Stars & The Universe

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The Sun as a Star

The Sun is a medium-sized star at the centre of the Solar System.
It consists mostly of hydrogen and helium.
It radiates most energy in the infrared, visible and ultraviolet regions.
In the core, nuclear fusion converts hydrogen into helium, releasing huge energy.
All stable stars are powered by nuclear fusion.

The Sun as a star

Scale of the Universe

The Universe is a large collection of billions of galaxies.
A galaxy is a large collection of billions of stars.
The Milky Way is our galaxy; the Sun is one of its billions of stars.
Other stars in the Milky Way are much further from Earth than the Sun.
A light-year is the distance light travels in one year (≈ 9.5 × 10¹⁵ m).
The Milky Way's diameter is about 100 000 light-years.

Hierarchy of the Universe

Star Formation

Stars form from a nebula (cloud of gas and dust).
Gravity pulls particles together to form a protostar; density and temperature increase.
When the core is hot enough, nuclear fusion begins → stable star.
In a stable star, inward gravitational forces balance outward thermal pressure.

Balanced forces in a stable star

Life Cycle of Low-Mass Stars (≤ 8 solar masses)

After stable phase: red giant → planetary nebula → white dwarf.
Red giant: hydrogen depleted, core shrinks and heats, helium fuses to carbon, outer layers expand.
Planetary nebula: outer layers ejected; core collapses.
White dwarf: hot, dense core that cools and dims over time.

Life cycle of a low-mass star

Life Cycle of High-Mass Stars (> 8 solar masses)

After stable phase: red supergiant → supernova → neutron star or black hole.
Red supergiant: fuses heavier elements up to iron; core collapses and expands repeatedly.
Supernova: core collapses rapidly, triggering a huge explosion; heavy elements (heavier than iron) are formed.
Neutron star: dense remnant at the centre; if massive enough, collapses to a black hole.
Nebulae from supernovae may form new stars and planets.

Life cycle of high-mass stars

Galactic Redshift

Redshift is the increase in wavelength of light from a source moving away (Doppler effect).
Light from distant galaxies is redshifted compared to hydrogen on Earth → galaxies are moving away.
Greater redshift means the galaxy is further away and moving faster.
This shows the Universe is expanding.

Redshift and blueshift of light

The Big Bang Theory

The Universe began about 14 billion years ago from a hot, dense single point.
Evidence: galactic redshift (galaxies receding → expansion) and cosmic microwave background radiation (CMBR).
CMBR is uniform microwave radiation from all directions, left over from the early Universe.
CMBR was originally high-energy gamma radiation; it has been redshifted as the Universe expanded.

The Big Bang Theory showing the expansion of the universe from a single point.

Age of the Universe (Extended)

Hubble's law: recession speed v is proportional to distance d: v = H₀ d.
Hubble constant H₀ = 2.2 × 10⁻¹⁸ per second (current accepted value).
Age of Universe ≈ 1 / H₀ ≈ 13.7 billion years.
Distance to galaxies measured using redshift and brightness of supernovae (standard candles).

Graph of galaxy recession velocity against distance

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Soal latihan

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1.Which of the following is the correct definition of a galaxy?
Easy
- AA large collection of billions of stars
- BA large collection of billions of planets
- CA large collection of billions of galaxies
- DA large collection of billions of moons
2.What is the approximate diameter of the Milky Way galaxy in light-years?
Easy
- A100 000 light-years
- B1 million light-years
- C10 000 light-years
- D1 billion light-years
3.What is the name of the force that pulls the outer layers of a star inwards during its stable phase?
Easy
- AGravitational force
- BThermal pressure
- CNuclear force
- DElectromagnetic force
4.What is the correct order of stages in the life cycle of a star similar in mass to the Sun after the stable star phase?
Easy
- ARed giant, planetary nebula, white dwarf
- BRed supergiant, supernova, neutron star
- CPlanetary nebula, white dwarf, red giant
- DWhite dwarf, red giant, planetary nebula
5.What does the observed redshift in the light from distant galaxies indicate?
Easy
- AGalaxies are moving away from Earth
- BGalaxies are moving towards Earth
- CGalaxies are stationary
- DGalaxies are shrinking
6.The Hubble constant H0 is approximately 2.2 × 10⁻¹⁸ per second. A galaxy is at a distance of 1.0 × 10²³ m. What is its recessional velocity?
Medium
- A2.2 × 10⁵ m/s
- B2.2 × 10⁻⁴¹ m/s
- C4.5 × 10⁴⁰ m/s
- D2.2 × 10⁻⁵ m/s
7.Which of the following is evidence for the Big Bang theory?
Easy
- AGalactic redshift and cosmic microwave background radiation
- BThe Sun's energy from nuclear fusion
- CThe existence of black holes
- DThe formation of planetary nebulae
8.What is the approximate age of the Universe according to the Hubble constant?
Easy
- A13.7 billion years
- B13.7 million years
- C4.5 billion years
- D100 billion years

