Cosmology Notes
Cosmology
Learning Intentions
Star brightness (how bright stars look)
Distance between stars (how far away they are)
How bright stars actually are
Star colour (what colour the star is)
How stars are born: gas clouds turning into baby stars, then normal stars
Life cycle of stars: how stars change over time (H-R Diagram)
Life cycle of Sun-like star: Red giant à Planetary Nebula à White Dwarf à Black Dwarf
Life cycle of big stars: Supergiant à Supernova à Neutron Star OR Black hole
Galaxies – Big groups of stars, like our Milky Way
Measuring distances to galaxies
How the universe started: Big Bang vs Steady State theory
Measuring how galaxies move
Red shift/blue shift and the Doppler effect (how light changes as things move)
Cosmic Microwave Background Radiation (leftover heat from the Big Bang)
Introduction to Cosmology
Cosmology is the study of everything in space, including:
Stars
Planets
Galaxies
Blackholes
Observing the Night Sky – Stars
Stars are different in age, size, and how they look.
Knowing why stars are different helps us understand the universe.
It can also show how life started in a star.
What is a Star?
A star is a big, hot ball of glowing gas.
Stars are mostly made of hydrogen and helium.
These gases are always reacting in the star's core, making energy through nuclear fusion.
When stars are born, they turn hydrogen into helium in their core.
This energy comes out as light.
Stars are different in size, mass, temperature, and brightness.
Example of a Star – Our Sun
The Sun has layers:
Inside:
Core (center)
Radiative Zone (energy moving out)
Convection Zone (hot stuff rises, cool stuff sinks)
Outside:
Photosphere – the “surface” we see
Granules (bumpy spots)
Sunspots (dark spots)
Chromosphere (layer above the surface)
Corona (outer layer)
Solar Winds (stuff flying off the Sun)
Anatomy of the Sun:
Core: Makes energy.
Radiative zone: Energy moves slowly.
Convective zone: Hot gas moves around.
Photosphere: The surface we see.
Chromosphere: Layer above the surface.
Corona: Outer atmosphere.
Solar wind: Stuff flying off.
Sunspots: Dark, cooler spots.
Properties of a Star
Two main things about stars:
Brightness – how much light.
Colour – the light’s frequency.
We use brightness and colour to find out how hot a star is.
Understanding the Brightness of a Star
Brightness is called magnitude.
Two kinds of magnitude:
Apparent magnitude – how bright it looks from Earth.
Absolute magnitude – how much light it really makes.
We can see stars down to a magnitude of 6.5.
Understanding Apparent Magnitude
Apparent magnitude goes from -30 to +30.
-30 is super bright.
+30 is super dim.
A change of 1 means the star is 2.5 times brighter or dimmer.
Example of Apparent Magnitude
Bright stars have low magnitudes, dim stars have high ones.
Two stars in Orion: Betelgeuse and Bellatrix.
Betelgeuse: 0.6 magnitude.
Bellatrix: 1.6 magnitude.
Bellatrix - Betelgeuse = 1. Betelgeuse is 2.5 times brighter.
Factors Affecting Apparent Magnitude
Two things change how bright a star looks:
How much light it makes.
How far away it is – farther away = dimmer.
Apparent magnitude doesn't tell the whole story, so we need a better way to measure.
Measuring the Universe
The universe is huge, so we need big units.
Three units we use:
Astronomical Unit (AU) – Earth to Sun distance.
Light Years (LY) – how far light travels in a year.
Parsec (PC) – 3.26 light years.
Understanding Astronomical Units (AU)
AU is the average distance from Earth to the Sun.
About 150,000,000 kilometers.
Understanding Light Years (LY)
Light years measure distances between stars.
A light year is how far light goes in a year.
About 9.5 trillion km.
Example of a Light Year
Betelgeuse is 650 light years away.
The light you see from it left 650 years ago!
Understanding Parsec (PC)
A parsec is another unit astronomers use.
1 parsec = 3.26 light-years
To change light years to parsecs, divide by 3.26
Example of converting to parsec
Betelgeuse is 650 light years away.
Understanding Stellar Parallax
Stars seem to move across the sky every night.
Stars ‘move’ in the sky, but it's hard to see because they are far away.
Friedrich Bessel first measured the distance to a star in 1837.
He used stellar parallax: a star looks like it's in a different place depending on where Earth is.
Understanding Parallax
Parallax – closer things seem to move more than far things when you move.
We need a baseline to use parallax.
Bigger baseline = better guesses.
We use the size of Earth’s orbit as the baseline.
Understanding Stellar Parallax
Parallax makes us see things differently. (close one eye, then the other)
We can measure the distance to stars with parallax.
As Earth goes around the Sun, stars look like they move a little.
This tiny movement is stellar parallax.
Understanding Stellar Parallax
Stellar Parallax:
Baseline – Earth’s orbit size.
Viewpoint A – where we see the star now.
Viewpoint B – where we see the star 6 months later.
We can find the parallax angle.
Smaller angle = farther away.
Calculating stellar parallax
Find the baseline.
Bigger baseline = better measurement.
We use Earth’s orbit size.
Understanding Colour of a Star
Parallax is good, but hard for far away stars.
We can also use colour to find the distance to a star.
Colour tells us how hot the star is.
If a star isn't as bright as it should be, it's far away.
Understanding Star Colour
Stars make light in different colours.
Some light we can see, some we can't (infrared, ultraviolet).
A star’s colour depends on how hot it is.
Cool stars are red.
Hot stars are blue.
Medium stars are orange, yellow, or white.
