The Sun and the life cycle of stars
The lives of stars
- Stars are born, shine for billions of years, and then die.
- Our Sun is one ordinary star among billions.
- Let's meet the Sun, the galaxies, and how a star lives and dies.
The Sun as a star
- The Sun is a medium-sized star, made mostly of hydrogen and helium.
- It radiates energy as infrared, visible light and ultraviolet.
- It is powered by nuclear fusion in its core: hydrogen nuclei join to make helium, releasing huge energy.
What powers the Sun?
In a stable star, hydrogen nuclei fuse into helium in the core, releasing huge amounts of energy.
Galaxies and the Universe
- A galaxy is billions of stars held together by gravity. Our Sun is in the Milky Way.
- Distances between stars are measured in light-years — the distance light travels in one year ($\approx 9.5 \times 10^{15}\ \text{m}$).
- The Milky Way is about $100\,000$ light-years across, and is just one of billions of galaxies that make up the Universe.
A galaxy is:
A galaxy is a huge collection of billions of stars bound by gravity. The Sun is one star in the Milky Way galaxy.
A light-year is a unit of:
A light-year is the distance light travels in one year — a measure of distance, not time.
The life cycle of a star
- A star forms from a nebula (a cloud of gas and dust).
- Gravity pulls it together and it heats up — a protostar.
- It becomes a stable star when gravity's inward pull is balanced by the outward push from its very hot core.
- When the hydrogen fuel runs low, what happens next depends on the star's mass:
- a medium star → red giant → throws off a planetary nebula → leaves a white dwarf;
- a massive star → red supergiant → explodes as a supernova → leaves a neutron star or a black hole.
Put the life stages of a Sun-like star in order.
A star forms from a nebula → protostar → stable star → (fuel runs low) red giant → white dwarf.
In a stable star, the inward pull of gravity is balanced by:
The hot core pushes outward, balancing gravity's inward pull, so the star stays a stable size.
A much heavier star ends its life as a supernova, leaving behind:
A massive star becomes a red supergiant, explodes as a supernova, and leaves a neutron star or a black hole.
You've got it
- the Sun is a medium star (H + He), powered by fusion of hydrogen into helium
- a galaxy = billions of stars held by gravity; we are in the Milky Way
- distances in light-years ($\approx 9.5 \times 10^{15}\ \text{m}$); billions of galaxies make the Universe
- life cycle: nebula → protostar → stable star → (red giant → white dwarf) or (supergiant → supernova → neutron star/black hole)