(c) Stellar evolution
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
8.7Star colour classificationUnderstand how stars can be classified according to their colour8.8Star colour and temperatureKnow that a star’s colour is related to its surface temperature8.9Sun-like star evolutionDescribe the evolution of stars of similar mass to the Sun through the following stages:• nebula• star (main sequence)• red giant• white dwarf8.10Massive star evolutionDescribe the evolution of stars with a mass larger than the Sun811P Absolute magnitudeUnderstand how the brightness of a star at a standard distance can be represented using absolute magnitude812P Hertzsprung-Russell diagramDraw the main components of the Hertzsprung–Russell diagram (HR diagram)
Classify stars by their colour
Stars can be classified by the colour of the light from their surfaces. Stars with similar observed colours belong in the same colour group.
Use the colour itself as the classification evidence: common groups include red, orange, yellow, white and blue-white. For example, two red stars share a colour classification even if they differ in size, brightness or evolutionary stage.
Colour classification describes one observable property; it does not identify a unique star and does not by itself mean that two stars have the same mass, brightness or age. Do not classify by apparent brightness when the requested property is colour.
Use colour to compare stellar surface temperature
A star's surface colour is related to its surface temperature. Moving from red towards blue-white corresponds to increasing surface temperature.
| Surface colour | Relative surface temperature |
|---|---|
| red or orange-red | coolest |
| orange | cooler |
| yellow | intermediate |
| white | hotter |
| blue-white | hottest |
The Sun appears yellow, while Betelgeuse appears red. The colour relationship therefore shows that the Sun has a higher surface temperature than Betelgeuse. Likewise, a blue-white star has a higher surface temperature than a yellow star.
This relationship concerns surface temperature, not the temperature of the star's core. Brightness, distance from Earth and mass are different properties and cannot replace colour when comparing surface temperature from this evidence.
Trace the life cycle of a Sun-like star
A star with a mass similar to the Sun follows the ordered path nebula → main sequence star → red giant → white dwarf.
- In a nebula, gravity pulls gas and dust together. The collapsing material heats and forms a protostar. When nuclear fusion begins, it becomes a main sequence star.
- During the main sequence, hydrogen fusion releases energy. When the core hydrogen supply is depleted, the star expands and its surface cools, forming a red giant. The outer layers are then lost; the hot, dense remaining core is a white dwarf, where fusion no longer occurs.
A Sun-like star does not become a red supergiant, supernova, neutron star or black hole. Those stages belong to the high-mass route. The required final stage here is white dwarf; later cooling beyond that stage is outside this objective.
Trace the life cycle of a high-mass star
A star with a mass much larger than the Sun begins like a Sun-like star but follows a different route after the main sequence. Its mass determines that later pathway.
The ordered sequence is: nebula → protostar → high-mass main sequence star → red supergiant → supernova → neutron star or black hole. After leaving the main sequence, the star expands into a red supergiant; its core then collapses and the outer layers explode as a supernova.
| Outcome after the supernova | Meaning |
|---|---|
| neutron star | an extremely compact stellar remnant |
| black hole | a remnant whose gravity is strong enough that light cannot escape |
The final outcome is neutron star or black hole—not both in sequence. A high-mass star does not normally finish as a white dwarf, and a supernova is an explosive stage rather than the final remnant.
Compare stellar brightness using absolute magnitude
Absolute magnitude represents how bright a star would appear if every star were viewed from the same standard distance: 10 parsecs, about 32.6 light-years.
Using one fixed distance removes the effect that nearby stars look brighter than identical distant stars. Absolute magnitude can therefore compare the stars' brightness on a common basis rather than their apparent brightness from Earth.
The magnitude scale runs backwards: a lower or more negative absolute magnitude means a brighter star. For example, a star with absolute magnitude −4 is brighter at the standard distance than one with absolute magnitude +6.
Absolute magnitude is not the brightness seen from the star's actual distance. Apparent brightness depends on distance from the observer; absolute magnitude deliberately places every star at the same standard distance for comparison.
Draw the main regions of an HR diagram
A Hertzsprung–Russell (HR) diagram classifies stars by surface colour or temperature on the horizontal axis and absolute magnitude or luminosity on the vertical axis.
Draw the horizontal axis with hot blue or white stars on the left and cool red stars on the right, so temperature decreases from left to right. Draw absolute magnitude with bright, low or negative values at the top and dim, high positive values at the bottom; a typical scale runs from −5 at the top to +15 at the bottom.
| Region | Position on the HR diagram |
|---|---|
| main sequence | diagonal band from upper left to lower right |
| red giants | upper right: cool but bright |
| white dwarfs | lower left: hot but dim |
Drawing method: label both axes and their directions first; add the main-sequence diagonal; then place and label the red-giant region above and to the right, and the white-dwarf region below and to the left. The Sun lies on the main sequence.
Do not reverse the temperature axis: the hottest stars are on the left. Do not infer brightness from temperature alone; red giants are cool yet bright, while white dwarfs are hot yet dim. An HR diagram is for stars, not planets or moons.