Stars and Their Life Cycle
Stars
The Sun
Sun Data
Age: 4.5 Billion Years Old
Sun Diameter: 109 Earth Diameters
Solar Mass:
Composition (by # of Particles)
92.1% Hydrogen, 7.8% Helium, 0.1% Others
Mean Temperature: 5800K (Surface), K (Core)
Luminosity: W (Energy emitted per second)
Orbital Period Around Milky Way Galaxy: 220 million years
Orbital Speed Around Milky Way Galaxy: 220 km/s
Sun's Location in the Milky Way Galaxy
How does the Sun generate energy
Nuclear fusion Simply Explained
Thermonuclear Fusion
The process of combining lighter elements into heavier ones
Primarily hydrogen atoms combining to helium atoms within the Sun
Thermonuclear Fusion is caused by the pressure of the Sun and the heat of the Sun fusing hydrogen atoms together
Thermonuclear Fusion
Step 1
Two Hydrogen atoms combine to form 1 Hydrogen molecule
(1H + 1H => 2H + n + e+)
or
(p + p => pn + n + e+)
Proton, p.
Neutron, n.
positron .
neutrino, v.
Step 2
A Hydrogen molecule combines with a hydrogen atom to form Helium ion
(2H + 1H => 3He + g)
or
(pn + pn => ppn + g)
g is radiation emitted from the interaction
Step 3
Two Helium ions combine to for Helium.
(3He + 3He => 4He + 2 1H)
or
(ppn + ppn => ppnn + 2p)
Total process
(4 1H => 4He + n + g + e+)
Note the positron and a free electron interact to emit more radiation (e- + e+ => 2g)
Energy Released from Nuclear Fusion
Going from hydrogen to helium energy is released through radiation.
We can calculate this energy loss by looking at the mass differences between the two atoms.
4 Hydrogen Atoms: kg
1 Helium Atoms: kg
4 Hydrogen mass subtracted from 1 helium mass means we lost some mass!
Lost Mass: kg
Energy Released from Nuclear Fusion
The mass must be accounted for.
The positron is annihilated with an electron releasing energy
Neutrino is basically massless
So, all this lost mass (about 7% of initial mass, 4 Hydrogen atoms) is converted into energy
How much energy?
E is the energy emitted, m is the mass lost, and c is the speed of light
Joules
Enough energy to light a 10W light bulb for s
Energy released from one Nuclear Fusion
Energy Released from Nuclear Fusion
The Sun emits a lot of energy so it must be doing Nuclear Fusion a lot.
How much fuel is the Sun consuming to emit its energy?
Sun's Luminosity
The Sun's Luminosity is a measure of how much energy per second the Sun emits.
We know how much mass is used in Nuclear Fusion
4 Hydrogen atoms are used per reaction
Using the Sun's Luminosity Joules per Second we can calculate how much hydrogen
The Sun consumes 600 million metric tons of hydrogen every second
pounds per second
This reaction occurs at the core.
Structure of the Sun
Core - Location of Nuclear Fusion
Radiative Layer
This layer contains the energy/ heat generated by the Nuclear Fusion.
It takes energy generated in the core 170,000 years to travel through this layer
Photons must past through dense matter which means it doesn't have a straightforward path to leave the layer
Convection Layer
This layer has cooler top layers falling into the Sun and bottom layers warming and rising to the top layers in a cycle.
Structure of the Sun
Photosphere – The visible layer of the Sun
Chromosphere – The plasma layer that is heavily manipulated by Magnetic Field and have solar flares.
Corona - Solar atmosphere or glow around the Sun.
Solar Flares – Large intense localized eruptions from the chromosphere that are thought to be irregularities in the magnetic field.
Magnetic Field of the Sun
With all this hot plasma full of charged particles the Sun creates its own Magnetic field
The field is so strong and big that it surrounds the entire solar system known as the Heliopause
The magnetic field despite having a large influence around the solar system it also has irregular rays on its surface.
Sometimes the irregular magnetic fields at the surface seem to cool the areas it interacts with.
These cooled areas are Sun Spots.
Sun Spot Activity is Periodic
Sun Spots are cooled spots on the Sun's photosphere that experience irregular magnetic fields.
