Chapter 15: The Milky Way Galaxy WHAT DO YOU THINK? How many stars does the Milky Way Galaxy contain? Where is our Solar System located in the Milky Way Galaxy? Is the Sun moving through the Milky Way Galaxy and, if so, about how fast? You will discover…
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Edge-on view showing the Milky Way’s disk
Two possible distributions of the spiral arms of our Galaxy. Our Galaxy has at least four major spiral arms and several shorter arm segments.
Built by the Earl of Rosse in 1845, a 1.8-m-diameter telescope.
Lord Rosse’s sketch of the spiral structure of the galaxy M51
A modern photograph of M51 (also called NGC 5194)
Wide-angle photograph spanning half the Milky Way, as seen from the equatorial latitudes
More than a million stars in the disk of our Galaxy fill this view, which covers a relatively clear window just 4º south of the galactic nucleus in Sagittarius.
Radio waves from various gas clouds exhibit slightly different Doppler shifts, permitting astronomers to sort out the gas clouds and map the Galaxy.
This map (left), based on radio telescope surveys of 21-cm radiation, shows the distribution of hydrogen gas in the Milky Way.
Taken by the COBE satellite in 1997
Radio image taken at the VLA
Infrared image showing the motion of six stars in the vicinity of the unseen massive object at the position of the radio source Sagittarius A* ().
NGC 4144, very similar to the Milky Way
Canis Major dwarf elliptical galaxy
Stars closer to the Galaxy’s center than the Sun are overtaking the solar system, while stars farther from the center are lagging behind us.
Gravitational fields cause light to change direction.
The light curve of the gravitational microlensing of light from a star in the Galaxy’s nuclear bulge by an intervening object.
dark matter (missing mass)
disk (of a galaxy)
halo (of a galaxy)
Milky Way Galaxy
nebula (plural nebulae)
rotation curve (of a galaxy)
spin (of an electron or
21-cm radio radiation