Thursday, December 02, 2010

The Galactipedia

kw: virtual astronomy, science, photographs

A telescope and a clear sky can just eat up the time. Now, even though I live where the sky is seldom worth looking at, and don't have much of a telescope, I can waste all the time I want on the virtual sky. Here I'll just touch on one resource, the SDSS SkyServer. The Sloan Digital Sky Survey has mapped about a quarter of the sky, primarily around the North Galactic Pole, so the deeper sky away from most of the Milky Way galaxy can be studied, and also simply viewed with the virtual telescope I'm about to describe.

To get the view shown just below, click on the link above, then near the lower left under SkyServer Tools, click the "Famous Places" link. Select almost any item from the page that shows up, which will present a selection, and select one of the items there. That will get you into the Navigator, as shown below. What I did next was enter the coordinates of the Coma Cluster of galaxies (ra 195 and dec 28) in the Parameters fields at the upper left. As the third parameter shows ("L"), and the check mark below, I've also turned on the Label option, which produced the four lines of white text on the image (Click on this screen shot to see it at full resolution).

A word on the coordinate system. If you look up an object in a sky catalog, its coordinates are traditionally shown as RA or Right Ascension and Dec or Declination. These are spherical coordinates on the sky. RA is given in time units from 00:00:00 to 23:59:59.9..., with the zero point being the First Point of Aries, where the Sun crosses the celestial equator at the vernal equinox. In the SDSS survey, rather than time units from 0-24, they use decimal degrees from 0-360 (or 0-359.9999...). Thus the traditional RA of the center of the Coma cluster is almost exactly 13h, so 13*360/24 yields 195 degrees.

Declination is in degrees from the celestial equator (Earth's orbit) as zero to +90 at the North celestial pole and -90 at the South one. The point on the celestial equator nearest the First Point of Aries has coordinates (0,0). By the way, because of the precession of the equinoxes, the zero point slowly moves along the celestial equator, about a degree every 64 years. Thus to specify very exact sky coordinates one must specify the Epoch, or the reference date. So far as I've been able to find out, the SDSS system coordinates are epoch 2000.

Once you are looking at something, you can shift the view by clicking on the N, S, E, W letters on the frame. To move in bigger steps, zoom out a couple of levels; each level is a factor of 4. For the second screen shot, I shifted the view left and a little down and zoomed in one level. Almost every object in this image is a galaxy. I clicked on the bluest one I could see, and its parameters are shown in the "Selected Object" window at upper right. The letters u, g, r, i, and z refer to light filters: ultraviolet, green, red, infrared, and z=farther infrared (but not very far). The "g" magnitude is closest to the visual magnitude often given as V in star catalogs. This galaxy is of 19th magnitude.

The little window to the right shows a closeup of the selected object. All the objects in this image are out of reach of nearly any amateur telescope. In fact, it is rather hard to show things you might look at visually, because any object brighter than 12th magnitude overloads the SDSS telescope's sensors! For reference, the faintest galaxies in the Messier Catalog, a popular list of visually appealing objects, are about magnitude 10, six times brighter than that.

There is so much more to this that I'll have to defer further discussion. I'll soon be reviewing a book about the making of the Sloan survey.

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