20. Intrepret the Hertzsprung-Russell diagram:
The diagram shows the various stages of stellar evolution. By far the most prominent feature is the main sequence stars, which runs from the upper left (hot, luminous stars) to the bottom right (cool, faint stars) of the diagram. Our Sun is a main sequence star with average size, medium temperature and luminosity. Similar stars (like our Sun) spend about 90% of their lives burning hydrogen into helium in their cores.
The giant and supergiant stars lie above the main sequence, and white dwarfs are found below it. Stars enter this evolutionary stage once they have exhausted the hydrogen fuel in their cores and have started to burn helium and other heavier elements.
Sun
White dwarf stars are the final evolutionary stage of low to intermediate mass stars, and are found in the bottom left of the HR diagram. These stars are very hot but have low luminosities due to their small size. 
 21. List some tools used for Space Exploration.
Satellites, Space probes, Space Shuttle, Hubble Space Telescope (HST), Other spacecraft like the Mars Rover, International Space Station (ISS), Rocket and Communication Tools.
NB: Space probes are spacecraft sending to the space for a purpose. Satellites are spacecraft that orbiting the Earth for many purposes.
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