The solar system, stars and the life cycle of matter

The solar system formed from a collapsing cloud of gas and dust about 4.6 billion years ago. Collapse concentrated most mass at the centre, which became the sun, while conservation of angular momentum flattened the remainder into a spinning disc, which is why the planets orbit in nearly the same plane and direction.

Temperature in the disc explains the layout. Near the sun only rock and metal could condense, producing small dense terrestrial planets. Beyond the frost line ices survived, allowing cores large enough to capture vast hydrogen and helium envelopes, producing the gas giants.

A star is a sustained balance between gravity pulling inward and pressure from fusion pushing outward. In the core, hydrogen nuclei fuse into helium, and the slight mass difference is released as energy according to the mass-energy relation.

Fusion requires enormous temperature and pressure because nuclei repel each other electrically. This is why only the core fuses, why the smallest failed stars never ignite, and why a star's mass determines almost everything else about it.

• Low-mass stars burn slowly and last tens of billions of years, ending as red giants that shed their outer layers and leave a white dwarf.
• Sun-like stars follow the same path on a scale of about ten billion years.
• High-mass stars burn fiercely and briefly, fusing progressively heavier elements in shells until iron forms, which absorbs rather than releases energy on fusion.
• That failure triggers core collapse and a supernova, leaving a neutron star or, above a threshold, a black hole.

Hydrogen, helium and a trace of lithium formed in the first minutes after the Big Bang. Everything heavier was made later: carbon, oxygen and nitrogen in the cores of stars, and the heaviest elements in supernovae and in collisions between neutron stars.

Those elements are scattered when stars die and become the raw material of later generations of stars and planets. The carbon in your cells and the iron in your blood were assembled inside stars that ended before the sun existed.

Distances come from a ladder of methods. Parallax, the apparent shift of nearby stars as the earth orbits, works for the closest. Standard candles, objects of known intrinsic brightness such as certain variable stars and a class of supernova, extend the reach much further.

Spectra do the rest. Light from a star carries absorption lines identifying its elements, and the shift of those lines reveals motion toward or away. The systematic redshift of distant galaxies, growing with distance, is the observation that established an expanding universe and led to Big Bang cosmology.