Year 10 Physics Astrophysics

How has our understanding of the universe changed?

• Early thoughts: Earth was the centre of the universe

• Galileo: Discovered the Earth orbited the Sun and therefore the Sun is the centre of the Universe

• Newton: Discovered the Sun was a star and Earth was one of 6 planets orbiting it

What is a star/sun?

A large luminous sphere of gas/plasma held together by gravity

What is a planet? What effect does it have on other objects?

A large body of matter that orbits a sun

• Its gravity is large enough to clear the surrounding area of other objects

What is a galaxy? What are the types of galaxies?

Galaxy: A group of solar systems that are held together by a gravitational force

Types: Spiral, elliptical, peculiar, irregular, barred spiral

• The Milky Way is a barred spiral

What is a solar system? What are the planets (in order) of our solar system?

Solar system: A series of planets and moons orbiting the same star/sun

Memory cue: My Very Educated Mother Just Served Us Nachos

• My = Mercury

• Very = Venus

• Educated = Earth

• Mother = Mars

• Just = Jupiter

• Served = Saturn

• Us = Uranus

• Nachos = Neptune

Why do the stars appear to move through the night?

The Earth's rotation makes the stars appear to rotate around a celestial pole each night

• The stars only appear to move as the Earth rotates on its axis

• In the Southern Hemisphere, the stars appear to rotate around the south celestial pole

How do astronomers detect new planets?

By observing the slight wobble of a star caused by the gravitational pull of an orbiting planet

• A planet's gravity pulls on its star which causes it to wobble

• Astronomers detect this wobble to infer the presence of an orbiting planet

• This detection method is called the radial velocity (Doppler) method

What is an Astronomical Unit?

The distance from the Earth to the Sun (AU)

• Used to measure distances within our solar system

• EG Distance between planets, distance from the Sun to other planets

• 1 AU: 1 AU in KM = 149,598,000 kilometers

What is a Parsec?

The distance to an object whose parallax angle is 1 arcsecond (pc)

• 1 arcsecond = 1/3600th of a degree

• Generally measured from the sun to the object

How does the distance of an Astronomical Unit, Parsec, Light Year and Megaparsec compare?

Unit	Distance	Used for
Astronomical Unit (AU)	149.6 million km	Distances within the Solar System
Light Year (ly)	9.46 trillion km	Distances to nearby stars
Parsec (pc)	3.26 light years or 31 trillion km	Distances to stars and star clusters
Megaparsec (Mpc)	1 million parsecs (≈ 3.26 million light years)	Distances to galaxies and galaxy clusters

• Smallest: AU → Light Year → Parsec → Megaparsec

What is parallax? How is parallax used to measure the distance to stars?

Parallax: The apparent change in the position of an object when viewed by an observer from a different position

• Used to measure the distance to nearby stars

• Nearby stars show a larger apparent shift than distant stars

• The larger the parallax angle, the closer the star

What is the largely discredited theory for the origin of the universe?

• The steady state theory

• States that the universe has no beginning and no end, where new matter is constantly created yet the universe’s appearance remains the same

• Discredited as there was little evidence to support it

What is the widely accepted theory of the creation of the universe?

• The Big Bang Theory

• States that the universe began about 13.8 billion years ago from an extremely hot, dense state and has been expanding and cooling ever since.

Describe the process of how the Big Bang Theory created the universe. How were the first stars formed from this?

• E = mc² shows that energy can be converted into matter (and matter into energy)

• In the early universe, energy formed matter and antimatter.

• Matter and antimatter annihilated, producing energy in the form of photons (energy particles)

• Slightly more matter than antimatter remained, which seperated from the photon as it cooled and expanded

• This allowed protons and electrons combined to form hydrogen atoms

• Gravity pulled hydrogen atoms together into clouds of hydrogen gas.

• Increasing mass → gravity → pressure → temperature.

• When the core became hot and dense enough, nuclear fusion began → the first stars formed

How does Einstien’s famous equation help support the Big Bang Theory?

• E = mc² shows that energy can be converted into matter and matter can be converted into energy

• In the early universe, vast amounts of energy formed matter and antimatter

• Matter and antimatter annihilated, converting back into energy

• Slightly more matter than antimatter remained, allowing the universe to form

• Explains how matter could have formed after the Big Bang, supporting the Big Bang Theory

What is a singularity?

A point where a huge amount of energy was condensed into an extremely small, hot, and dense state with no mass

What are the 4 piecesd of evidence that support the Big Bang Theory?

