The Milky Way from Earth
The Milky Way from Earth
Goals: Understand how the band of light in the sky relates to the 3-D picture of the Galaxy.
Before starting, turn on: nothing
You will be using: gall command, stars, mwVis, alpha command, radec, gc
If you're coming from the previous tour, let's clear the board. In Partiview's command line, type the command
gall offThis command tells Partiview to act on all groups with the off command. Of course, you can use this command to increase the brightness of all groups (gall slum *4) or turn them all red (gall color const 1 0 0) if you desire.
The Band of the Milky Way
Now turn on the stars and the Visible Milky Way all-sky survey groups. The mwVis data group is the band of light across the sky that, for thousands of years, we have called the Milky Way. That band is the disk of our Galaxy, bright toward Sagittarius, dimmer toward Orion.
Brighten up the Galaxy by increasing its alpha value. You can either use the Alpha Slider (choose alpha from the Slider Menu) or use the Command Line by entering alpha 0.8, for example. (Recall that the alpha value ranges from 0-1.)
Now you see some details that would not normally be seen. The bright clouds and the dark dust lanes reveal themselves when we brighten up the image. Upon scanning the entire band, you will notice a clear distinction between the bright areas toward the center of the Galaxy and the dim regions opposite the center.
Relate this to Earth by turning on the celestial coordinates. These are Earth's coordinates projected onto the sky. The bright line is the celestial equator, and the poles (where the lines converge) are those parts of the sky directly over the poles on Earth. From mid-northern latitudes, we see from about -40o to +90o declination throughout the course of the year. Notice the difference between the Milky Way visible in the Northern Hemisphere and that part visible at southern latitudes. The southern Milky Way is much brighter and more beautiful.
Globular Clusters Are Mostly Near Galactic Center
Let's perform the same experiment that the astronomer Harlow Shapley did 80 years ago. Born in 1885, Shapley was studying the sky at Mount Wilson Observatory outside Los Angeles when the country was involved in World War I. Wartime blackouts in the city provided astronomers an opportunity to see the sky without the lights of the city below.
Shapley had been looking at the globular clusters in the sky for some time. Turn on the globular clusters and see if you notice anything odd about where they are located by scanning the sky. See anything strange? Look toward the star Sirius, then look toward the opposite side of the sky along the Milky Way. It's easy to see that most of the clusters are found around the bright part of the Milky Way.
From this, Shapley deduced that the center of our star system must be in this direction. By estimating the distance to these clusters, he put to rest the debate about the size and structure of our Galaxy.
Earth lies within the disk of the Galaxy, making it difficult for astronomers to understand the structure of our star system. Not until astronomers began seeing the Milky Way in wavelengths outside the visible spectrum did the structure of the Galaxy become apparent.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott