6.2 Stars & The Universe (Cambridge (CIE) O Level Physics): Flashcards
Exam code: 5054
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- FrontThe Sun as a Star
Define the Milky Way.
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Define the Milky Way.
The Milky Way is the galaxy that contains the Sun, made up of billions of stars, some of which have orbiting planets.
Define the Sun.
The Sun is the star at the centre of the Solar System. It is a medium-sized star made mainly of hydrogen and helium.
Why do the planets orbit the Sun, rather than the Sun orbiting them?
The Sun contains over 99% of the mass of the Solar System, so its gravitational pull dominates and keeps the planets in orbit around it.
How does a star's colour relate to its surface temperature?
A star's colour depends on its surface temperature: a red star is the coolest (around 3000 K) and a blue star is the hottest (around 30 000 K).
The Sun radiates most of its energy in the ______, ______ and ______ regions of the electromagnetic spectrum.
The Sun radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum.
Which two isotopes of hydrogen fuse together in a star's core to form helium?
Deuterium (
) and tritium (
) fuse together to form helium (
) and a neutron (
).Why does nuclear fusion prevent a stable star from collapsing under gravity?
Fusion reactions release a huge amount of energy, creating an outward pressure force that balances the inward force of gravity, keeping the star stable.
True or False?
The Sun is one of the largest stars in the Milky Way.
False.
The Sun is only a medium-sized star — stars range from red dwarfs up to blue giants and red supergiants, many of which are far larger than the Sun.
Define nebula.
A nebula is a giant interstellar cloud of hydrogen gas and dust from which stars form.
Define protostar.
A protostar is a hot ball of gas formed when gravity pulls the particles of a nebula closer together, increasing its density and temperature.
What is meant by saying a main-sequence star is in equilibrium?
The inward force due to gravity is exactly balanced by the outward pressure force from nuclear fusion reactions in the core, so the star remains stable.
After the main sequence, a low-mass star becomes a ______, then ejects its outer layers as a ______, before its core collapses into a ______.
After the main sequence, a low-mass star becomes a red giant, then ejects its outer layers as a planetary nebula, before its core collapses into a white dwarf.
Why does a low-mass star's core shrink and heat up as it becomes a red giant?
As the hydrogen fuel runs out, fusion in the core dies down, so the inward force of gravity becomes greater than the outward pressure force, causing the core to shrink and heat up.
What happens to a white dwarf over time?
A white dwarf cools down and its energy output decreases; once it has lost a significant amount of energy, it becomes a black dwarf.
Describe the stages of a high-mass star's life cycle after the main sequence.
Red supergiant → supernova → neutron star (or a black hole for the most massive stars).
Define supernova.
A supernova is the gigantic explosion caused by the sudden collapse of a high-mass star's core, once fusion reactions can no longer continue after iron has formed.
True or False?
A neutron star can form from the collapse of a low-mass star.
False.
Neutron stars (and black holes) form only from the collapse of high-mass stars during a supernova; low-mass stars end their lives as white dwarfs and eventually black dwarfs.
Define light-year.
A light-year is the distance travelled by light through (the vacuum of) space in one year, equal to 9.5 × 10¹⁵ m.
The diameter of the Milky Way is approximately ______ light-years.
The diameter of the Milky Way is approximately 100 000 light-years.
Define redshift.
Redshift is an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies.
How does the Doppler effect change the wavelength and frequency of waves in front of and behind a moving source?
In front of the source, the wavelength decreases and the frequency increases; behind the source, the wavelength increases and the frequency decreases.
What is the relationship between a galaxy's distance from Earth and the redshift observed in its light?
The greater the distance to a galaxy, the greater the redshift — meaning more distant galaxies are moving away from us faster.
What evidence from distant galaxies supports the Big Bang theory?
Light from distant galaxies is redshifted, showing that the galaxies are moving away from us. This indicates the universe is expanding, which supports the Big Bang theory.
According to the Big Bang theory, how did the Universe begin?
Around 14 billion years ago, the Universe began from a very small region that was extremely hot and dense, then underwent a giant explosion (the Big Bang) and has been expanding and cooling ever since.
True or False?
The balloon analogy shows that galaxies move apart because space itself expands between them.
True.
As the balloon inflates, points (galaxies) move apart by the same amount because the space itself stretches between them, not because the galaxies move independently through fixed space.
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