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Visit http://kicp.uchicago.edu/nsta09 Produced for the NSTA Meeting 2009 The Milky Way Model Student Handout I. Background Thousands of years ago, people peering up into the sky from anywhere on Earth couldn’t possibly miss the broad, hazywhite streak stretching across the sky. This cloudy band of light was about 5 degrees wide - 10 times the width of the Moon in the sky - bulging brightest in the direction of the constellation Sagittarius, where it plumped out into a sphere. In some places, particularly around the Mediterranean, the cloudy band came to be known as the Milky Way. For centuries astronomers and philosophers debated the true nature of this cosmic smear: was it a cloud of gas and dust or a collection of closely packed stars, was it part of the Earth’s atmosphere or at great astronomical distances? These days, looking up at the sky from even a small city or suburb, the Milky Way isn’t even visible anymore due to light pollution and has lost its hold on imagination of most people on Earth. Yet at the same time scientists know more about it than ever before. Galileo is the first person credited with uncovering the most basic property of the Milky Way in the 17th century. Pointing his new invention - the telescope - in the direction of the Milky Way, he observed the seemingly uniform cloud dissolve into a great number of individual stars. He was able to separate (resolve) the stars that otherwise blended together when observed by the naked eye; much like putting on a pair of glasses helps a near-sighted person separate the letters on a blurry road sign. So it was revealed that what was once hypothesized to be a cloud of gas was actually an immense collection of tightly packed stars. By counting the stars in different directions across the sky it wasn’t too hard to determine that the Milky Way must have an edge with the Solar System located closer to it than the center (though the first attempt by William Herschel in 1785 wasn’t quite successful, see the diagram to the left, the sun is the dark spot near the center). Furthermore, since the Milky Way appeared only in a relatively thin band on the sky, it couldn’t be round like a sphere but must be flat like a disk. Astronomers in the centuries following Galileo’s initial observation found a variety of fuzzy cloud like objects (nebulae) scattered throughout the sky. The most famous of these was known as the Great Andromeda Nebula, named for the constellation it is found in. Andromeda, like many of the “spiral nebulae”, appeared to be a swirling cloud of gas; something like a storm cloud or tornado. A new debate came to the fore: were these cloudy nebulae located outside the Milky Way, so far away that the stars within them could not be resolved, or were these actually clouds of gas located within the Milky Way itself? Perhaps more provocatively, 1 Visit http://kicp.uchicago.edu/nsta09 Produced for the NSTA Meeting 2009 astronomers posed the cosmological question: was the Milky Way the whole entire universe or was it just one of many “island universes” located within a much greater system? Eventually a famous astronomer, Edwin Hubble was able to use a powerful telescope to resolve the stars in these spiral nebulae - ending the debate - and measure the great distances to them using Cepheid-variable stars. These nebulae were renamed Galaxies: the Milky Way being just one rather standard example. Through more detailed observations with conventional optical (visible wavelength) telescopes and across the entire electromagnetic spectrum of light - from the radio to x-rays - astronomers have determined that the Milky Way Galaxy is actually a complicated and busy collection of stars, gas and dust. Measurements of the distance and velocity of a variety of astronomical objects within the Galaxy have revealed that the bulk of the Milky Way appears to rotate around a central point. Perhaps most astonishingly, the buzzing hive of stars that populate the Galactic center reveal an incredibly massive but invisible object in the middle of it all: a super-massive black hole! II. Intro to Galactic Coordinates In this activity you will assemble a 3-D scale model of the Milky Way based on a handful of interesting objects, which have been observed at different wavelengths of light: radio, infra-red, visible, and x-ray. You will be given pictures of these objects along with their observation wavelength, galactic coordinates and distance, to place them appropriately in your model. Only two angular coordinates are needed to specify the