Measuring the distance to Galaxies

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Measuring the distance to Galaxies

• Henrietta Leavitt and the Cepheid Variable Star

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The Great Distances of Space

• Many of the challenges in understanding the
  universe have involved measuring distances
• How can you measure the distance to objects that
  are too far away to visit?

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Inverse Square Law

• The brightness of the star changes according to
  an inverse square law
• When the star is twice as far away, it is 1/4 as
  bright
• 3 times as far away, it will be 1/9 as bright

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Click animation to start
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Distance

• If you know the amount of light that a star
  emits, you can use the inverse square law to work
  out its distance from us by reading off the graph

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Stars of equal brightness

• The brightness of stars varies both because of
  the luminosity and distance
• Luminosity is a way of measuring the light
  emitted by a star, the more luminous the star,
  the more light it emits

Are the three bright stars equally distant - we
cannot tell from the photo unless we know that
they are equally luminous
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Harvard Computer Henrietta Leavitt

• Some stars vary in luminosity a bit like a
  lighthouse
• In 1912, Henrietta Leavitt discovered a type of
  variable star for which the more luminous the
  star, the longer the period of variation in
  brightness
• These stars are called Cepheid Variables

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Cepheid Variables

• The varying brightness of 2 Cepheid variable
  stars
• The bottom one (pink) is twice as luminous as the
  top one

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• These stars have equal maximum brightness
• Which of these two Cepheid Variables is more
  luminous?
• Which of these stars is further away?
• Can you work out how many times further away?

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Answers

• 1.Which of these two Cepheid Variables is more
  luminous?
• 2. Which of these stars is further away?
• 3. Can you work out how many times further away?

• The star on the right has a longer period so must
  be more luminous
• The star on the right is more luminous but
  appears only as bright as the one on the left. It
  must be further away
• The period is doubled so it is twice as luminous
  but its brightness is halved. It is less than
  twice as far away. (It will be 1.4 times as far
  away).

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If the two Cepheid Variables were equally distant
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A movie of variable stars in a globular cluster
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Finding the Distance to Cepheids

• The distance to nearby Cepheid variables can be
  determined by parallax (a method you will learn
  in this course)
• The inverse square law and the period-luminosity
  relationship of Henrietta Leavitt enables the
  distance of all observable Cepheid variables to
  be determined

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Measuring the distance to galaxies

• Cepheids are found in other galaxies
• Astronomers can therefore measure the distance to
  other galaxies.

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The Discovery of Galaxies

• In the 1920s astronomers questioned whether
  objects like this were small nebulae within our
  galaxy, or enormous galaxies great distances
  beyond our galaxy.
• Cepheid variables in the objects showed that they
  were huge distances from our galaxy