01:750:109 Chapter Notes - Chapter 14.1: Exotic Matter, Universe Today, Galaxy Rotation Curve
Evidence for Dark Matter
What is the evidence for dark matter?
Dark Matter in the Milky Way
●Natural to assume that galaxies and galaxy clusters are held together by combined gravity of
their stars and gas
●BUT they have too little mass to provide needed gravity
●Because we do not observe light of any wavelength from this mass, we call it dark matter
●Evidence comes from our galaxy and from others
●We can use the Sun's orbital velocity and its distance from the galactic center to determine the
mass lying within the Sun's orbit
●We can use the orbital motion of any other star or gas cloud to measure the mass of the galaxy
within that star or gas cloud's orbit
●Bc interstellar dust obscures our view of stars throughout most of our galaxy's disk. Most
measurements of motion in the Milky Way are based on observations of atomic hydrogen gas
clouds, which emit radio waves that penetrate interstellar dust
●Orbital speeds in our solar system decrease w distance from the Sun
○Bc virtually all the mass of the solar system is concentrated in the Sun
○The gravitational force holding a planet in its orbit decreases w distance rom Sun and
smaller force means a lower orbital speed
○Orbital speeds mist drop similarly w distance in any other astronomical system that has
its mass concentrated at its center
●Rotation curve for the Milky Way is nearly level as distance increases from center
○Speed remains large at large distances and not small
○Means most of the Milky Way's mass is not concentrated at its center
Dark Matter in Other Galaxies
●Can determine the amount of dark matter in a galaxy by comparing the galaxy's mass to its
luminosity
●For spiral galaxies:
○Detect atomic hydrogen gas clouds much farther from the galactic center than stars, so
most of our data comes from using Doppler Shifts to determine how fast the clouds are
moving away or towards us
○Then use orbital speeds and distances to get rotation curve
○Dark matter common in spiral galaxies
●For elliptical galaxies:
○Contain very little atomic hydrogen gas so measure masses of elliptical galaxies by
observing the motion of their stars
Document Summary
Natural to assume that galaxies and galaxy clusters are held together by combined gravity of their stars and gas. But they have too little mass to provide needed gravity. Because we do not observe light of any wavelength from this mass, we call it dark matter. Evidence comes from our galaxy and from others. We can use the sun"s orbital velocity and its distance from the galactic center to determine the mass lying within the sun"s orbit. We can use the orbital motion of any other star or gas cloud to measure the mass of the galaxy within that star or gas cloud"s orbit. Bc interstellar dust obscures our view of stars throughout most of our galaxy"s disk. Most measurements of motion in the milky way are based on observations of atomic hydrogen gas clouds, which emit radio waves that penetrate interstellar dust. Orbital speeds in our solar system decrease w distance from the sun.
