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Astronomy 315
Professor Lee Carkner
Lecture 5
 Blue star is hotter than red star
 From Wien’s law: short
wavelength means higher
 T  (1/l)
 Image looks larger in photograph
 How can the red star be brighter
even though it is cooler?
 Stefan-Boltzmann law
 P  AT4
 What colors are easy/hard to
 Eye is least sensitive to violet and
Star Colors
 What temperature are red, yellow and blue stars?
Wein’s Law: 3,000,000/T = l
 Multiply each side by T
 Divide each side by l
 Red: l = 650 nm
3,000,000/650 =
 Yellow: l = 575 nm
 3,000,000/575 =
 Blue: l = 475 nm
 3,000,000/475 =
History of Observing
The earliest observing was done with the naked eye
Around 1600 telescopes were invented
Around 1670 Isaac Newton invented a telescope
that used mirrors instead of lenses
In 1990 the Hubble Space Telescope was placed in
orbit, capping a decade of spaced-base observing
Galileo’s Observations
How Do Telescopes Work?
Light gathering ability (not
magnification) is the most important
attribute of a telescope
telescopes make faint things brighter
Need a lens
Lenses bend light (refraction) and focus
all of the light incident on the front to a
point (focus) a certain distance behind
the lens (focal length)
Lenses and Refraction
Refracting Telescope
If you put a second lens (eyepiece) behind the
first lens(objective), you can magnify the
Magnification is equal to the ratio of the focal
in practice the magnification you can achieve is
limited by the blurring effects of the Earth’s
Refracting Telescope
Refractors and Reflectors
It is hard to make large refracting
A curved mirror can be used to gather
and focus the light instead (reflecting
Reflecting Telescopes
Problem: The focal point is between
the mirror and the sky
Cassegrain Telescope -- secondary
reflects light through a hole in the
primary, most common type of large
Cassegrain Reflecting
Path of Light
Light beams enter from infinity and are
initially parallel
The eyepieces then magnifies the point
image by taking the divergent rays
from the focal and making them
parallel again
Telescope Misconceptions
Magnification is the most important
property of a telescope
Astronomers peer through an eyepiece
Telescopes stick out of the dome
Telescopes fold up like a giant pirate’s
Observing at Different
To fully understand the universe you need
telescopes that can observe all forms of
Space based astronomy began in the 1970’s and
became very important in the 1980’s-1990’s
The Electromagnetic Spectrum
Telescope Taxonomy
Radio and Millimeter -- penetrates atmosphere
and everything else
Infrared (IR) -- we feel as heat
Optical -- what our eyes can see
Example --
More Telescope Taxonomy
Ultraviolet (UV) -- high energy radiation,
causes sunburn
Example --
X-ray -- very high energy
Example --
Gamma Ray -- the highest energy
Example --
Hubble Space Telescope
What Can a Telescope Do?
Imaging -Photometry -Spectroscopy -A spectrum is the amount of light at each
The shape of the spectrum tells you about the
temperature, composition and motions of the
Types of Detectors
Eye --
Photographic plate -- allows you to measure
brightness and spectra
Charge Coupled Device (CCD) --
Today, light is moved around with fiber optic
cables and data is moved electronically
Next Time
Read Ch. 3.5, 4.1-4.3