P6.2.2P6. Space Physics
Stars — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 10 parts Lesson 5 of 6 1 quiz
Lesson summary
P6.2.2 Stars — COMBINED SCIENCE notes: Describe galaxies and the life cycles of low-mass and high-mass stars.Galaxy = a large collection of billions of starsThe Sun is one of billions of stars in the Milky Way.The Milky Way is about 100 000 light-years in diameter.Light takes 100…
- Distances to stars
- Equilibrium
- Fusion energy
- Later stages
- Low-mass star
- Heavy elements
Part 1 of 8Galaxies and stars
1Galaxy
Galaxy a large collection of billions of stars
- The Sun is one of billions of stars in the Milky Way.
- The Milky Way is about 100 000 light-years in diameter.
- Light takes 100 000 years to cross it, and the centre is about 30 000 light-years from Earth.
2Distances to stars
- Distances to stars other stars in the Milky Way are much further away than the Sun
- Some of these stars have planets that orbit them.
3Light-year
Light-year the distance travelled by light in one year, about 9.5 × 1015 m
Part 2 of 8Formation of a star
1
A nebula, an interstellar cloud of gas and dust, collects under gravitational attraction.
2
The particles are pulled together into a hot, dense ball of gas called a protostar.
3
Collisions become more frequent, so the temperature rises.
4
When the core is hot enough, hydrogen fuses to helium and the protostar becomes a stable star.
Part 3 of 8Stable star
1Equilibrium
- Equilibrium the inward gravitational force is balanced by the outward force from the high temperature in the core
2Fusion energy
- Fusion energy each fusion reaction releases heat and light, which keeps the core hot
3Later stages
- Later stages the final stages of the life cycle depend on the mass of the star
Part 4 of 8Low-mass stars
- Low-mass star a star of up to about eight times the mass of the Sun that ends as a white dwarf
1
After several billion years the hydrogen runs out and the rate of fusion falls.
2
The core shrinks and heats up because gravity now exceeds the outward pressure.
3
Helium in the core fuses to carbon and the outer layers expand and cool, giving a red giant.
4
When the helium runs out the core collapses and the outer layers are ejected as a planetary nebula.
5
The collapsed core is a white dwarf, which cools and emits less energy over time.
Part 5 of 8High-mass stars
- High-mass star a star of more than about eight times the mass of the Sun
1
After several million years the hydrogen in the core runs out.
2
The core shrinks and heats, and the outer layers expand and cool to form a red supergiant.
3
Helium fuses to carbon and then to successively heavier elements, with the core collapsing and expanding repeatedly.
4
Fusion stops once iron forms, so the core collapses rapidly and causes a supernova explosion.
5
A neutron star forms at the centre, and the outer layers are ejected as a nebula.
6
The most massive stars collapse further into a black hole.
Part 6 of 8Supernova and heavy elements
Heavy elements
- Heavy elements elements heavier than iron are formed in a supernova and ejected into space
New stars
- New stars the nebula from a supernova may form new stars with orbiting planets
Black hole
Black hole an extremely dense point in space from which not even light can escape
Part 7 of 8Life cycles compared
| Stage | Low-mass star | High-mass star |
|---|---|---|
| After the stable star | Red giant | Red supergiant |
| Ejection of outer layers | Planetary nebula | Supernova |
| Remnant | White dwarf | Neutron star or black hole |
Part 8 of 8More from the specification
More from the specification
Stellar parallax as a distance means
- Stellar parallax as a distance means measuring tiny angular shifts relative to distant stars provides a geometric means to calculate astronomical distances.
Stars
- Stars a star such as the Sun produces radiation by nuclear fusion, and its life cycle depends on its mass
Redshift
- Redshift the wavelength of light from distant galaxies is stretched because they are moving away from us
Hubble's law
- Hubble's law galaxies move away at a speed proportional to their distance, which supports an expanding universe
Cosmic microwave background
- Cosmic microwave background radiation of long wavelength left over from the early universe;
- the speed of light is constant at 3 x 108 m/s in vacuum
Nuclear fusion
- Joining of hydrogen nuclei, releasing energy.
Background radiation
- The radiation that exists around us all the time from natural and man-made sources.
Universe
- Everything that exists.
Redshift
- An increase in the observed wavelength of light from a receding source.
Radiation
- Thermal energy transfer by waves, with no particles needed.
Wavelength
- The distance between the same points on two neighbouring waves.
Recession
- Moving away.
Spectrum
- The range of colours in white light.
Hubble constant
- H0 = v / d, the ratio of a galaxy's recession speed to its distance, about 2.2 × 10-18 per second.
v = H0d
What the symbols mean: v = recession speed of the galaxy (km s−1) · H0 = Hubble constant (km s−1 Mpc−1) · d = distance (Mpc)
Key terms
| Term | Meaning |
|---|---|
| Galaxy | A large collection of billions of stars. |
| Nebula | An interstellar cloud of gas and dust. |
| Protostar | A hot ball of gas formed as gravity pulls a nebula together. |
| Red giant | A low-mass star that has expanded and cooled after its core hydrogen runs out. |
| White dwarf | The cooling collapsed core left after a low-mass star ejects its outer layers. |
| Supernova | The explosion of a high-mass star when fusion stops at iron. |
| Neutron star | A very dense body left at the centre of a supernova. |
| Black hole | A point of extreme density from which light cannot escape. |
Avoid these mistakes
1
Saying all stars become black holes — most high-mass stars become neutron stars and only the most massive form black holes
2
Saying the Sun will explode as a supernova — it is a low-mass star and ends as a white dwarf
3
Confusing a nebula with a galaxy — a nebula is a cloud of gas and dust and a galaxy is billions of stars
4
Saying a protostar already fuses hydrogen — fusion starts once it is hot enough and it becomes a stable star
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