Astrophysics Beginner

The Solar System formed about 4.6 billion years ago from a rotating cloud of gas and dust. The Sun contains 99.8% of the total mass; everything else — planets, moons, asteroids, comets — makes up the rest.

The eight planets

• Inner (rocky): Mercury, Venus, Earth, Mars — small, dense, close to the Sun.
• Outer (giant): Jupiter, Saturn (gas giants); Uranus, Neptune (ice giants) — massive, far out, many moons.

Other populations

• The asteroid belt between Mars and Jupiter
• The Kuiper Belt beyond Neptune (home of Pluto)
• The Oort Cloud — reservoir of comets at vast distances

Scale

If the Sun were the size of a basketball, Earth would be a pea 25 m away, and Neptune would be a marble 750 m away.

The Sun is a fairly ordinary middle-aged star — a ball of plasma 1.4 million km across, 150 million km from Earth. It will shine for another 5 billion years.

Energy source

The Sun's core (temperature ~15 million K) fuses hydrogen into helium by the proton–proton chain, converting ~4 million tonnes of mass into energy every second via E = mc².

Layers

• Core: fusion zone, 0–25% of radius
• Radiative zone: energy carried by photons (a photon takes ~170,000 years to escape!)
• Convective zone: churning convection carries energy to the surface
• Photosphere: visible surface, T ≈ 5,500 K
• Corona: thin outer atmosphere, T > 1 million K (the coronal heating problem is unsolved)

To understand stars, astronomers must measure how far away they are, how bright they really are, and what colours they emit.

Parallax

As Earth orbits the Sun, nearby stars shift slightly against the distant background. The angle is the parallax p; distance d (in parsecs) = 1/p (in arcseconds). The Gaia mission has measured parallaxes for over a billion stars.

Luminosity vs apparent brightness

A star's true luminosity L tells how much energy it emits; its apparent brightness b = L/(4πd²) tells how bright it looks from Earth. The magnitude scale dates to Hipparchus — a difference of 5 magnitudes means a factor of 100 in brightness.

Colour and temperature

A hotter star glows bluer (Wien's law: peak wavelength λ_max = 2.9 mm K / T). The colour index B − V is a temperature proxy.

Stars are born in giant molecular clouds, live burning hydrogen in their cores, and end their lives in ways that depend on their mass.

Birth

A gas cloud fragment collapses under gravity. As it heats, it becomes a protostar. When core temperature reaches ~10 million K, hydrogen fusion ignites — a star is born.

Main sequence

A star spends most of its life fusing hydrogen (main sequence). More massive stars burn brighter and hotter but live shorter lives: a 10 M_☉ star lives only ~30 million years; the Sun will last ~10 billion years total.

Death

• Low-mass stars (like the Sun): expand into a red giant, shed outer layers as a planetary nebula, leave a white dwarf.
• High-mass stars: explode as a supernova, leaving a neutron star or black hole.

Our Sun is one of about 300 billion stars in the Milky Way, a barred spiral galaxy about 100,000 light-years across. We sit about 26,000 light-years from the centre.

Structure of the Milky Way

• Bulge and bar: dense central region with an ancient stellar population and a 4-million-solar-mass black hole (Sgr A*).
• Spiral arms: sites of active star formation.
• Disk: ~1,000 light-years thick.
• Halo: spherical region of old stars and globular clusters, and dark matter.

Galaxy types

Elliptical (E), spiral (S), barred spiral (SB), irregular. The nearest large galaxy, Andromeda (M31), is ~2.5 million light-years away and is on a collision course with the Milky Way in about 4 billion years.

In 1929 Edwin Hubble discovered that galaxies are moving away from us, and the farther away they are, the faster they recede. The universe is expanding.

The Big Bang

If the universe is expanding, running the film backwards takes us to a hot, dense beginning ~13.8 billion years ago — the Big Bang. The universe has been expanding and cooling ever since.

Evidence

• Hubble's law: recession velocity v = H₀ d; today H₀ ≈ 70 km/s/Mpc.
• Cosmic microwave background: afterglow of the hot early universe, discovered 1965, T = 2.725 K today.
• Big Bang nucleosynthesis: the proportions of hydrogen, helium, deuterium produced in the first 3 minutes match observed abundances.

Ordinary matter (atoms) makes up only ~5% of the universe. The rest is mysterious.

Dark matter (27%)

Galaxies rotate too fast for the visible matter to hold them together — something unseen provides extra gravity. Candidate particles: WIMPs, axions, sterile neutrinos. Evidence: galactic rotation curves, gravitational lensing, structure formation.

Dark energy (68%)

In 1998 two teams studying distant supernovae discovered the universe's expansion is accelerating. A mysterious dark energy, possibly Einstein's cosmological constant Λ, is driving it. We don't know what it is.

The big picture

The universe on large scales is composed of a cosmic web of filaments and voids. Galaxy clusters mark the densest nodes. On the very largest scales the universe looks the same in every direction (isotropic) and from any position (homogeneous).

Almost everything we know about the universe beyond our Solar System comes from collecting and analysing light (and now gravitational waves and particles).

Types of telescope

• Optical/IR: Hubble Space Telescope, James Webb Space Telescope (JWST, launched 2021) — reveals galaxies just 300 million years after the Big Bang.
• Radio: Very Large Array (VLA), Event Horizon Telescope — imaged the M87 and Sgr A* black holes.
• X-ray: Chandra — hot gas, black hole accretion discs, galaxy clusters.
• Gamma-ray: Fermi — gamma-ray bursts, pulsars.
• Gravitational wave: LIGO, Virgo, KAGRA.

Space missions

Voyager 1 (launched 1977) is now the most distant human-made object, ~24 billion km away. New Horizons flew past Pluto in 2015. The Planck satellite mapped the CMB with exquisite precision.