Copyright © 2010 Pearson Education, Inc. Chapter 14 The Milky Way Galaxy

Copyright © 2010 Pearson Education, Inc. Chapter 14 The Milky Way Galaxy

Copyright © 2010 Pearson Education, Inc. Units of Chapter 14 Our Parent Galaxy Measuring the Milky Way Galactic Structure The Formation of the Milky Way Galactic Spiral Arms The Mass of the Milky Way Galaxy The Galactic Center

Copyright © 2010 Pearson Education, Inc. The Milky Way 200+ billion stars 100k light years across 10k light years thick at the galactic bulge 1k light years thick in the disc. 1 of 100s of billions

Copyright © 2010 Pearson Education, Inc. Women In Astronomy Williamina Fleming –Catalog of brightness and spectra Antonia Maury –Stellar spectra leading to H-R Diagram Annie Cannon –Spectra classification system Henrietta Leavitt –Cepheid variable stars

Copyright © 2010 Pearson Education, Inc. Williamina Fleming Maid to Curator of Astronomical Photographs

Copyright © 2010 Pearson Education, Inc. Antonia Maury Classification of stellar spectra leading to H-R Diagram

Copyright © 2010 Pearson Education, Inc. Annie Cannon spectral classes O, B, A, F, G, K, M

Copyright © 2010 Pearson Education, Inc. Henrietta Leavitt 'period-luminosity relationship‘ of Cepheids

Copyright © 2010 Pearson Education, Inc. Human Computers Harvard Observatory 1910

Copyright © 2010 Pearson Education, Inc. Cepheid and RR Lyrae Stars Two stars played a key role in creating a model of the universe. Variable stars allowed measurement of luminosity and thus distance. luminosity Apparent brightness   distance 2

Copyright © 2010 Pearson Education, Inc. The variability of these stars comes from a dynamic balance between gravity and pressure – they have large oscillations around stability. Measuring the Milky Way

Copyright © 2010 Pearson Education, Inc. This allows us to measure the distances to these stars. RR Lyrae stars all have about the same luminosity; knowing their apparent magnitude allows us to calculate the distance. Cepheids have a luminosity that is strongly correlated with the period of their oscillations; once the period is measured, the luminosity is known and we can proceed as above. Measuring the Milky Way

Copyright © 2010 Pearson Education, Inc. The usefulness of these stars comes from their period–luminosity relationship. Measuring the Milky Way

Copyright © 2010 Pearson Education, Inc. a) measuring distances with Cepheid variable stars. b) identifying the mass of the Galaxy’s central black hole. c) determining the masses of stars in an eclipsing binary system. d) using spectroscopic parallax to measure distances to stars. Question 4 The period – luminosity relationship is a crucial component of

Copyright © 2010 Pearson Education, Inc. a) measuring distances with Cepheid variable stars. b) identifying the mass of the Galaxy’s central black hole. c) determining the masses of stars in an eclipsing binary system. d) using spectroscopic parallax to measure distances to stars. Question 4 The period – luminosity relationship is a crucial component of Cepheid variable stars with longer periods have higher actual luminosities; short-period Cepheids are dimmer.

Copyright © 2010 Pearson Education, Inc. Many RR Lyrae stars are found in globular clusters. These clusters are not all in the plane of the galaxy, so they are not obscured by dust and can be measured. This yields a much more accurate picture of the extent of our Galaxy and our place within it. Measuring the Milky Way

Copyright © 2010 Pearson Education, Inc. We have now expanded our cosmic distance ladder one more step. Measuring the Milky Way

Copyright © 2010 Pearson Education, Inc. a) supernova remnants. b) white dwarf stars in the spiral arms. c) red giant variable stars in globular clusters. d) bright O and B stars in open clusters. e) X-ray sources. Question 1 The location of the galactic center was identified using

Copyright © 2010 Pearson Education, Inc. a) supernova remnants. b) white dwarf stars in the spiral arms. c) red giant variable stars in globular clusters. d) bright O and B stars in open clusters. e) X-ray sources. Question 1 The location of the galactic center was identified using Harlow Shapley used pulsating RR- Lyrae variables as distance indicators to the globular clusters. He then deduced the distance and direction of the Milky Way’s center.

Copyright © 2010 Pearson Education, Inc. a) about 30 Kpc from the center in the halo. b) 30,000 light-years from the center in a globular cluster. c) at the outer edge of the galactic disk, in the plane. d) about halfway from the center, in the spiral arms. e) in the bulge, near the Orion arm. Question 2 Our Sun is located in the Milky Way Galaxy

Copyright © 2010 Pearson Education, Inc. Question 2 Our Sun is located in the Milky Way Galaxy The Sun orbits the center of the Galaxy within the disk, taking about 225 million years to complete one orbit. a) about 30 Kpc from the center in the halo. b) 30,000 light-years from the center in a globular cluster. c) at the outer edge of the galactic disk, in the plane. d) about halfway from the center, in the spiral arms. e) in the bulge, near the Orion arm.

