Astronomy A BEGINNER S GUIDE TO THE UNIVERSE EIGHTH EDITION

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1 Astronomy A BEGINNER S GUIDE TO THE UNIVERSE EIGHTH EDITION CHAPTER 14 The Milky Way Galaxy Lecture Presentation

2 14.0 the Milky Way galaxy How do we know the Milky Way exists? We can see it even though we are inside of it!

3 14.1 Our Parent Galaxy From Earth, see Fig (a), we see few stars when looking out of the Galaxy (red arrows), but many in (blue and white arrows) the Galaxy plane. A composite of many photos from Earth shows the Milky Way Galaxy in Fig (b). Can you see dark dust clouds?

4 14.1 Our Parent Galaxy Our Galaxy is a spiral galaxy. Here are three similar spiral galaxies.

5 14.2 Measuring the Milky Way 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 variable stars have been found: RR Lyrae stars and Cepheids. These are of interest because they are very luminous AND we have a way to know their luminosity. We call such objects standard candles, i.e. light sources with known brightness!

6 14.2 Measuring the Milky Way The upper plot is the plot of the Luminosity VS time for a RR Lyrae star. All such stars have essentially the same luminosity curve and periods from 0.5 to 1 day. The lower plot is the Luminosity VS time for a Cepheid variable; Cepheid periods range from about 1 to 100 days (how convenient)!!

7 14.2 Measuring the Milky Way The variability of these stars comes from a dynamic balance between gravity and pressure curiously they have periodic, large oscillations around stability. Note that the Luminosity of these stars is 100x (or more) that of our Sun!

8 14.2 Measuring the Milky Way The usefulness of these stars comes from their period luminosity relationship.

9 14.2 Measuring the Milky Way The period of these stars tells us their Luminosity If we also measure their apparent brightness, then knowing their absolute brightness (Luminosity) we can deduce the distance to these stars using the brightness-distance relation : RR Lyrae stars all have about the same Luminosity; knowing their apparent brightness 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.

10 14.2 Measuring the Milky Way We have now expanded our cosmic distance ladder one more step. Note: the Milky Way extends about 30kpc, what is within 25Mpc (next Chapt 15)?

11 14.2 Measuring the Milky Way 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. Harlow Shapley (almost 100 years ago) used RR Lyrae stars to get the first true picture of the extent of our Galaxy and our place within it!

12 14.3 Galactic Structure This artist s conception shows the various parts of our Galaxy and the position of our Sun.

13 14.3 Galactic Structure This infrared view of our galaxy shows much more detail toward the galactic center than the visiblelight view does, as infrared is not as much absorbed by gas and dust. Can you see the galactic disk and the bulge?

14 14.3 Galactic Structure Star orbits in the galactic disk are in a plane and in the same direction; orbits in the halo and bulge are much more random. To help visualize: the galactic disk orbits are like the planet and Kuiper belt orbits. The halo orbits are like the Oort cloud orbits!

15 14.5 Galactic Spiral Arms Curiously the position and motion of gas clouds shows that the Milky Way has a spiral form more detail soon! Regions with ongoing star formation are limited to the disk as traced by hot young OB stars, emission (red) and reflection (blue) nebula.

16 14.3 Galactic Structure The globular clusters orbits define a spherical halo (analogous to the Oort cloud for our own solar system). All the stars in the halo are very old, and there is no gas and dust. Thus we infer that the galactic halo and globular clusters formed very early. The galactic disk is where the youngest stars are, as well as star formation regions: large clouds of gas and dust and emission nebulae. Thus the galactic disk may be a more recent galaxy feature. Surrounding the galactic center is the galactic bulge, which contains a mix of older and younger stars.

17 14.4 The Formation of the Milky Way Any theory of galaxy formation should be able to account for all the properties below.

18 14.4 The Formation of the Milky Way The formation of the halo of the galaxy likely involved the merger of smaller proto-galaxies. These clusters now comprise the stellar halo of the Milky Way Galactic disk formation may be similar to the formation of the solar system (cartoon right) or may not: e.g. a model simulation suggests a wild past history: most-detailed-computer-galaxy/ Additionally Chapt 16 suggests that spiral galaxy disks likely result from interaction of a small and large galaxy. Understanding galaxy formation is a work in progress!