1. Cosmic background radiation

2. Red-shifted light from galaxies

3. Presence of primordial elements

4. The formation of galaxies over time

How does cosmic background radiation provide evidence to support the Big Bang theory? (1 of 4)

• The energy from the initial expansion of the universe still exists

• It has cooled and now exists as electromagnetic microwaves throughout the universe

How it supports: It is evidence of leftover energy from the universe's beginning

How does red-shifted light from galaxies provide evidence to support the Big Bang theory? (2 of 4)

• Light from other galaxies is red shifted, showing that that galaxies are moving away from us

• The further away a galaxy is, the faster it is moving away

How it supports: Shows the universe is expanding

How does evidence of primordial elements provide evidence to support the Big Bang theory? (3 of 4)

• The first atoms to form were the simplest elements (hydrogen)

• Heavier elements formed when stars collapsed

• The current ratio of light to heavy elements supports the universe being about 13.7 billion years old

How it supports: The observed element ratios match its predictions

How does the process of galaxy formation provide evidence to support the Big Bang theory? (4 of 4)

• Galaxies formed and evolved over time as the universe expanded.

• The formation of heavier elements in stars is similar to the formation of the first elements during the Big Bang.

• This shows that galaxies continued to evolve over time, as predicted by the Big Bang Theory

What does the luminosity of a star depend on?

It’s actual brightness and its distance from Earth

What does the colour of a star depend on? What can be determined from the colour of a star?

• The colour of star depends on it’s surface temperature

• From colour, it’s temperature and element composition can be found

• Most stars are either red or orange

What is the dispersion of white light?

The process of white light dispersing into its different colours when it passes through a triangular glass prism

• White light contains all the colours of the visible spectrum

• Each colour is refracted (bent) by a different amount as it passes through the prism

• Causes the colours to spread out and form a spectrum

What colours does white light disperse into? How does their wavelength and frequency differ?

• 6 Colours: Red, Orange, Yellow, Green, Blue, Violet

• Frequency increases and wavelength shortens moving from Red - Violet

What is emission spectrum?

The unique pattern of narrow, discrete bands of coloured light produced when the light emitted by a heated element is separated

What is emission spectra? How is it viewed and what can be determined from it?

• When an element is heated, its atoms emit light which can e separated into narrow, discrete bands of colour

• The bands of colour are characteristic of that particular element

• An emission spectrum can be viewed using a spectroscope

• Since every element has a unique emission spectrum, it can be used to identify the elements present

What is absorption spectrum?

The unique pattern of dark lines produced when white light passes through a cold gas element, and the gas absorbs specific wavelengths

What is absorption spectra? What can be determined from it?

• A cool element absorbs the same colours of light that it emits when heated

• When white light passes through a cloud of cool gas, the gas absorbs specific wavelengths, leaving dark lines in the spectrum.

• Each element has a unique absorption spectrum.

• An absorption spectrum can be used to identify the elements present in a gas or the outer atmosphere of a star.

How do absorption and emission spectra differ?

Emission spectrum: The unique pattern of narrow, discrete bands of coloured light produced when the light emitted by a heated element is separated

Absorption spectrum: The unique pattern of dark lines produced when white light passes through a cold gas element, and the gas absorbs specific wavelengths

What do the dark bands of a star’s absorption spectrum tell you compared to the emission spectrum of the same star?

• The dark bands in a star's absorption spectrum occur at the same wavelengths as the bright lines in its emission spectrum.

• This confirms the star's elemental composition, as each element has a unique pattern of spectral lines.

What implication do red and blue shift have for the motion of stars relative to Earth?

Red shift and blue shift show whether a star or galaxy is moving away from or towards Earth.

• Red shift: Light is shifted towards the red end of the spectrum, indicating the star or galaxy is moving away from Earth.

• Blue shift: Light is shifted towards the blue end of the spectrum, indicating the star or galaxy is moving towards Earth.

• The greater the shift, the faster the object is moving.

• Red and blue shift are caused by the Doppler effect.

How does the speed of light affect what we see?

• Light travels at a fixed speed that takes time to travel across space

• Therefore we view distant stars/galaxies as they were in the past, not as they are now

• The further away an object, the further back in time we are seeing it due to the increased time taken for light to travel

What is the doppler effect? How does it explain the observations of red and blue shift?

Doppler effect: The apparent change in the wavelength and frequency of a wave caused by the relative motion between the source and the observer

• As a star or galaxy moves away from Earth its light waves are stretched, increasing their wavelength and causing a red shift.

• As a star or galaxy moves towards Earth its light waves are compressed, decreasing their wavelength and causing a blue shift

• The greater the red or blue shift, the faster the object is moving relative to Earth.

• Astronomers use the Doppler effect to determine the direction and speed of stars and galaxies.

