1. Refraction: light goes through lens. the light is bent as it passes from one transparent meduim to the next. this kinda telescope uses a convex len.
2. Reflection: perfered optical telescope type. uses curved mirrors in back of telescope to reflect light in to and eye piece. light touches nothing but air.     

•  light gathering power- reflectors work better at this. larger scopes have a greater Collecting Area (total area capable of gathering radiation.)
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•  Resolving Power- shapness of images, class example: when looking at a cluster if you have poor resoving power or resolution, two clusters could be mistaken as one.
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• Stability- must have a stable stan to keep telescope stable.    

(note: magnification is not on list, because magnification can be changed with an eye piece and does very little overall.) 

• is the failure of a lens to focus all colors to the same point. It occurs because lenses have a different refractive index for different wavelengths of light.
•  simply put chromatic Aberration is the seperating of light through each lens. this happens with Refracting Telescopes not Reflecting

1.) Radio telescopes

2.) Infrared (heat-sensing) telescopes

3.) UV telescopes

4.) X-ray telescopes

5.) gamma ray telescopes.

• is used for observing object far away.
• because radio waves are so long they are not easy scatterd through the atmosphere because of dust.
• they can be on the surface unlike gamma, xray and uv telescopes 

• is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light.

1.) Heavy Lens, hard to make and send to space 

because of their weight

2.) chromatic Aberration

3.) a lens has two surfaces that must be accurately polished, this is very difficult. 

• images whose colors represent measured intensities outside the visible portion of the electromagnetic spectrum.
• the radio wave do not show a picture.

• Very Large Array. this is when many radio telescopes are placed togather to cover a large portion of the sky.
•  the telescopes are sometime mounted on rail road tracks for easy moving.

• taking many small pictures and composing them in to a larger cohessive one.

1.) Can't be explained by Nebula Theroy because of Differential rotation

2.) has Spiral arms

3.) exhibit differential rotation (like the sun)

• A galaxy is a cluster of stars, nebulae, dark matter, and other astronomical objects.
• Most galaxies are tens of thousands of light years in diameter, and contain billions of stars

   Galaxies come in three primary shapes

1.) spiral galaxies: are thin disks, with spiral arms surrounding a central hub

2.) elliptical galaxies are uniform,

3.) oval-shaped agglomerations; and irregular galaxies have little or no definite structure.

• the spiral structure is a wave that moves through the disk causing the stars and gas to clump up along the wave, a density wave.
• The spiral arms are where the stars pile up as they orbit the center. 
• In a galaxy the spiral region of greater gravity concentrates the stars and gas. The spiral regions rotate about as half as fast as the stars move. as the gas pasess through more dense regions of the density wave it becomes compressed and sparks star formation.
• bright young blue stars are located along the outer edge while older redder stars are left behind. 

Differential rotation is the difference in the angular speeds of different parts of the galactic disk so stars closer to the center complete a greater fraction of their orbit in a given time. But differential rotation is too efficient in making the spiral arms. After only 500 million years, the arms should be so wound up that the structure disappears. Also, the spiral pattern should occupy only a small part of the disk. The observations of other galaxies contradicts this: the spiral arms in spiral galaxies rarely have more than two turns.

why is it wrong to say all the gas is in the arms of the milky way?

 (Pressure wave theory)

• because the gas is disturbted equally, we only see the spirals because of something happening to the gas in the spiral

• interstellar medium: is material between the stars systems, composed of gas and dust.
• 70% H 22% He

• Emissions nebula
• Dark Nebula
• Reflection nebula

Emission nebula

 how do we know whats in it

• Emissions nebula-gives of light
• if the nebula is reddish in color it's hydrogen  
• if it is greenish in color its oxygen
• place where stars are born
• also known as HII Region

Dark Nebular

• absorbs light
• denise cloud of dust
• no light can get through
• opaque because of their internal dust grains.
• The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes.

Dark Nebula

How do we know what in it?

• we know whats in a dark nebula by measureing the protons kicked off when hydrogen goes into a  super cold state.
• we mesauser the photon with radio Telescopes.