6.2.2 Stars
- Syllabus
- 0625–2026–2027
- Topic
- 6.2.2
- Level
- —
Learning objectives
6.2.2.1(a) galaxies contain many billions of• State: (a) galaxies contain many billions of stars (b) the Sun is in the Milky Way (c) other Milky Way stars are much farther from Earth than the Sun (d) astronomical distances can use light-years: distance travelled by light in space in one year6.2.2.2One light-year is equal to 9.5 × 1015• Know: one light-year is equal to 9.5 × 1015 m6.2.2.3Star life cycle: (a) forms from• Describe a star life cycle: (a) forms from hydrogen-rich gas/dust clouds (b) protostar collapses under gravity and heats up (c) stable star forms when inward gravity balances outward pressure from high core temperature (d) stars eventually run out of hydrogen fuel (e) most expand to red giants; more massive stars become red supergiants after central hydrogen becomes helium (f) less massive red giant forms a planetary nebula with a white dwarf at the centre (g) red supergiant supernova forms a nebula with hydrogen/new heavier elements and leaves a neutron star or black hole (h) supernova nebula may form new stars with planets
Describe galaxies and define the light-year
A galaxy is a vast collection containing many billions of stars. The Sun is one star in the galaxy called the Milky Way.
The Sun is much closer to Earth than every other star in the Milky Way. Even the nearest stars beyond the Sun are so far away that astronomical distance units are useful.
A light-year is the distance travelled by light through the vacuum of space in one year.
If a star is four light-years away, its light takes about four years to reach us. We therefore observe that star as it was about four years earlier.
A light-year is a unit of distance, not time. The Solar System is inside the Milky Way; the Milky Way contains many stars, while the Solar System contains only one star, the Sun.
Use the distance represented by one light-year
One light-year is approximately 9.5 × 10¹⁵ m, or 9.5 × 10¹² km.
Light travels at about 3.0 × 10⁸ m/s and one year is about 365 × 24 × 60 × 60 = 3.15 × 10⁷ s. Therefore d = ct ≈ (3.0 × 10⁸)(3.15 × 10⁷) ≈ 9.5 × 10¹⁵ m.
| Conversion | Operation |
|---|---|
| light-years to metres | multiply by 9.5 × 10¹⁵ |
| metres to light-years | divide by 9.5 × 10¹⁵ |
| light-years to kilometres | multiply by 9.5 × 10¹² |
A distance of 6.6 × 10²⁰ m corresponds to (6.6 × 10²⁰)/(9.5 × 10¹⁵) ≈ 6.9 × 10⁴ light-years.
Do not attach seconds to 9.5 × 10¹⁵: it is a distance in metres. Converting metres to kilometres divides by 1000, so the numerical power changes from 10¹⁵ to 10¹².
Describe the life cycle of a star
A star begins in an interstellar cloud of gas and dust containing hydrogen. Internal gravitational attraction makes part of the cloud collapse; as it contracts, its temperature rises and it becomes a protostar.
A protostar becomes a stable star when the inward force of gravitational attraction is balanced by an outward force due to the high temperature at its centre. Hydrogen fusion supplies the energy that maintains this hot stable state.
Eventually every star runs short of hydrogen fuel in its centre because much of that hydrogen has been converted to helium. The star expands: most stars become red giants, while more massive stars become red supergiants.
| Lower-mass route | Higher-mass route |
|---|---|
| red giant | red supergiant |
| outer layers form a planetary nebula | explodes as a supernova |
| white dwarf remains at the centre | nebula remains, with hydrogen and newly formed heavier elements |
| — | collapsed remnant is a neutron star or, for a sufficiently massive core, a black hole |
Material in the nebula from a supernova can later be pulled together by gravity to form new stars. Accretion discs around those forming stars may also produce orbiting planets, so stellar material is recycled into later systems.
Shared beginning: hydrogen-rich gas and dust → protostar → stable star → hydrogen fuel runs low. Initial mass then controls which end branch the star follows.
A red giant does not explode as a supernova in the lower-mass route, and a white dwarf does not normally become a neutron star or black hole. A planetary nebula is ejected stellar gas, not a nebula containing a newly formed planet.