Space Physics Notes

The Solar System

• The Earth is one of eight planets orbiting the Sun.
• Planets travel in elliptical paths.
• All planets except Mercury and Venus have at least one moon.
• Other objects orbiting the Sun include comets and asteroids.

Planets' Orbit

• Planets orbit the Sun in the same plane and direction.
• This is due to gravitational force between the Sun and the planets.
• Suggests they formed around the same time.
• Inner planets (Mercury, Venus, Earth, Mars) have rocky surfaces.
• Outer planets (Jupiter, Saturn, Uranus, Neptune) are gas giants, composed of dense gases like hydrogen, methane, and ammonia.

Satellites

• Moons are natural satellites of planets.
• Artificial satellites are human-made objects in space.
• Most orbit Earth, some orbit the Sun (e.g., Kepler), and others orbit other planets (e.g., Mars Reconnaissance Orbiter).
• Purposes of artificial satellites:
  • Communications
  • Earth observation (spying, monitoring rainforests, deserts, crops)
  • Astronomy
  • Weather monitoring

Comets

• Range in diameter from a few hundred meters to tens of kilometers.
• Often called 'dirty snowballs'.
• Composed of rock, ice, silicates, and organic compounds at their centers.
• Surrounded by a 'coma' of gases and dust.
• Orbit the Sun in very elliptical paths.
• As a comet approaches the Sun, solar radiation vaporizes some of the frozen gas, increasing the coma size.
• Dust and gas stream away, forming a long tail (millions of kilometers long) pointing away from the Sun due to solar radiation.
• Most astronomers believe comets originate in the Oort cloud, a region 2000-3000 times further from the Sun than Pluto.

Asteroids

• Large rocks in outer space, ranging from a few meters to very large.
• Too small to have enough gravity to form a spherical shape.
• Many are found in the Asteroid Belt between Mars and Jupiter.
• Leftover materials from the formation of the Solar System, not incorporated into a planet due to Jupiter's gravitational pull.

The Life Cycle of Stars

• Stars form in cold clouds of hydrogen gas and dust called stellar nebulae.
• Gravity causes gas particles to come together.
• Gravitational force overcomes outward pressure due to kinetic energy, causing gravitational collapse.
• Material at the center heats up as gravitational potential energy converts to thermal energy.
• The hot core is called a protostar.
• As the protostar accumulates more gas and dust, its density and temperature rise.
• Eventually, outward pressure balances gravitational force, and the protostar becomes luminous due to its high temperature.
• If the mass of the protostar is greater than about 8% of our Sun's mass, nuclear fusion begins.
• Equilibrium is reached between inward gravitational force and outward force from radiation pressure due to fusion.
• The star enters the main sequence phase.
• Smaller stars have longer main sequence lives. Our Sun will last about 10 billion years, while more massive stars last only a few million years.

Life Cycle of a Star: Death of a Star Like Our Sun

• Hydrogen in the core is converted into helium via nuclear fusion.
• Energy output reduces, causing the core to compress significantly.
• A layer of hydrogen surrounding the core undergoes nuclear fusion due to gravitational contraction.
• Outward pressure prevents collapse, causing the star to expand to hundreds of times its former size.
• Surface temperature falls, and the starlight becomes predominantly orange, turning the star into a 'red giant'.
• Within a red giant, helium can fuse to become carbon and oxygen.
• All naturally occurring elements up to iron are formed by nuclear fusion in stars.
• The outer layers of gas flow out, cool, and surround the core forming a nebula.
• The remaining core cools to become a white dwarf.
• Eventually, all fusion stops, and the star cools to become a black dwarf.

Death of a High Mass Star

• If a star is more massive than the Sun, helium fusion occurs more rapidly.
• The star turns into a red supergiant, which is among the largest stars by volume.
• Red supergiants require huge amounts of energy to sustain them and prevent gravitational collapse.
• They burn through their nuclear fuel quickly, lasting only a few tens of millions of years.
• A red supergiant successively fuses different elements, up to iron.
• The core can no longer sustain outward radiation pressure, and gravity causes the supergiant to collapse.
• This releases gravitational potential energy, heating the outer layers and throwing them off in a supernova explosion.
• A supernova emits more radiation than all the other stars in its galaxy for about a month, shining with the brightness of ten billion Suns.
• The interaction of elements exploding outwards with surrounding atoms produces elements with atomic masses larger than iron.
• The core is left behind as a neutron star or, for very massive stars, a black hole.
  • Neutron stars are composed almost entirely of neutrons and are the smallest and densest stars known.
  • Black holes are incredibly dense, with such enormous gravitational fields that nothing, not even light, can escape.

Experimental Evidence of Elements in the Sun

• Evidence comes from the absorption spectrum of sunlight.
• Joseph von Fraunhofer found dark lines (Fraunhofer lines) in the spectrum.
• When visible light passes through the Sun's atmosphere, atoms absorb particular wavelengths, creating black lines in the spectrum.
• Each black line corresponds to an element in the Sun's atmosphere.
• The major gases in the Sun are hydrogen (71%), helium (27%), and oxygen (1%).

