1. The Sun as a star
The Sun is a star of medium size. It consists mostly of hydrogen and helium.
Cambridge IGCSE0625
Stars and the Universe
Physics 0625 Chapter Notes
What this chapter covers
Stars and the Universe - The Sun as a starStars and the Universe - StarsStars and the Universe - The Universe
Stars and the Universe notes
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~16 min readIn this chapter
- 1The Sun as a star
- 2How the Sun radiates its energy
- 3Nuclear fusion in stable stars
- 4Galaxies and the Milky Way
- 5The light-year
- 6Light-years in metres
- 7Birth of a star
- 8A stable star
- 9Red giants and planetary nebulae
- 10Supernovae and new stars
- 11Galaxies and the Milky Way
- 12Redshift
- 13Evidence for an expanding Universe
- 14Cosmic microwave background radiation
- 15Finding speed and distance
- 16The Hubble constant
- 17The age of the Universe
2. How the Sun radiates its energy
The Sun radiates most of its energy in three regions of the electromagnetic spectrum: infrared, visible light and ultraviolet.
Name the regions. Write infrared, not simply heat.
Examiner insight
“Better performing candidates stated visible light and infra-red (not simply heat) as the types of radiation emitted.” “Common incorrect regions of the electromagnetic spectrum given were X-rays and gamma radiation.”
Worked example 12 marks
Complete the sentence about energy transferred from the Sun.
The Sun radiates most of its energy in the infrared, visible and ............ regions of the ............ spectrum.
- 1
ultraviolet [1]
- 2
electromagnetic [1]
3. Nuclear fusion in stable stars
Stars are powered by nuclear reactions that release energy.
In a stable star, such as the Sun, the reaction is nuclear fusion: hydrogen nuclei join to form helium.
Spell fusion correctly. ‘Fussion’ is not accepted, because it could mean fission.
Examiner insight
“Although most weaker candidates could recall that nuclear fusion takes place in stars, they were less successful at recalling that hydrogen was converted into helium.” “Another incorrect answer given was “combustion” which suggested that the Sun is powered by a chemical reaction.”
Worked example 13 marks
The Sun is an example of a stable star. Describe the process that releases energy in the Sun.
- 1
(nuclear) fusion [1]
- 2
hydrogen nuclei join [1]
- 3
(into / to form) helium [1]
Recap
- The Sun is a star of medium size, mostly hydrogen and helium.
- It radiates most of its energy as infrared, visible light and ultraviolet.
- Extended: stars are powered by nuclear reactions.
- Extended: in stable stars, hydrogen fuses into helium.
4. Galaxies and the Milky Way
Galaxies are each made up of many billions of stars. The Sun is a star in the galaxy known as the Milky Way.
The other stars in the Milky Way are much further away from the Earth than the Sun is. The diameter of the Milky Way is approximately 100 000 light-years.
Examiner insight
“Many candidates did not correctly distinguish between the Universe and galaxies.” “Very few candidates could recall that the diameter of the Milky Way galaxy was of the order of 100 000 light-years.”
Worked example 11 mark
Which description of a galaxy is correct?
A a collection of billions of stars · B a collection of gaseous planets orbiting a star · C a collection of rocky planets orbiting a star · D all of the stars that exist
- 1
A: a collection of billions of stars. [1]
5. The light-year
Astronomical distances can be measured in light-years.
One light-year is the distance travelled in (the vacuum of) space by light in one year. It is a unit of distance, not of time.
Examiner insight
“Common errors included stating that it was a speed or a time or even that it was a year with above average amounts of sunlight.”
Worked example 12 marks
State the meaning of the term light-year.
- 1
it is a distance [1]
- 2
travelled in (the vacuum of) space by light in one year [1]
6. Light-years in metres
One light-year is equal to 9.5 × 10¹⁵ m.
To change a distance in light-years to metres, multiply by this value. For kilometres, divide the metres by 1000.
1 light-year = 9.5 × 10¹⁵ m
Examiner insight
“Candidates would benefit from underlining key words and phrases in questions to help them focus.”
Worked example 11 mark
Proxima Centauri is one of the closest stars to the Earth and is located approximately 4.3 light-years away. What is this distance in kilometres?
A 1.3 × 10⁹ km · B 4.1 × 10¹³ km · C 2.2 × 10¹⁵ km · D 4.1 × 10¹⁶ km
- 1
distance in metres = 4.3 × 9.5 × 10¹⁵ = 4.1 × 10¹⁶ m
- 2
divide by 1000: 4.1 × 10¹³ km
- 3
B: 4.1 × 10¹³ km. [1]
7. Birth of a star
A star is formed from interstellar clouds of gas and dust that contain hydrogen.
A protostar is an interstellar cloud collapsing and increasing in temperature as a result of its internal gravitational attraction.
