Chapter_6 - Chapter 6 Our Galaxy Our study of stars began...

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Unformatted text preview: Chapter 6 Our Galaxy Our study of stars began with a definition of the word “star”. A sense of symmetry compels us to begin our study of galaxies with a definition of the word “galaxy”. A galaxy is a collection of stars (between a million and a trillion of them, in round numbers), plus gas, dust, and dark matter, held to- gether by gravity. A galaxy is bigger than a star cluster, such as the Pleiades, and smaller than a cluster of galaxies, such as the Virgo cluster. Although there are hundreds of billions of galaxies within the volume accessible to our telescopes, we will start by looking in depth at the galaxy in which we live: the Milky Way galaxy. 6.1 Overview: Morphology of our galaxy On a dark night, far from city lights, you can see a luminous band of light across the sky, forming a great circle on the celestial sphere (Figure 6.1). In English, this band of light is called the Milky Way , because it looks, to the naked eye, like a luminous white fluid. In ancient Greece, it was called the “galaktikos kuklos”, which literally translates as “milky circle”. The Greek word “galaktikos” is the origin of the English word “galaxy”. 1 Although the Milky Way looks as if someone spilled glow-in-the-dark milk across the celestial sphere, when Galileo examined it with his telescope, he found that it is actually composed of a very large number of stars, individually very faint. A hypothesis that explains the existence of the Milky Way is that 1 The Milky Way is tilted by 60 . 2 ◦ relative to the ecliptic, and by 62 . 6 ◦ relative to the celestial equator. 123 124 CHAPTER 6. OUR GALAXY Figure 6.1: The Milky Way seen from Mount Graham, Arizona the Sun is embedded in a thin disk of stars. When we look perpendicular to the disk, we see few stars, and the sky is dark. When we look in the plane of the disk, we see the many stars that make up the Milky Way. This disk of stars is a major component of the galaxy in which we live, which is therefore called the Milky Way galaxy . It is also called the Galaxy (with a capital ‘G’) or, if we’re feeling particularly possessive, our galaxy. The first method used to determine the size and shape of our galaxy was the method of star counts . To demonstrate how star counts work, let’s start with some simplifying assumptions. • All stars have the same absolute magnitude M . This is not true in general, but we can choose to look only at main sequence stars of a particular spectral type. • The number density of stars, n , is constant within our galaxy. • There is no absorption due to dust. (This is perhaps our most dubious assumption.) A star of absolute magnitude M , will have an apparent magnitude m when it is at a distance d = 10 . 2( m- M +5) pc . (6.1) 6.1. OVERVIEW: MORPHOLOGY 125 Every star closer than a distance d will be brighter than m . Thus, the total number of stars brighter than m will be N ( < m ) = 4 π 3 d 3 n = 4 π 3 10 . 6( m- M +5) n , (6.2) or, taking the logarithm,...
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This note was uploaded on 07/17/2008 for the course ASTRO 292 taught by Professor Ryden during the Winter '06 term at Ohio State.

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Chapter_6 - Chapter 6 Our Galaxy Our study of stars began...

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