Editors note: This is a companion project to the post on viewing the summer Milky Way found here.
OK – the universe beat you to it by roughly 13 billion years. But you can build a scale model of the Milky Way, and in doing so you’ll develop a better feel for its size, its relationship to other galaxies, and why the Milky Way looks like the Milky Way when you see it in your sky. Essentially, all this project entails is printing the image below and gluing it to a disc that is the appropriate thickness. How thick is that? About 2 mm – a little more than one-eighth of an inch. I glued it to cork, but cardboard of similar thickness would be fine. Two millimeters will seem mighty thin – but it is believed that our galaxy – at least in the spiral arms where our Sun is located, is actually just 1,000 light years thick. Since it is believed to be 100,000 light years across, that means the thickness is 1/100th the diameter. If your printed version of thegalaxy image is dramatically different in size – say 150 mm (6-inches) in diameter rather than 200 mm (8-inches), then simply reduce the thickness of your backing to 1.5mm or about 1/th of an inch. The exact size will depend on what image you print from and how your computer handles the printing. But don’t get all fanatical about these dimensions. They are much more than guesses, but something less than precise. After all, no one has ever been outside our galaxy to look in at it, and it would take millions of years to send a space probe out of the galaxy, take a picture, and send it back.
The finished scale model of our Milky Way Galaxy . . .
. . . and how to use it.
First, the green dot not only marks the approximate location of our Sun and solar system, but it also covers the approximate area where you will find just about all of the naked eye stars that you see. That alone should give you pause for thought.
But the main point of this model is to drive home the basic shape of the galaxy – not unlike a pizza pie – and to help you see why the Milky Way makes a thin cloud of stars across our sky. Imagine yourself at the area of the green dot. Now imagine you’re in the middle of it – that is, down one millimeter from the surface, or about 500 light years by this scale. Now if you look up, you are looking through a thickness of 1 mm – 500 light years of stars. And if you look down, the same thing. In fact, just about any direction you look, you don’t see many stars before you get to the surface – the outer reaches – of our galaxy.
BUT . . . if you look along the plane of the galaxy toward the core, you are now looking through about 75,000 light years of stars – so you see many more – but they are very distant and also very faint. They make a faint, hazy cloud across our sky – a thin line. a river – what we call the Milky Way. And if you look outward along the plane in the opposite direction, then you are looking through perhaps 25,000 light years of stars, so you also see a Milky Way – but I think of this one – the winter Milky Way – as skim milk, for it’s much thinner 😉
And this is where the tabs come in. One says “Teapot.” When you look from the green dot toward the “Teapot” tab, you are looking through the core of our galaxy – and this what you are doing when you look at the Teapot asterism in our summer sky. Similar tabs mark the “Cross” and the “W” asterisms and show you the direction you are looking when you see those in your sky. (If these asterisms are not familiar to you, be sure to read the post on looking at the Milky Way in August. )
In the winter time we also see the Milky Way, but it is not as bright. That’s because you are looking outward in the general direction of the constellation “Orion,” which is the other tab.
Whatever time of year it is, try this. Bring your scale model of the Milky Way outside under the stars with you. Find the Milky Way overhead. Then hold your model up and orient it so the edge of your model aligns with the Milky Way. That should give you a sense of the plane of the galaxy and where we all are in this vast river of stars.
What about M13 and many other objects you look at with binoculars and telescope? You can find many of them if you understand the galactic coordinate system explained below. But even if you don’t understand this system, there’s a wonderful – and free – computer program that works on any computer that will help. It’s called “Where is M13?” It will show you exactly where you are looking relative to our galaxy when you look at any of the Messier objects. It’s easy to download and install. Just go here.
How about the next nearest galaxy? Well, there are several very small galaxies that are very near, but usually what we think of as the next nearest galaxy is one that’s close to being our twin – the Great Andromeda Galaxy, M31. What’s most interesting to me is to consider these two questions in tandem: How far is it to the nearest star after our Sun? And how far is it to the Andromeda Galaxy? If we reduce our Sun to an eight-inch ball – about the diameter of our galaxy model – then the next nearest star – the next nearest eight-inch ball – is roughly the distance between Boston and Hawaii – essentially half an earth away! That’s a whole lot of empty space betwen stars. So where is the next large galaxy? Well, the Andromeda Galaxy is about 2.5 million light years away. That would be about 200 inches away on the scale of our model – a bit less than 17 feet! That’s mighty close – and, by the way, getting closer. In fact, scientists feel we’re heading for a collision with the Andromeda Galaxy – but don’t lose any sleep over it – it won’t happen for about 3 billion years! (But you can see a simulation of it right here and now!)
Galactic coordinate system
Editor’s note: The best explanation of the galactic coordinate system I have found is in the brief manual for “Where is M13.” Its author, Bill Tschumy, graciously gave permission to reproduce it here with its accompanying diagram.
The galactic coordinate system is the key to understanding where objects are located within the Galaxy. It was established in 1958 by the International Astronomical Union and is useful for specifying an object’s location relative to the Sun and the galactic core of the Milky Way.
The galactic coordinate system is a 2-D spherical coordinate system with us (or the Sun) at its center. It has latitude and longitude lines, similar to Earth’s. In fact, a good analogy is to imagine yourself standing at the center of a hollow Earth looking at the latitude and longitude lines on the Earth’s surface. The galactic coordinate system is similar except we are looking out at the celestial sphere.
There is a one-to-one mapping between the galactic coordinate system and the more familiar equatorial coordinate system. Relatively simple equations can be used to convert from one to the other.
The galactic equator (i.e., 0º galactic latitude) is coincident with the plane of the Milky Way Galaxy and is shown as the red circle in the image above. Galactic latitude is the angle above or below this plane (e.g. the yellow angle above). Thus, objects with a galactic latitude near 0º will be located within the Milky Way’s spiral arms. Objects with a positive galactic latitude will be above the arms in the northern galactic hemisphere.
Galactic longitude is measured from 0º to 360º, counter clockwise as seen from the north galactic pole. 0º galactic longitude is arbitrarily defined as the direction pointing to our galactic center. Within the plane of our galaxy (0º galactic latitude), the main points of longitude and the Milky Way constellations which lie in their directions are as follows:
- 0º is in the direction of Sagittarius
- 90º is in the direction of Cygnus
- 180º is in the direction of the galactic anti-center in Auriga
- 270º is in the direction of Vela
Now consider an object and its galactic coordinates. Any other object lying along the same line of sight will have the same coordinates but only differ in its distance component. An object’s distance is not part of its galactic coordinates. However, knowledge of an object’s galactic latitude, longitude and distance, does allow us to uniquely locate it within the 3-D space around the Milky Way. Where is M13? uses this information to plot deep sky objects in its Galaxy View.