The amount of Sun Spots varies on an ~11 year cycle.
The Life and Death of Stars
Birth Places for Stars
Stars are born from large dense clumps of gas and dust known as stellar nurseries.
Can also be known as a molecular cloud
As discussed in our solar system lecture it is believed that Stars form from these clouds
Gravity pulls the matter toward a center of mass and starts to spin due to this accretion creating a protostar.
Once enough gravity is acquired the pressure at the core becomes enough to start Nuclear Fusion then a star finally born
There are many types of stars
Stars have a few features that separate them
Color – Stars can be red, yellow, white and even blue
Size - Some stars are bigger, giants, and others are significantly small, dwarfs.
Fusion – Some stars no longer produce light from nuclear fusion.
Age – Stars change as they get older
Stars that are younger are known as main sequence stars.
These are stars that are still producing Hydrogen to Helium fusion except brown dwarfs
Once they fuse helium into heavy elements the star begins its old age and eventually dies.
After death, the stars leave behind remnants of what they used to be
Low Mass vs High Mass
The more mass the initial molecular cloud had the bigger the star.
As the protostar accumulates the dust and gas from the cloud it gets bigger gaining more gravity over time.
This leads to some stars being more massive and other being very small.
Low Mass vs High Mass
The smallest stars are known as dwarf stars or very low mass star
Sun like stars are just low mass stars (sometimes called dwarf stars too)
The largest stars are known as giants, high mass star or even Supergiants.
Our own star is not a giant, it is relatively "normal" size. However, it would be classified as a low mass star or a yellow dwarf star.
Low Mass vs High Mass
The threshold for each is the following:
Brown Dwarf Star - < 0.08 MSUN
Low Mass Star - 0.08 MSUN < star < 8 MSUN
High Mass Star - > 8 MSUN
This classification will determine how the star lives its life and even dictate how the star will die.
Brown Dwarfs
Brown dwarfs (<0.08 MSUN) do not perform nuclear fusion because it's not hot enough and their gravity is not high enough to create the pressure.
They appear brown because they do not create heat through fusion, but rather released infrared heat stored from the formation of the star.
The brown dwarf does not die like the other stars.
It will spend eternity cooling off never officially died.
Black Dwarf Star?
Red, Yellow, Orange, Blue and White Dwarfs
There are other dwarf stars, but we will introduce them based on the cycle of a star's life.
Brown dwarfs are created and live a different life from their other dwarf siblings.
We will introduce them as we see them
Low Mass Stars
Most stars are low mass stars (0.08MSUN - 8MSUN)
Low mass stars are different from Brown dwarfs since they were able to begin nuclear fusion due to their increased gravity and internal heat and pressure.
Low mass stars tend to live way longer some stars we have never seen die yet.
Stars closer to 8MSUN will last around 100s million years
Stars closet to 0.08 MSUN can last trillions of years
Red Dwarfs
Red dwarfs (0.08MSUN - 0.6MSUN) are close cousins of Brown dwarfs
Like brown dwarfs, they are small and low mass, but they had enough to start nuclear fusion.
These stars don’t use much fuel. The more mass a main sequence star has the more fuel it consumes.
Pressure and heat at the stars core determine how fast it converts hydrogen to helium which are a function of gravity/ mass
Red Dwarfs
Red dwarfs have less fuel than other low and mass stars just because its smaller, but it's very efficient and does consume fuel fast.
This means that red dwarfs outlive all other stars. In fact, the less mass a main sequence star has the longer it will survive
Red dwarfs are thought to live trillions of years
If the universe is 13.7 billion years old, then no red dwarfs have died yet.
When we get to how stars die, we will discuss what will happen when one finally does.
Yellow/Orange Dwarf Stars
The rest of the low mass stars (0.6 MSUN < star < 8MSUN) are yellow and orange dwarf stars.
The term dwarf is still used here since relative to how big stars can become they are still very small.
However, some astronomers call this range "normal stars"
The majority of stars are low mass and most are yellow and orange dwarf stars.
Our Sun is a yellow dwarf star
Yellow/Orange Dwarf Stars
These stars are low mass but higher mass than red dwarfs meaning that they consume their fuel faster than red dwarfs
Yellow and Orange dwarf stars live 100s of millions to billions of years old depending on their mass.
