1. Measuring the distance to Galaxies
Henrietta Leavitt and the Cepheid Variable Star

2. 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?

3. 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

4. Click animation to start

5. 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

6. 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

7. “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

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

9. 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?

10. 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).

11. If the two Cepheid Variables were equally distant

12. A movie of variable stars in a globular cluster

13. 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

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

15. 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