P7 Observing the Universe- Answers
P7.1 Observing the sky with the naked eye
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The Sun appears to travel across the sky once every 24 hours
The Sun appears to travel east-west across the sky
The stars appear to travel across the sky once every 23 h 56 min
The stars appear to travel east-west across the sky
The Moon appears to travel across the sky every 25 hours
The Moon appears to travel east-west across the sky
With the naked eye we can see Mercury, Venus, Mars, Jupiter and Saturn. They appear to
move with the stars but change their positions in complicated patterns
The planets appear to have complicated motions across the sky due to a phenomenon called
‘retrograde motion’
Retrograde motion is the apparent reversal in a planet’s usual direction of motion, as seen
from the Earth against the background of fixed stars. This happens periodically with all
planets beyond the Earth’s orbit
At any time, half of the Moon is lit up by the Sun’s rays, just like the Earth. The view from the
Earth depends on where the Moon is around its orbit. When the Moon is on the opposite
side of the Earth to the Sun, an observer on the Earth can see the whole of its illuminated
side. This is a full Moon. When the Moon is in the direction of the Sun, the side that is in
darkness faces the Earth. This is a new Moon.
Both the Earth and the Moon have shadows- areas where they block the sunlight. Because
the Sun is an extended source of light, these shadows do no have hard edges. There is a
region of total darkness (umbra) fringed by a region of partial darkness (the penumbra). The
Earth’s Shadow is much bigger than the Moon’s. When the Moon’s umbra touches the
surface of the Earth, a solar eclipse is seen from inside the contact area. When the Moon
passes into the Earth’s umbra, a lunar eclipse is seen.
The Moon’s orbit is tilted relative to the plane of the Earth’s orbit by about 5°. Usually, the
Earth, Sun, and Moon re not in a line so no eclipse occurs
The positions of astronomical objects are measured in terms of angles as seen from Earth.
The position is defined using two angles: right ascension and declination.
A sidereal day is the time taken for the Earth to rotate 360°
A sidereal day lasts 23 hours and 56 minutes
A solar day is the time taken for the earth to rotate so that it fully faces the Sun again.
A solar day lasts exactly 24 hours.
Different stars are seen in the night sky at different times of the year because the Earth
travels halfway round its orbit in six months. You see the stars that are in the opposite
direction of the Sun, so after six months you will see the opposite half of the sky.
P7.2 How do telescopes work?
1. True: both convex and converging lenses bring parallel light to a focus
2. More powerful lenses of the same material have more curved surfaces.
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P7 Observing the Universe
Answers
1
3. 𝑃𝑜𝑤𝑒𝑟 (𝑑𝑖𝑜𝑝𝑡𝑟𝑒) = 𝑓𝑜𝑐𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ (𝑚𝑒𝑡𝑟𝑒)
4. Astronomical objects are so distant that light from them is effectively parallel.
5. The more powerful lens is at the eyepiece;
6. 𝑀𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 =
𝑓𝑜𝑐𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑜𝑏𝑗𝑒𝑐𝑡𝑖𝑣𝑒 𝑙𝑒𝑛𝑠
𝑓𝑜𝑐𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑒𝑦𝑒𝑝𝑖𝑒𝑐𝑒 𝑙𝑒𝑛𝑠
7. Most astronomical telescopes have concave mirrors, not convex lenses, as their objectives
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P7.3 What are the objects we see in the night sky and how far away
are they?
1. Parallax is the apparent shift of an object against a more distant background, as the position
of the observer changes. This can be used to measure how far away an object is for example,
to measure the distance to stars.
2. The parallax angle of a star as half the angle moved against a background of distant stars in 6
months.
3. The further away the object is, the less it appears to shift , thus the smaller parallax angle is.
4. A parsec (pc) as the distance to a star with a parallax angle of one second of arc;
5. True: a parsec is similar in magnitude to a light-year;
6. Typical interstellar distances are a few parsecs.
7. The luminosity (intrinsic brightness) of a star depends on its temperature and its size.
8. The observed intensity of light (observed brightness) of a star (as seen on Earth) depends on
its luminosity (intrinsic brightness) and its distance from Earth because the light from a star
spreads outwards. So the more distant the observer, the smaller amount of light that they
receive.
9. The period of a Cepheid variable star’s pulse in brightness is related to their brightness;
10. Astronomers can estimate the distance to Cepheid variable stars by measuring its observed
brightness and period of variation, and from its period, read its luminosity of the graph
(constructed by Henrietta Leavitt). They then use the luminosity and observed brightness to
work out the distance.
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P7 Observing the Universe
Answers
11. The two things the exam board want you to remember that telescopes revealed are: that
the Milky Way consists of very many stars and led to the realisation that the Sun was a star
in the Milky Way galaxy; the existence of many fuzzy objects in the night sky, and that these
were originally called nebulae;
12. The main issue in the Curtis-Shapley debate was whether nebulae were objects within the
Milky Way or separate galaxies outside it.
