PHY 1060 Lecture Notes - Lecture 1: Infrared Astronomy, Depth Perception, Cosmological Principle
Astronomy PHY 1060
Chapter 1: Why Learn Astronomy?
• Modern astronomy is more than cataloging objects
• It also covers:
o How do stars (matter and energy) work?
o What is the universe?
▪ Our place in the universe
• Earth is a small planet, orbiting a medium sized star, in a galaxy of 100 billion stars,
which is just one of billions of galaxies in a universe that is 13.7 billion years old
• The universe is vast
• Billions of galaxies are separated by thousands of trillions of miles.
• Astronomers need to be able to talk about great distances
o Use the travel time of light to measure distances.
o We often use times to denote distances
o Compare distances by the amount of time it takes to travel them
• Speed of light
o Light travels 300,000 km ever second
▪ 186,000 mph
o Universe’s greatest speed
o Light-year: distance light travels in one year
o Light could travel around Earth in 1/7 second.
o Light takes:
▪ 1.25 seconds to arrive from the moon
▪ 8.3 minutes to arrive from the sun
▪ 5.5 hours to get to Pluto from the sun
▪ 4.2 years to arrive from the nearest star
▪ 100,000 years to cross the galaxy
▪ 2.5 million years to get to the nearest big galaxy
▪ 10 billion years to come from distant galaxies
• Studying the universe can reveal many things
o For example, all the atoms in our body were made in stars (except for the
hydrogen in water)
o Stars generate energy by making heavier elements out of light ones
▪ Seen through the spectrum of light
▪ Dying stars eject these elements into space in massive explosions
(stardust)
• Can be collected by other things
• All material on earth and anywhere is from other stars that have
died and ejected that material out into space
• Astronomy uses measurements to…
o Test scientific ideas by observation
o Space exploration has expanded our view of planets and the Solar System
o Telescopes, satellites, etc. extend our view
• Science involves the use of theories, hypotheses, and principles
o A scientific hypothesis is an idea that leads to testable or falsifiable predictions
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o The cosmological principle is a very important underlying assumption
▪ “There is nothing special about one singular place in the universe”
• Anything we calculate or measure should be able to be used
elsewhere
o This is how we can come up with testable predictions
• The cosmological principle:
o On one level:
▪ Our view from the earth is not special or unique
▪ Distant objects should behave like nearby ones, which we can study in
greater detail
o On another level:
▪ Matter and energy obey the same physical laws everywhere
▪ By doing calculations based on things here, we can study distant objects
o Essentially: the same rules work everywhere in the universe
• The scientific method
o Begins with an idea, forms a hypothesis, and tests it
o The test will support or invalidate the hypothesis
o The scientific method broken down:
▪ Start with an observation
▪ Suggest a hypothesis
▪ Make a prediction
▪ Perform an experiment or additional observation
• If false, we have to revise our hypothesis
• If true, we make more predictions and run more tests
o Concept quiz:
▪ What is a scientific way of viewing nature?
• Nature informs us about the usefulness of our ideas
• Mathematics works when used to describe nature and its patterns
o Basic tools:
▪ Scientific notation: handling large or small numbers based on powers of
10
▪ Ratios: comparison of measurements
▪ Geometry: shape and orientation
▪ Algebra: representation
▪ Proportionality: understanding the relationships between quantities
o Graphs are used to represent relationships between quantities
▪ Can be linear or nonlinear
▪ For linear, the slope is the change of the vertical axis divided by the
change of the horizontal axis
• Can you draw a straight line through all the points?
o ^ this is a way to tell if it is linear
Chapter 6: The Tools of the Astronomer
• The oldest and most important tool is the telescope
o To gather light of all kinds
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o Two kinds of optical telescopes:
▪ Reflecting and refracting
• When light encounters a new material it can either experience reflection or refractions
o In reflection, light bounces off of the material
o In refraction, the light will pass through the material and be bent depending on the
value of the index of refraction relative to the first material.
• Refracting telescopes use lenses
o Objective les: refracts the light
o Aperture: size of the objective lens (larger aperture gathers more light)
▪ The objective lens is placed in the aperture
o Focal length: distance between lens and the images (longer = larger image)
o Aperture sets the light-collecting power.
o Focal length determines the image size
• The largest refracting telescope has a 1-meter aperture
o It needs to be large to have a long focal length
o Lenses suffer from chromatic aberration
• Chromatic aberration
o As light travels through glass it slows down, which is one reason why it bends
o Different wavelengths of light will slow to different speeds, and will bend at
different angles
o When the light travels through the objective lens, the different wavelengths of
light will focus at slightly different points
o The different colors are misaligned, creating a blurry image
o Refraction depends on the wavelength—violet light is bent more than red
o Dispersion: the resulting spreading out of the wavelengths of light
▪ Causes chromatic aberration in lenses, which can be fixed by a compound
lens
• Reflecting telescopes use mirrors
o There are primary and secondary mirrors
o Focal length is determined by the path the light takes reflecting off the mirrors
• Reflectors have advantages over refractors
o No chromatic aberration
o Bigger telescopes due to increased focal length in the same amount of physical
space and no need for massive lenses
▪ Can have a longer focal length without being physically longer
o The largest telescopes in the world are reflectors
• Computers are essential for astronomy
o Help in data analysis
o Simulations and models help to investigate complex situations
• The light-gathering power of a telescope s proportional to the square of the aperture size
• A telescope’s magnification depends on the focal lengths of the objective lens or mirror
and the eyepiece.
o How large an image is in comparison to the object in the night sky
• Resolution: smallest details that can be separated. Modern telescopes are able to count the
number of lines in Roosevelt’s hair on a dime at 2.3 miles away
o The longer the focal length, the better the separation of two objects or features
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