Examiner insight
“Few candidates stated clearly that protostars are formed from clouds of gas and dust or stellar nebula.” “However, some candidates had the misconception that a stable star occurred before a protostar and chose option A.”
8. A stable star
A protostar becomes a stable star when the inward force of gravitational attraction is balanced by an outward force due to the high temperature in the centre of the star.
A stable star, such as the Sun, is powered by nuclear fusion: hydrogen nuclei join to form helium. All stars eventually run out of hydrogen as fuel for the nuclear reaction.
Examiner insight
“Answers that suggested a force is balanced by a pressure or an energy did not gain credit.” “Weaker candidates often described atoms fusing instead of nuclei.”
Worked example 13 marks
The Sun is an example of a stable star. Describe the process that releases energy in the Sun.
- 1
(nuclear) fusion [1]
- 2
hydrogen nuclei join [1]
- 3
(into / to form) helium [1]
9. Red giants and planetary nebulae
When most of the hydrogen in the centre has been converted to helium, most stars expand to form red giants. More massive stars expand to form red supergiants.
A red giant from a less massive star forms a planetary nebula with a white dwarf star at its centre.
Examiner insight
“A common misconception was to state that a red super giant forms a planetary nebula.” “Common incorrect responses for what is at the centre of a planetary nebula were either a black hole or dust and gas.”
Worked example 12 marks
Planetary nebulae form as part of the life cycle of stars.
(i) Complete the sentence about a planetary nebula.
Stars expand when they run out of hydrogen to fuel nuclear reactions. A ............ forms a planetary nebula.
(ii) State what is at the centre of a planetary nebula.
- 1
(i) red giant [1]
- 2
(ii) white dwarf (star) [1]
10. Supernovae and new stars
A red supergiant explodes as a supernova. This forms a nebula containing hydrogen and new heavier elements.
A neutron star or a black hole is left behind at its centre. The nebula from a supernova may form new stars with orbiting planets.
Examiner insight
“The most common errors included red giant instead red supergiant, not knowing that the nebula contains the heavier elements or white dwarf and black dwarf instead of neutron star and black hole.”
Recap
- Galaxies are made of many billions of stars. The Sun is a star in the Milky Way.
- A light-year is the distance travelled in (the vacuum of) space by light in one year.
- Extended: 1 light-year = 9.5 × 10¹⁵ m.
- Extended: a cloud of gas and dust collapses and heats up as a protostar. It becomes a stable star when inward gravitational attraction is balanced by an outward force.
- Extended: all stars run out of hydrogen. A red giant forms a planetary nebula with a white dwarf at its centre.
- Extended: a red supergiant explodes as a supernova. This forms a nebula with heavier elements and leaves a neutron star or a black hole.
11. Galaxies and the Milky Way
A galaxy is a collection of billions of stars. The Milky Way is the galaxy that contains the Sun.
The Milky Way is one of many billions of galaxies that make up the Universe. Its diameter is approximately 100 000 light-years.
A light-year is a distance, not a time. It is the distance travelled in space by light in one year.
Examiner insight
“Many candidates scored partial credit here, but a large number thought that a light year was a unit of time.”
Worked example 11 mark
The Sun is a star in the Milky Way galaxy. State the approximate diameter of the Milky Way. Give your answer in light-years.
- 1
diameter = 100 000 light-years [1]
12. Redshift
Redshift is an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies.
The light from distant galaxies appears redshifted when it is compared with light emitted on the Earth. Say that the wavelength of the light increases, not the wavelength of the galaxy.
Examiner insight
“Stronger answers clearly stated that the wavelength of light or electromagnetic radiation from a distant galaxy increased due to the galaxy receding (from Earth).” “Some candidates stated that when wavelength increases frequency decreases. This is correct but does not answer the question.”
Worked example 11 mark
Scientists on the Earth observe light emitted by a distant galaxy. The observed light is redshifted. State what is meant by redshifted.
- 1
the observed light has a longer wavelength [1]
13. Evidence for an expanding Universe
Redshift in the light from distant galaxies is evidence that the Universe is expanding. It supports the Big Bang Theory.
More distant galaxies show more redshift. This means they are moving away faster than nearer galaxies. The galaxies were closer together in the past.
Examiner insight
“A large number mentioned ‘redshift’ but did not link the redshift to a type of electromagnetic radiation. Some candidates talked about the redshift of stars or the redshift of planets.” “The strongest answers to this question stated that scientists’ measurements show that the more distant galaxies are moving away faster and that this is evidence of the Universe expanding.”
Worked example 14 marks
Describe the evidence that supports the idea of an expanding Universe. Use your ideas about redshift.