Copyright © 2010 Pearson Education, Inc. From Earth, we see few stars when looking out of galaxy (red arrows), many when looking in (blue and white arrows). Milky Way is how our Galaxy appears in the night sky (b). Our Parent Galaxy

Copyright © 2010 Pearson Education, Inc. a) a spiral galaxy. b) a barred spiral galaxy. c) an elliptical galaxy. d) a quasar. e) an irregular galaxy. Question 3 Detailed measurements of the disk suggest that our Milky Way is

Copyright © 2010 Pearson Education, Inc. a) a spiral galaxy. b) a barred spiral galaxy. c) an elliptical galaxy. d) a quasar. e) an irregular galaxy. Question 3 Detailed measurements of the disk suggest that our Milky Way is Measurements of stellar motion in and near the bulge imply that it is football shaped, about half as wide as it is long, characteristic of a barred spiral galaxy.

Copyright © 2010 Pearson Education, Inc. Andromeda which can be seen with the naked eye. 2.5Mly away Spiral Galaxies M101 NGC 4565 M31

Copyright © 2010 Pearson Education, Inc. Finding Andromeda near Pegasus

Copyright © 2010 Pearson Education, Inc. One of the first attempts to measure the Milky Way was done by Herschel using visible stars. Unfortunately, he was not aware that most of the galaxy, particularly the center, is blocked from view by vast clouds of gas and dust. Measuring the Milky Way

Copyright © 2010 Pearson Education, Inc. We have already encountered variable stars – novae, supernovae, and related phenomena – which are called cataclysmic variables. There are other stars whose luminosity varies in a regular way, but much more subtly. These are called intrinsic variables. Two types of intrinsic variables have been found: RR Lyrae stars and Cepheids. Measuring the Milky Way

Copyright © 2010 Pearson Education, Inc. The upper plot is an RR Lyrae star. All such stars have essentially the same luminosity curve, with periods from 0.5 to 1 day. The lower plot is a Cepheid variable; Cepheid periods range from about 1 to 100 days. Cepheid variable Measuring the Milky Way

Copyright © 2010 Pearson Education, Inc. Overlay of Images

Copyright © 2010 Pearson Education, Inc. This artist’s conception shows the various parts of our Galaxy, and the position of our Sun. Galactic Structure

Copyright © 2010 Pearson Education, Inc. The galactic halo and globular clusters formed very early; the halo is essentially spherical. All the stars in the halo are very old, and there is no gas and dust. The galactic disk is where the youngest stars are, as well as star formation regions – emission nebulae, large clouds of gas and dust. Surrounding the galactic center is the galactic bulge, which contains a mix of older and younger stars. Galactic Structure

Copyright © 2010 Pearson Education, Inc. This infrared view of our Galaxy shows much more detail of the galactic center than the visible-light view does, as infrared is not as much absorbed by gas and dust. Galactic Structure

Copyright © 2010 Pearson Education, Inc. Stellar orbits in the disk are in a plane and in the same direction; orbits in the halo and bulge are much more random. Galactic Structure

Copyright © 2010 Pearson Education, Inc. Any theory of galaxy formation should be able to account for all the properties below. The Formation of the Milky Way

Copyright © 2010 Pearson Education, Inc. The formation of the galaxy is believed to be similar to the formation of the solar system, but on a much larger scale. The Formation of the Milky Way

Copyright © 2010 Pearson Education, Inc. a) the spiral arms formed first. b) the globular clusters formed first. c) the disk component started out thin and grew. d) spiral density waves formed first. e) the bar in the bulge formed first. Question 5 In the formation of our Galaxy

Copyright © 2010 Pearson Education, Inc. a) the spiral arms formed first. b) the globular clusters formed first. c) the disk component started out thin and grew. d) spiral density waves formed first. e) the bar in the bulge formed first. Question 5 In the formation of our Galaxy Globular clusters contain very old stars, no gas or dust, and orbit around the center randomly.

Copyright © 2010 Pearson Education, Inc. Measurement of the position and motion of gas clouds shows that the Milky Way has a spiral form. Galactic Spiral Arms

Copyright © 2010 Pearson Education, Inc. The spiral arms cannot rotate along with the galaxy; they would “curl up.” Galactic Spiral Arms

Copyright © 2010 Pearson Education, Inc. Rather, they appear to be density waves, with stars moving in and Galactic Spiral Arms out of them much as cars move in and out of a traffic jam.

Copyright © 2010 Pearson Education, Inc. As clouds of gas and dust move through the spiral arms, the increased density triggers star formation. This may contribute to propagation of the arms. The origin of the spiral arms is not yet understood. Galactic Spiral Arms

Copyright © 2010 Pearson Education, Inc. The orbital speed of an object depends only on the amount of mass between it and the galactic center. The Mass of the Milky Way Galaxy

Copyright © 2010 Pearson Education, Inc. a) the Sun’s mass and velocity in orbit around the galactic center b) the rotation of the bulge and disk components c) the Sun’s age and age of globular cluster stars d) the motion of spiral arms and the mass of the central black hole e) the Sun’s orbital period and distance from the center Question 6 What two observations allow us to estimate the Galaxy’s mass?

