Topic 4: StarsTopic 4: Stars

Sun = star à scientists analyse surface +radiation

Hotter areas of sun = white                                         Warmer areas = orange

Spectrum shows the elements that are present in the sun.

Coronal mass ejection – bubbles of gas bursting out of the sun and flying into space

  • contains charged particles à damage electrical communication + power supply
  • can be monitored by space “weather forecast” = warning

How hot is the sun? 4A


Hot objects emit radiation. The also emit a continuous range of frequencies = continues spectrum

BUT always emit more of one frequency = peak frequency (green)

Peak frequency depends on temperature:

  • Hotter = more energy given to photons (packets of radiation) = higher peak frequency
  • As the temperature increases, the amount of radiation at higher frequency increases.

Graphs show intensity of radiations

  • Intensity = energy radiated from surface

Luminosity – amount of energy emitted form surface a.k.a. glow

  • Taller = greater luminosity (red)



Stars (hot object) produce lots of frequencies across EM spectrum (usually in UV, Visible and infrared section).

  • Stars shine with different coloured lights e.g. reddish, yellowish

Colour à temperature of star’s surface

E.g. burn metal à red à orange à yellow à white = order of spectrum

Red = low frequency =cool                                          Blue = higher frequency = hot

Spectrometer attached to telescope à starlight is broken up into different colours à spectrum produced à see frequencies present (from the colours present)

What is the sun made of? 4B

Emission spectrum – A few coloured lines but the rest is black spaces (not a continuous band from red-violet)

  • This only applies to chemicals/elements
  • Each element has a unique spectrum à this is used to identify elements

Absorption spectrum – has some black lines which shows that some wavelengths of frequencies are missing

  1. The suns photosphere emits white light which contains all wavelengths of radiation (continuous spectrum)
  2. White light passes through atmosphere
  3. Some of the wavelengths in the white light is absorbed by the atoms in the atmosphere
  • These atoms are also present in the photosphere
  1. So by the time it reaches us some of the wavelengths of lights has been absorbed à black missing lines in the spectrums = absorption spectrum

The missing wavelengths in the absorption spectra correspond to the lines in the emission spectra (it is an exact fit).

Absorption    ß Emission


Electrons + Energy levels

Atoms contains positive nucleus, electrons and energy levels (shells)

  • The lowest energy level is nearest to the nucleus

Electrons move between energy levels if they gain/lose energy.

Emission spectra

If they lose energy they may go to a lower shell

When electrons lose energy it emits a single photon of light

Electrons are unstable in higher energy levels à fall from high to low levels = losing energy b/c emits single photon of light à single lights are code for particular frequency à specific coloured lines in emission spectra

Energy of photon = energy difference between the two energy levels

  • Greater energy gap = greater energy of photon
  • High energy photon = high frequency, short wavelength light (violet)

Absorption spectra

If the electrons gain energy they may go to a higher shell

If they gain enough energy they can repel the attractive force of the nucleus and be removed from the atom à atoms has no charge = ionisation

  1. White light consists of photons of all energies
  2. Only certain energy levels that an electron can occupy

Low energy electrons can absorb a particular frequency of radiation to get to a higher level

  1. When it absorbs the particular photon, jumps to higher level
  2. The white light is now missing photons
  3. These black lines correspond to emission spectra

Discovering helium

Each element spectra is unique.

Scientists observed edge of sun during solar eclipse. Use spectrometer à found a new line in spectrum which didn’t correspond to any known element à helium discovered.

Studying helium spectra àfound that sun consists mainly of helium and hydrogen

What fuels the sun? 4C                                                                                          

Early discovery

The sun cannot burn its own material b/c been burning for too long – should have run out of fuel long time ago

Nuclear Fusion

Einstein said that mass could be converted to energy:    E = MC2

  • Mass gets lost when hydrogen in sun changes to helium
  • The mass changes to energy which fuels the sun

During nuclear fusion, the nuclei lose mass and energy is released

Small positive hydrogen nuclei fuse together à bigger hydrogen nuclei

  • Need enough energy (heat) from sun’s core for the nuclei to get close enough to fuse; electrostatic repulsion will be pushing them away
  • The strong force makes them react with each other


  1. Two hydrogen nuclei (proton) fuse à deuterium nucleus (one proton and one neutron) + energy released
  • the neutron forms when the proton decays à releases positron
  1. The deuterium reacts with another hydrogen nucleus àHELIUM + energy released
  2. Helium + Helium à another isotope of Helium but larger + two hydrogen nuclei + energy released
  • The total mass of particles after the reaction is less than the total mass before; missing mass à energy released

Word equation steps

  1. Hydrogen + hydrogen à Deuterium + positron + energy
  2. Hydrogen + deuterium à helium + energy
  3. Helium + helium à helium (bigger) + hydrogen + hydrogen + energy

Calculating steps

  1. Top : 1 + 1 =2 à 2 + 0 = 2 Bottom: 1 + 1 + 2 à 1 + 1 = 2
  2. Top: 1 + 2 =3 à 3 Bottom: 1 + 1 = 2 à 2
  3. Top: 3 + 3 = 6 à 4 + 1 + 1 = 6 Bottom: 2 + 2 = 4 à 2 + 1 + 1 = 4

The equation needs to be balanced. The number of top numbers must be equal in both sides. This applies to bottom numbers too.