location of any object on the sky, for instance common coordinates used in astronomy are: 1. Azimuth (az) - the angle the object can be found along the horizon 2. Elevation (el) - the angle the object can be found above the horizon These two coordinates are perpendicular to each other just like left-right (az) and up-down (el) are. The Milky Way does not lie along or perpendicular to the horizon, so az/el coordinates are not convenient when describing the position of objects in the Galaxy. This is why astronomers often use coordinates aligned with plane of the Galaxy; galactic coordinates. These are specified by: 1. Galactic Longitude (l) - the angle along the plane of the Milky Way. 2. Galactic Latitude (b) - the angle away from the plane of the Milky Way. 2 Visit http://kicp.uchicago.edu/nsta09 Produced for the NSTA Meeting 2009 These two coordinates are left-right and up-down in the Galaxy as seen from the Earth. Now, since the Milky Way is a pretty much just a flat disk, most of the objects we observe that are associated with it lie in the plane, i.e. zero Galactic latitude (b = 0). So we can ignore the up-down and get away with only using the leftright longitude coordinate. Finally, for the three-dimensional space we live in, these two-dimensional coordinates are incomplete. The angular coordinates only tell us which direction; the final coordinate that locates an object in space (not just on the sky) is its distance. Astronomical distances are most often specified in light-years, which are very large. Galactic Center III. Setting Up the Model Galactic Coordinate System Before building your model of the Milky Way Galaxy you must determine the dimensions of your workspace and the appropriate scale - i.e. feet per light-years - to use in your model; the entire room you are working in will be filled with the Milky Way model. 1. Begin by measuring the size of your room. Locate and mark the center: this will be the location of the Galactic Center (you can place those pictures here now). 2. The narrowest width of the room is the widest you can make the model. Record this width; round down to the nearest meter or foot: _________________________ The Milky Way is approximately 100,000 light-years across (but only about 1,000 light years thick, hence why we can neglect the up-down dimension). You will need to fit these 100,000 light years into the width you recorded above. 3. Calculate the scale in feet (or inch, meter, centimeter, etc.) per light-year; divide the width the narrowest width of your room by the width of the Galaxy: _________________________ This scale will now tell you how to convert a distance given in light-years to the model units you have chosen. Now, galactic coordinates are not measured from the Galactic Center (GC), they are measured from the location of the Earth, or rather the Sun (Sol). So you must first place the Sun in its proper location before you can proceed to locate any other objects. 4. Calculate the distance between the Sun and the GC in the units of your model. Find the distance to the GC on its picture and multiply it by the scale you determined in step 2: (distance to GC in light-years) x (scale) = ________________________ 5. Place the Sun in its proper position. It doesn’t matter where around the center you choose, only the correct distance is important (since the Galaxy is a round disk). 3 Visit http://kicp.uchicago.edu/nsta09 Produced for the NSTA Meeting 2009 Now there’s only one last step left before we can populate the rest of the galactic disk. The scale calculated in step 2 allows us to work with the distance coordinate, but we must establish the angular coordinate as well. 6. Place a large sheet of poster board at the location of the Sun. Make sure to secure it to the floor with copious amounts of tape; the paper should never move! 7. Place a protractor on the paper so that: a. the center point of the protractor is located at the exact location of the Sun. b. the 0-degree line points directly from the Sun to the GC; this is the definition of zero galactic longitude (l = 0) 8. Use the protractor to draw out a circle around the position of the Sun and mark the angle at intervals of 5 degrees from 0 to 360; remember 0 points to the GC. Congratulations, you have successfully established your very own galactic coordinate system and distance scale! Now you can read off the objects’ galactic longitude and distance coordinates to place them on the floor and assemble your model. Go ahead and put together your very own Milky Way. Galactic Center Obscuring Dust Center The Plane The Bulge 