The Milky Way. Finding the Center. Milky Way Composite Photo. Finding the Center. Milky Way : A band of and a. Milky Way

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1 The Milky Way Milky Way : A band of and a The band of light we see is really 100 billion stars Milky Way probably looks like Andromeda. Milky Way Composite Photo Milky Way Before the 1920 s, astronomers used a model for the galaxy Tried to estimate our location in the galaxy by counting stars in different in the center Dark strip in the middle, from Because some stars are by dust, the true shape of this group of stars was unclear. Finding the Center Harlow Shapely studied. He theorized that they must orbit the true of the galaxy Finding the Center Shapely plotted the of the globular star clusters. He found that they are are not centered on the Sun. but are centered on a point about light years from the Solar System. A Globular Cluster 1

2 The Milky Way Size: The Milky Way is roughly light years across, and about light years thick. Stars: The Milky Way is comprised of over stars! Almost everything visible with the naked eye is inside the Milky Way Parts of Our Galaxy Parts of Our Galaxy Disk: The Resides in the Disk Nuclear Bulge: The dense region Halo: Spherical region surrounding the disk where the live. Questions: Milky Way Scales Lecture Tutorial: Page 123 Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Discuss each question and come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. How big is the Milky Way? Where are stars forming (or not forming)? How much mass is in the Milky Way? What s going on at the center? 2

3 Milky Way: A Spiral Galaxy Our galaxy seems to be : it has spiral arms These are dense concentrations of and. Stars orbit the, pass through the spiral arms as they go. Stars and pile up in the spiral arms, like cars in a traffic jam. Star Formation in the Milky Way The Disk contains, so stars are still forming there. (Population I stars) The Halo has very little, and no new stars are forming there. The halo of the galaxy is populated by stars. (Population II stars) Stellar Populations Pop. I: Newer, disk & spiral arm stars, with percentage heavy elements Pop. II: Older, bulge and halo stars, with percentage of heavy elements Heavy elements (metals): anything that isn t H, He, or Li Measuring Distances To map the Milky Way Galaxy, we need to measure to stars. Parallax only works for nearby stars (within about light years) For more distant stars, we use Standard Candles Standard Candles Standard Candles We can easily measure how bright a star appears ( magnitude) If we knew how bright the star really was (its magnitude) then we could calculate its distance. We need a star whose absolute magnitude is always the same, wherever we observe it. Car Headlights are standard candles: We use them to determine the car s distance Such a star is called a standard candle 3

4 Cepheid Variables In 1908, astronomer Henrietta Leavitt discovered a new standard candle using stars These stars are called They are named for δ Cephei, the first example of such a star. Measuring Distances with Cepheids Cepheid stars change in brightness. They pulsate in a very regular way. Large, bright Cepheids pulsate, while small, dim Cepheids pulsate. Delta Cephei Henrietta Leavitt If we observe the period of pulsation, we can figure out the absolute magnitude & luminosity. If we compare this to the apparent magnitude, we find the distance! The Structure of the Milky Way By measuring the distances to various parts of the Milky Way Galaxy, we map out its structure Mapping the Milky Way The Sun is about out from the center The Milky Way is a Galaxy It has a straight structure at the center called a Bar A modern map of the Milky Way (computer-generated diagram) Measuring the Mass of the Milky Way We use the Sun s around the center of the Milky Way The greater the mass inside the orbit, the the Sun has move around the center. This way we can measure the mass of the Milky Way. Mass of the Milky Way The mass of the Milky Way is between billion and M Sun and billion M Sun Stars & Gas we see in the Milky Way can only account for a fraction of the total mass. -What is it? - Why can t we see it? Total mass: about M Sun 4

5 The Center of the Milky Way The Center of Our Galaxy The of stars in the Galactic Center is much greater than in the Sun s neighborhood. They appear to be orbiting a black hole at the center of the galaxy Its mass is over M Sun! Chapter 13 We now realize that our galaxy is only one of billions of galaxies we can see. These galaxies come in three main types: Galaxies Spiral, & Spiral Galaxies Typically very bright, in color Look like (sometimes with ) M 100 NGC 300 5

6 Elliptical Galaxies are, not flat like spirals They are typically in color. Less gas and dust than spirals. Irregular Galaxies Lack any distinct shape Are generally than spirals and ellipticals The Andromeda Galaxy Our Sister Galaxy 6

7 Hubble Tuning Fork Galaxies (S): Classified according to spiral arms (a,b,c) and presence of a bar ( B ) Galaxies (E): Classified according to shape (E0-E9) Galaxies (Irr): Basically anything funkylooking! A Barred Spiral Galaxy with only 2 arms. Measuring Distances to Galaxies Too far for! Standard Candles: Cepheid Variables (for Local Group) Type Ia Redshifts Type Ia Supernovae These are another used to measure distances to galaxies Matter from large companion falls onto a white dwarf, causing its mass to exceed M sun The resulting explosion is a Type Ia supernova. Supernovae Types Type Ia: Exploding White Dwarf in Binary Type II: ordinary supernovae caused by an exploding massive stars Supernovae are Good Standard Candles They are all the same Galaxies in Motion Motion of galaxies is measured using the effect. Spectrum will be redshifted if it is moving, blueshifted if it is moving us. Supernova in galaxy NGC4526 (HST Image) They can be seen at very large (1000x farther than Cepheids) Non-moving galaxy spectrum Redshifted Spectrum 7

8 Galaxy Redshifts When we look at the spectrum of most galaxies, they are. This means they are moving from us. Only a few exceptions: is moving towards the Milky Way (may collide with the Milky Way in 3 billion years) Hubble s Law E. Hubble measured the to some of these galaxies (using Cepheid variable stars) He discovered that the redshift is proportional to the galaxy s. The relationship is called Hubble s Law Hubble s Original Data Hubble s Law and Distances Hubble s Law can be used to determine the to galaxies which are too far away for us to see Cepheid Variables or Type Ia supernovae v r = H 0 x d d = v r / H 0 Hubble Law d = v r / H 0 d = distance to galaxy (Mpc) v r = radial velocity of galaxy (km/s) H 0 = Hubble constant (70 km/s/mpc) Hubble Law Example: Find the distance to a galaxy that has a radial velocity of 35,000 km/s. d = d = d = v r / H 0 Techniques for Measuring Distances Review (nearby stars) (nearest galaxies) 1. Measure angle, p. D = 1/p 2. (standard candle) Measure Period, get luminosity (distant 3. Type Ia (standard candle) galaxies) 4. (whole universe!) Measure velocity V r. Use: V r = H o x D 8

9 Classifying Galaxies Lecture Tutorial: Page 127 Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Discuss each question and come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. 9

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