Spiral arms Using hydrogen emission spectra as a guide, astronomers have identified four major spiral arms and numerous minor arms in the Milky Way. The Sun is located in the minor Orion spiral arm and follows an orbital path around the nuclear center.

  15. Section 30.1 Mass of the Milky Way The Milky Way Galaxy Mass of the halo Evidence of the movement of outer disk stars and gas suggests that as much as 90 percent of the galaxy’s mass is contained in the halo. A galactic black hole When the center of the Milky Way is observed at infrared and radio wavelengths, several dense star clusters and supernova remnants stand out. Among them is a complex source called Sagittarius A (Sgr A), with sub-source called Sgr* (Sagittarius star), which appears to be an actual point around which the whole galaxy rotates.

  16. Section 30.1 Mass of the Milky Way The Milky Way Galaxy A galactic black hole The formation of a supermassive black hole begins with the collapse of a dense gas cloud. The accumulation of mass releases photons of many wavelengths, and perhaps even a jet of matter.

  17. Section 30.1 Mass of the Milky Way The Milky Way Galaxy Stellar populations in the Milky Way Population I stars have small amounts of heavy elements and are found in the disk and arms of a galaxy. Population II stars contain even smaller traces of heavy elements and are found in the halo and bulge of a galaxy.

  18. Section 30.1 Spiral Arms The Milky Way Galaxy The Milky Way is subject to gravitational tugs by neighboring galaxies and is periodically disturbed by supernova explosions from within, both of which can create or affect spiral arms. There are several hypotheses about why galaxies keep this spiral shape.

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