4 Visit http://kicp.uchicago.edu/nsta09 Produced for the NSTA Meeting 2009 The Milky Way Model Teacher Guide Much of the information presented here has been adapted from “The Milky Way Model”, by R.B. Friedman, AER 7, 176 (2009) (http://aer.noao.edu/cgi-bin/article.pl?id=289). Please see that article for additional information. Make certain you visit http://kicp.uchicago.edu/nsta09 to download accompanying resources! See the attached source list for suggested objects to use in the model. I. Basic Setup and Supplies The nice thing about the Milky Way model is that it is rich in information and teachable concepts, but cheap on resources! In principle, you can do this entire exercise on graph paper or with Excel and displaying the images on a computer screen or with a projector. However, a large-scale (e.g. gym sized) model is much more compelling and engaging for students. Here are some tips for constructing your BIG model: 1. The Milky Way is vast but most of the beautiful objects we have imaged with high enough resolution to be impressive to use here are relatively nearby and clustered. For this reason a lot of space is necessary to properly setup and view the model. A gymnasium floor, parking lot, or playing field is ideal. For example we used the 60-foot diameter floor in the dome at Yerkes Observatory. 2. To hold up the astronomical images in this model, we attached wooden clothespins to the ends of ~3 foot wooden dowels using hot glue and cable ties. The images can then be easily clipped to the dowels as the model is assembled. To use these indoors, we fashioned a small wooden block base (drilling out a hole for the dowel); modeling clay or playdoh may also work well. If constructing this model in a field, stick the dowels right into the ground. 3. To measure out the large distances, just measure the appropriate length of twine, string or yarn using standard rulers or yardsticks. 4. Print out the images and laminate them! This isn’t cheap, but definitely worth it. Make sure to either print or hand-write all the important information about each object on the back of the image. These can be found at: http://kicp.uchicago.edu/nsta09 (left) Students measuring the Galactic Longitude from the center of the coordinate system (Sol) and the distance with string. (right) Instructor leading a discussion of the model. The dowel, clothespin and block base display setup should be clearly seen in these photos. 5 Visit http://kicp.uchicago.edu/nsta09 Produced for the NSTA Meeting 2009 II. Milky Way Talking Points The Milky Way model can simply be used as unique way to teach coordinate systems or the concept of scale, but it can also be used as a springboard for an exploration of astrophysical concepts. There are a number of important astronomical and physical that this model can be used to illustrate: 1) coordinate systems, 2) morphology of the Milky Way, 3) distribution of objects in the Milky Way, 4) the electromagnetic spectrum, 5) the life cycle of stars, etc. While this isn’t intended to be an extensive review, below are three examples: ON THE INSIDE LOOKING OUT: The most important thing to get across to students is that we have (and will) NEVER photograph or observe the Milky Way from outside of it. We are forever bound within our Galaxy and getting an exterior perspective would take many thousands of light-years, traveling at the speed of light of course. Everything we know about the Milky Way has been learned sitting right here on Earth looking through it. Any exterior “image” of the Milky Way Galaxy is an artist’s conception based on interior data and comparisons with other galaxies we can see. THE SPIRAL ARMS: Once the model is constructed, the spiral arm structure of the Milky Way Galaxy should be visible in the distribution of objects - this was intended to be so. The number of Milky Way arms is still a hot topic of astronomical research and under some debate. The current convention is that there are 5: Orion, Perseus, Sagittarius, Scutum-Crux and Norma - in that order from outside in. The sun is believed to reside in the Sagittarius arm. THE LIFE CYCLE OF STARS: This is a very rich astrophysical discussion topic that can be nicely coupled to the Milky Way model. Dividing the Galaxy into 3 components representing stages of the life cycle - birth, life and death - correctly frames it as a living, evolving, causally connected system. Furthermore, each of the 3 components discussed below is observed in some signature electromagnetic spectral range, or band. This is a convenient way to demonstrate the existence and necessity