6.2.2 Stars
What Is a Star?
A star is a huge, self-luminous sphere of hot gas that shines due to nuclear fusion. Our Sun is one such star in the Milky Way.
- Galaxies are made of billions of stars.
- Other stars in the Milky Way are far more distant from Earth than the Sun.
- Astronomical distances are often measured in light-years (1 ly ≈
9.5 × 1015 m).
Birth of a Star
- Interstellar clouds (nebulae), mainly hydrogen, begin to collapse under gravity.
- Collapsing clumps heat up by gravitational energy → a protostar forms.
- A star becomes stable when inward gravitational pull balances outward gas pressure from a hot core (hydrostatic equilibrium).
- At core temperatures > ~10 million K, hydrogen fuses to helium → the star enters the main sequence.
Old Age — Giants and Supergiants
- When core hydrogen is mostly used up, stars leave the main sequence.
- Lower-mass stars (up to ~1.5× Sun) expand into red giants.
- More massive stars (≈1.5–>3× Sun and above) become red supergiants.
- In this phase, fusion shifts to shells around the core (hydrogen-shell fusion).
Stellar “Endings” and Remnants
| Initial Star Type | End of Life | Remnant | Notes |
|---|---|---|---|
| Sun-like / lower-mass | Sheds outer layers → planetary nebula | white dwarf | White dwarf cools over time toward a black dwarf (very long timescales). |
| High-mass (supergiant) | supernova | neutron star or black hole | Supernova spreads heavy elements; debris may seed new stars and planets. |
Why Stars Stay Stable (for a Time)
Stability in the main sequence is the balance of inward gravity and outward pressure from fusion-heated gas — a state called hydrostatic equilibrium. As fuel is depleted, this balance changes, driving the later life stages.
Key Takeaways
- Stars form from collapsing hydrogen-rich nebulae; fusion “turns them on”.
- Main sequence = hydrogen-to-helium fusion in the core; stability via hydrostatic equilibrium.
- Fate depends on mass: giants → planetary nebula + white dwarf; supergiants → supernova → neutron star / black hole.
- 1 light-year is a distance unit (≈
9.5 × 1015 m), not a time unit.
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