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Unformatted text preview: Chapter 1 Properties of Stars The word astronomy is derived from the Greek roots astron, meaning star, and nomos, meaning law. Although astronomers now study a wide range of objects, from dust grains to superclusters of galaxies, the study of stars, and the laws dictating their behavior, is still a key part of astronomy. A star is best defined as a luminous ball of gas powered by nuclear fusion in its interior. This definition distinguishes stars from smaller objects like planets and brown dwarfs, that are too cool inside for fusion to take place. It also distinguishes stars from stellar remnants like white dwarfs and neutron stars, that were stars once but which no longer host a fusion reactor in their interiors. 1 It was not apparent, in the pre-Copernicus era, that the Sun and the stars in the night sky were members of the same class of objects. Consider Genesis 1:16 And God made two great lights: the greater light to rule the day, and the lesser light to rule the night; he made the stars also. The Sun and Moon, from this viewpoint, are both great lights, bright and large (about thirty arcminutes across). The stars are mere afterthoughts, dim and small (much less than an arcminute across). However, once astronomers were aware of the immense distance to stars other than the Sun, they realized that the stars were all glowing spheres like the Sun. The Sun appears much brighter than the other stars we see simply because it is much closer to Earth than the other stars. 1 Despite their name, neutron stars are not gaseous and not fusion-powered; thus, they are not stars by the strict definition we have adopted. 1 2 CHAPTER 1. PROPERTIES OF STARS 1.1 How Far is a Star? A recurring problem in astronomy is the measurement of distances. When you look up at the night sky, you have no sense of depth. In Figure 1.1, for instance, it is not immediately clear that the fuzzy blob on the left is a galaxy two million light-years away and the fuzzy blob on the right is a comet a few light-minutes away. Simply gawking up at the sky with your Figure 1.1: Left to right the Andromeda galaxy, Comet Hale-Bopp. [Image credit: J. C. Casado (Astronomy Picture of the Day, 1999 March 14)] naked eyes gives you little sense of how far away astronomical objects are. There are a few clues you can pick up for instance, the Moon occults stars, so the stars must lie beyond the Moon but quantitative measurement of distances is difficult. Astronomers have developed many methods of estimating distances. I will only review a few of the more useful techniques. Within the Solar System, the distances between planets can be accurately measured using radar . A brief, powerful burst of radio waves is sent toward a planet, using the dish of a large radio telescope to collimate the radiation. After a time t , a radio echo is detected; the time t , which can be measured with great accuracy, is the round-trip travel time for a photon. The one-way distanceaccuracy, is the round-trip travel time for a photon....
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