Measuring the distance to Measuring the distance to GalaxiesGalaxies

Henrietta Leavitt and the Cepheid Henrietta Leavitt and the Cepheid Variable StarVariable Star

The Great Distances of SpaceThe Great Distances of Space

Many of the challenges in Many of the challenges in understanding the universe have understanding the universe have involved measuring distancesinvolved measuring distances

How can you measure the distance to How can you measure the distance to objects that are too far away to visit?objects that are too far away to visit?

Inverse Square LawInverse Square Law

The brightness of The brightness of the star changes the star changes according to an according to an inverse square lawinverse square law

When the star is When the star is twice as far away, it twice as far away, it is 1/4 as brightis 1/4 as bright

3 times as far away, 3 times as far away, it will be 1/9 as it will be 1/9 as brightbright

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Distance Distance

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

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Stars of equal brightnessStars of equal brightness

The brightness of stars varies both The brightness of stars varies both because of the luminosity and distancebecause of the luminosity and distance

Luminosity is a way of measuring the Luminosity is a way of measuring the light emitted by a star, the more light emitted by a star, the more luminous the star, the more light it emitsluminous 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

““Harvard Computer” Henrietta LeavittHarvard Computer” Henrietta Leavitt

Some stars vary in luminosity a bit like a Some stars vary in luminosity a bit like a lighthouselighthouse

In 1912, Henrietta Leavitt discovered a In 1912, Henrietta Leavitt discovered a type of variable star for which the more type of variable star for which the more luminous the star, the longer the period of luminous the star, the longer the period of variation in brightnessvariation in brightness

These stars are called Cepheid VariablesThese stars are called Cepheid Variables

Cepheid VariablesCepheid Variables

The varying The varying brightness of 2 brightness of 2 Cepheid Cepheid variable starsvariable stars

The bottom The bottom one (pink) is one (pink) is twice as twice as luminous as luminous as the top onethe top one

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These stars have equal maximum brightnessThese stars have equal maximum brightness Which of these two Cepheid Variables is Which of these two Cepheid Variables is

more luminous?more luminous? Which of these stars is further away?Which of these stars is further away? Can you work out how many times further Can you work out how many times further

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AnswersAnswers

1.Which of these two 1.Which of these two Cepheid Variables Cepheid Variables is more luminous?is more luminous?

2. Which of these 2. Which of these stars is further stars is further away?away?

3. Can you work out 3. Can you work out how many times how many times further away?further away?

The star on the right has a The star on the right has a longer period so must be longer period so must be more luminousmore luminous

The star on the right is more The star on the right is more luminous but appears only as luminous but appears only as bright as the one on the left. It bright as the one on the left. It must be further awaymust be further away

The period is doubled so it is The period is doubled so it is twice as luminous but its twice as luminous but its brightness is halved. It is less brightness is halved. It is less than twice as far away. (It will than twice as far away. (It will be 1.4 times as far away).be 1.4 times as far away).

If the two Cepheid Variables If the two Cepheid Variables were equally distantwere equally distant

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A movie of variable stars in a A movie of variable stars in a globular clusterglobular cluster

Finding the Distance to Finding the Distance to CepheidsCepheids

The distance to nearby Cepheid The distance to nearby Cepheid variables can be determined by parallax variables can be determined by parallax (a method you will learn in this course)(a method you will learn in this course)

The inverse square law and the period-The inverse square law and the period-luminosity relationship of Henrietta luminosity relationship of Henrietta Leavitt enables the distance of all Leavitt enables the distance of all observable Cepheid variables to be observable Cepheid variables to be determineddetermined

Measuring the distance to Measuring the distance to galaxiesgalaxies

Cepheids are found in other galaxiesCepheids are found in other galaxies Astronomers can therefore measure the Astronomers can therefore measure the

distance to other galaxies.distance to other galaxies.

The Discovery of GalaxiesThe Discovery of Galaxies

In the 1920s astronomers In the 1920s astronomers questioned whether objects questioned whether objects like this were small nebulae like this were small nebulae within our galaxy, or within our galaxy, or enormous galaxies great enormous galaxies great distances beyond our distances beyond our galaxy. galaxy.

Cepheid variables in the Cepheid variables in the objects showed that they objects showed that they were huge distances from were huge distances from our galaxyour galaxy