Physics (P73)

Physics (P73)

1. Parallax is the apparent motion of an object against a background. The motion of the observer causes parallax motion.

2. Parallax angle of a star is half the angle moved against the background of distant stars in 6 months.

   • An object that's further away from Earth will have a smaller parallax angle.

3. Parallax is measured in a interstellar distance using parsecs (the distance to a star with a parallax angle of one second of an arc.

4. Megaparsecs are used to measure intergalactic distances even though these objects are so far away tat the parallax angle is too small to measure.

5. Astronmers can measure distanced by observing how bright stars are.

   • The closer the star, the brighter it will appear. However, many stars don't have the same luminosity.
     • A stars luminosity depends on its: size and temperature.However, observed intensity depends on: luminosity and distance
       • A large or hot star will emit more light than a small or cool star. It may appear brighter even if its further away.
       • A star with low luminosity may appear dull even if its close to earth. A high luminosity star may appear brighter even if its further away.

6. Cepheid Variable doesn't have constant luminosity. It pulses and its luminosity depends on the period of pulses. The period= the frequency of pulses.

   • Its changing frequency can be used to work out distance to Cepheid Variable stars.
   • The distance of a star can be worked out by: how bright the star is and how bright it appears.

7. Curtis-Shapely Debate 1920

   • Telescopes revealed the Milky Way contained stars including the Sun. There were also fuzzy objects in the night sky called nebulae.
     • Curtis believed that the universe contained many galaxies and fuzzy objects were distant galaxies. The universe was 300,000 light years in diameter.
     • Shapley believed that the universe contained one big galaxy and the nebulae were nearby gas clouds within the milky way. The universe was 30,000 light years in diameter.
   • Edwin Hubble Mid 1920s
     • He observed a Cepheid variable star in one nebula and found it was furthest star in Milky Way. It proved evidence that Curtis was right because the nebula was in a separate galaxy; there were many galaxies.

8. Hubble constant

   • From Edwin Hubble's observations from Cepheid variables, he found the universe was expanding. The further away a star is, the faster its moving away.
   • Cepheid variables in distant galaxies have been used to accurately measure the Hubble constant because we know how far they are.
   • Speed of recession: Hubble constant x distance

9. The expanding universe

   • Red shift shows that the further galaxy is moving away, the faster it is expanding.
   • The Big Bang occurred 14 million years ago.
   • Extrasolar planets orbit stars and astronomers have found hundreds of planets with nearby stars. The first ones were large gas planets and smaller planets (similar size to Earth) have been found.