Astrophysics flashcards

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What are the main bodies in the Solar System?
The Sun, eight planets, dwarf planets, moons, asteroids and comets. The planets orbit the Sun; moons orbit planets.
What is the force that keeps planets in orbit?
Gravity. The Sun's gravitational attraction provides the centripetal force that keeps a planet moving in a near-circular path.
What is gravitational field strength and its unit?
The force per unit mass on an object in a gravitational field: g = W ÷ m. It is measured in N/kg. On Earth it is 9.8 N/kg.
What does the gravitational field strength of a planet depend on?
The mass of the planet (larger mass gives larger g) and the distance from its centre. It decreases with distance.
A planet has g = 3.7 N/kg. What does a 60 kg astronaut weigh there?
W = mg = 60 × 3.7 = 222 N. The astronaut's mass is still 60 kg.
Why is your weight different on the Moon but your mass is the same?
Mass is the amount of matter and does not change. Weight = mg, and the Moon's g is about one-sixth of Earth's, so weight is smaller.
State the equation for the orbital speed of an object in a circular orbit.
v = 2πr ÷ T. v is orbital speed (m/s), r is orbital radius (m) and T is the time for one orbit (s). The distance in one orbit is the circumference.
The Earth orbits at r = 1.5 × 10¹¹ m with T = 3.16 × 10⁷ s. Find its orbital speed.
v = 2πr ÷ T = 2π × 1.5 × 10¹¹ ÷ 3.16 × 10⁷ ≈ 3.0 × 10⁴ m/s.
How does orbital speed change with orbital radius and why?
Planets nearer the Sun move faster. Gravity is stronger closer in, so a greater speed is needed to stay in orbit, and the orbital period is shorter.
Why is an object in a circular orbit accelerating even though its speed is constant?
Its direction keeps changing, so its velocity changes. The resultant force, gravity, is directed towards the centre of the orbit.
Describe the orbit of a comet compared with a planet.
Comets follow very elongated elliptical orbits. They speed up as they approach the Sun and slow down far away, so they spend most of the time in the cold outer Solar System.
What is a moon and what is an artificial satellite?
A moon is a natural satellite orbiting a planet. An artificial satellite is a human-made object in orbit, used for communications, weather, navigation or observation.
What is a geostationary orbit and why is it useful?
A satellite orbits over the equator with a period of 24 hours, so it stays above one point on Earth. It is used for TV and communications because dishes need not move.
What is a light-year?
The distance light travels in one year, about 9.5 × 10¹⁵ m. It is a unit of distance, not time.
How long does light take to reach the Earth from the Sun (1.5 × 10¹¹ m, light speed 3.0 × 10⁸ m/s)?
t = s ÷ v = 1.5 × 10¹¹ ÷ 3.0 × 10⁸ = 500 s, about 8 minutes.
What is a galaxy and what galaxy is the Sun in?
A galaxy is a huge group of billions of stars held together by gravity. The Sun is in the Milky Way, a spiral galaxy.
How does a star form?
Gravity pulls a cloud of dust and gas (a nebula) together. It heats up as it contracts, and when the core is hot and dense enough, hydrogen fusion starts and the protostar becomes a star.
Why is a main-sequence star stable?
The inward pull of gravity is balanced by the outward pressure from the thermal energy released by fusion in the core.
What happens to a star like the Sun at the end of its life?
It runs low on hydrogen, swells to a red giant, sheds its outer layers as a planetary nebula, and the core shrinks to a white dwarf that cools and fades.
What happens to a much more massive star at the end of its life?
It becomes a red supergiant, then explodes as a supernova. The core becomes a neutron star or, if massive enough, a black hole.
Where were the elements heavier than iron made?
In supernova explosions. Lighter elements up to iron form by fusion in stars. These elements are scattered into space and can form new stars and planets.
What is a black hole?
A region where gravity is so strong that nothing, not even light, can escape from it. It forms when a very massive star collapses.
(Paper 2) What is red-shift?
The wavelength of light from a receding source is stretched, shifting it towards the red end of the spectrum. The greater the shift, the faster the source is moving away.
(Paper 2) What does the red-shift of distant galaxies tell us?
Almost all galaxies are moving away from us, and the more distant the galaxy, the larger the red-shift. This shows that the Universe is expanding.
(Paper 2) State the Big Bang theory.
The Universe began about 13.8 billion years ago from an extremely hot, dense, tiny point, and has been expanding and cooling ever since.
(Paper 2) What is cosmic microwave background radiation (CMBR)?
Low-energy microwave radiation coming from all directions in space. It is thought to be left over from the early Universe, stretched by expansion, and supports the Big Bang theory.
(Paper 2) Why does the expansion of the Universe support the Big Bang theory?
If galaxies are moving apart now, then in the past they were closer together, and tracing back leads to a single point from which everything started.
Name the stages in the life cycle of a star the size of the Sun, in order.
Nebula, protostar, main-sequence star, red giant, planetary nebula, white dwarf.
How can the colour of a star tell you about it?
A star's colour is linked to its surface temperature. Blue-white stars are the hottest and red stars are the coolest, so stars can be classified by colour.

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