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Mindblown: a blog about philosophy.

  • The Expanding Universe

    Learning Objectives By the end of this section, you will be able to: We now come to one of the most important discoveries ever made in astronomy—the fact that the universe is expanding. Before we describe how the discovery was made, we should point out that the first steps in the study of galaxies came…

  • The Extragalactic Distance Scale

    Learning Objectives By the end of this section, you will be able to: To determine many of the properties of a galaxy, such as its luminosity or size, we must first know how far away it is. If we know the distance to a galaxy, we can convert how bright the galaxy appears to us…

  • Properties of Galaxies

    Learning Objectives By the end of this section, you will be able to: The technique for deriving the masses of galaxies is basically the same as that used to estimate the mass of the Sun, the stars, and our own Galaxy. We measure how fast objects in the outer regions of the galaxy are orbiting the center,…

  • Types of Galaxies

    Learning Objectives By the end of this section, you will be able to: Having established the existence of other galaxies, Hubble and others began to observe them more closely—noting their shapes, their contents, and as many other properties as they could measure. This was a daunting task in the 1920s when obtaining a single photograph or…

  • The Discovery of Galaxies

    Learning Objectives By the end of this section, you will be able to: Growing up at a time when the Hubble Space Telescope orbits above our heads and giant telescopes are springing up on the great mountaintops of the world, you may be surprised to learn that we were not sure about the existence of…

  • Thinking Ahead

    In the last chapter, we explored our own Galaxy. But is it the only one? If there are others, are they like the Milky Way? How far away are they? Can we see them? As we shall learn, some galaxies turn out to be so far away that it has taken billions of years for…

  • The Formation of the Galaxy

    Learning Objectives By the end of this section, you will be able to: Information about stellar populations holds vital clues to how our Galaxy was built up over time. The flattened disk shape of the Galaxy suggests that it formed through a process similar to the one that leads to the formation of a protostar…

  • Stellar Populations in the Galaxy

    Learning Objectives By the end of this section, you will be able to: In the first section of his chapter, we described the thin disk, thick disk, and stellar halo. Look back at Table 25.1 and note some of the patterns. Young stars lie in the thin disk, are rich in metals, and orbit the Galaxy’s center…

  • The Center of the Galaxy

    Learning Objectives By the end of this section, you will be able to: At the beginning of this chapter, we hinted that the core of our Galaxy contains a large concentration of mass. In fact, we now have evidence that the very center contains a black hole with a mass equivalent to 4.6 million Suns and that…

  • The Mass of the Galaxy

    Learning Objectives By the end of this section, you will be able to: When we described the sections of the Milky Way, we said that the stars are now known to be surrounded by a much larger halo of invisible matter. Let’s see how this surprising discovery was made. Kepler Helps Weigh the Galaxy The…

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