a1 20 milky way
DESCRIPTION
Miller's Astronomy 1 lecture notes on the Milky Way galaxyTRANSCRIPT
Discovering our Milky WayLACC: §24.1, 3, 5
• Where are we in the Universe?
• What are “spiral nebulae”?
• What is our Milky Way galaxy like?
An attempt to answer the “big questions”: where are we? how did we get here?
1Thursday, May 6, 2010
The Discovery of the Milky Way
http://cass.ucsd.edu/physics/ph162/lect1.html
• 1755 Immanuel Kant speculates that there may exist "Island Universes" like our Milky Way.
• 1785 William Herschel studies star counts along several hundred lines of sight in the galaxy.
2Thursday, May 6, 2010
Harlow Shapley (1915) and Globular Clusters)
http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit4/milkyway.html
Sketch based on Shapley's original data, uncorrected for interstellar absorption. The Sun
is located at the center of the axes (looking roughly side-on), and the center of the Milky
Way inferred by Shapley is marked by the red X.
3Thursday, May 6, 2010
“Spiral Nebulae”
http://ottawa-rasc.ca/features/marchHubble/index.html
• 1920 Shapley Curtis DebateHarlow Shapley took the position that the universe consisted
only of our Galaxy, which was very large -- about 300,000 light-years in diameter. The spiral nebulae, while distant, were still part of our galaxy
Heber Curtis argued for a smaller galaxy - about 30,000 light-years in diameter - that was one of a vast number of similar systems. The spiral nebulae, he said, were separate star systems similar to our own galaxy, and at great distances "from 500,000 to 10,000,000 light-years away"
• 1923 Edwin Hubble discovers Cepheid Variable stars in Messier 31 - the Great Nebula in Andromeda, estimating its distance as nearly 0.3Mpc (modern value is about 0.7Mpc), well outside our Galaxy.
4Thursday, May 6, 2010
Milky Way - Spiral Arms (via radio observations)
http://www.astro.wisc.edu/goat/article/7/the-milky-way-using-real-data
5Thursday, May 6, 2010
The Size of the Milky Way
http://zebu.uoregon.edu/~imamura/123/lecture-2/lecture-2.html
6Thursday, May 6, 2010
What is a parsec?
http://www.daviddarling.info/encyclopedia/P/parsec.html
An astronomical unit of length, equal to the distance at which the radius of the Earth's orbit subtends an angle of one arcsecond. The name is a contraction of "parallax-second."
1 parsec = 3.259 light-years = 206,265 AU = 30.83 trillion km = 19.16 trillion miles.
The parsec is generally used by astronomers in preference to the light-year. For larger distances, the kiloparsec (kpc) = 1,000 pc or megaparsec (Mpc) = 1,000,000 pc are used.
7Thursday, May 6, 2010
The Size of the Milky Way
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated
and corrected version.
8Thursday, May 6, 2010
Milky Way - Bulge: A Barred Spiral
http://apod.nasa.gov/apod/ap050825.html
A recent survey of stars conducted with the Spitzer Space Telescope is convincing astronomers that our Milky Way Galaxy is not just your ordinary spiral galaxy anymore. Looking out from within the Galaxy's disk, the true structure of the Milky Way is difficult to discern. However, the penetrating infrared census of about 30 million stars indicates that the Galaxy is distinguished by a very large central bar some 27,000 light-years long. In fact, from a vantage point that viewed our galaxy face-on, astronomers in distant galaxies would likely see a striking barred spiral galaxy suggested in this artist's illustration. While previous investigations have identified a small central barred structure, the new results indicate that the Milky Way's large bar would make about a 45 degree angle with a line joining the Sun and the Galaxy's center. DON'T PANIC ... astronomers still place the Sun beyond the central bar region, about a third of the way in from the Milky Way's outer edge.
9Thursday, May 6, 2010
Milky Way: What Do We See?
https://sites.google.com/site/earthsplaceintheuniverse/
Our sun orbits every 225–250 million years
10Thursday, May 6, 2010
Milky Way: What Do We See? https://sites.google.com/site/earthsplaceintheuniverse/
Here's the Universe within 5000 light years, our little arm of the Milky Way galaxy, the Orion Arm. Virtually every star we can see with the naked eye from Earth is within this distance.
The stars on the plot are all thousands of times brighter than the Sun. The brightest star here is Rho Cassiopeia (ρ Cas,) some 100,000 times brighter than the Sun. At 4000 light years away it is barely visible to the naked eye.
