3.4 Evolution Of Stars Flashcards
How are stars formed
Stars are formed from huge clouds of gas and dust called nebulae
Found mainly in spiral arms of Galaxy
e.g. Orion’s nebula
Gas cloud begins to collapse and breaks up into protostars
Rise in temperature and pressure as GPE (gravitational potential Energy) KE (kinetic energy).
Temperature at core reaches 15 million k.
Fusion starts
Outward pressure from radiation prevents any further collapseMain sequence star is formed.
Star reaches stable size
Describe a stars death
Star runs out of Hydrogen
No radiation countering gravitational collapse
Star collapses
Temperature rises further
Fusion restarts in a shell surrounding the core which is now rich in helium.
Outer layers of the star expand and cool to form red giant.
Temperatures at core rise to 100 million kelvin
Helium nuclei fuse to form carbon.
Helium is depleted
Red giant looses it’s outer layers in an expanding shell of gas.
Planetary nebula
Core collapses to form White Dwarf.
Cools eventually to brown and then black.
Mass of star affects how star dies
Super giants can form iron through fusion
Supernova!
Then Neutron star or Black Hole.
What is a supernova
A massive explosion at the end of a supergiant star’s life. (greater than 8 solar masses)
Outer layers blown away at speeds of 5000km/s
What’s a neutron star
A remnant of a supernova
Mass of sun collapsed into a sphere of 20km diameter.
Intense gravitational fields cause them to spin.
Emit radio waves from their polar regions
Pulsar
How can we deduce the composition of a star
Each element has it’s own unique set of spectral line wavelengths.
Analysis of starlight can tell us what the star is made of.
How can we deduce the temperature of a star
The temperature of a star determines it’s colour so spectral analysis can also be used to determine its temperature.
How can we determine the radial velocity of a star
Analysis of the Doppler shift of spectral lines.
Describe the Harvard classification of spectral type
OBAFGKM.
O is hottest and M is coolest.
What’s an HR diagram
This is a plot of luminosity against spectral type.
Draw a HR diagram
Check image
Bottom left - White dwarfs
Nearly top right - Giants
Very top right - supergiants
Middle line bottom right to top left - main sequence
Which stars run out of fuel sooner
Massive stars (top left of HR Diagram). 61 Cygni is a low mass star and will spend 100’s of billions of years on the main sequence.
Give evidence for the existence of neutron stars
Regular pulsating radio signals from Pulsars.
Give 3 pieces of evidence for the existence of black holes
Gravitational lensing
Light bent by black hole’s gravity
Speed of rotation of our galaxy and its size hints that the mass at its centre is great enough to be a black hole.
Charged particles spinning around a Black hole produce x rays which we can detect.
How are stars formed
By gravitational collapse of huge clouds of gas and dust - found in spiral arms of galaxies
How big could a cloud that forms stars be
15kpc across - contain enough raw material for several thousand stars
What’s an emission nebula
Eg the Orion Nebula - site of star formation
They contain glowing gas that has been excited by newly formed stars and emits light by fluorescence
What happens when a gas cloud stars to collapse
A protostar forms
How does a protostar form
When an emission nebula collapses, breaks into smaller knots - protostars
Tell me characteristics of a protostar
Rise in temp
As GPE connected to KE
What happens after a protostar forms
The central temperature gets to 15 million k so fusion begins - hot enough for hydrogen nuclei to turn into helium
What happens when fusion occurs in a protostar
The outward pressure generated halts further collapse and star settles to stable size
Begins to radiate energy from its hot surface and becomes a MAIN SEQUENCE STAR
Tell me about the main sequence phase of a star
Very stable
Low mass star can remain in phase for 1 million million years
Large mass star can remain so for 100 million years
What happens when hydrogen runs out to a main sequence phase of a star
Hydrogen in core runs out
Outward radiation pressure is now no longer present - star collapses once more under own gravity
This causes a further temp rise
Until outer layers can also do fusion like the core - cause outer layers to expand and cool to form a RED GIANT OR SUPERGIANT
What happens when a star is in the red giant phase
With further hydrogen depletion and contraction - core temp can get 100 million k
So helium nuclei can form carbon
Red giant loses its outer layers in an expanding shell of gas - PLANETARY NEBULA
Tell me about planetary nebula
Inner part of star collapses to form a dense hot white dwarf
Tell me what happens after a star is a white dwarf
White dwarfs cool to become red brown and eventually black dwarfs
Instead of supergiants becoming a planetary nebula what happens?
Further fusion involving elements up to iron
Once these are depleted a supernova (violent explosion) happens at outer part of core and supergiant blows away outer layers at speeds of 5000km/s VRMMM VROOOM
What happens after a supernova
The remaining dense core is a supernova remnant and can either be a neutron star or black hole
Tell me about neutron stars
Small diameter
High density
Rotate rapidly on axis due to strong gravitational effects
Emit high radio waves from polar regions - pulses of radio waves enable us to detect them as pulsars
Tell me about black holes
Formed when a massive massive supergiant has a supernova
- greater than 3 solar masses
Very high densities of supernova remnants have high densities and gravitational pull and so light and Em radiation is unable to escape
How are black holes detected
By X Ray’s emitted from accreting matter from a nearby companion star in a binary system
