الشمس ودورة حياة النجوم

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 聚变 in its core: hydrogen nuclei join to make helium, releasing huge energy.

The Sun, our nearest star, seen in ultraviolet light

Galaxies and the Universe 宇宙

• A galaxy 星系 is billions of stars held together by gravity. Our Sun is in the Milky Way 银河系.
• Distances between stars are measured in light-years 光年 — 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 spiral galaxy: billions of stars held together by gravity

The life cycle of a star

• A star forms from a nebula 星云 (a cloud of gas and dust).
• Gravity pulls it together and it heats up — a protostar 原恒星.
• 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 红巨星 → throws off a planetary nebula → leaves a white dwarf 白矮星;
• a massive star → red supergiant → explodes as a supernova 超新星 → leaves a neutron star 中子星 or a black hole 黑洞.

The life cycle of a star: all stars start the same way, but their mass determines how they end. Medium stars become white dwarfs; massive stars explode as supernovae and leave neutron stars or black holes.

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