Nuclear Fusion in Our Sun

• Stars like our Sun get their energy mainly from the fusion of light hydrogen nuclei into heavier helium nuclei.
• Fusion requires a temperature of at least 13 million °C and a density of $100 \frac{g}{cm^3}$.
• These conditions are met at the core of our Sun.

The Doppler Effect

• The pitch of a sound changes as a source moves relative to an observer.
• When a source approaches, the pitch is higher (shorter wavelength), and when it recedes, the pitch is lower (longer wavelength).
• A similar effect happens with light. Light from approaching sources is 'blue shifted' (shorter wavelength), and light from receding sources is 'red shifted' (longer wavelength).
• Red shift from distant galaxies is interpreted as evidence that space itself is expanding.

The Origin of Our Universe

• Most physicists believe the universe began about 14 billion years ago with a 'Big Bang'.
• Evidence comes from red shift.
• Galaxies are huge collections of star systems.
• Red shift in the light from distant galaxies indicates that they are moving away from us.
• The fact that we almost always get red shift from the distant galaxies tells us that nearly all galaxies are moving away from us.

Red shift Example

• The red shift in the absorption spectrum for calcium in galaxies Nubecula and Leo tells us they are moving away, with Leo moving faster.
• A further piece of evidence supporting the Big Bang is the discovery of cosmic microwave background radiation (CMBR).
• In 1964, Penzias and Wilson detected microwaves (wavelength 7.35 cm) evenly spread over the sky, coming from outside our galaxy.
• CMBR is considered the signature or 'afterglow' of the Big Bang that occurred 14 billion years ago.

The Big Bang Theory

• The argument suggests that all galaxies originated from a common point, called a singularity.
• About 14 billion years ago, the Universe came into existence suddenly with an enormous explosion (Big Bang).
• It underwent a short period of very rapid growth (inflation).
• As the Universe expanded, it cooled down and became less dense.
• Cooling allowed subatomic particles (neutrons, protons, and electrons) to form.
• Protons and neutrons combined to form simple nuclei.
• Around 380,000 years after the Big Bang, the first stars came into existence.

Exoplanets

• Exoplanets are planets outside our Solar System.
• There are over 3600 known exoplanets in over 2600 known solar systems.

Detection Methods

Radial Velocity Method

• A star and planet have a common center of gravity (center of mass).
• Both the star and the planet orbit this common center.
• The star's movement around the common center leads to small variations in its velocity as observed from Earth.
• These variations cause tiny variations in the wavelengths of the light from the star due to the Doppler effect.
• Variations as small as 1 m/s can be detected to infer the existence of an exoplanet.

Transit Method

• An exoplanet passes between us and the star (transit).
• This causes a very small (typically about 1%) but detectable drop in the brightness of that star as seen from Earth.
• Transits are used to infer the existence of exoplanets.

Planetary Atmospheres

• Techniques similar to those used to identify elements in the Sun are used to identify elements in planetary atmospheres.
• Planets scatter sunlight from their surfaces, and that sunlight passes through the planetary atmosphere.
• The black lines in the absorption spectra of this light allow us to identify the elements in the atmosphere.

Table of Atmospheres of the rocky planets

The Speed of Light

• Nothing can travel faster than light.
• Light travels at $300,000 \frac{km}{s}$ ($3 \times 10^5 \frac{km}{s}$ or $3 \times 10^8 \frac{m}{s}$).
• Light can travel:
  • More than 7 times around the Earth in 1 second.
  • From the Earth to the Moon and back in less than 3 seconds.
  • From the Sun to Earth in about 500 seconds.
  • From Earth to Neptune in less than 5 days (when planets are closest).

Light Year

• Astronomical distances are measured in light years (ly).
• A light year (ly) is the distance light travels in one year.
• $distance = speed \times time$
• $= 3 \times 10^8 \frac{m}{s} \times (60 \times 60 \times 24 \times 365) s$
• $= 9.46 \times 10^{15} m$
• $= 9.46 \times 10^{12} km$
• Over 63,000 times the distance from the Earth to the Sun.
• The nearest star system to Earth (other than our Sun) is Alpha Centauri, which is 4.2 ly away from Earth.
• Our fastest spacecraft can travel at around $70,000 \frac{m}{s}$. At this speed, it would take a staggering 18,000 years to reach Alpha Centauri or any exoplanet orbiting it.

Challenges of Interstellar Travel

• The vast distances to the stars mean that with our present technology, it is not currently feasible to visit any planet outside our Solar System.

• There are many difficulties:

  • Flight time: The distance is so great that the flight would last for many generations.
  • Engineering: Our spacecraft are just too slow.
  • Logistics: It is not clear how the spacecraft could carry enough fuel, oxygen, and water.
  • Ethics: The chance of failure would be high, with no possibility of return to Earth or rescue.