SCI Earth+Space T2 26

What is a star

giant ball of hot glowing gases, mostly hydrogen and helium, produces energy through nuclear fusion in core —> released as light, heat, other electromagnetic radiation.

Characteristics of stars

• Mass – greater mass, stronger gravitational force which compresses core, increasing rate of nuclear fusion.

• Size – Stars vary greatly in size, from small stars to enormous supergiants.

• Temperature –

• Brightness

Nuclear Fusion

Nuclear fusion is the process that powers stars.

• ¾ of a star is hydrogen.

• core to 15 million°C.

• atoms become plasma, where electrons separate from nuclei.

• Gravity forces positively charged hydrogen nuclei together- overcome repulsion.

• fuse together, forming helium.

• some mass is converted into energy

• pushes outward against gravity, keeping the star stable during the Main Sequence stage.

difference between absolute and apparent magnitude

Apparent Magnitude

• Measures how bright a star appears from Earth.

• Depends on the star's distance from Earth.

• The more negative the magnitude, the brighter the star appears.

Absolute Magnitude

• Measures a star's true brightness (luminosity).

• Compares all stars if they were 10 parsecs from Earth.

• Allows astronomers to compare stars fairly regardless of distance.

A star may have a high absolute brightness but appear dim because it is very far away.

how parallax is used to identify the relative position of stars from two different positions in space

Parallax is the apparent shift in the position of a nearby star when viewed from two different positions in Earth's orbit around the Sun.

• Astronomers observe a nearby star six months apart.

• The nearby star appears to move against the background of much more distant stars.

• The closer the star is, the greater the parallax angle.

• The farther away the star is, the smaller the parallax angle.

Parallax allows astronomers to calculate the distance to nearby stars.

A star with a parallax angle of 1 arcsecond is 1 parsec away.

Convert between light-years and parsecs.

A light-year is the distance light travels in one year.

• Light travels at 300 000 km/s.

A parsec (pc) is the distance to a star with a parallax angle of 1 arcsecond.

Relationship

1 parsec = 3.26 light-years

Formulae

Light-years → Parsecs

Parsecs = Light-years ÷ 3.26

Parsecs → Light-years

Light-years = Parsecs × 3.26

Describe the relationship between temperature and the colour of a star

colour of a star depends on its surface temperature.

Colour	Temperature
Blue	Hottest
White	Very hot
Yellow	Medium (e.g. Sun)
Orange	Cooler
Red	Coolest

Scientists use a spectrometer to analyse the light from stars.

A star's spectral class shows:

• Colour

• Temperature

• Elements present

As temperature increases, the star's colour changes from red → orange → yellow → white → blue.

Interpret a Hertzsprung-Russell Diagram.

graph showing the relationship between a star's temperature and absolute magnitude (brightness).

Axes

X-axis: Temperature (or spectral class/colour)

• Hot (blue) on the left

• Cool (red) on the right

Y-axis: Absolute magnitude (brightness)

• Bright at the top

• Dim at the bottom

Main Sequence

Most stars lie along the Main Sequence, running diagonally from the top left to the bottom right.

Main Sequence stars are stable because:

• Gravity pulls inward.

• Radiation pressure from nuclear fusion pushes outward.

acts as a map of a star's life cycle.

Describe how galaxies are formed.

a gravitationally bound collection of stars, gas and dust.

Galaxies form when:

• Large clouds of gas and dust collapse under gravity.

• Nebulae form stars.

• Billions of stars become gravitationally bound together.

• Galaxies can cluster together into superclusters.

The three main galaxy shapes are:

• Spiral

• Elliptical

• Irregular

Our Solar System is located in the Milky Way, which is a spiral galaxy.

Describe the stages in the life cycle of stars.

Nebula – large cloud of gas and dust where stars form.

Protostar – A collapsing cloud of gas and dust that is heating up but has not started nuclear fusion.

Main Sequence – A stable stage where gravity and radiation pressure are balanced.

Red Giant – A low-mass star that expands and cools after using up hydrogen in its core.

Supergiant – A massive star that expands greatly and fuses heavier elements up to iron.

Supernova – The explosive death of a massive star after core collapse.

Neutron Star – An extremely dense remnant left after a supernova.

Planetary Nebula – The glowing shell of gas released by a dying low-mass star.

White Dwarf – The hot, dense core left after a low-mass star sheds its outer layers.

Describe the life cycle of Low-Mass Star

Nebula

↓

Protostar

↓

Main Sequence

↓

Red Giant

↓

Planetary Nebula

↓

White Dwarf

Describe the life cycle of High-Mass Star

Nebula

↓

Protostar

↓

Massive Main Sequence Star

↓

Supergiant

↓

Supernova

↓

Neutron Star or Black Hole

Describe how black holes are formed

forms after the death of a very massive star.

• The star becomes a supergiant.

• It runs out of fuel.

• The core collapses under its own gravity.

• A supernova explosion occurs.

• If the remaining core is extremely massive, gravity becomes so strong that not even light can escape.

This forms a black hole, one of the densest objects in the universe.

Construct a timeline of the Big Bang Theory.

Time	Event
0 s	Big Bang occurs. Space and time begin. Universe rapidly expands (inflation).
0.000001 s	Universe cools. Fundamental forces, protons, neutrons and electrons form.
3 s	Hydrogen, helium and trace lithium nuclei form.
10 000 years	Radiation cools and stretches into visible light and microwaves.
300 000 years	Electrons combine with nuclei to form hydrogen and helium atoms.
300 million years	First stars and galaxies form.
9 billion years	Our Solar System begins to form.
13.7–13.8 billion years	Present-day universe.

Big Bang theory key evidence

• Cosmic Microwave Background (CMB): The afterglow of the Big Bang, discovered by Penzias and Wilson.

• Expanding Universe: Hubble found that galaxies are moving away from each other, with more distant galaxies moving faster (Hubble’s Law).

• Red Shift and Doppler Effect: Light from distant galaxies is shifted towards the red end of the spectrum, showing they are moving away as the universe expands.

• Abundance of Light Elements: The universe is made mostly of hydrogen and helium, matching predictions of the Big Bang Theory.