Soal lainnya

9.What is the name of the galaxy that contains the Sun?
Easy
- AThe Milky Way
- BAndromeda
- CTriangulum
- DVirgo A
10.What is a protostar?
Easy
- AA hot ball of gas formed from a nebula before nuclear fusion begins
- BA star that has run out of hydrogen fuel
- CThe remnant of a supernova
- DA star in its stable phase
11.What is the ultimate fate of a very massive star (more than 8 solar masses) after a supernova?
Easy
- ANeutron star or black hole
- BWhite dwarf
- CPlanetary nebula
- DRed giant
12.A galaxy is at a distance of 9.0 × 10⁷ light-years from Earth. The Hubble constant is 2.2 × 10^−18 per second. What is the recessional speed of this galaxy in m/s? (1 light-year = 9.5 × 10¹⁵ m)
Medium
- A1.9 × 10⁶ m/s
- B2.0 × 10⁵ m/s
- C1.9 × 10⁸ m/s
- D2.0 × 10⁷ m/s
13.The spectrum of light from a distant galaxy shows a dark line at a wavelength of 656 nm. The same line from a laboratory source on Earth is at 656 nm. What can be concluded about the galaxy?
Medium
- AThe galaxy is moving away from Earth.
- BThe galaxy is moving towards Earth.
- CThe galaxy is stationary relative to Earth.
- DThe galaxy is moving perpendicular to the line of sight.
14.The graph shows recession velocity vs distance for several galaxies. What does the gradient of the best-fit line represent?
Medium
- AThe age of the Universe
- BThe Hubble constant
- CThe speed of light
- DThe distance to the farthest galaxy
15.The Sun is a stable star. Which two forces are in equilibrium during this stage?
Easy
- AGravitational force pulling inwards and thermal pressure pushing outwards
- BGravitational force pushing outwards and thermal pressure pulling inwards
- CMagnetic force and electric force
- DNuclear force and electromagnetic force
16.A star with a mass 15 times that of the Sun is in its stable phase. What will be its final evolutionary stage?
Easy
- AWhite dwarf
- BNeutron star
- CBlack hole
- DRed giant
17.The Hubble constant is 2.2 × 10^−18 per second. What is the approximate age of the Universe in seconds?
Medium
- A4.5 × 10¹⁷ s
- B2.2 × 10¹⁸ s
- C4.5 × 10¹⁸ s
- D2.2 × 10¹⁷ s
18.Which of the following is NOT evidence for the Big Bang theory?
Easy
- AGalactic redshift
- BCosmic microwave background radiation
- CThe existence of black holes
- DThe relative abundances of light elements
19.A distant galaxy has a recessional velocity of 1.5 × 10⁶ m/s. Using H0 = 2.2 × 10^−18 per second, what is its distance from Earth in metres?
Medium
- A6.8 × 10²³ m
- B3.3 × 10²⁴ m
- C6.8 × 10²² m
- D3.3 × 10²³ m
20.What is the main source of energy in a stable star like the Sun?
Easy
- ANuclear fission of uranium
- BNuclear fusion of hydrogen into helium
- CChemical burning of hydrogen
- DGravitational collapse
21.Cosmic microwave background radiation (CMBR) is detected in all directions. What is its current wavelength region?
Easy
- AGamma rays
- BX-rays
- CMicrowaves
- DRadio waves
22.A student investigates the relationship between the recessional velocity of galaxies and their distance from Earth. Which two measurements are needed to determine the Hubble constant?
Medium
- ARedshift of light from a galaxy and the brightness of a supernova in that galaxy
- BDistance to a galaxy and the mass of the galaxy
- CRedshift of light from a galaxy and the temperature of the galaxy
- DBrightness of a supernova and the age of the galaxy
23.A galaxy is observed to have a recessional velocity of 1.2 × 10⁶ m/s. Using H0 = 2.2 × 10^−18 s^−1, what is the distance to this galaxy in metres?
Medium
- A5.5 × 10²³
- B2.6 × 10²⁴
- C5.5 × 10²²
- D2.6 × 10²³
24.The diagram shows the spectrum of light from a distant galaxy compared to a laboratory spectrum. The dark lines are from hydrogen. What does the shift in the dark lines indicate?
Medium
- AThe galaxy is moving away from Earth
- BThe galaxy is moving towards Earth
- CThe galaxy is stationary relative to Earth
- DThe galaxy is rotating
25.The Hubble constant is estimated to be 2.2 × 10^−18 s^−1. What is the approximate age of the Universe in years? (1 year = 3.15 × 10⁷ s)
Medium
- A1.4 × 10¹⁰ years
- B4.5 × 10⁹ years
- C1.4 × 10⁹ years
- D4.5 × 10¹⁰ years
26.Which of the following is the correct order of stages in the life cycle of a star with a mass similar to the Sun after the stable star phase?
Easy
- ARed giant → planetary nebula → white dwarf
