Star Planets Manual

Author: (British) Ian Morrison, Margaret Preston
Publisher:
Publish Date: 2006-09-01
Features: The interstellar medium (ISM) is composed of gas and dust between stars and nebulae. Most of the interstellar medium is invisible to the naked eye, but in some places, you can see "emission nebulae," which emit light themselves; or "dark nebulae," which can be seen against a background of stars, revealing these dust clouds. The spectacular emission nebula is the Great Orion Nebula (see Table A, M42), a birthplace of stars where young, hot stars emit ultraviolet light, activating hydrogen in the nebula's center to form a quadrilateral. A prominent example of dark nebulae is the Coal Bag Nebula, located near the Southern Cross. Some nebulae glow by reflecting the light of nearby bright stars and are called reflection nebulae. The star Rigel near the Pleiades is a typical reflection nebula. This star appears blue because the hot star emits blue light. Additionally, these nebulae more readily reflect blue light than red light (which is also why the sky appears blue).
The size and structure of the Milky Way The size of the Milky Way was measured by Harlow Shapley, who used Cepheid variables to determine the distances of 100 globular clusters within the galaxy. He found that these clusters were distributed spherically, and logically, their center should also be the center of the Milky Way, leading him to estimate that the Sun is 30,000 light-years away from the galaxy and that the Milky Way's diameter is approximately 100,000 light-years. The ubiquitous element hydrogen in the universe provides a method for studying the spatial structure of the Milky Way. Neutral hydrogen (H0) emits radio waves with specific wavelengths. Observations of these radio waves show that the gas in the central region of the Milky Way is densely concentrated in nebulae. Using the Doppler effect, the velocity of the nebulae can be determined, and their positions can then be identified. The Milky Way's nebulae are spiral-shaped, with spiral arms located within the galactic disk. Now, it is known that the Sun is 28,000 light-years away from the galactic center. Using spectrographs, the motion of the Sun relative to the globular clusters (whose positions are relatively fixed) can be observed, and it can be calculated that the time it takes for the Sun to complete one orbit around the galactic center at a speed of approximately 200 kilometers per second is equivalent to 250 million years on Earth. The central part of the Milky Way rotates like a solid body, with rotational speeds increasing progressively outward. Further research has shown that as stellar rotation slows, their lifespans increase. By measuring the rotational speeds of stars in other galaxies relative to the galactic center, a "galactic rotation curve" can be plotted.
The way stars orbit the galactic center is puzzling and attributed to two important questions. One question relates to the spiral structure of the Milky Way. The Sun has already completed about 20 orbits around the galactic center, yet the spiral arms are invisible. The answer to this question can be seen in visual diagrams. The spiral arms of other galaxies are very prominent because they are composed of many bright blue stars—each of which is 50,000 times brighter than the Sun. Since these stars have short lifespans, the spiral structure observed today may not be the same as what was observed in the past. Swedish astronomer Bertil Lindblad first proposed that the existence of spiral arms is temporary and that they are formed by spiral density waves surrounding