19 14.5 Galactic Spiral Arms Measurement of the position and motion of gas clouds shows that the Milky Way has a spiral form. [Curious why might that be?]

20 14.5 Galactic Spiral Arms For star speeds that are constant (or decrease) with distance from the galaxy center, the spiral arms should curl up over time; but they haven t!

21 14.5 Galactic Spiral Arms Rather, they appear to be density waves, with stars moving in and out of them much as cars move in and out of a traffic jam.

22 14.5 Galactic Spiral Arms Once the gas and dust are in a plane, wave patterns naturally form; we see them as the spiral arms! 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.

23 14.6 The Mass of the Milky Way Galaxy The orbital speed of an object depends only on the amount of mass between it and the galactic center. Thus: measuring the Galactic orbital speed allows astronomers to calculate the Galactic mass contained within the orbit.

24 14.6 The Mass of the Milky Way Galaxy Once all the Galaxy is within an orbit, the velocity should diminish with distance, as the dashed (Keplerian) curve shows. It doesn t; more than twice the mass of the Galaxy must be outside the visible part to reproduce the observed curve. This additional mass is invisible and is given the name Dark Matter.

25 14.6 The Mass of the Milky Way Galaxy The invisible Dark Matter occupies a volume much larger than the Milky Way visible matter:

26 14.6 The Mass of the Milky Way Galaxy What could this dark matter be? It is dark at all wavelengths, not just the visible. Stellar-mass black holes (e.g. from past hypernova events in the Milky Way i.e. no new physics is needed)? Brown dwarfs, faint white/black dwarfs, and red dwarfs (all of which form naturally again no new physics is needed)? Something totally new e.g. new subatomic particles? This is currently the favored solution but no Earth based experiments have yet to detect these new particles!

27 14.6 The Mass of the Milky Way Galaxy The bending of spacetime can allow a large mass to act as a gravitational lens: Observation of such events suggests that low-mass e.g. brown dwarfs could account for about half of the mass needed but only in the inner (luminous) part of the Milky Way. The rest is still a mystery.

28 14.7 The Galactic Center Left image looks toward the galactic center in visible light. The two arrows in the inset indicate the location of the center. Even at IR wavelengths (right), the galactic center is obscured by dust.

29 14.7 The Galactic Center These images are in Infrared (I), radio (R), and X-ray (X). The radio and X-ray images can see into the galactic center.

30 14.7 The Galactic Center The galactic center appears to have a stellar density (i.e. stars/pc 3 ) 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 radio and X-ray source at the center Apparently, there is an enormous Black Hole at the center of the Galaxy, which is the source of these phenomena. The radio source, presumed the location of the supermassive Black Hole, is named Sagittarius A* (pronounced "Sagittarius A-star", Sgr A*).

31 14.7 The Galactic Center These objects are very close to the galactic center. The orbit parameters for star S2 are consistent with a central Black Hole of about 3.7 million solar masses! Fortunately this supermassive Black Hole is not being fed much matter!

32 14.7 The Galactic Center Later this year (2018) star S0-2 will be at perihelion. This will allow another test of Einstein s General Relativity! When a star gets very close to a very massive object, like Sagittarius A*, the light from the star has to climb out of a gravity well in space-time. This takes energy from the photons of starlight, which stretches the light toward longer wavelengths. This causes it to appear more red: gravitationally redshifted (see Chapter 13). Then will the observed gravitational redshift agree with the predictions of General Relativity?

33 Summary of Chapter 14 A galaxy is stellar and interstellar matter bound by its own gravity. Our Galaxy is spiral (actually a barred-spiral, next Chapt 15). Variable stars can be used for distance measurement, through period luminosity relationship. True extent of a galaxy was first mapped out using globular clusters. Star formation occurs in disk, but not in the halo or in the central bulge.

34 Summary of Chapter 14, cont. Spiral arms are probably 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.

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