Stars and Stellar Evolution
Starts convert gravitational energy into radiation energy (e.g. light) , and in the process create the heavy elements of the universe
How they do that?
The Formation of a Star
- Stars are born out of an interstellar cloud that gravitationally collapses over a time span of a few million years
- As the the cloud collapses the temperature and pressure at the core increases. Eventually the temperature and pressures is high enough to fusion hydrogen into helium and a star is born
Interstellar clouds fragment due to gravitational instabilities and multiple stars are born as the fragments of the cloud collapse due to their own gravity
How stars are born
The Eagle nebula is one of the most active star formation regions in the Milky Way
Fusion
Nuclear Fusion
In nuclear fusion, energy is released because the mass of child atoms is less than the sum of mass of the parents
Nuclear fusion is the process of binding lighter atoms/element into heavier ones
How nuclear fusion occur?
For fusion to occur, nuclei must collide at very high speeds to overcome electrical repulsion.
Temperature
Temperature is the average kinetic energy (energy due to motion) of atoms and molecules in matter
When something feels hot is because its atoms and molecules are hitting your skin with a lot of kinetic energy
Thermonuclear Fusion
For fusion to occur, nuclei must collide at very high speeds to overcome electrical repulsion.
Fusion brought about by high temperatures is called thermonuclear fusion
Hydrogen -> Helium
Helium -> Carbon
In the central part of the sun, about 657 million tons of hydrogen are converted into 653 million tons of helium each second
The missing 4 million tons of mass is discharged as radiant energy
Stars are Natural Thermonuclear Fusion Reactors
Pressure v.s. Gravity
Gravitational equilibrium in a Star: At each point inside, the pressure pushing outward balances the weight of the overlying layers
Properties of Stars
Temperature and Color
The temperature of a star determines its color
All objects continuously emit radiation. Objects at low temperatures emit long waves. Higher-temperature objects emit waves of shorter wavelengths
Sun T~6000 K, emits visible radiation
Thermal Radiation
Thermal Radiation
All objects continuously emit radiation. Objects at low temperatures emit long waves. Higher-temperature objects emit waves of shorter wavelengths
Human body T~300 K, emits infrared radiation
Intensity
Wavelength
Luminosity
Luminosity is a measure of a star’s rate of energy production (or hydrogen fuel consumption).
The amount of energy a star emits each second is its luminosity (usually abbreviated as L)
The luminosity of 100-watt bulb is: 100 watts
The Sun’s luminosity is: ~ 4 x 10^26 watts
The Magnitude Scale
About 150 B.C., the Greek astronomer Hipparchus measured apparent brightness of stars using units called magnitudes
- Brightest stars had magnitude 1 and dimmest had magnitude 6
- The system is still used today and units of measurement are called apparent magnitudes to emphasize how bright a star looks to an observer
Apparent Magnitude
A star’s apparent magnitude depends on the star’s luminosity and distance – a star may appear dim because it is very far away or it does not emit much energy
Absolute Magnitude
Absolute magnitudes are a better measure a star’s luminosity
- The absolute magnitude of a star is the apparent magnitude that the same star would have at 10 parsecs
- A comparison of absolute magnitudes is now a comparison of luminosities with no distance dependence
Radius
If two stars have the same temperature but one has a larger radius, the one with the lager radius will be more luminous
Classification of Stars
Historically, stars were first classified into four groups according to their color (white, yellow, red, and deep red), which were subsequently subdivided into classes using the letters A through M
Annie Jump Cannon discovered that classes were more orderly in appearance if rearranged by temperature – Her reordered sequence became O, B, A, F, G, K, M (O being the hottest and M the coolest) and are today known as spectral types
Classification of Stars
The Hertzsprung-Russell Diagram
If you make a plot of the Luminosity v.s. Temperature of known stars, regions for different types of stars appear
The Hertzsprung-Russell Diagram
Stellar Destiny
Stellar Evolution
Stellar Evolution
Evolution in the H-R diagram for low mass stars
Stellar Evolution
Evolution in the H-R diagram for high mass stars
Supernova explosions ignite second generation stars in an enriched interstellar medium of heavier elements
Black Holes
Scape Velocity
Neglecting air resistance if you throw a rock with a velocity greater than 11.2 Km/s (25000 miles/h), it wont come back
The size of stars is the result of a “tug of war” between two opposing processes: nuclear fusion (pressure) and gravitational contraction
If the fusion rate increases, the star gets hotter and bigger. If the fusion rate decreases, the star gets cooler and smaller
When a star runs out of fusion fuel (hydrogen and helium), gravity dominates and the star starts to collapse
Stellar Remnats
If the mass of the star is > ~1.5x the mass of the sun it becomes black holes
White Dwarfs
Neutron starts can pack up ~1.4x the mass of the sun on a radius that is the size of a city
In order to achieve such a high density they combine electrons and protons to form neutrons
Neutron Stars
The Components of Atoms
The strong gravity in Neutron stars combine electrons and protons to form neutrons
As the radius of a star decreases the gravitational force at its surface increases by
This results in a higher escape velocity!
Black Holes
At the surface of a black hole (the event horizon) the force of gravity is so strong that not even light moves fast enough to scape
Black Hole Observations
Black hole shadow at the center of Abell 2597 galaxy cluster
Active Galactic Nuclei in NGC 383
Black Hole Observations
The End
Temperature and Pressure
Temperature Scales
The three scales we use to measure temperature are Centigrade (Celsius), Farenheit and Kelvin
Who the heck uses Celsius anyway?
Everyone but US!
Pressure
Pressure is force per unit area
Metric unit of pressure is Pascal.
1 Pascal = 1 Newtons per square meter
A pressure < B pressure
A pressure < B pressure
Checkpoint 4
How does the pressure in A) compares to the pressure in B)?
A) has 3x more force over 3x more area, so The pressure in A) is the same as in B)
Pressure in Fluids
Pressure in a fluid depends on depth.
As with bricks, weight of what’s above determines pressure
Atmospheric Pressure
We live at the bottom of an ocean of fluid—the fluid is air & the “ocean” is the atmosphere.
The density and pressure of the air in the atmosphere is greatest at the surface of Earth and decreases with increasing altitude
Atmospheric Pressure
We live at the bottom of an ocean of fluid—the fluid is air & “ocean” is the atmosphere.
Atmospheric column of air
Base: 1 square meter
Height: 10 km
Volume: 10,000 m^3
Mass: 10,000 kg
Weight: 100,000 N = 10 tons
Pressure: 100,000 Pascals
Atmospheric Pressure is Really Strong
~15 pounds per square inch
Pressure, Volume and Temperature Relation
Pressure, Volume and Temperature Relation
When the number of molecules is constant
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