Essentials I Discovering Astronomy

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Essentials IDiscovering Astronomy
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Astronomers do much more than just make
observations

• Most astronomers do NOT spend much time behind a
  telescopes eyepiece
• Astronomers observe and analyze observations
• Astronomers create theories about what is seen
  and what might exist yet unseen
• Astronomers devise computer models that simulate
  what occurs in the cosmos

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The importance of using physical laws and
laboratory measurements

• Planets are too far away to visit easily
• Stars are much too far away to visit
• Galaxies are impossibly far away to visit
• THEREFORE, astronomers rely on the premise that
  the laws of physics consistently govern the
  nature and behavior of everything in the universe

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Scientific Method

• observe
• theorize
• test
• modify

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Science and PseudoScience
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Astrology or Astronomy?

• Uses the positions of planets to predict human
  behavior
• Is a branch of Physics
• Uses the scientific method to interpret
  observations
• Is considered a pseudoscience
• A detailed study of all the celestial bodies in
  the Universe, including planets, moons, stars,
  galaxies, etc.
• Associates the signs of the zodiac with
  personality traits

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A Few Definitions

• planet A celestial body of substantial size
  (more than 1000 km across), basically
  non-radiating and of insufficient mass for
  nuclear reactions to ever begin, ordinarily in
  orbit around a star.

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A Few Definitions

• star A self-luminous, gravitationally bound,
  ball of gas that shines or has shone because of
  nuclear reaction in its interior

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A Few Definitions

• solar-system A collection of celestial objects
  (e.g. planets, comets, asteroids) orbiting around
  a common star

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A Few Definitions

• galaxy A collection of stars, gas, and dust
  that are gravitationally bound together

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A Few Definitions

• Universe Consists of everything around us

Includes Everything!
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Concept Question
Which one of the following is listed in
the correct order from smallest to largest in
size? A) star, planet, solar-system, galaxy,
Universe B) planet, star, solar-system, Universe,
galaxy C) star, planet, galaxy, solar-system,
Universe D) planet, star, solar-system, galaxy,
Universe
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Cosmic Timescales
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Scientific Notation

• express very small or very
• large numbers in powers of ten
• Format . X 10
• Rule count the number of zeros to the
• left or right of the decimal point

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Scientific Notation
Examples
Average Distance from the Earth to the Sun
149,600,000.0 km ?
1.496 X 108 km
Mass of ant
0.004 g ?
4 X10-3 g
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Concept Question
Write -0.00000315 in scientific notation A)
3.15 X 10-6 B) 3.15 X 106 C) -3.15 X106 D)
-3.15 X 10-6
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The Finite Speed of Light
Average Distance from the Earth to the Sun ?
Astronomical Unit
1 A.U. 1.5 X 108 km
Distance that light travels in a year ? Light Year
speed of light in a vacuum c 3 x105 km/s
(186,000 mi/s) so one light second 3x105 km and
one light year 9.46x1012 km
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Looking Backwards in Time
The distance from the moon to the Earth is 1.3
light seconds away i.e. it takes light from the
moon traveling at a velocity of 3x105 km/s 1.3
seconds to reach the Earth. Thus, you are seeing
the moon as it was 1.3 seconds ago since it
took that amount of time to travel to the earth.
Because of the finite speed of light, we are
able to look backwards in time!!!
Furthermore, the time that light travels is
directly proportional to the distance

The more distant the object, the longer it takes
for the light to reach us and the further back
in time we are looking!!!
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Astronomical Distance Scales are Astronomical!
Proxima Centauri 4.2 light years away
The Moon 1.3 light seconds away
The Sun 8.3 light minutes away
Andromeda Galaxy 2.4 million light years away
Galaxies in the Hubble Deep Field as far as
billions of light years away
The finite speed of light gives us a fossil
record of the history of the Universe!!!!