a few definitions light year (ly): distance light travels in one years (about 10 trillion km)...
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A few definitionsLight year (ly): Distance light travels in one years (about 10 trillion
km)
Andromeda Galaxy – 2.4 million light years away.
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A Few Definitions
Star: A self-luminous, gravitationally bound ball of gas that shines (or used to shine) because of nuclear reactions in its core. The Sun is a typical star (and by far the nearest star).
Planet: A body massive enough to be round due to gravity (≥ 600 km diameter, but not massive enough for nuclear reactions to begin), orbiting a star, and far more massive than all other objects in its vicinity.
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A Few DefinitionsPlanetary system: A collection of planets and smaller bodies
orbiting a star (solar system).
Galaxy: Large (typically 5,000 to 200,000 ly across), gravitationally bound system of hundreds of millions (up to a trillion) stars
Universe: “All that there is.” (Actually, there could be many other, physically disjoint universes in a “super-Universe” or “multiverse”!)
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Galaxy
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The Universe
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Distance (AU)Astronomical Unit (AU): the average distance between the Earth
and Sun 150,000,000 km (90,000,000 miles). Stars are much farther away.
AU and Light years are convenient ways to measure distance.
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Distance (Light Years)
We use light years to measure distances outside of our solar system. The distances are far to big to measure using the Earth – Sun distance as a reference. So we use the distance light travels in one year (a constant value). 1 light year ≈ 6 trillion miles. Alpha Centauri is 4.2 ly away (easier than imagining 25,200,000,000,000 miles!).
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Distance
If stars and galaxies are SO FAR, FAR away...
How do we know so much about them?
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The Electromagnetic Spectrum
The supreme informant of our knowledge of the universe is from the electromagnetic spectrum
that the stars emit. (gamma, x-ray, UV, visible light, IR, radio)
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Electromagnetic Field
Electric field of a stationary charge. Magnetic field of a stationary magnet.
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James Clark Maxwell (1831-1879)
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The Electromagnetic Wave
Visible light is one type of electromagnetic radiation. Different colors of light correspond to E&M waves having different wavelengths (λ). The waves consist of self-propagating, oscillating electric and magnetic fields that are perpendicular to each other and to the direction of motion.
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The Electromagnetic Wave
Gamma RaysX RaysUltraviolet (UV)Visible (optical)Infrared (IR)Radio