Galaxies and Stars

The Milky Way• The galaxy we live in is called The Milky Way. It is disc-

shaped.

The Milky Way• Looking into the night sky, in one direction we see many

sky objects since we are looking through the arms of our galaxy. In other directions we see fewer sky objects since we are not looking through our galaxy’s arms.

100 Billion : The Stars in The Milky Way• The number of stars in the Milky Way is about the same

number as the number of sand grains in a full dump truck of sand.

Astronomers Estimate 125 Billion Galaxies in Universe• In 1925, Hubble identified the first galaxy besides the Milky

Way, the Andromeda galaxy, our nearest neighbor galaxy. Since that time 125 billion galaxies have been identified.

An Empty Sky By the Big Dipper Handle?• Earth telescopes showed darkness in the sky around the Big

Dipper handle. When the Hubble telecope in space was pointed to the Big Dipper handle, it revealed a whole universe of galaxies and sky objects where we only saw a dark sky before.

Three Kinds of Galaxy• Galaxies can be spiral, elliptical (egg-shaped) or irregular.

Stars• A star is made up of extremely hot gases (millions of

degrees celsius) that give off light. The force of gravity pulls the gases of a star together, raising their temperature to the point where they begin the thermonuclear reaction called fusion, the reaction where nuclei begin to join, releasing huge quantities of energy.

More Stars than Sand Grains on Earth• Astronomers estimate that the Observable Universe we are

a part of has about 9000 billion billion stars.

Interstellar Matter• Space is filled with gases (mostly hydrogen gas) and dust

(only 1 % of the matter in space). A cloud of gas/dust is called a nebula.

The Birth of a Star• A star begins as a nebula, a matter cloud of gas/dust. Any

particle of matter (gas/dust) exerts an attraction force of gravity on any other particle of matter so the cloud begins to shrink as it is attracted together by forces of gravity between all the matter particles in the cloud.

The Birth of a Star• As gravity pulls a nebula into a smaller and smaller cloud,

temperatures rise within the nebula cloud. If the nebula is massive enough (with enough gravity force), the temperature rises to 10 000 000 celsius and at this temperature, atomic fusion of atom nuclei begins, releasing huge amounts of energy.

A Nebula with Too Little Mass?• When a nebula has too little mass, it does not have enough

gravity to raise its temperature to 10 000 000 degrees celsius. In this case, the nebula contracts into a giant gas planet and never becomes a star – like jupiter, saturn, neptune and uranus.

A Star is Born• If the nebula cloud is massive enough, gravity within the

cloud will be large enough to shrink it and to raise its temperature above 10 000 000 degrees celsius, a temperature at which atomic fusion happens, releasing huge amounts of energy in the form of all kinds of electromagnetic radiation (gamma rays, x-rays, ultraviolet rays, visible light, infrared rays, microwaves, radio waves)

Star Size• Stars can be classified as low mass stars, intermediate

mass stars or high mass stars.

Low Mass Stars• These small stars spend their long lives (100 billion years)

slowly burning their hydrogen fuel. They are called red dwarfs which describes their lower temperatures. Eventually they change into much smaller, hotter and dimmer white dwarfs and burn themselves out.

Intermediate Mass Stars (Like Our Sun)

• Intermediate mass stars burn their hydrogen fuel faster, lasting only 10 billion years. After a long stable period, intermediate mass stars expand into red giants, expelling much of themselves into space. Eventually after losing much of its mass to space, the red giant collapses into a small, dim white dwarf. The white dwarf then cools into a black dwarf made up of mostly carbon and oxygen.

Intermediate Mass Stars (Like Our Sun)• Our sun has about 5 billion years left before it will expand

into a red giant.

High Mass Stars• Stars over 12 times the mass of the sun are classified as

high mass stars. These stars consume their hydrogen fuel faster than any star, lasting only 7 billion years. They become red giants, growing rapidly.

Stars as Factories Making New Elements• As stars continue the process of fusion, they make heavier

and heavier elements which settle towards the centre of the star.

High Mass Stars : Supernova Explosions

• As high mass stars continue to make heavier and heavier elements that sink, they get layers of various elements. Eventually they reach the stage where they make iron and nickel, heavier elements that settle to their core. At this stage high mass stars become unstable, collapse in on themselves and form an explosion called a supernova.

Supernova Explosions of High Mass Stars

Supernovas: Suppliers of New Elements• Some Supernovas are so bright that they can be seen in the daytime sky.

The extra energy of Supernova explosions fuses the heavier elements past iron and shoots them out into space. The elements of He to Fe on earth are thought to have come from former stars and the elements heavier than iron are thought to have come from earlier supernova(s).