- 1
redshift is the increase in the observed wavelength of light from galaxies [1]
- 2
light from distant galaxies is observed on Earth [1]
- 3
the redshift is caused by the galaxies moving away from Earth [1]
- 4
more distant galaxies have more redshift [1]
- 5
also credited: more distant galaxies are moving away faster than nearer galaxies
14. Cosmic microwave background radiation
Microwave radiation of a specific frequency is observed at all points in space around us. It is called cosmic microwave background radiation (CMBR).
The CMBR was produced shortly after the Universe was formed. As the Universe expanded, the radiation was expanded into the microwave region of the electromagnetic spectrum.
Examiner insight
“Answers just stating “Big Bang” without any reference to time were not creditworthy.” “Stronger answers stated that due to expansion of the Universe, redshift had occurred, or the wavelength of the radiation had increased.”
Worked example 11 mark
As the Universe expands, the wavelength of electromagnetic waves reaching the Earth from distant galaxies increases. This can cause waves that are emitted in one part of the electromagnetic spectrum to be detected in another part of that spectrum. Which change could be caused by expansion of the Universe?
A radio waves to microwaves · B microwaves to ultraviolet · C visible light to microwaves · D microwaves to gamma rays
- 1
expansion makes the wavelength longer, so the waves move to a longer-wavelength region
- 2
C: visible light to microwaves. [1]
15. Finding speed and distance
The speed v at which a galaxy is moving away from the Earth can be found from the change in wavelength of its starlight due to redshift.
The distance d of a far galaxy can be found from the brightness of a supernova in that galaxy.
Examiner insight
“This question required the answer of speed (that galaxy is moving away from the Earth) as this is the quantity that can be directly calculated from the amount of redshift.” “Some candidates identified ‘distance’ or ‘distance from Earth’ but these were too vague to gain credit.”
16. The Hubble constant
The Hubble constant H₀ is the ratio of the speed at which the galaxy is moving away from the Earth to its distance from the Earth.
The current estimate is H₀ = 2.2 × 10⁻¹⁸ per second.
H₀ = v/d
Examiner insight
“Although many candidates were able to identify the key, a large number chose option B, that the speed was inversely proportional to the distance that the galaxy is from the Earth.” “Candidates who defined H0 as the subject of the equation H0 = v / d without defining v and d gained some credit.”
Worked example 12 marks
A distant galaxy is 1.6 × 10⁸ light-years away from the Earth. The current estimate for the Hubble constant is 2.2 × 10⁻¹⁸ per second. Calculate the speed at which this galaxy moves away from the Earth. Give your answer in m / s.
- 1
1 light-year = 9.5 × 10¹⁵ m, so v = H₀ d = 2.2 × 10⁻¹⁸ × 1.6 × 10⁸ × 9.5 × 10¹⁵ [1]
- 2
= 3.3 × 10⁶ m/s [1]
17. The age of the Universe
The equation below gives an estimate for the age of the Universe.
This is evidence for the idea that all the matter in the Universe was present at a single point.
d/v = 1/H₀
Examiner insight
“This question required either an equation with age of the Universe as the subject or the equation 1 / H0 = d / v. Common insufficient answers here included writing an equation with H0 as the subject or writing the term 1 / H0 on its own.” “The command word was ‘calculate’ and the answer line had the unit s so there was no credit for recalling a value for the age of the Universe in e.g., years.”
Worked example 12 marks
The Hubble constant is 2.2 × 10⁻¹⁸ per second. Estimate the age of the Universe in seconds.
- 1
age of the Universe = 1/H₀ [1]
- 2
= 1/(2.2 × 10⁻¹⁸) = 4.5 × 10¹⁷ s [1]
Recap
- A galaxy is a collection of billions of stars. The Milky Way is one of many billions of galaxies.
- The diameter of the Milky Way is approximately 100 000 light-years.
- Redshift is an increase in the observed wavelength of light from receding galaxies.
- Redshift is evidence that the Universe is expanding and supports the Big Bang Theory.
- Extended: CMBR is microwave radiation seen at all points in space, produced shortly after the Universe formed.
- Extended: H₀ = v ÷ d, with H₀ = 2.2 × 10⁻¹⁸ per second. Speed comes from redshift, distance from a supernova's brightness.
- Extended: age of the Universe ≈ d ÷ v = 1 ÷ H₀.
End-of-chapter exercise
Test yourself on the whole chapter. Work through these before moving on.
