The pale band across a clear country sky, the Milky Way, is our own galaxy seen edge-on from inside. A galaxy is a vast system of hundreds of billions of stars, gas and dust held together by gravity, and by shape galaxies are sorted into spirals, ellipticals and irregulars.
Almost all distant galaxies are moving away from us. This is not because they fly through space but because the space between galaxies is itself stretching.
Increasing the time in the second diagram, the other galaxies move away whichever galaxy you watch from, and the farther a galaxy is, the longer its arrow and the faster it recedes.
The third graph shows that relationship as a straight line of distance against speed, Hubble's law, and the evidence for it is redshift, the stretching of the wavelength of light.
What Is a Galaxy?
🌌 A Vast City of Stars
①A galaxy is a huge system of hundreds of billions of stars, gas and dust bound by gravity
②The Milky Way, home of the Solar System, is a barred spiral galaxy with about 200 billion stars
③Galaxies are classified by shape into spirals, ellipticals and irregulars
④The observable universe contains more than hundreds of billions of galaxies
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The shapes of spiral, elliptical and irregular galaxies
Spiral galaxy
📝 Galaxy Shapes
①Spiral: arms wind around a bright center; the Milky Way and Andromeda
②Elliptical: shaped like a ball or a rugby ball, mostly old stars, with almost no new stars forming
③Irregular: no definite shape; the Large and Small Magellanic Clouds
The Universe Is Expanding
🎈 Dots on an Inflating Balloon
①Draw dots on a balloon and blow it up: every distance between the dots grows
②From any dot, all the others move away; the universe has no fixed center
③In the universe, the space between galaxies itself is stretching in the same way
④Galaxies that started farther away move away by more in the same time, so they appear to recede faster
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The distances between galaxies growing over time, with farther galaxies receding faster
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Hubble's Law and Redshift
🔴 What Redshift Tells Us
①Light from an object moving away is stretched to longer, redder wavelengths (redshift)
②Hubble found that the farther a galaxy is, the greater its redshift
③So the farther a galaxy is, the faster it is moving away
④This is evidence that the universe is expanding
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Graph of Hubble's law: a galaxy's recession speed is proportional to its distance
The more the light is stretched, the faster the galaxy is receding
Big Bang theory
About 13.8 billion years ago the universe began expanding from a very hot, dense state
Running the expansion backward, the universe was once far smaller and hotter
🎯 Common Test Points
①A galaxy is a vast system of stars, classified as spiral, elliptical or irregular
②The Milky Way is a spiral galaxy, and the Solar System sits in a spiral arm about 26,000 light-years from the center
③Redshift is evidence that galaxies are moving away
④Hubble's law: v = H₀d; the farther a galaxy, the faster it recedes
⑤Big Bang: the universe began expanding about 13.8 billion years ago
Examples and Unit Review
Example 1
Light from a distant galaxy is observed to have a longer wavelength than normal (redshift). What does this say about how the galaxy is moving?
1
Light from an object moving away is stretched to longer, redder wavelengths.
2
So a galaxy showing redshift is moving away from us.
▸ The galaxy is moving away from us
An object approaching us shows the opposite, a blueshift with shorter wavelengths; the nearby Andromeda galaxy is an example.
Example 2
What relationship did Hubble find between a galaxy's distance and its recession speed? How fast do more distant galaxies recede?
1
By Hubble's law, the recession speed v is proportional to the distance d (v = H₀d).
2
So the more distant a galaxy is, the faster it moves away.
▸ Recession speed is proportional to distance, so more distant galaxies recede faster
A galaxy 100 Mpc away recedes at about 70 × 100 = 7,000 km/s.
Unit Test
Which statement about galaxies and the expanding universe is correct?
①Redshift is evidence that a galaxy is moving toward us
②By Hubble's law, the farther a galaxy is, the more slowly it recedes
③The farther away a galaxy is, the faster it moves away
④Our galaxy is an elliptical galaxy
⑤The universe stays a constant size and does not change
▸ ③ The farther away a galaxy is, the faster it moves away
1
Redshift is evidence that a galaxy is receding (① wrong), and by Hubble's law a farther galaxy recedes faster (② wrong).
2
Our galaxy is a spiral galaxy (④ wrong), and the universe is expanding (⑤ wrong); by Hubble's law v = H₀d, a farther galaxy recedes faster (③ correct).