1. Hubble’s Law

2. After the “Great Debate”:
After resolving the ‘Curtis-Shapley’ debate. Hubble measured the distance to Cepheid variable stars in the Andromeda Galaxy:
2.5 million light years!
Hmm...Much further than I suspected! I also noticed something else…..

3. Moving Galaxies
Hubble’s observations showed that the light from distant galaxies was ‘red-shifted’. This was due to the ‘Doppler Effect’.
Light from distant galaxies
Light from the Sun
By Golly, just look at that! The dark lines in the spectrum on the right hand side are ‘red shifted’.

4. Recession Velocity
Hubble also noticed that the further away the galaxy, the greater the red-shift. The greater the red-shift the greater the speed of recession.
My startling conclusion was that the further away the galaxy, the faster is was moving. Amazing! It also led me to develop what later became known as Hubble’s Law. After Me!

5. Hubble’s Law
The red-shift of distant objects is easy to measure therefore the recession velocity can also be easily calculated.
We can use the recession velocity to calculate the distance to a galaxy (or the velocity if the distance is known). This is known as Hubble’s Law.
Speed of recession (km/s) = Hubble Constant (s-1) x distance (km)
The value of the Hubble Constant is 2x10-18 s-1 (this value is still being researched).
E.g. A galaxy is 1.5 x 1020 km away. How fast is it moving?
Recession Velocity = Hubble Constant x 1.5 x 1020
= 2x x 1.5 x = 300 km/s

6. Telescopes

7. Telescopes
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation (such as visible light).
Within a few decades, the reflecting telescope was invented, which used mirrors.
In the 20th century many new types of telescopes
were invented, including radio telescopes in the 1930s
and infrared telescopes in the 1960.

8. Refracting telescope .
                                                                                  .

9. Refracting telescope
A refracting telescope works bending light through a lens so that it forms an image. There are a few problems with refracting telescopes:
some of the light reflects off the lens so the image is very faint
large lenses are needed to improve the magnification – this can be difficult to do perfectly.

10. Reflecting telescope

11. A reflecting mirror.
How it works……
In a reflecting telescope the image is formed by reflection from a curved mirror.
It is then magnified by a secondary mirror.

12. The first telescopes
The first known practical telescopes were invented in the Netherlands at the beginning of the 17th century, using glass lenses. They found use in terrestrial applications and astronomy.

13. Optical telescopes
Optical telescopes observe visible light from space. Small ones allow amateurs to view the night sky relatively cheaply but there are very large optical telescopes sited around the world for professional astronomers to use.
Optical telescopes on the ground can only be used at night and they cannot be used if the weather is poor or cloudy.

14. Radio telescopes
Radio telescopes detect radio waves coming from space. Although they are usually very large and expensive, these telescopes have an advantage over optical telescopes.
They can be used in bad weather because the radio waves are not blocked by clouds as they pass through the atmosphere. Radio telescopes can also be used in the daytime as well as at night.

15. Earth Orbiting telescopes
Objects in the universe emit other electromagnetic radiation such as infrared, X-rays and gamma rays.
These are all blocked by the Earth's atmosphere, but can be detected by telescopes placed in orbit round the Earth.

16. Space telescopes
Telescopes in space can observe the whole sky and they can operate both night and day. However, they are difficult and expensive to launch and maintain. If anything goes wrong, only astronauts can fix them.