Neutron Stars and Black Holes• A high mass star (after a supernova explosion) can

collapse into either a neutron star or a black hole. Stars 12 to 15 times as massive as the sun collapse from a diameter of a million km to a diameter of just 10 km. The core of neutron star is as hot as 100 000 000 degrees celsius and takes trillions of years to cool.

How Can High Mass Stars Collapse?• Stars are made up of atoms and atoms are mostly empty

space. If the large force of gravity in high mass stars pulls the electrons, protons and neutrons of atoms together, the atoms collapse into a space over 99.99% smaller.

Black Holes

• High mass stars over 25 times as massive as the sun become black holes after a supernova explosion. A black hole is even more massive than a neutron star and has so much gravity force that it pulls light back into itself which is why it is black (no light can escape from it).

Extreme Mass of Black Holes• If the sun were to collapse into a black hole, its diameter of

1.39 x 106 km would collapse to a diameter of 3 km. This 3 km diameter black hole would have a mass of 1.989 x 1030 kg, the same mass as its full size. 1 cm3 of this black hole (the size of a sugar cube) would have a mass of 1.41 x 1014 kg. A piece of this black hole the size of a grain of sand would have a mass of 17, 580 kg (Like fitting the mass of 4 elephants into a grain of sand).

How do Astronomers Know Black Holes Exist?• 1. Material pulled towards black holes (because of their

extreme gravity pull) emits electromagnetic radiation that can be measured.

How do Astronomers Know Black Holes Exist?• 2. The extreme gravity of black holes has effects on stars

and galaxies that pass close to the black holes. These effects have been seen even though the black hole itself is not visible.

How do Astronomers Know Black Holes Exist?• 3. Computer models show that super-dense black holes

should distort light coming from distant stars and these predicted distortions have actually been observed.

Two High Mass Stars in the Constellation, Orion• The constellation, Orion (The Hunter), found in the

southern sky during winter has two high mass stars.

Betelgeuse and Rigel in Orion• Betelgeuse and Rigel are two high mass stars found in the

constellation, Orion.

Colour and Temperature• As an object is heated more and more, it first glows red,

then with more heat it glows yellow, then white and finally blue. Celsius temperature + 273 = Kelvin temperature

Star Patterns for Brightness and Colour• In 1910, a Dutch astronomer, Ejnar Hertzsprung and an

American astronomer, Henry Norris Russell, both independently plotted star luminosity (brightness) versus star temperature (colour) for thousands of stars. They both observed important patterns in their diagram, a diagram that came to be called the Hertzsprung-Russell Diagram.

The Hertzsprung-Russell Diagram• There are 4 regions in the H-R diagram. 90% of all stars

(including our own sun) are found along the curved line called the main sequence. The other types of stars fit into the white dwarf region, red giant region or super giant region. Super giants are cool and bright while white dwarfs are hot and dim.

The Hertzsprung-Russell Diagram• The H-R diagram was used to determine the life cycles of the

various stars. Stars begin as nebuae, ignite due to fusion and most settle into the main sequence. Later in life they become red giants which then become white dwarfs.

Another Form of the H-R Diagram

The Colors of Stars• The colours of stars relate to their temperatures and the

stage of their life cycle that they are in.

How Can Astronomers Tell What Stars Are Made Of?

• Astronomers look at the spectra of stars to determine what they are composed of.

Light Waves and Wavelength• White light is a mixture of many colours of light : ROYGBIV (red,

orange, yellow, green, blue, indigo, violet).• The different colours of light have different wavelengths.• Violet light has shorter wavelength while red light has longer

wavelength.

Light Waves and Energy

• Light towards the red end of the spectrum has longer wavelengths and lower energies.

• Light towards the violet end of the spectrum has shorter wavelengths and higher energies.

Bright and Dark Line Spectra

• When an element is energized (by heating it or passing electricity through it), it gives off a spectrum which is made up of just specific energies of light.

• When white light (made up of all light energies) is passed through a cool gas of an element, a spectrum with dark lines is produced.

Element Spectra : Unique Like Fingerprints

• Each element has its own unique bright line and absorption spectrum which identify it like different fingerprints identify different people.

Hydrogen’s Visible Light Spectrum: Balmer Lines

• A Swiss teacher and mathematician, Johann Balmer discovered a mathematical pattern in bright lines of hydrogen’s visible light spectrum. The visible light lines in hydrogen’s spectrum are called the Balmer series.

What Are Stars Made Of ?

• Spectra of stars are used to determine what they are made of and what gases surround them.

A• A