3.4 Evolution Of Stars

Question 1

How are stars formed

Answer 1

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

Question 2

Describe a stars death

Answer 2

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.

Question 3

What is a supernova

Answer 3

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

Question 4

What’s a neutron star

Answer 4

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

Question 5

How can we deduce the composition of a star

Answer 5

Each element has it’s own unique set of spectral line wavelengths.
Analysis of starlight can tell us what the star is made of.

Question 6

How can we deduce the temperature of a star

Answer 6

The temperature of a star determines it’s colour so spectral analysis can also be used to determine its temperature.

Question 7

How can we determine the radial velocity of a star

Answer 7

Analysis of the Doppler shift of spectral lines.

Question 8

Describe the Harvard classification of spectral type

Answer 8

OBAFGKM.

O is hottest and M is coolest.

Question 9

What’s an HR diagram

Answer 9

This is a plot of luminosity against spectral type.

Question 10

Draw a HR diagram

Answer 10

Check image

Bottom left - White dwarfs

Nearly top right - Giants

Very top right - supergiants

Middle line bottom right to top left - main sequence

Question 11

Which stars run out of fuel sooner

Answer 11

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.

Question 12

Give evidence for the existence of neutron stars

Answer 12

Regular pulsating radio signals from Pulsars.

Question 13

Give 3 pieces of evidence for the existence of black holes

Answer 13

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.

Question 14

How are stars formed

Answer 14

By gravitational collapse of huge clouds of gas and dust - found in spiral arms of galaxies

Question 15

How big could a cloud that forms stars be

Answer 15

15kpc across - contain enough raw material for several thousand stars

Question 16

What’s an emission nebula

Answer 16

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

Question 17

What happens when a gas cloud stars to collapse

Answer 17

A protostar forms

Question 18

How does a protostar form

Answer 18

When an emission nebula collapses, breaks into smaller knots - protostars

Question 19

Tell me characteristics of a protostar

Answer 19

Rise in temp

As GPE connected to KE

Question 20

What happens after a protostar forms

Answer 20

The central temperature gets to 15 million k so fusion begins - hot enough for hydrogen nuclei to turn into helium

Question 21

What happens when fusion occurs in a protostar

Answer 21

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

Question 22

Tell me about the main sequence phase of a star

Answer 22

Very stable

Low mass star can remain in phase for 1 million million years

Large mass star can remain so for 100 million years

Question 23

What happens when hydrogen runs out to a main sequence phase of a star

Answer 23

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

Question 24

What happens when a star is in the red giant phase

Answer 24

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

Question 25

Tell me about planetary nebula

Answer 25

Inner part of star collapses to form a dense hot white dwarf

Question 26

Tell me what happens after a star is a white dwarf

Answer 26

White dwarfs cool to become red brown and eventually black dwarfs

Question 27

Instead of supergiants becoming a planetary nebula what happens?

Answer 27

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

Question 28

What happens after a supernova

Answer 28

The remaining dense core is a supernova remnant and can either be a neutron star or black hole

Question 29

Tell me about neutron stars

Answer 29

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

Question 30

Tell me about black holes

Answer 30

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

Question 31

How are black holes detected

Answer 31

By X Ray’s emitted from accreting matter from a nearby companion star in a binary system