space physics (p8)

Question 1

describe the planets in the solar system, and the sun:

Answer 1

• at the centre is the sun, which is a star.
• mercury, venus, earth and mars are relatively small rocky planets.
• jupiter, saturn, uranus and neptune are much larger.
• beyond the orbit of neptune, we have a number of dawrf planets, one of which is pluto.
• the dwarf planets are extremely small.

Question 2

what else does the solar system contain?

Answer 2

a number of other objects besides planets. for example, moons, asteroids and comets.
- moons are natural satellites which orbit planets.
- most planets in the solar system have at least one moon, some planets have a large number of moons.

Question 3

describe the milky way galaxy:

Answer 3

• the solar system is just a tiny part of the milky way galaxy.
• a galaxy is a massive group of stars - i.e. the milky way contains hundreds of billions of stars.
• in the universe, there are hundreds of billions of galaxies.

Question 4

how do stars form?

Answer 4

• stars form from clouds of dust and gas - this is called a nebula.
• in a nebula, the gas is mainly hydrogen.
• in the first stage, gravity causes the cloud of dust and gas to collapse.
• as the dust particles move faster, the temperature rises to millions of degrees celsius.
• this collapsing cloud of dust and hot gas is called a protostar.
• if the temperature of the protostar gets high enough, the hydrogen nuclei undergo nuclear fusion to form helium.
• at this point, the protostar has turned into a main sequence star.
• nuclear fusion is now releasing a great deal of energy, so stars can stay in the main sequence for quite a long time.

Question 5

describe what happens to a star when it undergoes nuclear fusion:

Answer 5

• there are two opposing forces acting within the star.
• the force of gravity acts inwards, and tends to make the star collapse.
• the energy from nuclear fusion creates a force acting outwards. this tends to make the star expand.
• the force of gravity acting inwards is balanced due to the force of fusion acting outwards.
• because these two forces are balanced, scientists would say that the star is in equilibrium.

Question 6

what happens to main sequence stars that are the same size as our Sun?

Answer 6

• the hydrogen nuclei had been fusing together to form helium nuclei.
• at some stage the hydrogen in the star will begin to run out.
• therefore the outward force due to fusion is less than the inward force due to gravity, causing the star to collapse inwards.
• the collapse of the star causes its temperature to increase.
• now helium nuclei fuse together to create heavier elements, and the star expands to form a red giant.
• at some point, the red giant stops fusing helium, and shrinks to form a white dwarf.
• as the white dwarf is no longer carrying out fusion, it gradually cools down, and eventually stops releasing any energy and becomes a black dwarf.

Question 7

what happens to main sequence stars that are much bigger than our Sun?

Answer 7

• once these main sequence stars run out of hydrogen, they leave the main sequence stage and expand into red super giants.
• once again, helium nuclei fuse together to produce heavier elements.
• once again, the red super giant stops carrying out nuclear fusion.
• at this stage, the star explodes - this is called a supernova.
• the temperature in a supernova explosion is incredibly high, creating elements heavier than iron.
• these elements are then distributed throughout the universe.
• after the supernova, the remains of the star can either form a neutron star (consists of neutrons densely packed together) or a black hole (this has such a large gravity that not even light can escape).

Question 8

what are the limitations of nuclear fusion, and how are these limitations overcome?

Answer 8

• nuclear fusion, in any of the red giant stages, and in any of the stages up until red super giant, cannot create any elements heavier than iron.
• the temperature of a supernova explosion is high enough to create elements heavier than iron.

Question 9

what is the name of the Earth’s orbit around the Sun?

Answer 9

• a virtually circular orbit.
• virtually follows the path of a circle around the Sun.
• technically, the Earth orbits the sun elliptically.

Question 10

describe how the Earth and the moon stay in orbit of the Sun and the Earth, respectively:

Answer 10

• the force of gravity acting between the Sun and the Earth holds the Earth in its orbit.
• the moon also has a circular orbit around the Earth. this is called a natural satellite, and once again, the force of gravity holds the moon in its orbit.

Question 11

how does the force of gravity lead to a change in velocity but not a change in speed in satellites?

Answer 11

as objects are orbiting the Earth in a circular motion, this means their direction is constantly changing. therefore even if its speed is remaining the same, as the direction is constantly changing, the velocity must also be constantly changing (vector quantity).