Our Sun is 4.6 billion years old, and has about 6 billion years left before we expect it to die.
High Mass stars
High mass stars (>8MSUN) these stars are big and appear white and blue.
These stars are given the name giants given that can be extremely big compared to the dwarf stars.
These stars are very high mass and therefore eat through their fuel very fast.
The largest of the giants will last only a few millions of years
Since they burn large amounts of fuel, they are very bright and emit large amounts of heat which is why they are white and blue.
Colors of Stars
Red main sequence stars are smaller is mass and don’t consume much fuel.
While yellow main sequence stars are bigger and consume more fuel.
Giant stars are even more big and consume ever more fuel and are white and blue (blue being the largest of stars)
Notice a trend here.
There exists a relationship between the mass of the star, the amount of fuel it consumes and a color change.
Colors of Stars
Bigger main sequence stars burn more fuel per second which means its brighter and gives off more heat.
More heat means higher temperature.
High temperature means a change in color
Stars at low temperatures are red and go from orange, yellow, white then blue as their temperature increases.
High mass main sequence stars have high temperatures and appear bluer
Low mass main sequence stars have low temperatures and appear redder
Hertzsprung- Russel Diagram
This diagram related the Luminosity, temperature and mass of stars together.
Focusing on the main sequence we see lower temperature stars are red, have small masses, live long lives and are less bright (low luminosity)
Higher temperature stars are bluer, large masses, live short lives and are very bright
Hertzsprung- Russel Diagram
The other star categories are stars when they get old and die
Main sequence stars follow this relationship until they get into their old age
What happens to main sequence stars when they get old?
Dwarf Stars
Low mass stars (aka dwarf stars except brown dwarfs) conduct nuclear fusion.
Fusing hydrogen into helium.
Once they run out of hydrogen, they are only left with helium. It has begun its old age and begins the process of dying.
To understand what happens we first must discuss how a star maintains itself during its younger phase. (Main sequence phase)
Hydrostatic Equilibrium
For a star to maintain its shape and size it must be in balance.
Energy is created at the core which generates heat that travels through the layers creating an internal pressure.
Hydrostatic equilibrium is when the gravity of the star is in balance with the outward pressure due to its hot internal gases.
Without the internal pressure gravity will compress the star into its core crushing all materials together (Sneak Peak: A blackhole)
Example
Let's say a star suddenly consumes more fuel.
It generates more heat and therefore more gas.
The internal pressure will be stronger than the force of gravity
The core will expand making the star bigger in size
The spreads the material out making it less dense and lowering internal temperatures
The star eventually rebalances at its new larger size.
This system prevents the system from under generating and over generating fusion.
What happens when the pressure decreases due to no longer producing hydrogen fusion?
Hydrogen Fusion Stops
At one point the core will run out of hydrogen fuel for its hydrogen fusion.
The internal pressure will dimmish gravity will shrink the star.
As the star shrinks its temperature will increase.
There is still hydrogen in the shell around the core. The increase in temp causes the shell to fuse hydrogen.
This causes the atmosphere of the star to expand increasing the size of the star.
This increase in size changes a dwarf star into a giant and a giant star into a supergiant.
Hydrogen Fusion Stops
As the star gets bigger its surface spreads out causing surface temperatures to decrease
This causes brighter stars to low mass stars become redder.
Giant stars to become white, then yellow then red as well.
Note as it gets redder the star gets bigger.
Helium Fusion
While the surface of the star spreads out the core temperatures rise as gravity pushes down.
At some point the temperatures become high enough to fuse helium into heavier elements.
This continues in the same fashion.
Helium runs out it shrinks the star causing helium fusion in the shell
Expands the atmosphere turning the star redder.
Gravity shrinks the core until it reaches the temperature to fuse carbon.
Stars in Old Age
Stars are in their old age when they stop fusing hydrogen
Stars in the main sequence maintain Hydrostatic Equilibrium
When hydrogen fusion stops the star grows and surface temps decrease/ becomes redder.
Stars in their old age have increased core temperatures that cause the fusion of helium into heavier elements.
