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

The missing 4 million tons of mass is discharged as radiant energy

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

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

First Images From the Event Horizon Telescope (April 2019)

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)?

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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