- BRed supergiant → supernova → neutron star
- CRed giant → supernova → black hole
- DPlanetary nebula → red giant → white dwarf
27.A supernova in a distant galaxy has the same peak brightness as a known standard candle. Its apparent brightness is measured to be 1/100 of the standard candle's brightness at a distance of 10 parsecs. What is the distance to the galaxy? (Assume brightness follows inverse square law)
Hard
- A100 parsecs
- B1000 parsecs
- C10 000 parsecs
- D1 000 000 parsecs
28.What is the approximate diameter of the Milky Way galaxy?
Easy
- A100 000 light-years
- B1 000 000 light-years
- C10 000 light-years
- D1 000 light-years
29.The table shows the distance of three galaxies from Earth. | Galaxy | Distance from Earth (m) | |---|---| | A | 1.0 × 10²³ | | B | 5.0 × 10²² | | C | 2.0 × 10²³ | | D | 8.0 × 10²² | According to Hubble's law, which galaxy has the greatest recessional velocity?
Medium
- AGalaxy A
- BGalaxy B
- CGalaxy C
- DGalaxy D
30.What is the definition of a light-year?
Easy
- AThe distance travelled by light in one year
- BThe time taken for light to travel from the Sun to the Earth
- CThe distance from the Earth to the Sun
- DThe time taken for light to travel across the Milky Way
31.The Sun is a star. Which two elements does it consist mostly of?
Easy
- AHydrogen and helium
- BHydrogen and carbon
- CHelium and oxygen
- DCarbon and nitrogen
32.A galaxy is at a distance of 5.0 × 10²² m from Earth. The Hubble constant is 2.2 × 10^−18 per second. What is the recessional velocity of this galaxy?
Medium
- A1.1 × 10⁵ m/s
- B1.1 × 10⁴ m/s
- C2.2 × 10⁴ m/s
- D1.1 × 10⁶ m/s
33.A student measures the spectrum of light from a distant galaxy. A hydrogen absorption line that is normally at 656 nm on Earth is observed at 662 nm. What can be concluded about the galaxy?
Hard
- AIt is moving towards Earth at a speed of about 2.7 × 10⁶ m/s
- BIt is moving away from Earth at a speed of about 2.7 × 10⁶ m/s
- CIt is stationary relative to Earth
- DIt is moving away from Earth at a speed of about 3.0 × 10⁸ m/s
34.The Sun consists mostly of hydrogen and helium.
Easy
True or false?
35.A light-year is a unit of time.
Easy
True or false?
36.What is the initial stage in the life cycle of all stars?
Easy
- AProtostar
- BRed giant
- CNebula
- DWhite dwarf
37.The greater the observed redshift of a galaxy, the closer it is to Earth.
Easy
True or false?
38.Cosmic microwave background radiation (CMBR) is evidence for the Big Bang because it is the redshifted radiation from the early hot Universe.
Medium
True or false?
39.The age of the Universe can be estimated as 1/H₀, where H₀ is the Hubble constant.
Easy
True or false?
40.Arrange the following stages in the life cycle of a low-mass star (like the Sun) in the correct order, starting from the earliest stage.
Medium
- white dwarf
- red giant
- planetary nebula
- stable star (main sequence)
41.Match each object with its correct description.
Easy
- Universe
- Galaxy
- Solar System
- a large collection of billions of stars
- a large collection of billions of galaxies
- a star and the planets that orbit it
42.Which of the following are pieces of evidence for the Big Bang theory? (Select all that apply.)
Medium
- AGalactic redshift shows that distant galaxies are moving away from us.
- BThe cosmic microwave background radiation is detected uniformly in all directions.
- CThe Sun is composed mostly of hydrogen and helium.
- DThe further away a galaxy is, the faster it is moving away from us.
43.The Hubble constant H₀ is 2.2 × 10⁻¹⁸ per second. A galaxy is at a distance of 1.5 × 10²³ m. Calculate the recessional velocity of the galaxy in m/s.
Easy
44.Complete the sentence about star formation.
Easy
A star begins as a cloud of gas and dust called a ____. Gravitational attraction pulls particles together to form a hot ball of gas called a ____. When the core becomes hot enough for ____ reactions to begin, the star becomes a stable star.
45.Describe and explain what can be deduced about the history of the Universe from the cosmic microwave background radiation (CMBR).
Hard
46.The Hubble constant H₀ is 2.2 × 10⁻¹⁸ per second. A galaxy is at a distance of 1.5 × 10²³ m. Calculate the recessional velocity of the galaxy in m/s.
Easy
- A330000 m/s
- B396000 m/s
- C363000 m/s
- D297000 m/s

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