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Earth and Space Science

Unit 10

Stars, Galaxies and the Universe

7 topics · 27 practice questions · Free responseNot yet reviewed

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Notes

Stars

  • A light-year is a distance (about 9.5 trillion km). Star color shows temperature: blue is hot, red is cool.
  • The H-R diagram plots luminosity against temperature. The main sequence holds most stars; giants are cool but luminous; white dwarfs are hot but dim.
  • Life cycle: nebula, protostar, main sequence, red giant. Low-mass stars end as white dwarfs; massive stars end in a supernova and become a neutron star or black hole.

Galaxies

  • Galaxies are spiral, elliptical or irregular. The Milky Way is a barred spiral. Gravity (and dark matter) holds them together.

The Big Bang

  • The universe is about 13.8 billion years old and expanding. Hubble's law: $v = H_0 d$.
  • Evidence: redshift, the cosmic microwave background and the ratio of hydrogen to helium (about 75% to 25%).

Topics in this unit

Each topic has its own short notes and practice.

Practice

Which star is likely the coolest?
A star is cool but extremely luminous. On the H-R diagram it is a:
Which object will the Sun become after its red giant stage?
A galaxy is 300 Mpc away. With $H_0 = 70$ km/s per Mpc, its recession speed is:
The cosmic microwave background supports the Big Bang because it is:
Elements heavier than helium were mostly made by:

Free response

Write your answer first, then open the scoring guide.

Free-response question

A star is 8.6 light-years from Earth. Astronomers also measure a distant galaxy 150 Mpc away with a redshift. Use $H_0 = 70$ km/s per Mpc.

(a) How long ago did the light we see from the star leave it?

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8.6 years ago, because one light-year is the distance light travels in a year.

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(b) Calculate the recession speed of the galaxy.

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$v = 70 \times 150 = 10{,}500$ km/s.

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(c) Explain what redshift tells us and name one other piece of evidence for the Big Bang.

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Redshift means the galaxy's light is stretched to longer wavelengths because it is moving away, showing the universe is expanding. Another evidence is the cosmic microwave background (or the hydrogen to helium ratio).

Check off the points you earned:

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