Doppler Shift/Effect of EM Waves
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1 Doppler Shift/Effect of EM Waves A quick and dirty intro Not quite the same as Doppler shift of sounds (acoustic waves) but has similar physical principles Ø Speed of emitting source changes the wavelength/frequency of lights (photons) Ø approaching wavelength is squeezed shorter (frequency higher). Light will be blue shifted Ø receding wavelength stretches longer (frequency lower). Light will be red-shifted
2 Doppler Shift in Spectra Shift from H-alpha line, at 656 nm, is demonstrated for two recessional velocities Velocity can be calculated from this information for most
3 Let s get oriented with the Milky Way Almost everything we see in the night sky belongs to the Milky Way. We see most of the Milky Way as a faint band of light across the sky.
4 Over Crater Lake Deneb Altair Sagittarius - Antares
5 Almost everything The we see in the night sky belongs Way to the Milky Way. We see most of the Milky Way as a faint band of light across the sky. Andromeda Galaxy
6 The Milky Way Almost everything we see in the night sky belongs to the Milky Way. We see most of the Milky Way as a faint band of light across the sky.
7 Another view Andromeda
8 A Panoramic Mosaic Polaris Andromeda Galaxy Cepheus & Deneb & Altair Antares Small Magellanic Cloud & Large Magellanic Cloud Procyon & Sirius Q: Where are Ursa Major, Orion and Sagittarius?
9 Rho Ophiuchus dark nebulae emission nebulae reflection nebulae Antares (Seems distinct) M4 A Globular Cluster Clear view! No dust or gas In foreground Rho Ophiuchi
10 First Studies of the Galaxy First attempt to unveil the structure of the galaxy by William Herschel (1785), based on optical observations and counting. The shape of the Milky Way was believed to resemble a grindstone, with the sun close to the center (seems to have spiral arms to me )
11 From what we see let s get serious and try to do some science and explore.
12 (Modern) Strategies to explore the structure of our Milky Way I. Select bright objects that throughout the Milky Way and trace their directions and distances. II. Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration), and catalog their directions and distances. III. Trace the orbital velocities of objects in different directions relative to our position. But it was not that easy in the beginning..
13 It all started with Cepheid Variables and Henrietta Swan Leavitt The Cepheid Variables analysis is one of the earliest and the most important work of astronomy of the modern age - Instrumentation, Observation, Analysis, Discovery, New science (See also Annie Canon Spectral Class )
14 Henrietta Swan Leavitt Photographic Plates Leavitt worked under Pickering to study the variable stars in the Large Magellanic Cloud Plate b26816 of Large Magellanic Cloud taken on December 18, 1900 from Arequipa, Peru, with the 8 inch Bache Doublet, Voigtlander, reworked by Clark. The exposure was 60 minutes centered on 5h09m47s R.A. and -67d22m51s Declination. We have not removed the annotations made on the back of the plate because this plate is referenced in the Henrietta Leavitt logbooks made during her research on Cepheid stars.
15 Cepheid Variables Some stars pulsate with regularity (periods are tens of days nearly optimal for nightly observations) exhibiting Period-Luminosity relation but distance unknown 1908 Paper Wikipedia, Harvard
16 1912 Cepheids Plots Original graphs from Leavitt s 1912 paper describing the period-luminosity relationship for a select group of SMC variable stars. The graphs show the stars apparent magnitudes for their maxima and minima versus their periods in days (fig.1) and the logs of their periods (fig.2) leavitt04.asp
17 What are Cepheids? Why do they pulse like that? Cepheid variable stars have periods from 1 to 60 days. They lie in a region of the H-R diagram known as the (TO BE CONTINUED!). They are giant stars. Yellow (super)giants 5-20 times solar mass Composition manifests in pulsation characteristics (Out of main-sequence into the giant stage) (In later stage of life)
18 (HST animation Let s play it)
19 He doubly ionizes Underneath the outer H layer Ionization temperature matches interior stellar temperature à Interact with photons He ionizations absorb radiation or photons ( Opacity ) Outer expanding layer cools He layer heats up He ionization maximizes He absorption stops Radiation breaks out He layer cools Outer layer contracts
20 Pulsation, vibrations are very common and are of basic occurrences. The physics of Cepheids pulsation is nothing too special it has the right condition to greatly amplify the push-pull mechanism in an already massive star (high luminosity).
