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Galaxies&

Stars

What is a galaxy?

• A galaxy is a huge collection of gas, dust and billions of stars.

• These stars are held together by the force of gravity.

• Earth and our solar system are part of the Milky Way Galaxy

• Most of the stars you see at night are part of the Milky Way Galaxy

THE MILKY WAY GALAXY

Our Home...

• The Milky Way is a spiral galaxy

• Our solar system is located on the Orion Arm of the Milky Way Galaxy

The galaxy is believed to be home to 100 billion stars!!!!

Our Galactic Neighbour

• If you look out towards the constellation of Andromeda you are looking towards our neighbouring galaxy Andromeda which is 29 million light years away!!!

Star clusters

• Star clusters are groups of stars that are close together and that travel together

Pleiades

M15 Globular Cluster

More…M15

Some popular star clusters!

Galaxies are divided into five different types, determined by their shape:

• Elliptical (shape similar to a football)– Most common

• Spiral (shape similar to a flat pinwheel)– Arms are composed of a lot of gas and dust and

young blue stars (I.e. star formation is ongoing)

More elliptical galaxies

Some spiral galaxies

Types of Galaxies (Cont’d)

• Lenticular galaxies - Disc-shaped with a bulge in the middle

Types of Galaxies (Cont’d)

• Irregular (no particular shape)– Smaller and less common than spiral or

elliptical

• Barred-spiral galaxy– A type of spiral galaxy

Irregular GalaxySpiral Galaxy

AB

C D

Spiral Galaxy

Elliptical Galaxy

• Galaxies are moving outwards in the Universe. Sometimes two galaxies collide and it appears that the big galaxy eats the smaller galaxy.

Unusual Galaxies

• Quasars– Very distant objects,

look like faint stars– Brightest objects in

the universe– Give off 100 times

more energy than Milky Way

Life and Death of Stars

Stars

• The biggest things in the universe are the stars like our sun

What makes it go

• Stars are giant balls of gas that burn by a process called nuclear fusion

• This process combines two hydrogen atoms into one helium atom giving off a lot of heat.

Star Colour and Temperature

• The temperature of a star determines its colour.

Red → Yellow → White → Blue

Coolest Hottest

• Our Sun is a yellowish colour so it is considered a medium-cool star.

Life and Death of a Star

Draw this diagram!!!

Star Birth

• Stars are created in a nebula as the gas contracts because of gravity

• As they become larger they heat up until they reach a temperature when fusion begins and they “turn on”

Star Life

• Our star is now in what is called the main sequence where it is stable and consistent.

• A cooler smaller star like our sun can last for about 8 billion years

• Fast burning blue stars only last for a million years

The Fate of Stars

Stellar Nebula – cloud of interstellar gas and dust

Red Giant- a large, cool, bright star

Red Supergiant- an extremely large, cool, bright star

Planetary Nebula- a bright shell of gas ejected from an old, low-mass star

Supernova- stellar explosion where a star becomes about a million times brighter

White Dwarf- a low-mass star that has run out of fuel and condensed to the size of

about Earth

Neutron Star- what’s left of a star, is very compact and consists of mostly neutrons

Star Death

• Stars live until the hydrogen in the core that fuels them runs out

• After that they collapse inward since there is nothing opposing gravity any more.

• The new heat in the core fuses the helium into carbon

• The outer shell still has some hydrogen and burns expanding to form a red giant or supergiant star

• Once the core fuses into lead fusion stops and the star collapses.

Dwarf Stars

• When the star core reaches lead it can not undergo fusion any longer

• This white hot ball of lead “white dwarf” cools to form a “black dwarf” star

Death of a Giant Star

Neutron Stars

• Sometimes a star is so heavy that even lead cannot support its weight.

• The atoms collapse even further creating a ball of neutrons.

• This forms a rapidly spinning neutron star that is only a few kilometers in size.

• We see neutron stars from the radiation that shoots out of either end.

Black Hole- an object whose gravity is so strong that nothing can escape it

Black holes

• Sometimes the star is so heavy that not even the neutrons stay apart and crash together forming a black hole

• After that we are not sure as this creates a black hole

• A black hole is an object so dense that not even light can escape it.

• We can find black holes by looking for objects in space orbiting seemingly empty space.

The End???