of multi-frequency observation in astronomy. Broadly speaking, higher energy phenomena produce higher energy emission both in intensity and spectral properties, i.e. higher frequency (shorter wavelength) light. Birth: The first component - stuff that will be stars - is mainly cold gas in the form of molecular clouds (composed of molecules) and dust; observed in the radio to far infrared or as an obstruction to optical radiation. Examples of these objects are W 44 and W 51, though it is very important to note that the star forming regions (second component) are embedded within molecular clouds. The dark regions surrounding and obscuring many of the nebulae - such as M16 - are regions of molecular gas. Life: The second component - stars themselves - is represented by star forming regions containing protostars or open and globular clusters; observed from radio to 6 M16 Visit http://kicp.uchicago.edu/nsta09 Produced for the NSTA Meeting 2009 ultraviolet. There are many examples of star forming regions (most of the nebulae) and star clusters in the galactic object set. The star cluster M16, embedded (center-left) within the Eagle Nebula is a particularly nice example. The dark surrounding regions are composed of cold molecular gas. The dark fingers (popularly known as the Pillars of Creation) protruding into the nebula are also composed of molecular gas. In both cases the cold gas is being stripped away by powerful ionizing radiation from the young stars in the cluster M1 Death: The third and most dramatic component - the stuff that was once stars - is populated by supernova remnants, planetary nebulae, neutron stars and black holes; observed in the visible to x-ray range. The best representative example in the object set is M1 - the Crab Nebula. The Crab Nebula is a famous supernova remnant - the diffuse remains of a star’s atmosphere - produced by the violent explosive end of the star’s life; actually observed by Chinese astronomers in 1054. At the center of the remnant is the Crab Pulsar - the remains of the star’s interior - a compact neutron star that emits radio waves in a focused pulsating beam, much like a lighthouse. III. Suggested Questions 1. If the Milky Way is approximately 1,000 light years wide and 100,000 light years across and the Earth is located approximately 25,000 light years from the center, what are the approximate angular dimensions of the Milky Way on the sky? 2. While the extent of the solar system is just barely accessible to humans using space probes, it is insignificant compared to the scale of the Milky Way. The solar system is approximately 100 AU wide; use your model scale to estimate how big it would be in the Milky Way model. (1 AU = 1.5 x 1011 meters, 1 light year = 9.5 x 1015 meters). For comparison a human hair is 10-4 meters wide, a red blood cell is 10-6 meters wide and an atom is 10-11 meters wide (approximately). 3. The objects in the Milky Way model are preferentially clustered about the location of the Sun/Earth, why? The answer is NOT that the sun is located in some particularly interesting location, or that the Milky Way has some crazy shape. What kinds of objects are seen farthest from the sun, in which observations frequencies? 4. Without measuring the distances to any objects in the sky, how might you determine the relative location of the sun in the Galaxy? What would the sky look like if we were in the center? What would it look like if we weren’t located in a galaxy? IV. Recommended Sources for Milky Way Information If you’d like to learn more about the topics above, consider consulting the following references. Even if you think you know everything, think again! The astronomical community is constantly developing a more thorough picture of the Milky Way and new discoveries are made regularly. 7 Visit http://kicp.uchicago.edu/nsta09 Produced for the NSTA Meeting 2009 • • • • • • The Wikipedia Milky Way entry is actually quite thorough with great references: http://en.wikipedia.org/wiki/Milky_Way. For a history of Galactic astronomy, check out "Coming of Age in the Milky Way" by Timothy Ferris. For up-to-date Milky Way news, which is actually rapidly developing, see space.com (http://www.space.com/milkyway/). A now outdated but still very cool page dedicated to multi-wavelength observations of the Milky Way: http://mwmw.gsfc.nasa.gov/ Wikipedia entries on star formation and evolution are also a good place to start: http://en.wikipedia.org/wiki/Star_formation and http://en.wikipedia.org/wiki/Stellar_evolution. For detailed information on galactic and stellar astronomy, consult