• Galaxies are sprawling space systems composed of dust, gas, and countless stars. 

• The number of galaxies cannot be counted—the observable universe alone may contain 100 billion. Some of these distant systems are similar to our own Milky Way galaxy, while others are quite different.

• Galaxies with less than a billion stars are considered "small galaxies." In our own galaxy, the sun is just one of about 100 billion stars!!

• Spiral galaxies
• Elliptical galaxies and 
• Irregular galaxies.

The Sun's Corona: 

• This colourful NASA image was only taken on April 25, 2010.
• This photography technique allows us to see the spectrum of light coming off the sun's surface.
• Here we can see 'jets' of coloured energy, known as 'spicules'. 

• In 1913, successful models were developed to really explain the interiors of stars and the birth of stars. The colourful light or spectrum coming from stars even showed the scientists what chemicals were in the stars.

• With the exception of supernovae, individual stars have been observed in the visible part of the Milky Way.

• Astronomers are able to find the mass, age, chemical composition and many other properties of a star by observing its spectrum (colours), luminosity (brightness) and how it moves through space.

• Almost all naturally occurring elements heavier than helium were created by stars!!

•  Either via stellar nucleosynthesis during their lifetimes, or by supernovae nucleosynthesis when stars explode. 

• The Periodic Table below shows the placement of helium compared to all the other heavier elements.

A STAR IS BORN:

• A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements.
• Once the stars core is dense enough, some of the hydrogen is converted into helium through the process of nuclear fusion.
• The star's internal pressure prevents it from collapsing further under its own gravity.
• Once the hydrogen fuel at the core is exhausted it expands to become a red giant.

• Over a period of a few hundred thousand years, much of the star's mass is expelled at a relatively slow speed of about 50,000 miles per hour. A hundred million years after the red giant phase all of the star's available energy resources will be used up. 
• The exhausted red giant will puff off its outer layer leaving behind a hot core. This star has a surface temperature of about 50,000 degrees Celsius and is furiously boiling off its outer layers in a "fast" wind traveling 6 million kilometers per hour.
• This mass loss creates a more or less spherical cloud around the star and eventually uncovers the star's blazing hot core.
• The radiation from the hot star heats the slowly moving red giant atmosphere and creates a complex and graceful filamentary shell called a Planetary Nebula.
• X-ray images reveal clouds of multi-million degree gas that have been compressed and heated by the fast stellar wind. 

PLANETARY NEBULA: 

• Planetary Nebula - so called because some of them resemble a planet when viewed through a small telescope - are produced in the late stages of a sun-like star's life.

• Dynamic elongated clouds envelop bubbles of multi-million degree gas produced by high-velocity winds from dying stars.

• The first white dwarf was observed by optical telescopes in the middle of the 19th century. They do not emit very much light.
• White dwarfs represent an intriguing state of matter. 
• Most stars, including our Sun, will become white dwarfs when they reach their final, burnt-out collapsed state!!

WHITE DWARF STARS:

• Eventually the central star will collapse to form a white dwarf star.

• In the white dwarf state, all the material contained in the star, minus the amount blown off in the red giant phase, will be packed into a volume one millionth the size of the original star. 

• That would mean an object the size of an olive made of this material would have the same mass as a car! 

• For a billion or so years after a star collapses to form a white dwarf, it is "white" hot with surface temperatures of about twenty thousand degrees Celsius.

NEW STARS ARE BORN:

• When the stars evolve into their degenerate forms, they recycle a portion of their matter out into space.