You can use this model as an aid in describing the shape of our galaxy and its features, to show the Earth’s approximate location within it, and to show its orientation with respect to the celestial sphere. (See demonstration 92.06 -- Celestial globe.) The model is about 15 inches in diameter, and its scale is 1 inch = 6,750 light-years.

A galaxy is a fantastically large, independent cluster of various kinds of stars (with planets, if they have them), black holes, gas and dust, all held together by gravity. Our solar system is located within a galaxy called the Milky Way. Galaxies come in several shapes, for which Edwin Hubble devised a system of classification. Based on certain characeristics of their shapes, he divided them into four types — Spiral, Barred Spiral, Elliptical and Irregular. People have made modifications and refinements to this system, but the Hubble Classification Scheme is still widely used.

Spiral galaxies, denoted “S,” have a galactic disk – a flattened disk of stars, with spiral arms, which thickens to a central galactic bulge. The structure of the spiral arms varies, and so does the thickness of the central bulge. Though it is not perfect, there is good correllation between the thickness of the central bulge and the structure of the arms. Sa galaxies have large central bulges, and typically have tightly bound, almost circular arms. Sb galaxies have medium-sized bulges, and their arms tend to be more open than those of Sa galaxies. Sc galaxies have the smallest bulges, and their spiral arm structure is often loose and poorly defined. Surrounding the disk and bulge is a roughly spherical cloud of faint old stars, which is called the glactic halo. Spiral galaxies contain both young and old stars, with only old stars in the halo.. Their disks contain much gas and dust, but their halos don’t contain much of either. Stars are continually being formed in the spiral arms. Gas and stars in the disk follow circular orbits around the center of the galaxy, and halo stars follow random orbits in all directions.

Barred spiral galaxies are spiral galaxies that have an elongated central “bar” of stars and gas. (These are designated “SB.”)

Elliptical galaxies, designated “E,” do not have a disk, but have their stars smoothly distributed throughout an ellipsoidal volume. Their shape runs from nearly spherical (E0) to highly flattened (E7). They have a dense portion in the center. They contain only old stars, and little or no gas and dust. They have not had significant star formation over the last 10 billion years. Their stars orbit randomly in all directions.

Irregular galaxies, designated “Irr” have no obvious structure. Irr II galaxies often appear as if they are exploding. These galaxies contain both young and old stars, contain a lot of gas and dust, and exhibit vigorous continual star formation. Their stars and gas move in highly irregular orbits.

Astronomers believe that the Milky Way is a barred spiral galaxy, either type SBb or SBc. It is roughly 100,000 light-years (9.46 × 10¹⁷ km) in diameter, and about 13,000 light-years (1.2 × 10 ¹⁷ km) thick at the central bulge. Our solar system lies near a feature called the Orion Spur, between the Perseus Arm on the outside, and the Sagittarius Arm on the inside. It sits about 26,000 light-years (2.46 × 10¹⁷ km) from the galactic center. At its location, the galactic disk is about 1,000 light-years ((9.46 × 10¹⁵ km) thick.

The position and orientation of our solar system within the Milky Way determines the orientation of the Milky Way with respect to the celestial sphere, and thus the way it appears in the sky. The line indicated by the two arrows in the photograph above is where the Milky Way intersects the celestial equator. The north-south axis of the celestial sphere passes through the point on the top surface of the model shown by a red dot, in the north, and through the point on the bottom surface whose location is indicated by a white dot, in the south. (These locations are labeled on the surfaces of the model.) With the model sitting as shown, to put it in the orientation in which we see it on Earth, then, we must rotate it clockwise about the line of the celestial equator until the line through the red and white dots is vertical. This should put the disk of the galaxy at an angle of about 62 degrees, with the intersection near the back of the table at a right ascension of 18 hours, 40 minutes (1900). Since it is difficult to define the mean plane of the galaxy precisely, astronomers arrived at these coordinates by convention. (The “1900” in parentheses refers to the equinox for which these coordinates were chosen. Because of the orientation of the Earth’s axis of rotation with respect to its orbit around the Sun, and because the Earth is slightly oblate, except at the equinoxes the Sun exerts a torque on it in a direction that would bring the equatorial bulge closer to the plane of its orbit. The plane of the Moon’s orbit lies at an angle of 5° to the plane of the Earth’s orbit, so it also exerts a torque on the Earth in the same direction. This torque causes the Earth’s axis to precess, which results in a westward drift of the equinoxes of about 50.3 arc seconds per year. The zero reference for right ascension is the location at which the Sun crosses the celestial equator from south to north at the vernal equinox, around March 21. The slow drift of the equinoxes results in corresponding changes in the right ascension of celestial objects. Hence the need for a reference equinox, which is updated every 50 years.) A joint in the stem that supports the model, which you can loosen and tighten via a wing nut, allows you to set the model at the angle that orients it as described above (north celestial pole and south celestial pole on a vertical line,with the celestial equator horizontal).

Locations of objects with respect to the Milky Way are expressed in galactic coordinates. If we view the galaxy edge on as the model sits in the photograph, the north and south galactic poles are at the top and bottom, respectively, of the central bulge. The galactic equator runs along the outer edge of the disc, in the central plane. Galactic longitude lines run from diametrically opposite points on the galactic equator through the galactic poles. The line marked with an asterisk at each end in the photograph is the galactic longitude line that corresponds to the (flattened) great circle that marks zero degrees and 180 degrees longitude. Zero degrees longitude goes from the center toward the back of the table, and 180 degrees longitude goes from the center toward the front of the table.

You can find a copy of a manual for this model (or a very similar one) at the this link: https://sciencefirst.com/wp-content/uploads/2017/05/030578-M-W100-653-8020-Milky-Way-Galaxy-Model.pdf

 

References:

1) Chaisson, Eric and McMillan, Steve. Astronomy Today (Upper Saddle River, New Jersey: Prentice-Hall, Inc., 1999), pp. 520-28, 548-53.