Space physics (physics only)

Explanation

• The solar system contains the Sun, eight planets and dwarf planets orbiting it, plus natural satellites orbiting planets; it is a small part of the Milky Way galaxy.
• A star begins when gravity pulls gas and dust in a nebula together into a protostar; compression raises the temperature until fusion reactions start.
• During the stable main-sequence stage, the inward effect of gravitational collapse is in equilibrium with the outward expansion caused by energy from fusion.
• A common error is to call the Milky Way the universe or the solar system: the solar system lies within the Milky Way, which is one galaxy in the universe.

Explanation

• All stars follow nebula $\rightarrow$ protostar $\rightarrow$ main-sequence star; their later path is determined by the star's mass.
• A Sun-sized star becomes a red giant, then a white dwarf and finally a black dwarf; a much more massive star becomes a red supergiant, then a supernova, leaving a neutron star or black hole.
• Successive fusion reactions in stars produce elements up to iron; elements heavier than iron are made in a supernova, whose explosion distributes elements through the universe.
• A common error is to give one ending for every star: only the much-more-massive route includes a supernova and can leave a neutron star or black hole.

Explanation

• Gravity supplies the inward force that keeps planets, natural satellites and artificial satellites in circular orbits.
• Classify by what is orbited and origin: planets orbit the Sun, natural satellites are moons orbiting planets, and artificial satellites are human-made objects placed in orbit.
• Higher only: in a circular orbit gravity changes velocity direction while speed can stay constant; if speed changes in a stable orbit, the orbital radius must also change.
• A common error is to say orbiting objects have no force acting on them; without the inward gravitational force they would not follow a circular path.

Explanation

• Light from most distant galaxies is observed at longer wavelengths than expected; this red-shift is the qualitative signature of galaxies receding from us.
• Compare spectral lines with laboratory wavelengths: a shift towards longer wavelengths indicates recession, and a larger shift indicates a greater recession speed.
• More distant galaxies generally recede faster and show greater red-shift, providing evidence that the universe is expanding and supporting a hot, dense beginning in the Big Bang model.
• A common error is to present the Big Bang as unchangeable fact: scientific theories are built from observations and may be refined as new evidence appears; dark mass and dark energy remain incompletely understood.