Astronomy Flashcards
What is a comet?
- Lumps of ice and dust that orbit the sun
- Usually highly elliptical orbits
What is a geocentric model?
- The theory that suggested the Sun, moon, planets and stars all orbited the Earth in perfect circles
What is the heliocentric model?
- Said the Sun was the centre of the solar system
- Evidence was found when Galileo found that the moons of Jupiter did not orbit the Earth, showing that there were things that did not orbit earth, disproving the geocentric model
What happens the closer a planet gets to orbiting a star?
- In circular motion, the stronger the force, the larger the instantaneous velocity needed to balance it
- So closer to the star (where the gravitational field strength is stronger) the planet will need to move faster to stay in a stable orbit.
- Faster moving objects will move in a stable orbit with a smaller radius than slower moving ones
Life cycle of a star (stages that both supergiants and giants go through)
1) Nebula: Stars initially form from a cloud of dust and gas called a nebula
2) Protostar: The force of gravity pulls the dust and gas together to form a protostar.
- The temp. rises as the protostar gets denser. - - When the temp. gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei, giving out huge amounts of energy keeping core hot.
3) Main Sequence Star: The star enters a long stable period. During this period, the outward pressure caused by thermal expansion balances the force of gravity pulling everything inwards.
- Lasts several billion years
4) Red Giant/Supergiant: Hydrogen core begins to run out and force of gravity is greater than thermal expansion.
- Core of the star is compressed, until it is dense and hot enough that energy released (and pressure created) makes the outer layers of the star expand.
- The surface cools and it becomes a giant/supergiant
End life cycle specific to stars about the size of the Sun
1) White dwarf: a small-to-medium-sized star like the Sun then becomes unustable and ejects its outer layers.
- This leaves behind a hot, dense solid core - a white dwarf
End life cycle specific to stars much bigger than the sun
1) Supernova: Instead of decreasing in size, big stars start to glow brightly again as they undergo nuclear fusion to make heavier elements.
- They expand an contract several times, shifting balance between gravity and thermal expansion. Eventually, they explode in a supernova
2) Neutron Star/Black hole: The exploding supernova throws the outer layers of dust and gas into space, leaving a very dense core called a neutron star. I
- If the star is massive enough, it will collapse and become a black hole - a super dense region of space that not even light can escape from
What is the Doppler effect?
- If a wave source is moving away from an observer, the observed wave frequency will be slightly lower and the wavelength slightly longer than the wave orginially emitted by the source
For example: - The sound waves from a stationary car are equally spaced
- But for a moving car, the wavlengths are longer behind the car are longer than the ones at the front
- This is why the noise sounds lower pitched when a racing car or ambulance is moving away from you
What does red shift suggest about the universe?
- Measurements of red shift of distant galaxies suggest they are all moving away from us very quickly in every direction
- Measurements also show most galaxies seem to be moving away from each other
How do you measure red shift?
- You can measure red shift by comparing absorption spectra:
1) Different elements absorb different frequencies (or wavelengths) of light
2) As a result, each element produces an absorption spectrum - a specific pattern of dark lines at the frequencies it absorbs in the visible part of the EM spectrum
3) By taking a particular pattern and looking at how far each absorption line (caused by absorption in a distant galaxy) has moved compared to light produced in a lab, you can measure the change in wavelength and calculate red-shift
4) More distant galaxies have greater red shifts than nearer ones - they show a bigger observed increase in wavelength
5) This means that more distant galaxies are moving away faster than nearer ones. This provides evidence that the whole universe is expanding
What are the theories for the creation of the universe?
Steady State theory:
- says that the universe has always existed as it is now, and it always will do
- based on the idea that the Universe appears pretty much the same everywhere
- as the universe expands, new matter is constantly created
- there is no beginning or end to the Universe
Big Bang theory:
- initially all matter occupied a very small space which was very dense and hot
- then it ‘exploded’; space started expanding, and the expansion is still going on.
- theory gives a finite age of the Universe (13.8 billion years)
What other evidence proves the big bang theory?
- Red shift can explain both the Steady State and Big Bang theory as both suggest the universe is expanding
- However, scientists have also detected low frequency EM radiation from all parts of the universe known as CMB (cosmic microwave background) radiation
- the presence of CMB shows that the Universe was once much hotter and denser
- suggests the Universe had a beginning, so CMB radiation only supports Big Bang model
How do you improve the quality of an image through a telescope?
- Increase the aperture of the telescope (diameter of its objective lens)
- Use a higher quality lens
Where should a telescope be placed to get the clearest view of space from Earth?
- a telescope should be on top of a mountain where there is less atmosphere to absorb the light coming from space
- should be in a dark place where there is less light pollution, as it can make it hard to pick out dim objects
- the best place, however, is in space
Why do we have different telescopes to detect different EM waves?
- much of the universe isn’t detectable using visible light, so other types of EM telescopes are needed to observe them
- help us to ‘see’ parts of the universe we couldn’t see before and learn about the structure of the Universe
- X-ray telescopes are used to ‘see’ violent, high-temperature events in space, like exploding stars
- Radio telescopes were responisble for the discovery of CMB which helped scientists to learn more about the origin of the universe