Copyright © 2010 Pearson Education, Inc. Question 6 What two observations allow us to estimate the Galaxy’s mass? Use the modified form of Kepler’s law to find the mass: Total mass = (orbital size) 3 / (orbital period) 2 a) the Sun’s mass and velocity in orbit around the galactic center b) the rotation of the bulge and disk components c) the Sun’s age and age of the globular cluster stars d) the motion of spiral arms and mass of the central black hole e) the Sun’s orbital period and distance from the center

Copyright © 2010 Pearson Education, Inc. Once all the galaxy is within an orbit, the velocity should diminish with distance, as the dashed curve shows. It doesn’t; more than twice the mass of the galaxy would have to be outside the visible part to reproduce the observed curve. The Mass of the Milky Way Galaxy

Copyright © 2010 Pearson Education, Inc. Question 7 a) 21-cm maps of the spiral arms b) the rotation curve of the outer edges of the Galaxy c) orbits of open clusters in the disk d) infrared observations of new star- forming regions e) X-ray images of other galaxies What suggests that the mass of our Galaxy extends farther than its visible disk?

Copyright © 2010 Pearson Education, Inc. a) 21-cm maps of the spiral arms b) the rotation curve of the outer edges of the Galaxy c) orbits of open clusters in the disk d) infrared observations of new star- forming regions e) X-ray images of other galaxies Question 7 What suggests that the mass of our Galaxy extends farther than its visible disk? The outer edges of the Galaxy’s disk rotate much faster than they should. Most of the mass of the Galaxy must be dark matter.

Copyright © 2010 Pearson Education, Inc. What could this “dark matter” be? It is dark at all wavelengths, not just the visible. Stellar-mass black holes? Probably no way enough could have been created Brown dwarfs, faint white dwarfs, and red dwarfs? Currently the best star-like option Weird subatomic particles? Could be, although no evidence so far The Mass of the Milky Way Galaxy

Copyright © 2010 Pearson Education, Inc. The bending of spacetime can allow a large mass to act as a gravitational lens: Observation of such events suggests that low-mass white dwarfs could account for about half of the mass needed. The rest is still a mystery. The Mass of the Milky Way Galaxy

Copyright © 2010 Pearson Education, Inc. This is a view toward the galactic center, in visible light. The two arrows in the inset indicate the location of the center; it is entirely obscured by dust. The Galactic Center

Copyright © 2010 Pearson Education, Inc. These images, in infrared, radio, and X ray, offer a different view of the galactic center. The Galactic Center Infrared RadioX-ray Radio

Copyright © 2010 Pearson Education, Inc. The galactic center appears to have a stellar density a million times higher than near Earth a ring of molecular gas 400 pc across strong magnetic fields a rotating ring or disk of matter a few parsecs across a strong X-ray source at the center The Galactic Center

Copyright © 2010 Pearson Education, Inc. a) tidal forces from the Andromeda Galaxy. b) accretion disks around neutron stars. c) gamma-ray bursts. d) gravitation from globular clusters. e) a supermassive black hole. Question 8 High-speed motion of gas and stars near the Milky Way Galaxy’s center is explained by

Copyright © 2010 Pearson Education, Inc. Question 8 High-speed motion of gas and stars near the Milky Way Galaxy’s center is explained by Recent observations estimate the black hole to be 4 million solar masses. a) tidal forces from the Andromeda Galaxy. b) accretion disks around neutron stars. c) gamma-ray bursts. d) gravitation from globular clusters. e) a supermassive black hole.

Copyright © 2010 Pearson Education, Inc. Apparently, there is an enormous black hole at the center of the galaxy, which is the source of these phenomena. An accretion disk surrounding the black hole emits enormous amounts of radiation. The Galactic Center

Copyright © 2010 Pearson Education, Inc. These objects are very close to the galactic center. The orbit on the right is the best fit; it assumes a central black hole of 3.7 million solar masses. The Galactic Center Evidence

Copyright © 2010 Pearson Education, Inc. A galaxy is stellar and interstellar matter bound by its own gravity. Our Galaxy is spiral. Variable stars can be used for distance measurement, through period–luminosity relationship. True extent of a galaxy can be mapped out using globular clusters. Star formation occurs in disk, but not in halo or bulge. Summary of Chapter 14

Copyright © 2010 Pearson Education, Inc. Spiral arms may be density waves. Galactic rotation curve shows large amounts of undetectable mass at large radii; called dark matter. Activity near galactic center suggests presence of a 3.7-million-solar-mass black hole. Summary of Chapter 14, cont.