Space Physics: The Solar System and Beyond
Our Solar System
Our solar system consists of the Sun (a main sequence star), eight planets, dwarf planets (such as Pluto), moons, asteroids and comets. The four inner planets (Mercury, Venus, Earth, Mars) are small, rocky and relatively close to the Sun. The four outer planets (Jupiter, Saturn, Uranus, Neptune) are much larger gas giants, further from the Sun.
All planets orbit the Sun in roughly the same plane, in the same direction, in slightly elliptical orbits. The Sun contains over 99% of the mass of the solar system. Gravity provides the centripetal force needed to keep planets in orbit around the Sun, and moons in orbit around planets.
For a stable circular orbit, the gravitational force provides exactly the right centripetal force. A planet closer to the Sun orbits faster because the gravitational pull is stronger and the orbital radius is smaller.
Natural satellites (moons) orbit planets. Earth has one moon, while Jupiter has over 90 known moons. Asteroids are small, irregular rocky bodies that mostly orbit the Sun in the asteroid belt between Mars and Jupiter. Comets are bodies of ice and dust with highly elliptical orbits. When a comet approaches the Sun, it heats up and develops a glowing tail of gas and dust that always points away from the Sun (due to the solar wind).
The speed of a comet changes throughout its orbit. It moves fastest when closest to the Sun (perihelion) because the gravitational pull is strongest, and slowest when furthest from the Sun (aphelion).
The Life Cycle of Stars
All stars begin as a nebula — a large cloud of hydrogen gas and dust. Gravity causes the cloud to collapse and compress. As it contracts, it heats up. When the temperature at the core reaches about 15 million °C, hydrogen nuclei begin to fuse into helium — nuclear fusion begins and a star is born. The star enters its main sequence phase, where it spends most of its life.
During the main sequence, the outward pressure from nuclear fusion is balanced by the inward pull of gravity. This balance keeps the star stable. The Sun is currently in its main sequence phase and has been for about 4.6 billion years.
The fate of a star depends on its mass. Stars about the size of the Sun become red giants, then white dwarfs. Stars much more massive than the Sun become red supergiants, then explode as supernovae, leaving behind a neutron star or black hole.
Sun-sized stars: When hydrogen fuel runs low, the core contracts and the outer layers expand and cool, forming a red giant. The star then sheds its outer layers as a planetary nebula, leaving behind a dense, hot white dwarf that gradually cools over billions of years.
Massive stars: These swell into red supergiants, then collapse catastrophically in a supernova — an explosion so bright it can briefly outshine an entire galaxy. The remnant core may form a neutron star (incredibly dense, about 10 km across) or, if massive enough, a black hole. The elements created inside stars and scattered by supernovae form the raw materials for new stars, planets and even life.
Red-Shift and the Big Bang
When we observe light from distant galaxies, we find that the absorption lines in their spectra are shifted towards the red end of the spectrum compared to where we would expect them. This is called red-shift. It occurs because the galaxies are moving away from us, stretching the wavelength of the light. The further away a galaxy is, the greater its red-shift — meaning it is moving away faster. This relationship is known as Hubble’s Law.
The observation that all distant galaxies show red-shift (and that more distant galaxies show greater red-shift) is strong evidence that the universe is expanding. If we imagine reversing this expansion, all matter would converge to a single point. This leads to the Big Bang theory — the idea that the universe began from an extremely hot, dense point approximately 13.8 billion years ago and has been expanding ever since.
The cosmic microwave background radiation (CMBR) is low-energy microwave radiation that fills the entire universe in every direction. It was first detected in 1965 by Arno Penzias and Robert Wilson. The CMBR is the cooled remnant of the intense radiation released shortly after the Big Bang, now at a temperature of about 2.7 K (−270.5°C). Its existence provides strong supporting evidence for the Big Bang theory.
Together, red-shift and the CMBR provide two independent lines of evidence for the Big Bang. The discovery of the CMBR was predicted by Big Bang theory before it was detected — a successful prediction that greatly strengthened confidence in the theory.
Dark Matter and Dark Energy
Observations of galaxy rotation suggest that galaxies contain much more mass than we can see — stars at the edges of galaxies rotate faster than expected. Scientists propose that this “missing mass” is dark matter, which does not emit or absorb electromagnetic radiation but does exert gravitational effects. Observations also show that the expansion of the universe is accelerating. The unknown cause of this accelerating expansion is called dark energy. Together, dark matter and dark energy make up about 95% of the total mass-energy of the universe.