13. Hubble’s observations of Cepheid variables in one nebula indicated that it was much further
away than any star in the Milky Way, and hence that this nebula was a separate galaxy.
14. Intergalactic distances are typically measured in megaparsecs (Mpc);
15. Cepheid variable data in distant galaxies has given accurate values of the Hubble constant.
𝑘𝑚
)
𝑠
16. 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑟𝑒𝑐𝑒𝑠𝑠𝑖𝑜𝑛 (
𝑘𝑚
𝑝𝑒𝑟
𝑠
= 𝐻𝑢𝑏𝑏𝑙𝑒 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (
𝑀𝑝𝑐) × 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 (𝑀𝑝𝑐)
P7.4 What are stars?
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All hot objects (including stars) emit a continuous range of electromagnetic radiation
Luminosity and peak frequency increase with temperature
The removal of electrons from atoms is called ionisation
Electron movement within atoms produces line spectra
Specific spectral lines within the spectrum of a star evidence of the chemical elements
present in it
When the volume of a gas is reduced its pressure increases
As the temperature of a gas increases, the particles within it start to move quicker. This
results in more collisions with the edge of the container the gas is in (pressure rise). If the
container is not fixed in size, the higher kinetic energy of the gas molecules will cause it to
expand (increase in volume).
Absolute zero is the theoretically lowest possible temperature, -273°, calculated by
extrapolating from the behaviour of gases at different temperatures.
To convert to Kelvin (K) from degrees Celsius you add 273. Eg 27° = (27 + 273)K = 300K
A cold cloud of gas and dust starts to contract, pulled together by gravity. It breaks up into
several smaller clouds and each continues to contract. Within a contracting cloud, each
particle attracts every other particle, so that the cloud collapses towards its centre. It forms
a rotating swirling disc. As the gas particles are attracted towards the centre, they move
faster, which means the gas gets hotter. Eventually, the temperature of this material is hot
enough for fusion reactions to occur and a star is born. Material further out in the disc
clumps together to form planets.
Before the 20th Century, they did not know about nuclear fission so could not explain the
source of the Sun’s energy
Compressing a gas will raise its temperature
The results of the Rutherford-Geiger-Marsden alpha particle scattering experiment indicated
that a gold atom contains a small, massive, positive region (the nucleus);
The nucleus of an atom contains positive protons and neutral neutrons.
Protons are held together in the nucleus by a strong force much greater than the repulsive
electrical force between them.
Hydrogen nuclei can fuse into helium nuclei, releasing energy, if brought close together
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P7 Observing the Universe
Answers
17. A star contains (in order from the centre outwards): a hotter core, where fusion takes place;
a radiative zone, where energy is transported towards the surface by photons of radiation; a
convective zone, where energy is transported to the surface by convection; the
photosphere, where energy is radiated into space
18. Small stars, like our Sun, become red giants when the core hydrogen is depleted, while
larger stars become red supergiants
19. Red giants and red supergiants liberate energy by fusing helium into larger nuclei such as
carbon, nitrogen and oxygen
20. Red giants lack the mass to compress the core further at the end of the helium fusion, and
they then shrink into hot white dwarfs, which gradually cool
21. Fusion in red supergiants continues to larger nuclei due to the higher pressures in the core
22. Fusion in large stars ceases when the core has been largely converted into iron, and the star
then explodes in a supernova, leaving a dense neutron star or black hole
P7.5 How do astronomers work together?
1. Major optical and infrared astronomical observatories on Earth are mostly situated in Chile,
Hawaii, Australia and the Canary Islands
2. Large telescopes are needed to collect the weak radiation from faint or very distant sources
3. Radiation is diffracted by the aperture of a telescope, so the aperture must be very much
larger than the wavelength of the radiation detected by the telescope to produce sharp
images
4. The main advantages and disadvantages of using telescopes outside the Earth’s atmosphere
are: avoids absorption and refraction effects of atmosphere; can use parts of
electromagnetic spectrum that the atmosphere absorbs; cost of setting up, maintaining and
repairing; uncertainties of space programme.
5. There is a need for international collaboration in terms of economy (to save money and use
the same resources) and to allow scientists to pool their expertise (and peer review).
6. The European Sothern Observatory (ESO) is an example showing how international
cooperation is essential for progress in expensive ‘big science’ projects such as astronomy. It
is a set of three observatories, run by 14 European countries in the Atacama Desert, high up
in the Andes Mountains of Chile.
7. Cost, environmental and social impact near the observatory, and working conditions for
employees are important considerations in planning, building, operating, and closing down
an observatory.
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