21 With proper Calibration, Cepheid Variable Stars and the distance method are immensely useful! Hertzsprung calcualted the distance to SMC lyrs away Edwin Hubble and modern cosmology Hubble Key Project and the Hubble constant (More on some topics later )
22 Silent Sky South Coast Repertory
23 The next few slides may be skipped until Galaxies
24 Measuring Distances: The Cepheid Method Instability Strip Observing the period yields a measure of its luminosity and thus its distance! The more luminous a Cepheid variable, the longer its pulsation period.
25 The Cepheid Method Allows us to measure the distances to star clusters throughout the Milky Way
26 The Size of the Milky Way? Challenges: 1)We have an inside view 2)Distance measurements are very difficult 3)Gas and dust are aplenty Harlow Shapley ( ) noticed that different kinds of star clusters have different distributions in the sky. There are two kinds of star clusters open clusters and globular clusters. Open clusters are concentrated along the Milky Way and globular clusters are scattered all over the sky. Shapley noticed that globular clusters were strongly concentrated toward the constellations Sagittarius & Scorpius. (Hmmm )
27 Exploring the Galaxy Using Clusters of Stars Two types of clusters of stars: 1) Open clusters = young clusters of recently formed stars; within the disk of the Galaxy Open clusters η and χ Persei 2) Globular clusters = old, centrally concentrated clusters of stars; mostly in a halo around the galaxy Globular Cluster M 19
28 Globular Clusters Globular Cluster M80 Dense clusters of 50,000 a million stars Old (~ 11 billion years), lower-main-sequence stars Approx. 200 globular clusters in our Milky Way
29 Rho Ophiuchus (Revisit) dark nebulae emission nebulae reflection nebulae Antares (Seems distinct) M4 A Globular Cluster Clear view! No dust or gas In foreground Rho Ophiuchi
30 Shapley assumed the cloud of globular clusters was controlled by the gravitational field of the galaxy. He could find the size and center of the galaxy by studying these globular clusters, however, he needed to know how far away they were
31 Shapley s challenge was to find the distances to the globular clusters. To do this he used Cepheid variables stars. The problem was that the luminosity-period relationship of the Cepheids was not calibrated. Shapley analyzed the proper motion of some Cepheids and measured the distances to the globular clusters and thus find the position of the center of the galaxy. ****Shapley CALIBRATED the Cepheids and established a base-rung of the modern day distance ladder system an enormous step foward
32 Locating the Center of the Milky Way Distribution of globular clusters is not centered on the sun, but on a location which is heavily obscured from direct (visual) observation. Why are globular clusters good measures of the size of the Milky Way? What are inherently assumed?
33 Class Exercise!! What are some assumptions or scientific knowledge other then ones mentioned supporting Shapley s analysis?
34 But not so fast. There are unresolved errors to be cleaned up at later times.. We will return to this in following lectures 1.Types of variables stars 2.Distance calibration method
35 The Structure of the Milky Way Our galaxy consists of two parts, the disk and the sphere. The sphere is made up of two parts, the nuclear bulge and the halo. Shapley s globular clusters were located in the halo. Since the halo is centered on the galaxy
36 An Analysis of the Galaxy Two main components disk & sphere Disk: stars, open star clusters Nearly all the gas and dust of the MWG Site of star formation tends to look blue Sun about 8.5 kpc (kiloparsec) from center (2/3 of the way to the edge of disk) About 75,000 ly in diameter
37 Spherical Component: All of the matter in the galaxy scattered in a roughly spherical distribution about the center Includes the halo and the nuclear bulge Halo: Spherical cloud of thinly scattered stars and globular clusters Very little gas and dust No new star formation, most of the stars in the halo are old
38 Nuclear Bulge: Dense cloud of stars that surround the center of our galaxy. Radius of about 2 kpc Slightly flattened Little gas and dust, no new star formation
39 The Milky Way South Sagittarius North - Orion
40 The Milky Way South Sagittarius North - Orion
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