- Name the two elements that make up most of the Sun.2 marks
- The Sun is a star. State whether it is a small, medium or very large star.1 mark
- Name the three regions of the electromagnetic spectrum in which the Sun radiates most of its energy.3 marks
- A student says that the Sun radiates 'heat and light'. Give the names of the two regions of the electromagnetic spectrum that these words describe.2 marks
- Describe the Sun in terms of what type of object it is, its size and what it consists of.3 marks
- A student says that the Sun radiates most of its energy as X-rays and gamma radiation. Explain whether the student is correct.2 marks
- A stable star is powered by nuclear reactions that release energy. Name the element that is used up in these reactions and the element that is formed.2 marks
- A student says that the Sun gives out energy because it is burning hydrogen. Explain what is wrong with this idea.2 marks
- A question asks for the process that releases energy in a stable star. A candidate writes: ‘Helium atoms are joined by nuclear fussion to form hydrogen.’ (a) Identify three faults in the candidate's sentence. (b) Write a sentence that would earn full credit.4 marks
- Star K is a stable star. It radiates its energy in the same way as the Sun. A satellite records the energy that star K radiates each second in six regions of the electromagnetic spectrum. The total for the six regions is 40.0 × 10²⁵ J. The table gives five of the six results. (a) Calculate the energy radiated each second in the sixth region, and name that region. (b) Calculate the percentage of star K's energy that is radiated in the three regions in which the Sun radiates most of its energy. (c) A student says: ‘Star K is powered by burning helium.’ Give two corrections. | region | energy radiated each second / 10²⁵ J | | --- | --- | | gamma rays | 0.1 | | X-rays | 0.4 | | ultraviolet | 4.0 | | visible light | 17.2 | | radio waves | 0.3 |6 marks
- State what a galaxy is made up of.1 mark
- The Sun is a star in the Milky Way. Name the type of object the Milky Way is. State how far away the other stars in the Milky Way are compared with the Sun.2 marks
- A student says: "A light-year is the time light takes to travel across space." Explain why this is wrong.2 marks
- A star is 8.0 light-years from the Earth. State how many years its light takes to reach the Earth.1 mark
- A star is 2.0 light-years from the Earth. Calculate this distance in m.2 marks
- Describe how a protostar is formed and what happens to its temperature.3 marks
- A star is stable. (a) Name the inward force on the star. (b) State what causes the outward force on the star. (c) Name the fuel used up in the nuclear reaction in the star.3 marks
- The Sun is a less massive star. Describe what happens to the Sun after most of the hydrogen in its centre has been converted to helium.3 marks
- Scientists plan to send a pulse of light to a mirror on a planet that orbits another star. The mirror reflects the pulse straight back to the scientists. The planet is 1.14 × 10¹⁴ km from the scientists. Calculate the time, in years, between sending the pulse and receiving it. Use 1 light-year = 9.5 × 10¹⁵ m.4 marks
- Astronomers observe two nebulae. Nebula X has a white dwarf star at its centre. Nebula Y has a neutron star at its centre. (a) For each nebula, name the type of star that formed it and state what type of nebula or event was involved. (b) State which nebula came from the more massive star. Give a reason. (c) State what nebula Y contains and what it may form.7 marks
- State the approximate diameter of the Milky Way. Give your answer in light-years.1 mark
- State what a galaxy is. Name the galaxy that contains the Sun.2 marks
- Light from a distant galaxy reaches the Earth. Its wavelength is longer than that of light emitted on the Earth. Name this effect. State what it shows about the motion of the galaxy.2 marks
- A galaxy emits light of wavelength 5.0 × 10⁻⁷ m. On the Earth the light has wavelength 5.2 × 10⁻⁷ m. Is the light redshifted? Explain.2 marks
- Explain how redshift supports the Big Bang Theory.3 marks
- Cosmic microwave background radiation (CMBR) is observed at all points in space. State which region of the electromagnetic spectrum it is in. Explain why it is in that region.3 marks
- A galaxy is 3.0 × 10²⁴ m from the Earth. The Hubble constant is 2.2 × 10⁻¹⁸ per second. Calculate the speed at which the galaxy moves away from the Earth.2 marks
- A galaxy is moving away from the Earth at 1.1 × 10⁷ m/s. The Hubble constant is 2.2 × 10⁻¹⁸ per second. Calculate its distance from the Earth.2 marks
- An astronomer studies one far galaxy. The change in wavelength of its starlight shows that it is moving away from the Earth at 7600 km / s. The brightness of a supernova in the galaxy shows that it is 4.0 × 10⁸ light-years from the Earth. 1 light-year = 9.5 × 10¹⁵ m. 1 year = 3.15 × 10⁷ s. (a) Use only these two measurements to calculate a value for the Hubble constant. Give the unit. (b) Use your value from (a) to estimate the age of the Universe. Give your answer in years.5 marks
- Galaxy P is 2.0 × 10⁸ light-years from the Earth. Galaxy Q is twice as far from the Earth as galaxy P. The Hubble constant is 2.2 × 10⁻¹⁸ per second. 1 light-year = 9.5 × 10¹⁵ m. (a) Calculate the speed at which galaxy P is moving away from the Earth. Give your answer in m / s. (b) A student says that galaxy Q is moving away at half the speed of galaxy P because it is further away. Calculate the speed of galaxy Q. State whether the student is correct. (c) Calculate d ÷ v in seconds for each galaxy. State what this value estimates and what idea it is evidence for.7 marks
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