What are other stars like? 4D

Hertzsprung-Russell Diagram

  • Mainstream: most of the stars, including the sun, fall along here
  • White Dwarfs: not very luminous (bright) but very hot
  • Red Dwarfs: Not as hot but very luminous b/c have bigger surface area to emit light


Temperature decreases as you go along, instead of increases.

Luminosity increases.

They used this chart to explain that perhaps stars started off as mainstream stars for most of their life. But they spend a small part of their life as a Red Giant or White Dwarf.

Scientists can only speculate b/c we cannot observe a star through its lifetime as it would take millions of years.

Instead they observe star populations and link it to models.

Interstellar Medium: the space between stars filled with low-density gas

  • Mainly hydrogen, helium, supernovae remnants
  • Gasses may be coloured b/c perhaps the star had more of one element (refer to spectrum)

Dense clouds – very cold

  • Not dense by Earth standards

How gases behave 4E

Measurable quantities

  • Temperature – degrees Celsius / kelvin
  • Pressure – 1 atmosphere = 100kPa
  • Volume – m3
  • Mass – kg

Absolute Zero

Absolute Zero is at the point where the atoms have the smallest amount of kinetic energy possible. All gasses condense to form liquids at this point. Absolute Zer0 = -273 degrees Celsius. This is the start of the Kelvin scale.

Converting scales

  • Degrees à Kelvin: Add 273 g. 2 Kelvin = 546 degrees
  • Kelvin à Degrees: Subtract 273    g. 30 degrees = 303 Kelvin




Volume and pressure (+ constant temperature)

  • When volume increases, pressure decreases = INVERSELY proportional relationship
  • When volume decreases, pressure increases.

E.g. if you double the volume to 100cm3, and the pressure was previously 5 atmospheres, it would become 10 atmospheres now.  So, P is directly proportional to 1/V:

This is because if there is less space the particles will collide with the wall more frequently. A small force is released each time the particles hit the wall.

More collisions = more force = more pressure.

If you were pushing on a syringe with a blocked end at first it would be easy to push it. But then it gets more difficult to push. The pressure is increasing.

If you reduce your force on the syringe so that you’re not pushing as hard, the air will push back and expand. The pushing part of the syringe will pull out slightly from the tube.

Formula:                              Pressure x Volume                      P1 x V1 = P2 x V2

E.g. a gas at constant temperature in a 50ml container has a pressure of 1.2 atmospheres. Find the new pressure if the container volume is reduced to 40ml.

1) 50 x 1.2 = 40 x P2           2) 50 x 1. 2 = 60        3) 60 / 40 = 1.5 atmospheres        4) 1.5 x 100 = 150kPa

Volume and temperature (+ constant pressure)

  • When temperature decreases, volume decreases = DIRECTLY proportional relationship

This is because the particles have less kinetic energy. This means that the collisions are less frequent and has less force. The volume gets smaller to compensate for this lack of collisions.

If you halve the temperature, the volume will also halve.

Formula:        Volume/Temperature           V1 / T1 = V2 / T2

E.g. a gas at constant pressure with a temperature of 270K has a volume of 24 litres.  Find the new volume if the temperature increased to 315K.

1) 24 / 270 = V2 / 315                       2) (24 / 270) x 315 = 28 litres

Pressure and temperature (+ constant volume)

  • When temperature increases, pressure increases = DIRECTLY proportional relationship

If you heat particles they gain more kinetic energy. Thus they have hit the walls more frequently and with more force, which increases the pressure.

If you double the temperature, the pressure also doubles.

Formula:              Pressure / Temperature                                    P1 / T1 = P2 / T2

E.g. a container has a volume of 30 litres. It is filled with a gas at a pressure of 1 atmosphere and a temperature of 290K. Find the new pressure if the temperature is increased to 315K.

1) 1 x 100 = 100kPa          2) 290K = 17 degrees (290 – 273)               3) 315K = 42 degrees      4) (100 / 17) x 42 = 25 litres

How do stars form? 4F

Protostar stages

  1. Clouds and dust in space
  2. Gravity pulls dense clouds so that they contract into clumps
  3. When clumps get dense, they break up into Protostars
  4. Protostars internally collapse under gravity.
  5. They become a swirling disk shape
  6. The volume of Protostar reduces as the dust collapses inwards
  • Decrease in volume à particles squashed b/c less space à increased pressure
  1. As particles get closer their speed increases àtemperature at the centre gets hotter à NUCLEAR FUSION à star is born
  • Material further out forms into clumps à planets
  1. The internal fusion releases vast amounts of energy and creates an outward pressure to stop gravitational collapse. The forces as in balance.
  2. The star has reached main sequence. The luminosity and temperature will remain unchanging.