11Thursday, May 6, 2010
Discovering our Milky WayLACC: §24.1, 3, 5
• Where are we in the Universe? We are about 2/3 away from the center of our galaxy’s core (Hershel and Shapley). Our galaxy is nowhere special.
• What are “spiral nebulae”? They are other galaxies; Curtis suspected this (Shapley didn’t), Hubble proved it using a Cepheid variable
• What is our Milky Way galaxy like? It is a barred-spiral galaxy about 30 kpc wide with a Bulge (barred), Disk (spiral arms), Halo (globular clusters)
An attempt to answer the “big questions”: where are we? how did we get here?
12Thursday, May 6, 2010
LACC HW: Franknoi, Morrison, and Wolff, Voyages Through the Universe,
3rd ed.
• Ch 24: Tutorial Quizzes accessible from: http://www.brookscole.com/cgi-brookscole/course_products_bc.pl?fid=M20b&product_isbn_issn=9780495017899&discipline_number=19
Due at the beginning of next week’s first class period (unless there is a test that week, in which
case it’s due the same period as the test).
Be working on your Distance Ladders.
13Thursday, May 6, 2010
Our Milky Way GalaxyLACC: §24.1, 3, 5
• Formation and Evolution
• Composition
• Recent Discoveries
An attempt to answer the “big questions”: where are we? how did we get here?
14Thursday, May 6, 2010
The Formation of the Milky Way
http://rst.gsfc.nasa.gov/Sect20/A2.html
A cloud of hydrogen/helium gas begins to form myriad stars. As this continues, the cloud may contract somewhat and the assemblage of stars begin to rotate around a common center.... With rotation, there is a tendency for the cloud to assume a more oblate ellipsoidal shape and begin to spin. The spinning produces strings of stars in at least several distinct arms. When well developed, the stars have organized into a spiral galaxy.
http://www.youtube.com/watch?v=n0jRObc7_xo&feature=related
15Thursday, May 6, 2010
Spiral Arms are Density Waves
http://abyss.uoregon.edu/~js/ast123/lectures/lec10.html16Thursday, May 6, 2010
Spiral Arms are Density Waves
http://abyss.uoregon.edu/~js/ast123/lectures/lec10.html17Thursday, May 6, 2010
Composition of the Milky WayThis page was copied from Nick Strobel's Astronomy Notes. Go to his site at
www.astronomynotes.com for the updated and corrected version.
The disk is the thin pancake-shaped part about 400 parsecs thick and 15 to 20 thousand parsecs in radius with the Sun 8000 parsecs from the center. The disk contains over 98% of the dust and gas in the Galaxy and has a few hundred billion stars. Some stars continue to form so the disk has some young metal-rich stars. The gas and dust are found in a layer that is thinner than the star layer (the gas/dust layer is the thin dark line at the midplane of the disk in the picture above and the star layer is the thicker light band).
The components merge smoothly into each other with the stellar halo among the disk and the inner part of dark matter halo and the dark matter halo among the disk, stellar halo, and bulge, etc. The bulge is the elliptical-shaped center part of the Galaxy about 1000 to 2000 parsecs in radius. It had lots of star formation early on, so now it is made of tens of billions of old, metal-rich! stars.
The stellar halo is a roughly spherical distribution of hundreds of millions of old, metal-poor stars that has increasing concentration of stars toward the center of the galaxy. It is about 20 to 30 thousand parsecs in radius and it may contain small amount of hot gas, but the disk contains the vast majority. Most of the globular clusters are found in the halo and, like the halo stars, the number of them increases toward the galactic center. If the solar system was at the center, you would see approximately the same number of globular clusters in any direction you looked in the sky. Since the globulars are found bunched up in one part of the sky, i.e., they are swarming around some other point in the Galaxy, and we are not at the center. The dark matter halo is denser toward the center. It extends further out than the stellar halo.
18Thursday, May 6, 2010
Stellar Population: Population I Stars
http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/milkywayparts.htm
Population I includes the younger stars in the disk/plane of the galaxy. Because these stars formed recently, they have all be enriched in heavy elements produced in previous generations of stars.
19Thursday, May 6, 2010
Stellar Population: Population II Stars
http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/milkywayparts.htm
Population II is the older stars that tend to lie around the center and in globular clusters, and hence have orbits that take them well out of the disk/plane. Many of these stars were among the first to form, and hence they tend to be almost pure hydrogen and helium, not enriched by previous generations of stars because there were no previous generations. From Gene Smith, http://
casswww.ucsd.edu/public/tutorial/Galaxies.html
20Thursday, May 6, 2010
Milky Way Satellite Galaxies
http://8minutesold.com/?p=135
Not shown here is the Canis Major Dwarf Galaxy--discovered in 2003, it is 42 000 ly from our galactic center (and about 25 000 ly from us).
