Space Physics

6.1.1 The Earth

• The Earth is a planet that orbits the Sun once in about 365 days (one year). Its orbit is an ellipse that is very nearly a circle.
• The Earth rotates on its axis once in about 24 hours (one day). This gives day and night. The axis is tilted, which is the cause of the seasons.
• The Moon orbits the Earth about once a month.
• Light takes about 500 s to reach the Earth from the Sun.

Orbital speed

The distance travelled in one orbit is the circumference of the orbit, $2\pi r$, and the time taken is the orbital period $T$.

$T = 365 \times 24 \times 3600 = 3.15 \times 10^{7}\ \text{s} \qquad v = \frac{2\pi \times 1.5 \times 10^{11}}{3.15 \times 10^{7}} = 3.0 \times 10^{4}\ \text{m/s}$

That is about $30\ \text{km/s}$.

$d = 3.0 \times 10^{8} \times 500 = 1.5 \times 10^{11}\ \text{m}$

6.1.2 The Solar System

The Solar System contains:

• one star, the Sun;
• the eight planets, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune;
• minor planets that orbit the Sun, including dwarf planets such as Pluto and the asteroids of the asteroid belt (between Mars and Jupiter);
• moons, which orbit planets;
• smaller bodies, including comets (balls of ice and dust in very elongated orbits) and other natural satellites.

Planetary data

Trends to look for when you interpret data:

• The further a planet is from the Sun, the longer its orbital period, and the slower its orbital speed.
• The four planets nearest the Sun (rocky planets) are dense and the four furthest (gas giants and ice giants) have a low density.
• Planets further from the Sun are generally colder. (Venus is hotter than Mercury because its thick atmosphere traps heat.)

$T = 687 \times 24 \times 3600 = 5.94 \times 10^{7}\ \text{s} \qquad v = \frac{2\pi \times 2.28 \times 10^{11}}{5.94 \times 10^{7}} = 2.4 \times 10^{4}\ \text{m/s}$

This is less than the Earth's speed of about $3.0 \times 10^{4}\ \text{m/s}$, as expected for a planet further from the Sun.

Gravitational fields in the Solar System

• The strength of the gravitational field at the surface of a planet depends on the mass of the planet. A planet with more mass has a stronger field, which is why Jupiter's surface gravity is much larger than the Earth's.
• Around any planet, the field gets weaker as the distance from the planet increases.
• The Sun contains most of the mass of the Solar System, so the gravitational field at its surface is much greater than at any planet.
• The force that keeps a planet in orbit is the gravitational attraction of the Sun. This is the force towards the centre that makes the planet go in a circle (see circular motion in chapter 1).
• The Sun's gravitational field becomes weaker with distance, so planets further out orbit more slowly.

6.2.1 The Sun as a star

The Sun is a star of medium size. It is made mostly of hydrogen and helium. Most of the energy it radiates is in the infrared, visible and ultraviolet parts of the electromagnetic spectrum.

Stars are powered by nuclear reactions. In a stable star such as the Sun, the nuclear reaction is the fusion of hydrogen into helium in the core, which releases a huge amount of energy.

6.2.2 Stars

• A galaxy is made of many billions of stars.
• The Sun is one star in the galaxy called the Milky Way.
• The other stars in the Milky Way are much further from the Earth than the Sun is.
• Astronomical distances are measured in light-years. One light-year is the distance travelled by light in a vacuum in one year.

$3.0 \times 10^{8} \times 3.15 \times 10^{7} = 9.5 \times 10^{15}\ \text{m}$

The life cycle of a star

1. A star forms from a cloud of gas and dust (an interstellar cloud) that contains hydrogen.
2. The cloud collapses because of its own gravitational attraction, and its temperature rises. This is a protostar.
3. When the centre is hot enough for hydrogen to fuse into helium, the star becomes stable: the inward force of gravity is balanced by an outward force from the high temperature in the core.
4. All stars eventually run out of hydrogen in the core as fuel.
5. Most stars then expand to form red giants, and the more massive stars expand to form red supergiants.

After that, the path depends on the mass of the star.

The nebula from a supernova can collect together and form new stars, with orbiting planets.

6.2.3 The Universe

The Milky Way is one of many billions of galaxies that make up the Universe. Its diameter is about 100 000 light-years.

Redshift

The wavelength of the light gets longer, so it shifts towards the red end of the spectrum. (Sound does the same: the pitch of a siren drops as it moves away from you.)

• The light from distant galaxies is redshifted, so they are moving away from us.
• The further away a galaxy is, the greater its redshift, and the faster it is moving away from the Earth.

The Big Bang

The redshift of galaxies shows that the Universe is expanding. If the galaxies are all moving apart now, then in the past they were closer together. Tracing the expansion back, everything was once in a very small, hot, dense place. The Big Bang theory says that the Universe began from this state about 14 billion years ago and has been expanding ever since. Redshift is evidence for the Big Bang theory.