Mindblown: a blog about philosophy.
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Thinking Ahead
“There are countless suns and countless earths all rotating round their suns in exactly the same way as the planets of our system. We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller and non-luminous. . . . The unnumbered…
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Interstellar Matter around the Sun
Learning Objectives By the end of this section, you will be able to: We want to conclude our discussion of interstellar matter by asking how this material is organized in our immediate neighborhood. As we discussed above, orbiting X-ray observatories have shown that the Galaxy is full of bubbles of hot, X-ray-emitting gas. They also…
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The Life Cycle of Cosmic Material
Learning Objectives By the end of this section, you will be able to: Flows of Interstellar Gas The most important thing to understand about the interstellar medium is that it is not static. Interstellar gas orbits through the Galaxy, and as it does so, it can become more or less dense, hotter and colder, and change its…
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Cosmic Rays
Learning Objectives By the end of this section, you will be able to: In addition to gas and dust, a third class of particles, noteworthy for the high speeds with which they travel, is found in interstellar space. Cosmic rays were discovered in 1911 by an Austrian physicist, Victor Hess, who flew simple instruments aboard…
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Cosmic Dust
Learning Objectives By the end of this section, you will be able to: Figure 20.9 shows a striking example of what is actually a common sight through large telescopes: a dark region on the sky that appears to be nearly empty of stars. For a long time, astronomers debated whether these dark regions were empty “tunnels”…
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Interstellar Gas
Learning Objectives By the end of this section, you will be able to: Interstellar gas, depending on where it is located, can be as cold as a few degrees above absolute zero or as hot as a million degrees or more. We will begin our voyage through the interstellar medium by exploring the different conditions…
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The Interstellar Medium
Learning Objectives By the end of this section, you will be able to: Astronomers refer to all the material between stars as interstellar matter; the entire collection of interstellar matter is called the interstellar medium (ISM). Some interstellar material is concentrated into giant clouds, each of which is known as a nebula (plural “nebulae,” Latin for “clouds”).…
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Thinking Ahead
Where do stars come from? We already know from earlier chapters that stars must die because ultimately they exhaust their nuclear fuel. We might hypothesize that new stars come into existence to replace the ones that die. In order to form new stars, however, we need the raw material to make them. It also turns…
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The H–R Diagram and Cosmic Distances
Learning Objectives By the end of this section, you will be able to: Variable stars are not the only way that we can estimate the luminosity of stars. Another way involves the H–R diagram, which shows that the intrinsic brightness of a star can be estimated if we know its spectral type. Distances from Spectral…
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Variable Stars: One Key to Cosmic Distances
Learning Objectives By the end of this section, you will be able to: Let’s briefly review the key reasons that measuring distances to the stars is such a struggle. As discussed in The Brightness of Stars, our problem is that stars come in a bewildering variety of intrinsic luminosities. (If stars were light bulbs, we’d say…
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