21Thursday, May 6, 2010
A panoramic X-ray view, covering a 900 by 400 light year swath, shows that the center of the Galaxy is a teeming and tumultuous place. There are supernova remnants: SNR 0.9-0.1, probably the X-ray Thread, and Sagittarius A East. There are many bright X-ray sources, which astronomers believe are binary systems—or pairs of orbiting objects—that contain a black hole or a neutron star (the 1E sources). There are hundreds of unnamed point-like sources that scientists think are solo neutron stars or white dwarfs, which all light up the region. In addition, the massive stars in the Arches and other star clusters (the DB sources) will soon explode to produce more supernovas, neutron stars, and black holes.
Sgr A*, the supermassive black hole that marks the center of the Milky Way Galaxy. Sgr A* contains about 3 million times the mass of the Sun, and is gaining weight daily as it pulls in more material.
The Galactic Core
http://xrtpub.harvard.edu/edu/gcenter/
22Thursday, May 6, 2010
The Galactic Core: A Supermassive Black Hole
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for
the updated and corrected version.
High-resolution infrared measurements of the orbits of the stars at the center show that a very compact mass---a super-massive black hole---with about 3.7 million solar masses lies at the center. The picture below (courtesy of Andrea Ghez and the UCLA Galactic Center Group) shows the orbits of the stars around the black hole from the years 1995 to 2006. At a distance of 8 kpc for the Sun, the 0.2 arc second scale bar in the figure corresponds to about 0.025 light years or 1600 AU. The object is too compact to be a dense cluster of stars---the Chandra X-ray Observatory's observations of X-ray bursts from the object place an upper limit of the diameter of the object of the size of the Earth's orbit. An expanding ring is also seen about 9000 light years from the center. Other galaxy cores have supermassive compact objects (the Andromeda Galaxy, M32, Sombrero Galaxy, M87, and many others).
23Thursday, May 6, 2010
Rotation Curves
http://abyss.uoregon.edu/~js/ast122/lectures/lec25.html24Thursday, May 6, 2010
Rotation Curves of Milky Way
http://abyss.uoregon.edu/~js/ast122/lectures/lec25.html
The surprising thing is there is very little visible matter beyond the Sun's orbital distance from the center of the Galaxy. So the rotation curve of the Galaxy indicates a great deal of mass, but there is no light out there. We call this the dark matter problem, and states that the halo of our Galaxy is filled with a mysterious dark matter of unknown composition and type.
To determine the rotation curve of the Galaxy, stars are not used due to interstellar extinction. Instead, 21-cm maps of neutral hydrogen are used. When this is done, one finds that the rotation curve of the Galaxy stays flat out to large distances, instead of falling off as in the figure above. This means that the mass of the Galaxy increases with increasing distance from the center.
25Thursday, May 6, 2010
What is Dark Matter?
http://relativity.livingreviews.org/open?pubNo=lrr-2002-4&page=node9.html
Basically, according to careful observations, we know that up to 90% of the matter in galaxies must be in the form of "dark matter" to account for the dynamics we observe. On top of that, the dark matter appears to be distributed in a spherical halo around the Milky Way, while the luminous matter is located largely in the flat disk.
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970417c.html
26Thursday, May 6, 2010
Our Milky Way GalaxyLACC: §24.1, 3, 5
• Formation and Evolution: protogalactic cloud, globular clusters, population I and II stars
• Composition: Bulge--population I, random orbits; Disk--population I, “normal” orbits; Halo--population II, random orbits
• Recent Discoveries: Canis Major Dwarf Galaxy, our nearest neighbor; Sagitarius A*, a supermassive black hole in the galactic center (ir and x-ray); dark matter halo (rotation curve)
An attempt to answer the “big questions”: where are we? how did we get here?
27Thursday, May 6, 2010
LACC HW: Franknoi, Morrison, and Wolff, Voyages Through the Universe,
3rd ed.
• Ch. 24, pp. 554-555: 11 (a--3 choices, b--1 choice, c--pick 2,
d--pick 1, pick 2).
• Ch 25: Tutorial Quizzes accessible from: http://www.brookscole.com/cgi-brookscole/course_products_bc.pl?fid=M20b&product_isbn_issn=9780495017899&discipline_number=19
Due at the beginning of next week’s first class period (unless there is a test that week, in which
case it’s due the same period as the test).
Be working on your Distance Ladders.
28Thursday, May 6, 2010