Question 12

describe geostationary satellites and general satellites:

Answer 12

• artificial satellites are man-made.
• geostationary satellites orbit once every 24 hours, so they point at the same part of the Earth.
• other artificial satellites orbit more frequently.

Question 13

describe orbits:

Answer 13

• the force of gravity holds objects in their orbit, and satellites are no exception.
• in the case of circular orbits, the force of gravity is leading to a change in velocity, but not a change in speed. this is because the direction of the satellite is constantly changing, therefore the velocity is too, despite the speed staying constant.
• if the speed of a satellite changes, the orbit radius must also change.
• if the speed of the satellite increases, the radius of its orbit decreases. this is because at a higher speed, the object requires a higher force of gravity to prevent it from flying off into space. by staying closer to the Earth, the gravitational force on the object is greater, and the object remains in stable orbit.

Question 14

describe the discovery of red-shift:

Answer 14

• in the 1900s, astronomers were looking at the light emitted from different galaxies.
• they detected that the light from very distant galaxies has an increased wavelength compared to light from closer galaxies.

Question 15

describe red-shift:

Answer 15

• if we take the light from the Sun and pass it through a prism, then we get a spectrum (rainbow).
• there are dark lines in the spectrum, and this is where certain elements in the sun absorb light of specific wavelengths, so they only appear as dark lines.
• a spectrum of light from a distant galaxy shows that the dark lines have shifted towards the red end of the spectrum (the right). this is called red-shift.
• there is an observed increase in the wavelength of light from most
  distant galaxies. the further away the galaxies, the faster they are
  moving and the bigger the observed increase in wavelength. this
  effect is called red-shift

Question 16

what does red shift tell us?

Answer 16

• the red-shift tells us that the galaxies are moving away from each other.
• because the galaxies are moving away, the light waves are stretched (their observed wavelength is increased).
• galaxies that are further away have a bigger red-shift, so these galaxies are moving faster than galaxies that are closer.

e.g. nearby galaxies have a small red-shift, showing that they’re moving away. more distant galaxies have a larger red-shift. they are moving away faster. very distant galaxies have a very large red-shift. they are moving away very fast.

Question 17

how can we use the idea of red-shift to provide evidence of the Big Bang Theory?

Answer 17

• the fact that distant galaxies are moving faster than nearby galaxies provides evidence that the Universe is expanding.
• scientists believe that the Universe began from a very small region that was extremely hot and dense.
• this region then expanded into the Universe we see today. this is the Big Bang Theory.
• around 1998, astronomers noticed something very strange. they’d assumed that gravity would cause the expansion of the Universe to gradually slow down, but observations of supernovae showed that the rate of expansion is actually increasing.
• in recent years, scientists have suggested that the Universe contains matter and energy that we cannot detect - these are called dark matter and dark energy.
• this could explain why the expansion of the universe is speeding up.

Question 18

how do fusion processes lead to the formation of new elements?

Answer 18

nuclear fusion is a nuclear reaction in which two or more atomic nuclei collide at nearly the speed of light to join to form a new type of nucleus.
- this creates new elements.
- hydrogen and helium nuclei can then be fused into heavier elements.

Question 19

what is the difference between centrifugal force and gravitational force?

Answer 19

• centrifugal force is the apparent outward force experienced by an object moving in a curved path. it arises due to an object’s inertia, which resists it being pulled to the centre of the orbit.
• gravitational force pulls the object towards the centre of the massive body it is orbiting.
• when an object is in orbit, the centrifugal forces and gravitational forces acting on it must be equal, to prevent it from flying into space or crashing into the body.

Question 20

how does the radius of the object’s orbit adjust to accommodate new speeds?

Answer 20

• when the speed of an object in a stable orbit changes, the balance between the centrifugal force and the gravitational force is disrupted
• if the object’s speed increases, the centrifugal force will become stronger. To counteract this increased outward force, the gravitational force needs to become stronger as well. This can only be achieved by moving the object closer to the centre of the orbit, reducing the radius
• if the object’s speed decreases, the centrifugal force will weaken. To restore the balance, the gravitational force must also weaken. This is accomplished by moving the object farther away from the centre of the orbit, increasing the radius