Astronomical Distances

Stars that seem to be close may actually be very far away from each other.

•The universe is immense in size

•Astronomers use units and methods of measuring that we are not used to on Earth

•Parallax is used to measure the distance to nearby stars

•Parallax: when objects appear to move or shift due to earth’s orbit

★This method uses trigonometry

★nearby stars = 300 light years or less

Parallax

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•Inverse square law of light is used for distant stars

•the intensity of light falls off as the square of the distance from the source

•Light year is a unit of distance, NOT time

•Light Year: the distance light travels in one year (9.5 trillion km!!!)

★Kilometers are too small to use in astronomy

★one light-year is equal to 9,500,000,000,000 kilometers.

Light Year

•Parsec is used to measure greater distances between stars and galaxies

•1 parsec = 3.26 light years

•Astronomical Units (AUs) measure distances between planets (our solar system)

•AU is the average distance between the Earth and Sun (1 AU)

★1 AU is about 150 million km

★Mercury is the closest at 0.39 AUs while Neptune is the farthest at 30.06 AUs

1 AUSun E

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60 second summary

Review#1 What is parallax used for?

•to measure a star’s temperature

•to measure a star’s distance from Earth

•to measure a star’s mass

•to measure a star’s magnitude

๏parallax measures a star’s distance from earth

Review#2 What does a light year

measure?•temperature

•time

•distance

•magnitude

๏distance

Reviw#3 An AU is used for distances

between...•stars

•galaxies

•solar systems

•planets

๏planets

Review#4 Which measurement is used for distant

stars?•AU

•parallax

•inverse square law of light

•light year

๏inverse square law of light