Evolution of Stars Flashcards
Nebulae
Stars are formed from the gravitational collapse of gas and dust mainly in the spiral arms of galaxies. These clouds can be 15kpc across and can have enough raw materials to form several thousand stars.
What is the Orion Nebula?
A stellar nursery, as it is an emission nebula.
How big can nebulae be?
15kpc across
Protostars
The clouds of gas and dust begin to collapse and form smaller collapsing ‘knots’.
Energy
Gravitational energy is converted into kinetic energy, which is converted into thermal energy. The star’s central temperature begins to rise and can reach as high as 15 million K.
Nuclear fusion
Due to these high temperatures, nuclear fusion of hydrogen into helium can begin to take place. This releases energy at all stages.
Stabilisation
The outwards radiation pressure from nuclear fusion balances the inwards pressure from gravity. This makes the star stay at a stable size and halts further collapse.
Main sequence
The star can stay at this stable size for millions of years depending on its mass. If the star is large, such as Spica, it stays at this stage for 100 million years. If the star is small, such as Barnard’s star, the star can spend a million million years as a Main Sequence star.
How far is the sun through its main sequence?
The sun is approximately halfway through its main sequence of 10,000 million years.
Star collapses
The star then runs out of hydrogen fuel. With no more radiation pressure, the star collapses under its own gravity.
Further hydrogen fusion
This heats up the shell around the star’s core (which is already rich in helium), meaning that more hydrogen fusion can take place.
Supergiant / Red Giant stage
The star expands and cools to become a red giant or supergiant.
Star collapses again
The hydrogen fuel eventually depletes again, causing further collapse.
Helium fusion
because the star collapses, temperatures rise to as high as 100 million K. This allows helium fusion into carbon to take place.
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The helium fuel depletes again and the red giant loses its outer layers in an expanding gas layer. This forms a planetary nebula.
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The core is left behind after this. It cools to become a white dwarf, then a brown and black dwarf. A white dwarrf has the size of the earth but contains approximately the mass of the sun.
(>8 sol) Fusion continues
Fusion continues with further collapse as the star fuses elements down to iron.
(>8 sol) Supernova
The core collapses once more. Temperature and density increases. Then, a final burst of fusion takes place as iron combines to make helium and electrons and protons combine to make neutrons. This causes an explosion as temperatures reach 100 billion K. 10% of the mass is converted into energy, causing collapse at speeds up to 70,000km/s and the outer layers to be blown away at speeds of up to 5,000km/s. This leaves behind a supernova remnant.
What nebulae are associated with the birth of stars?
Emission and absorption.
What are also associated with the birth of stars?
Open clusters.
How do emission nebulae glow?
Stellar radiation excites the hydrogen gas, which makes it appear red in colour.
Give an example of an emission nebulae.
The Eagle Nebula
Why do absorption nebulae appear as they do?
They block out the light from the stars behind them.
Give an example of an absorption nebula.
Dark Horse Nebula
Why do reflection nebula appear as they do?
Dust scatter the light from stars. They appear blue in colour.
Give an example of a reflection nebula.
Witch Head Nebula
What are planetary nebula?
The outer layer of gas from a dying star of less than 8 solar masses. They are dust and gas ejected into space.
Give an example of a planetary nebula.
Ring Nebula
What are open clusters? Relate to the birth of stars.
Open clusters are groups of around 1000 stars.
They are bright, implying that they are young.
Close to each other in space, thus they are formed in the same nebula from the same material.
Found around the galactic plane.
Describe the nature of neutron stars.
Have the mass of the sun enclosed in a sphere of 20km diameter.
Due to mass they have extremely high forces of gravity.
This gravity causes them to rotate on their own axis.
They emit radio waves in regular pulses from their poles, so are detected as pulsars.
What do neutron stars emit?
Radio waves
Describe the nature of black holes.
Extremely dense supernova remnant with extremely strong gravity.
Gravity so strong that no electromagnetic radiation can escape.
Emit X-rays from event horizon.
Impossible to view directly.
May account for much of the hidden mass in the universe.
Give evidence for the existence of black holes.
- Spiralling charged particles from accreting matter heat up due to friction and emit X-rays, which can be detected.
- Gravitational lensing: observed multiple or distorted images of galaxies due to light being bent by gravity.
- Galaxy’s speed of rotation is faster than expected, there must be hidden mass in black holes near its centre.
What do black holes emit?
X-rays
Give evidence for the existence of neutron stars.
They are pulsars that regularly emit radio waves from their polar regions.
