8. Astrophysics Flashcards
8.5 Describe the differences in the orbits of comets, moons and planets
- Comets have highly elliptical orbits, with the Sun at one focus. When they come in close to the Sun they speed up, due to the larger gravitational force on them. They also develop bright tails that point AWAY from the centre of the Sun. These are caused by tiny ice crystals that melt and break off from the comet and reflect the bright light of the Sun.
- Moons have almost circular orbits
- Planets have elliptical orbits, however are not as elliptical as those of comets
8.6 Use the relationship between orbital speed, orbital radius and time period:
Orbital radius = 2 x π x radius / time period
v = 2πr/t
8.7 know that a star’s colour is related to its surface temperature
Stars are classified into 7 groups according to their colours (which is due to their surface temperature); O,B,A,F,G,K,M
O are hottest (> 33 000 K and blue), M are coolest (2000 – 3700K and red)
8.09 describe the evolution of stars of similar mass to the Sun through the following stages: nebula, star (main sequence), red giant, white dwarf
- nebula: stars form from large clouds of dust and gas particles (nebulae) that are drawn together by gravitational forces over millions of years. As the particles get closer the temperature and pressure becomes so large that nuclear fusion of hydrogen nuclei to form helium nuclei occurs. This releases enormous amounts of energy in the form of heat and light.
- star (main sequence): Fusion produces forces that make the star expand outwards, but gravitational force is always pulling the particles within the star inwards. When these two opposing forces become balanced a star is stable (in equilibrium) and called a main sequence star. It should stay this way for millions of years, at a constant size and temperature.
- red giant: Eventually hydrogen fusion stops as the star runs out of fuel. At this point, the gravitational force pulling the star in is now bigger than the outward fusion force, which causes the star to collapse inwards. This causes it to heat up to even higher temperatures so that fusion of helium nuclei begins. The increased power output causes the star to expand greatly. The surface area is so large that it is cooler than before, so its colour changes to red and the star is called a red giant.
- white dwarf: Eventually fusion stops when the star runs out of helium nuclei and the gravitational force causes the star to collapse inwards and compress again. This heats it up, so it changes colour to emit white light. The star is squashed so greatly by the gravitational force to become a small and very dense white dwarf. (They are so dense that a teaspoon full would weigh more than a cruise liner). A white dwarf eventually cools down and change colour as it does so, eventually becoming black.
What is a protostar
A collapsing cloud of dust and gas
8.10 describe the evolution of stars with a mass larger than the Sun
- Red supergiant: After the stable period, a massive star expands into a red supergiant. (It produces all the heavier elements up to iron during nuclear fusion).
- Supernova: When it finally runs out of nuclei to fuse it collapses due to the gravitational force, and then explodes – an exploding star is called a supernova. Temperature during a supernova are hot enough to make metals heavier than iron, but only lasts for a SHORT TIME PERIOD. Heavy elements are distributed throughout the universe
- Neutron Star: A dense core remains – called a neutron star, because it is made entirely from neutrons densely packed together.
- Black hole: If its mass is large enough it can compress further to become a black hole. (Their gravity is so strong that not even light can escape!)
8.11P understand how the brightness of a star at a standard distance can be represented using absolute magnitude
Apparent magnitude: how bright a star appears from a particular point in space
Absolute magnitude: the apparent magnitude if the star was viewed from exactly 10 parsecs away (32.6 light years)
What does the Hertzsprung–Russell diagram (HR diagram) show?
This diagram shows the relationship between a star’s luminosity (its brightness or power output) and its surface temperature. A star moves to different positions in the diagram during its life, as its internal structure and temperature change.
What was the Big Bang Theory
A theory that states that the universe expanded outwards from a single point
Evidence 1 for Big Bang Theory
● Red shift
- The red shift of light from galaxies shows that they are all moving AWAY from us and that those which are the furthest away are moving the fastest, suggesting that the universe was formed from an explosion at a single point, evidence for the Big Bang. This is the principle of the Doppler effect.
- If we examine the light spectra for distant galaxies we can see that the wavelengths of the light have become longer. We call this stretching of the waves ‘red-shift’. It tells us that the galaxies producing the light are moving away from us. The further away a galaxy is, the greater its red-shift, so it is moving even faster.
Evidence 2 for the Big Bang Theory
● Cosmic microwave background radiation (CMBR)
- Microwave radiation can be detected EVERYWHERE in the Universe. These are the stretched remains of high energy gamma radiation that would have been produced in the explosion that created the Universe. They have stretched and now have longer wavelengths because the Universe is expanding.
Evidence 2 for the Big Bang Theory
● Cosmic microwave background radiation (CMBR)
- Microwave radiation can be detected EVERYWHERE in the Universe. These are the stretched remains of high energy gamma radiation (originally had short wavelength) that would have been produced in the explosion that created the Universe. They have stretched and now have longer wavelengths because the Universe is expanding.
8.15 describe that if a wave source is moving relative to an observer there will be a change in the observed frequency and wavelength
Change in the observed frequency and wavelength
is due to the Doppler effect. If something that emits a wave is coming towards you, then the wavelength will become shorter, increasing the frequency, and stretching the wavelength as it is moving away, which decreases the frequency, and therefore increases the wavelength. Doppler’s effect is also responsible for the red-shift of light from galaxies which are moving away from Earth, which is when the wavelength of the light coming from them (distant galaxies) is increased. The faster it is moving, the more its light is red-shifted.
8.17P describe the red-shift in light received from galaxies at different distances away from the Earth
The further away a galaxy is the greater its red-shift and hence the faster it is moving away from us.
8.18P explain why the red-shift of galaxies provides evidence for the expansion of the universe
More distant galaxies have greater red-shift and therefore are receding faster because the space between all galaxies is stretching as the universe expands. (which proves expansion theory)
