Space Physics: Physics 0625 (Cambridge O Level / IGCSE)

A student living in Nairobi keeps a weather diary. Every day she notes that the Sun appears low in the eastern sky at dawn, climbs to its highest point around midday, and disappears below the western horizon by evening, and the whole pattern repeats after about 24 hours.

Which of the following correctly explains why Nairobi experiences this repeating day-and-night cycle?

A student living in Wellington, New Zealand keeps an astronomy journal. She writes to a pen-friend, Lars, who lives in Tromsø, Norway, a city at a similarly high latitude, but in the Northern Hemisphere.

(a) In July, the student in Wellington records short, cold days, while at the very same time Lars in Tromsø records long, warm days. Explain, in terms of the tilt of the Earth's axis and the Earth's orbit around the Sun, why the two friends experience opposite seasons in July. [3]

(b) Over the next four weeks, the student also sketches the changing shape of the illuminated part of the Moon that she can see each night from Wellington. Describe, in terms of the changing relative positions of the Sun, Earth and Moon, why the illuminated shape of the Moon that she observes changes during this time, and state approximately how long one complete cycle of phases takes. [3]

Mission control for a space agency operates a probe named Meridian-6, which is currently orbiting Neptune, one of the giant planets that lie far beyond the asteroid belt.

(a) State the force that keeps Meridian-6 in orbit around Neptune, and state why Neptune itself orbits the Sun rather than the Sun orbiting Neptune. [2]

(b) State how the strength of Neptune's gravitational field changes as Meridian-6 moves from an orbit close to Neptune's cloud tops to a much wider orbit further away, and state the one property of Neptune that determines the strength of its gravitational field at any fixed distance from its centre. [2]

(c) Mission control sends a radio signal to Meridian-6. At the moment the signal is sent, Meridian-6 is $1.20\times10^{12}\text{ m}$ from Earth. Radio signals, like all electromagnetic waves, travel at the speed of light, $3.00\times10^8\text{ m/s}$. Calculate the time taken for the signal to reach Meridian-6, giving your answer in minutes. [3]

Astronomers studying a distant star, Talara, have identified two planets in its system: Vesa, the closer of the two, and Corim, which orbits much further out.

(a) Vesa orbits Talara at an average orbital radius of $2.00\times10^{11}\text{ m}$, completing one full orbit every $3.00\times10^7\text{ s}$. Determine the average orbital speed of Vesa. [3]

(b) Corim orbits Talara at a much greater average orbital radius than Vesa. Without doing any further calculation, state how the average orbital speed of Corim compares with that of Vesa, and explain your answer in terms of the strength of Talara's gravitational field. [2]

(c) Vesa's orbit is noticeably elliptical rather than perfectly circular, so its distance from Talara varies during one orbit. State and explain, using the idea of conservation of energy, how Vesa's orbital speed changes as it moves from the point in its orbit closest to Talara to the point furthest away. [2]

A star much more massive than the Sun has just used up all of the hydrogen fuel in its core, ending its time as a stable (main-sequence) star.

Which of the following gives the correct sequence of stages this massive star will go through next, ending with a compact remnant left behind?

An astronomy club is building a scale model of the Solar System. Before choosing materials for each planet, the members research how the eight planets differ from one another.

(a) State how the four planets closest to the Sun differ from the four planets furthest from the Sun, in terms of their typical size and composition. [2]

(b) The club also reads about the accretion model, which explains how the Solar System formed from a collapsing interstellar cloud of gas and dust. Describe this model, including the role played by gravity, the range of elements present in the cloud, and the formation of a rotating disc of material, and use these ideas to explain why the model leads to small rocky planets forming close to the Sun and large gas/ice giants forming much further out. [4]

Two newly discovered exoplanets, Peris and Doven, orbit the same distant star, Halvor. Peris has a much greater mass than Doven. A comet named Ashkel also orbits Halvor, but its path is a long, narrow ellipse rather than a circle.

(a) State which of Peris or Doven produces the stronger gravitational field at a fixed distance from its centre, and explain your answer in terms of the property of a planet that determines the strength of its gravitational field. [2]

(b) The comet Ashkel's elliptical orbit is drawn on a diagram, with the star Halvor positioned inside the ellipse but noticeably off to one side, rather than in the middle. State the astronomical term for this off-centre point, and state the one special condition under which a star instead lies at the exact centre of an orbit. [2]

(c) State the difference between a minor planet, such as a dwarf planet or an asteroid, and a natural satellite (moon), giving an example of where in the Solar System each is typically found. [2]

Engineers tracking the ice-giant exoplanet Coruna monitor two objects orbiting it: its moon, Tavel, and an uncrewed probe, Ranger-9.

(a) Tavel orbits Coruna at an average orbital radius of $4.50\times10^{8}\text{ m}$, completing one full orbit every $6.00\times10^{5}\text{ s}$. Calculate the average orbital speed of Tavel. [3]

(b) Ranger-9 is placed in a circular orbit around Coruna with an average orbital speed of $3.20\times10^{3}\text{ m/s}$, taking $9.00\times10^{5}\text{ s}$ to complete one full orbit. Calculate the average orbital radius of Ranger-9's orbit. [4]

Astronomy students are researching a star named Ophira, which has a mass very similar to the Sun's. Ophira formed inside a large, slowly-collapsing cloud of gas and dust called the Ilvenna Nebula, and is currently a stable star part-way through its life.

(a) Describe how gravity acting on the material of the Ilvenna Nebula led to the formation of Ophira as a protostar, and state what begins inside a protostar once it becomes hot and dense enough for it to become a stable main-sequence star. [3]

(b) Once Ophira eventually uses up the hydrogen fuel in its core, describe the sequence of stages it will go through, given that its mass is similar to the Sun's, ending with the compact object it leaves behind. [3]

(c) State the one property of a star that determines whether it follows this pathway, or instead follows the alternative pathway that ends in a supernova and a neutron star or black hole. [1]

Engineers monitor two objects orbiting the dwarf planet Ostrena: its small natural moon, Ithel, and an uncrewed probe, Wayfarer-3, which orbits much further out.

(a) Wayfarer-3 moves in a circular orbit around Ostrena with an average orbital radius of $4.00\times10^{7}\text{ m}$ and an average orbital speed of $5.03\times10^{2}\text{ m/s}$. Calculate the orbital period of Wayfarer-3, giving your answer in seconds, and then convert your answer to hours. [4]

(b) Ithel orbits Ostrena at a much smaller average radius than Wayfarer-3. Without doing any further calculation, explain why Ithel's orbital period is much shorter than Wayfarer-3's. Refer to both (i) the distance travelled during one orbit, and (ii) how Ithel's orbital speed compares with that of Wayfarer-3. [3]