Planck Curve
The blackbody spectrum depends only on temperature.
Understanding Star Colour
A spectrometer breaks starlight into colours.
We can see what elements are in the star by the colours it makes.
We use a spectral class system. This tells us what's in the star, how hot it is, and its colour.
Example of Spectral Class – Our Sun
The sun is 5700°C and yellow.
It's a G-type class.
Understanding Absolute Magnitude
A close light bulb looks brighter than a far one.
The sun looks brightest because it's close, not because it's the brightest star.
Apparent magnitude tells us how bright it looks from Earth.
But it doesn't tell us how bright the star really is.
So, we use absolute magnitude – how much light the star makes.
Understanding Absolute Magnitude
To study stars, we need to ignore distance.
Absolute magnitude measures how bright a star really is.
It's how bright the star would look if it were 32.6 light years away.
This helps us compare stars in different places.
It's like lining them up to see who's brighter.
Understanding Absolute Magnitude
The table shows how brightness changes when you use absolute magnitude.
The Sun:
Looks brightest from Earth (apparent magnitude: -26.7).
But is dimmest when we compare real brightness (absolute magnitude: 4.8).
Example Table:
Sun:
Apparent Magnitude: -26.7
Absolute Magnitude: 4.8
Sirius:
Apparent Magnitude: -1.47
Absolute Magnitude: 1.4
Canopus:
Apparent Magnitude: -0.72
Absolute Magnitude: -2.5
Alpha Centauri:
Apparent Magnitude: -0.27
Absolute Magnitude: 4.4
Betelgeuse:
Apparent Magnitude: 0.42
Absolute Magnitude: -7.2
Pollux:
Apparent Magnitude: 1.14
Absolute Magnitude: 0.7
Andromeda Galaxy:
Apparent Magnitude: 3.44
Absolute Magnitude: -27.5
Check Your Understanding
Which star has the greatest actual brightness? RIGEL
Which star is the faintest as seen from the Earth? ALDEBARAN
List the three stars in order of brightest to dimmest when viewed from the Earth. CANOPUS → RIGEL → ALDEBARAN
Understanding H-R Diagrams
Ejnar Hertzsprung and Henry Norris Russell made a way to show star data.
It's called the Hertzsprung-Russell diagram (H-R diagram).
It shows how bright a star is (absolute magnitude) compared to its colour and temperature.
The H-R diagram shows how brightness and temperature are related.
Understanding H-R Diagrams
The middle line is the main sequence.
Most stars (90%) are on the main sequence.
Other areas have dwarf planets, giants, and supergiants.
Understanding the Life of a Star
The H-R diagram shows how stars change over time.
Most stars, like our sun, are on the main sequence.
Stars stay together because of two forces:
Gravity - pulls stuff towards the center.
Radiation pressure - pushes stuff out because of heat from nuclear fusion.
How Stars are Formed
Space has matter, gravity, and small differences in matter.
Gas and dust clouds are moved by gravity.
Hotter areas clump matter together.
The clump spins and flattens into a disc.
Gravity pulls matter together faster.
The disc spins faster, making a hot core called a protostar.
Atoms crash, making heat.
Hydrogen atoms fuse, making hellum and energy:
Nuclear fusion turns matter into energy - a new star.
After millions of years, gas and dust are blown away.
Understanding the Life of a Star
In a main sequence star, gravity and radiation pressure are balanced.
This lasts for billions of years, until the hydrogen runs out.
The heavier the star, the hotter and brighter it is.
More mass = more gravity = faster fusion = more heat and light.
Stars on the top left are brighter, hotter, and bigger than the sun.
Stars start on the bottom right and move to the top left.
When hydrogen runs out, they change a lot.
Where they go depends on their mass.
Stars on the bottom right are dim, cool, and small
The sun is in the middle of its life.
When it uses all the hydrogen, it will become a red giant.
Then it will become a white dwarf.
Example of HR Diagram and the Life of a Star
A star is 5000K.
If it's on the main sequence, it's absolute magnitude is +5.
Understanding Luminosity
Luminosity is how much energy a star makes.
Absolute magnitude is how bright it looks from 32.6 light years away.
Luminosity depends on:
(a) the size of the star
(b) the temperature of the star
Understanding Luminosity
Size:
Bigger star = more energy.
A star twice as big is four times more luminous.
Temperature:
Hotter star = more energy.
A star twice as hot is sixteen times more luminous.
A cool red giant can be more luminous than the Sun because it is bigger.
Understanding the Life Cycle of a Star
Let's sum up what we know about the life cycle of a star.
Understanding the Life Cycle of Stars – The Birth
Interstellar Cloud:
Made of hydrogen.
Collapses because of gravity.
Gas heats up.
If it has enough mass, it starts nuclear fusion.
Fusion turns hydrogen into helium.
Protostar:
Formed when fusion starts.
Hard to see because of a disc of stuff around it.
Stays about the same brightness, but gets hotter and smaller.
Joins the main sequence.
Now it's like the Sun.
Understanding the Life Cycle of a Star – Death of a Star
Stars don't last forever.
They run out of hydrogen.
Helium gets very hot and makes heavier elements.
The star gets hot and big, almost 100 times bigger than the Sun.
It becomes a Red Giant.
The red giant gets bigger and smaller.
It ejects its outer layers, making a colorful planetary nebula.
Planetary nebulas happen when helium runs out.
Gravity pulls the outer layer away.
About half the star's mass escapes.
This makes a cloud of gas around the star.
The core is a white dwarf star.
Understanding the Life Cycle of a Star – Death of a Star
The white dwarf is