Space physics flashcards

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Why do we have day and night on Earth?
Earth rotates on its axis once every 24 hours, so a place faces the Sun by day and away from it by night. The Sun appears to move east to west.
What causes the seasons on Earth?
The Earth's axis is tilted at about 23.5° to its orbit. The hemisphere tilted towards the Sun gets more direct, longer-lasting sunlight (summer); the other has winter.
What is the length of a year, and what is the Moon's orbital period?
Earth takes about 365 days to orbit the Sun. The Moon takes about 27.3 days to orbit the Earth, and about 29.5 days between full moons.
Why do we see phases of the Moon?
We see the Moon by reflected sunlight. As it orbits the Earth, different amounts of its sunlit half face us, giving crescent, half and full phases.
What are the main objects in the Solar System?
The Sun, eight planets, dwarf planets (such as Pluto), moons, asteroids (mainly between Mars and Jupiter), comets and smaller debris.
Name the planets in order from the Sun.
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The first four are small and rocky; the last four are gas or ice giants.
What is the shape of planetary orbits, and how do speed and period change with distance from the Sun?
Orbits are nearly circular ellipses. Planets further out move more slowly and have much longer orbital periods, because the Sun's gravitational pull is weaker.
What force keeps planets and moons in orbit?
Gravitational attraction towards the Sun or planet. It acts as the centripetal force, constantly changing the direction of velocity but not the speed in a circular orbit.
What is gravitational field strength and how does it vary between planets?
The force per unit mass (N/kg). It depends on a planet's mass and radius: Earth about 9.8 N/kg, Moon 1.6, Jupiter about 25. Weight differs but mass does not.
Give the equation for the orbital speed of a planet in a circular orbit.
v = 2πr/T. Orbital speed v (m/s) equals the circumference of the orbit (2πr, r the orbital radius in m) divided by the orbital period T (s).
Earth's orbital radius is 1.5 × 10¹¹ m and its period is 3.15 × 10⁷ s. Find its orbital speed.
v = 2πr/T = 2π × 1.5 × 10¹¹ ÷ 3.15 × 10⁷ ≈ 3.0 × 10⁴ m/s (30 km/s).
How does a comet's orbit differ from a planet's?
A comet follows a very elongated elliptical orbit, moving fast near the Sun and slowly far away. Its tail of gas and dust points away from the Sun.
What is the Sun and what powers it?
A star made mostly of hydrogen. Nuclear fusion of hydrogen into helium in its core releases energy that is radiated out as light and heat.
What is nuclear fusion and why does it need very high temperatures?
Light nuclei join to form a heavier nucleus, releasing energy. High temperatures and pressures give nuclei enough energy to overcome their electrostatic repulsion.
Describe how a star forms.
A cloud of gas and dust (nebula) is pulled together by gravity. It heats up as it contracts, forming a protostar. When the core is hot enough, fusion starts and the star is stable.
Why is a main-sequence star stable?
The inward pull of gravity is balanced by the outward pressure from the energy released by fusion in the core.
Describe the life cycle of a star similar in mass to the Sun.
Nebula, protostar, main-sequence star, then it expands into a red giant, sheds its outer layers and the core shrinks to a white dwarf, which cools.
Describe the life cycle of a star much more massive than the Sun.
Nebula, protostar, main sequence, red supergiant, then a supernova explosion. The core is left as a neutron star or, if massive enough, a black hole.
Where do the heavier elements in the universe come from?
Light elements are made by fusion in stars, with heavier elements formed in the later stages of massive stars and in supernova explosions, then spread into space to form new stars and planets.
What is a galaxy, and what is the Milky Way?
A galaxy is a vast collection of billions of stars held together by gravity. The Milky Way is our spiral galaxy, with the Solar System in one of its outer arms.
What is a light-year and how does it compare with other units?
The distance travelled by light in one year, about 9.5 × 10¹⁵ m. It is used for the huge distances between stars and galaxies.
How long does light take to reach Earth from the Sun?
Distance 1.5 × 10¹¹ m ÷ speed 3.0 × 10⁸ m/s = 500 s, about 8 minutes 20 seconds.
What is the Big Bang theory?
The universe began about 13.8 billion years ago from an extremely hot, dense point and has been expanding ever since. Space itself is expanding, carrying galaxies apart.
What is cosmic microwave background radiation (CMBR) and why does it support the Big Bang?
Microwave radiation received equally from all directions in space. It is the stretched remains of the intense radiation from the early universe, as predicted by the Big Bang theory.
What is red-shift and what does it tell us about distant galaxies?
The wavelength of light from distant galaxies is stretched towards the red end of the spectrum. It shows they are moving away from us, and further galaxies are receding faster.
(Extended) State the Hubble constant equation and what it shows.
H₀ = v/d, where v is the recession speed of a galaxy and d its distance. It is the rate of expansion of the universe; a constant means speed is proportional to distance.
(Extended) How can the age of the universe be estimated from H₀? Calculate it for H₀ = 2.2 × 10⁻¹⁸ per second.
Age ≈ 1/H₀. 1 ÷ 2.2 × 10⁻¹⁸ = 4.5 × 10¹⁷ s, which is about 14 billion years (÷ 3.15 × 10⁷ s per year).

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