Resource: Spin a Spiral Galaxy

Scientists look for order and patterns by classifying or systematically grouping organisms and objects according to their characteristics. From the time Edwin Hubble first established that other galaxies exist outside our own Milky Way, and created the galaxy classification system, astronomers have classified galaxies based on their general appearance.

A spiral galaxy, viewed from the side, may appear as a long, thin strip, interrupted only by a bright central bulge midway across. Viewed face-on, spiral galaxies have two easily discernible components. A spherical component contains the bright central (or nuclear) bulge and a halo that contains clusters of old stars, but is largely devoid of gas and dust. A disk component contains most of the galaxy's gas, dust, and stars. The stars move around the bulge in a nearly circular orbit within spiral arms that emerge directly from the central bulge or from a distinct bar that runs through the bulge. These spiral arms contain many fully formed stars but are also the nurseries in which new stars are formed. New star formation results from the compression of gases and dust and yields the hottest and brightest stars. Hot stars emit most of their light at short wavelengths -- in the blue and ultraviolet part of the spectrum -- which explains the bluish tint of the arms.

The Milky Way is a spiral galaxy. Our Sun is positioned about two-thirds of the way from the center to the edge. Gas and dust prevent observations inward or outward along the plane of the disk using visible light, so it is difficult to know its precise shape, whether it has two or more spiral arms, or what lies at its center. Radio and infrared telescopes provide some understanding because they can detect gases, objects, and heat using wavelengths outside the visible part of the spectrum.

More common than spirals-- though less easily detected -- are the elliptical galaxies. These galaxies contain no visible gas and dust and lack the hot, bright stars that indicate new star formation. The presence of older, giant stars may give these galaxies a reddish tint. They are classified according to numerical index range of E0 (round) to E7 (highly elliptical). Stars in ellipticals lack the well-defined rotation axis of spirals, although their orbital paths are not entirely random.

Irregular galaxies, as their name suggests, have no specific form and come in two types. Type I irregulars contain many young stars and, much like spirals, contain bright nebulae (clouds of gas and dust). Type II irregulars are known as interacting or disrupting galaxies. They contain a large amount of gas and dust (i.e., star-forming potential) and result from the merger, collision, or near-miss of two or more galaxies.

To view an animation of a spinning black hole located inside a spiral galaxy, check out Monster Black Hole in Galaxy M84.

To learn more about Hubble's discovery that galaxies existed outside of our own, check out Hubble's Expanding Universe.

To learn more about the size of the universe, check out How Big Is Our Universe?.

To learn more about what different types of telescopes can "see" in space, check out Astronomical Images in Different Wavelengths.

• What is a galaxy?
• What is the shape of a spiral galaxy?
• What other shapes do galaxies form? Do we know why they come in different shapes?
• What do the white and black areas in these images represent?
• Why does the image of the Milky Way galaxy have to be a composite of several images?