This step-by-step tutorial documents how we make use of the model contained in sdss_parsecs.zip.
Requirements: This should be run on a computer with at least 1.5 Gigs of RAM.
If you click on SDSS_startatsun.bat, and wait a while (between two seconds and two minutes), you find yourself looking at the picture below. (For the GeoWall version, click on SDSS_startatsun_geowall.bat)
This has two windows, called the picture window and the command windows. The picture window is the one with the sun and stars, and the command window is the one below:
Minimize the command window by pressing the "_" button near its top right corner. Don't click on the "x" button yet, since that removes the window completely, and we need that later.
Press the right mouse button down (don't bring it up yet), pull the mouse back, and , while the mouse is still moving, release the button. You should be moving back, away from the sun... it may take a while for you to get the hang of moving with a good speed. You may or may not want to change the speed of moving as you keep zooming out.
The screenshots below describe what you see as you keep zooming out.
As you move further back, you pass through some nearby stars; at positions given by the Hipparcos satellite.
The stars form a small part of the Milky Way (which is, of course, not represented by a real photograph of our galaxy...)
The Milky Way has some galaxies nearby. These galaxies are represented by actual photographs taken from various astronomy websites. Like everything else here, only positions are accurate, sizes are not; galaxies have been blown up in size here so you can see them.
Zooming further out, we see some galaxies found by the SDSS. The apparently large gap between the local group and galaxies found by the SDSS is due to the ever increasing speeds at which we move out.
Lots more galaxies.
Notice the structures formed by the galaxies...
The pink spots represent quasars, which are much brighter than galaxies. That explains why quasars from much further away can be seen.
Now let's bring that command window back. Restore it, and click on the first big button, called Cosmic Microwave Background. The image from WMAP should now turn on. You can minimize the command window now.
Zooming back, we see that the CMB forms a sphere 13 800 megaparsecs in radius; in comoving coordinates.
This tool emphasize the scales we are dealing with here. You can turn on spheres of radius 1000, 1000 000 and 1000 000 000 light years. We'll just demonstrate the last one for now.
Restore the command window, turn the CMB off, and click on the button called BILLION, and zoom back in. This turns on a wireframe sphere of radius a billion light years.
One of the first things you notice is that the galaxies and quasars found bythe SDSS form a funny fan shape. Why is this? The quick answer is simple; the SDSS is an ongoing project, and what you see is what they've found so far.
The SDSS project scans the sky in slices. And sometimes they miss spots, as shown below, for filling in later. The picture below shows a stripe as seen from near the Milky Way. It may be a little confusing at first, but you'll realize what you're looking at later.
Okay, that just explains why there are fan shapes; not why they are these particular ones. Here are some reasons. The first is that the survey is being done - at the moment (pending further funding, hint hint) - from one site in the Northern Hemisphere, namely Apache Point Observatory near the US-Mexico border.
The screenshot below shows the stripes rotated so that the horizontal plane is in line with the earth's equator. There is only one small stripe being scanned in the sky of the earth's Southern hemisphere.
A second reason is that the Milky Way gets in the way of finding galaxies further out. There are a hundred billion stars in the Milky Way, and most lie in a plane that is the horizontal plane in the picture below. That means that we cannot survey the sky in the plane of the Milky Way with present methods.
When the SDSS is complete, the top portion of the above image will be filled in, a quarter of the sky.
One of the computers we use had an inadequate graphics card, which meant we ran into clipping plane problems. We got around it in a couple of ways; the one that's easier to explain is that we created a version of the above where you start out looking at the Milky Way and local group instead of the sun; this version starts by clicking on SDSS_startatlocalgroup.bat or SDSS_startatlocalgroup_geowall.bat.