a general astronomy textbook; “Universe” by Freedman and Kaufman is an excellent example. V. Finding More Astronomical Data Finding the appropriate objects to use - those which best represent the components of the Galaxy you wish to highlight and have interesting or publicly available images to accompany them - is no easy task. Here are some resources that may help you: • • • • • • Hubble Heritage (http://heritage.stsci.edu/): Hubble is the go to for beautiful astronomical images. This site features the cream of the crop and is by no means an exhaustive gallery of Hubble images or a thorough survey of the Milky Way. Astronomy Picture of the Day (http://antwrp.gsfc.nasa.gov/apod/astropix.html): If you’re not already checking this with your cup of coffee in the morning, start. Use their index link at the bottom of the page to search by astronomical object type. The best part is you get a short astronomy lesson with the images! Chandra Image Gallery (http://chandra.harvard.edu/photo/): Chandra is to X-ray what Hubble is to Optical; search for “images by category”. NRAO Image Gallery (http://images.nrao.edu/): There is no Hubble for radio observations, but there is the National Radio Astronomy Observatory, which is really a US association of radio telescopes. They seem to know enough to default to a category search page. IPAC Missions Image Gallery (http://coolcosmos.ipac.caltech.edu/image_galleries/missions_gallery.html): these guys work on a number of different Infra-Red telescopes and have an extensive collection of Infra-Red images for you to peruse. SIMBAD Astronomical Database (http://simbad.u-strasbg.fr/simbad/): come here once you’ve found some nice images; you can find an extensive collection of astronomical information for a vast number of objects. You can search for the position (and available data) by object name: go to SIMBAD, click on queries - by identifier and enter “Crab Nebula” into the identifier field (as an example). Doing so actually brings up the Crab pulsar, located in the center of the nebula (see the essential note below the coordinates). There’s plenty to play with here. You can also launch the Aladin application from here to view an image of this object. 8 Visit http://kicp.uchicago.edu/nsta09 Produced for the NSTA Meeting 2009 Object Band Type Arm Distance (ly) Longitude (deg) Sgr A* IR SMBH Sgr A* Radio SMBH Galactic Center 26,000 0 Galactic Center 26,000 0 Sgr A* X-Ray SMBH Galactic Center 26000 0 M76 M36 Optical PNe Perseus 4000 131 Optical OC Perseus 5100 175 M38 Optical OC Perseus 5300 172 M1 Optical SNR Perseus 6500 185 M103 Optical OC Perseus 8000 128 NGC 7538 Radio GMC Perseus 9100 112 M52 Optical OC Perseus 9500 113 Sol Optical Star Orion 0 0 M45 Optical OC Orion 380 167 M44 Optical OC Orion 577 206 NGC 1333 IR Reflection Neb Orion 720 158 M78 Optical Reflection Neb Orion 1600 205 M42 Optical HII Orion 1600 209 M97 Optical PNe Orion 2100 149 M57 Optical PNe Orion 2300 63 IC 1396 IR Emission Neb Orion 2450 99 IC 1396 IR Emission Neb Orion 2450 99 NGC 7129 IR Reflecction Neb Orion 3300 105 RCW 38 IR SFR Orion 6000 268 M29 Optical OC Orion 6000 77 DR 21 IR SFR Orion 6200 82 Carina Optical SFR Orion 7500 288 RCW 49 IR SFR Orion 13,700 284 M18 Optical OC Sagittarius 4900 14 M26 Optical OC Sagittarius 5000 24 M20 IR Diffuse Neb Sagittarius 5200 7 M8 IR Diffuse Neb Sagittarius 5200 6 M20 Optical Diffuse Neb Sagittarius 5200 7 M8 Optical Diffuse Neb Sagittarius 5200 6 M17 Optical Diffuse Neb Sagittarius 5500 15 M16 Optical OC Sagittarius 7000 17 M16 Optical Emission Neb Sagittarius 7000 17 W 44 Radio GMC Sagittarius 9000 35 W 51 Radio GMC Sagittarius 22,000 58 M24 Optical OC Scutum-Crux 12,000 13 M22 Optical GC Scutum-Crux 12,400 10 M55 Optical GC Norma 17,600 9 M28 Optical GC Norma 18,600 8 S201 Radio SFR Unknown 7500 138.2 9 Visit http://kicp.uchicago.edu/nsta09 Produced for the NSTA Meeting 2009 Object Band Type Arm Distance (ly) Longitude (deg) S206 Radio SFR Unknown 11,400 150.7 W30 Radio SNR Unknown 14,700 8.6 K3-35 Radio PNe Unknown 16,300 56.1 W50 Radio SNR Unknown 17,900 39.7 GS62.1+0.2-18 Radio Galcatic Shell Unknown 30,000 62.8 M80 Optical GC Unknown 32,000 353 M56 Optical GC Unknown 32,900 63 W49A Radio GMC Unknown 37,200 43.2 Object Type Legend: • SFR - Star Forming Region • SNR - Supernova Remnant • PNe - Planetary Nebula • GC - Globular Cluster (largest group of stars, gravitationally bound together) • OC - Open Cluster (smaller group of unbound stars) • GMC - Giant Molecular Cloud • SMGH - Super Massive Black Hole • Neb - Nebulae (Reflection, Emission, Diffuse, etc.) 10