The stars form in clusters and planets form at the same time as stars.

Hot enough for fusion?

  • Gas idea: gas is compressed by gravity, and this increases the temperature
  • Particle idea: Particles in the cloud attract other particles and they all collapse in on themselves. The particles collide and share energy. Energy enables them to move faster. This increases the temperature.

Computer models

Some models predict that the protostar spins faster and faster until it blows out jets of hot gas at right angles to planetary discs. If telescopes observe a jet, they can locate the planets.

Main sequence stars

Main sequence stars have steady luminosity and temperature.

Any differences are due to masses. More massive = hotter core = more fusion occurs.

Also they are more luminous because there is a bigger surface area, so more energy is released and it looks brighter.

Internal structure of the sun

Sun’s surface is too “cold” for nuclear fusions to take place. Therefore, it must take place somewhere else where the temperature is a few million degrees.

  • Core – nuclear fusions takes place. It is hotter and denser. The nuclei fuse together = fuel
  • Radiative zone – photons (packets of radiation) travel outwards
  • Convective zone – carries heat energy to photosphere. The heat travels up and then circles down so that it cools.
  • Photosphere – EM radiation emitted and radiated through atmosphere

How do stars end? 4G

Fuel will run out eventually and nuclear fusion will stop. The star will then change from the main sequence.

Red Giant

  1. Fusion slows down so the core cools down.
  2. There is less internal pressure so the outwards pressure gets too much (unbalanced) and makes core collapse.
  3. The outer layers containing hydrogen also fall inwards.
  4. As the hydrogen gets closer, it starts to fuse and makes the outer shell expand.
  5. The surface temperature falls (b/c now heat is spread over a larger surface).
  6. The surfaces changes color from yellow to red.
  7. It has become a Red Giant


White Dwarf

  1. Outer layers of Red Dwarf is expanding whilst the core is contracting
  2. It gets so hot that helium fusion starts
  3. Helium nuclei have a greater positive charge than hydrogen nuclei. Thus, the electrostatic repulsion is greater and they need more energy to overcome it.
  4. When helium fuse, they form heavier elements e.g. carbon, nitrogen, oxygen
  5. Soon the fuel runs out.
  6. The outer layer cools down and is thrown off into space
  7. The core remains and shrinks into itself
  8. It has now become a White Dwarf.
  9. The White Dwarf will eventually cool and fade.


Super Giants + Supernovae

Sun is a small star so the core won’t get hot enough for complex fusion to occur and make heavier elements than carbon. But supergiant cores exceed billions of degrees. This is hot enough for complex fusion. This leads to heavier elements being formed and more energy being released.

However, when the nuclei start forming iron there is a problem. When iron fuses they absorb energy. Energy is not being released so there is no decrease in mass. This means that there is in increase in the Iron’s mass.  Lack of heat means a lack of pressure inside the core. The outward pressure gets too much and the outer layers collapse inwards. However, because the core is very dense, when the outer material collides with it, they bounce off and fly outwards. This is a supernova.

During the explosion, temperatures rise even higher. The heat causes medium-weight elements in the surrounding atmosphere to fuse (before only lightweight hydrogen was able to fuse). These medium-weight elements fuse together and form the heaviest elements, up to Uranium. The different colors in the image show the different elements present (spectra).

The remnants of the supernovae may become part of ISM and then form in clumps until a Protostar is formed again. The cycle repeats.

Neutron star and Pulsars

The core remains after a supernova. If it is a low mass core it becomes a neutron star. The neutrons are compressed together like a giant nucleus.

They can help to explain Pulsars (distant objects that send out radio waves that vary with a regular pulse)

  1. The core collapses à neutron star
  2. The neutron star spins faster and faster
  3. The magnetic field gets concentrated
  4. Beams of radio waves are emitted from the magnetic poles on either end of the star
  5. As the star rotates at a regular speed, the beam also rotates and sweeps across space
  6. It is detected as a series of Pulsars


Black hole

If the core of the star is a larger mass it will collapse into itself more under the pull of gravity. It becomes a black hole. The gravity pull has become so strong that it pulls everything into it – even light can’t escape.

Are we alone? 4H

Remnants of supernovae contain traces of elements from the periodic table. If these particles contract and form into clumps and then into planets, life could be formed on them after evolution, as they would have all the necessary elements to survive.

Arguments for extra-terrestrial

Lots of stars thus there must be lots of planets formed at the same time

Life has evolved on the planets

Intelligent organisms may be communicating with us by sending out signals

SETI is an organization where people use computers to process data from radio telescopes to see whether it contains any “intelligent” signs.

Scientists have discovered Exo-planets, which are all more massive than the Earth.

No extra-terrestrial life has been detected so far.