What are 3 important conclusions related to the movement of galaxies that Edwin Hubble made from observations of the universe?

Edwin Hubble's observations provided evidence that the universe is expanding

His 3 important conclusions on the movement of galaxies:

• Distant galaxies are receding from Earth.

• Space itself is expanding

• The universe is expanding faster than the speed of light at great distances, meaning light from these galaxies will never reach Earth

What does the Hertzsprung-Russell Diagram describe?

The Hertzsprung–Russell (H–R) Diagram plots a star's absolute brightness (luminosity) against its surface temperature, which is determined from its colour

• Groups stars into the following categories:

  • Main sequence

  • Giants

  • Supergiants

  • White dwarfs

What does the Hertzsprung-Russell Diagram show?

• Blue stars have hotter surface temperatures than red stars.

• Most stars are main sequence stars, where they spend most of their lives.

• Low-mass main sequence stars have cooler surface temperatures.

• Stars can be grouped into categories: main sequence, giants, supergiants, and white dwarfs

What colour stars have hotter surface temperatures? What mass stars have hotter surface temperatures in the main sequence?

High-mass stars have hotter surface temperatures than low-mass stars

• High-mass stars have stronger gravity, causing higher core temperatures

• This increases the rate of nuclear fusion, producing more energy

• Therefore, high-mass stars are hotter, brighter, and blue, while low-mass stars are cooler, dimmer, and red

What is the lifecycle of a star?

• All stars: Nebula → Protostar → Main Sequence

• Low-mass stars: Main Sequence → Red Giant → Planetary Nebula → White Dwarf

• High-mass stars: Main Sequence → Supergiant → Neutron Star or Black Hole

• The mass of a star determines the path it follows after the main sequence stage

What is a nebula? How does it form a protostar?

Nebula: A giant cloud of gas (mostly hydrogen) and dust

• Gravity causes denser regions of the nebula to collapse and clump together

• Density and temperature increase as matter is pulled inward

• As the clump contracts further, it heats up and forms a protostar

What is a protostar? How does it form a main sequence star?

Protostar: The earliest stage star created from the collapse of gas and dust within a nebula

• Gravity continues pulling matter inward, causing the protostar to become more dense and hotter.

• Pressure and temperature in the core continue to increase.

• When the core becomes hot and dense enough, hydrogen nuclear fusion begins.

• The protostar becomes a main sequence star

What stage do stars spend most of their lives in? What outcomes follow this stage of a stars life?

Main sequence star: A stable star where hydrogen nuclei fuse into helium in the core, releasing energy

• Hydrogen fusion produces energy that creates outward pressure

• This balances the inward force of gravity, keeping the star stable

• The star remains in this stage for most of its lifetime

• When hydrogen in the core is depleted, the star evolves depending on its mass (low = red giant, high = supergiant)

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What is a red giant? How do low mass stars become this?

Red giant: A low-mass star that expands after most hydrogen in its core has been used

• Hydrogen fusion in the core slows and eventually stops.

• Gravity causes the core to contract, increasing its temperature.

• The outer layers expand and cool, causing the star to become larger and redder.

What are supergiants? How do high mass stars become this?

Supergiants are extremely large, luminous stars formed from high-mass main sequence stars.

• High-mass stars use up their hydrogen fuel in the core.

• The core contracts, causing temperature and pressure to increase.

• The outer layers expand significantly, forming a supergiant.

• The core begins fusing heavier elements.

What are black holes? How do they form following supergiants?

A black hole is an object with immense gravity where light can’t escape.

• A very high-mass star becomes a red supergiant.

• The core runs out of fuel and can no longer support itself against gravity.

• The core collapses after a supernova explosion.

• If the remaining core is extremely massive, it collapses into a black hole.

What are neutron stars? How do they form following supergiants?

A neutron star is an extremely dense star remnant made mostly of neutrons.

• A high-mass star becomes a red supergiant and eventually undergoes a supernova.

• The remaining core collapses under extreme gravity.

• The pressure forces protons and electrons to combine, forming neutrons.

• This creates a neutron star.

What is a planetary nebula? How do they form following red giants?

A planetary nebula is a glowing cloud of gas released from a low to medium-mass star.

• A red giant becomes unstable near the end of its life.

• It ejects its outer layers of gas into space.

• Radiation from the remaining hot core causes the gas to glow, forming a planetary nebula.

What stage follows a star becoming a planetary nebula?

A white dwarf follows the planetary nebula stage.

• The remaining hot, dense core is left behind after the outer layers are expelled.

• Nuclear fusion has stopped.

• The white dwarf slowly cools over time.