Red Giants and Supergiants
Are stars in the end of their old age.
They are continuously expanding their atmosphere and their cores are fusing into heavy elements.
At some point the star's structure will hit a breaking point this will lead it to finally becoming a dead star.
At the point it becomes dead the type of death it experiences depend on mass and its surroundings.
How low mass stars die.
Planetary Nebula
Low mass stars (yellow/ orange dwarfs) the stars now red giants will continue to blow their atmospheres farther away from the core
Eventually far enough that the gas is expelled from the star. The stellar wind from the core's fusion of heavy elements blow away the atmosphere
At the center of the nebula is a hot dense core fusing heavy elements.
Note: despite having the work planet it has nothing to do with planets themselves.
White Dwarf
Once the atmosphere is blown away the planetary nebula phase ends and the core is just left.
Eventually the core stops fusion, and it becomes a hot very dense.
Most white dwarfs are made of carbon and oxygen since the temperatures never got hot enough to fuse further
The leftover core is a white dwarf. This is the remnant of the star now.
From here on the white dwarf remits residual heat making it less bright but still at a hot temperature
White Dwarf
The core no longer conducts fusion so what is keeping the star from collapsing farther
Electron - degenerate pressure.
Gravity is attempting to compress the white dwarf further, but it's made of dense metal with many free electrons
Gravity is not strong enough to overcome the electron repulsion of other electrons thus a white dwarf comes into balance
The white dwarf will continue to give off residual heat until its temperature decreases eventually becoming a black dwarf.
Black Dwarf
A black dwarf is a theoretical star that is what happens when white dwarfs give off all their heat.
With no more heat the star does not give off light and becomes a dark piece of dense rock and metal floating in space.
There are likely no black dwarf stars.
White dwarfs are theorized to take a hundred million billion year to run out of residual heat.
Binary Star Systems
Nothing about star formation suggests that stars must form by themselves.
Stars can form in pairs or even threes. Imagine two or three Suns in the sky.
Binary stars develop from a shared localized cloud of gas and dust that is large enough for two or more stars to develop in the same system
Binary Star Systems
The binary stars orbit around a Center of Mass or a balance point between the two stars
They live their main sequence phase in peace/ balance.
Each star can have a different life: Ex. One could be a high mass and the other a low mass. Or both high mass.
Binary White Dwarf System
If each star can be different than what happens when one star dies first
The more interesting scenario is when one star is a low or high mass star with a white dwarf.
If a supergiant dies first, then the entire system is destroyed in its death leaving no binary white dwarf system behind.
A low mass star dies first leavings a white dwarf together with a high/ low mass star companion. Binary white dwarf system.
Binary White Dwarf System
The white dwarf and the other star will live in balance orbiting around a center point.
Once the second star, enters its old age its atmosphere is spread out
The white dwarf gravity is strong attracting this atmosphere toward it.
What happens next depends on how much mass the dwarf star steals.
Supernova Type Ia
If the white dwarf accumulates enough mass on its surface it heats up the Hydrogen collected.
This causes enough heat to cause a thermonuclear explosion. If the mass collected is high this explosion is enough to destroy the white dwarf
This explosion is called a Supernova type Ia
The companion star will often be ejected.
Nova
If the mass accumulated by the white dwarf is enough to cause a thermonuclear explosion but not enough to destroy the dwarf star, then it causes a Nova
A Nova is a thermonuclear explosion due to the heating of hydrogen, but not a large enough explosion to destroy the star.
It burns up the mass accreted and the cycle starts again.
White dwarf collect mass from the companion star then it explodes then repeats
Final Stage of Binary System
If the binary system does not get destroyed and all the stars are reduced to white dwarfs, then eventually merge into one final star system
Often the result is one white dwarf by itself.
Thermonuclear Fusion in Supergiants
Recall that Red giants have enough mass to fuse hydrogen then helium, then …. to Oxygen/ Carbon.
Low mass stars will not have enough heat and pressure to cause further fusion to Iron.
However, supergiants do have the pressure and heat to fuse all the way to Iron.
Each element heavier than the previous on form layers in the star.
Supergiants Die
As the star fuses to Iron the binding