The View from EarthThe View from Earth

Chapter 12 Lesson 1 p414-420Chapter 12 Lesson 1 p414-420

VocabularyVocabulary• Spectroscope (417) – an instrument that spreads Spectroscope (417) – an instrument that spreads

light into different wavelengthslight into different wavelengths• Astronomical Unit (418) – the average distance Astronomical Unit (418) – the average distance

between Earth and the Sun, about 150 million kmbetween Earth and the Sun, about 150 million km• Light-year (418) – the distance light travels in 1 Light-year (418) – the distance light travels in 1

yearyear• Apparent Magnitude (419) – a measure of how Apparent Magnitude (419) – a measure of how

bright it an object appears from Earthbright it an object appears from Earth• Luminosity (419) – the true brightness of an Luminosity (419) – the true brightness of an

objectobject

Looking at the Night SkyLooking at the Night Sky

• If you look at the stars for a long time they If you look at the stars for a long time they seem to move.seem to move.

• Why does this happen?Why does this happen?• Polaris is a star almost directly above the Polaris is a star almost directly above the

north pole.north pole.• As Earth spins Polaris stays in place as stars As Earth spins Polaris stays in place as stars

near it seem to circle around it.near it seem to circle around it.

• Polaris is commonly referred to as the Polaris is commonly referred to as the Northern Star, because it and the stars near it Northern Star, because it and the stars near it never set when viewed from the northern never set when viewed from the northern hemisphere.hemisphere.

Naked-Eye AstronomyNaked-Eye Astronomy

• Naked-eye astronomy means gazing at the sky Naked-eye astronomy means gazing at the sky using just your eyes; no binoculars or using just your eyes; no binoculars or telescopes.telescopes.

• Before telescopes, people used the stars to Before telescopes, people used the stars to tell time, seasons, and finding directions.tell time, seasons, and finding directions.

ConstellationsConstellations

• When ancient cultures gazed at the night sky, When ancient cultures gazed at the night sky, they saw patterns.they saw patterns.– They represented people, animals, and objectsThey represented people, animals, and objects

• The Greek astronomer Ptolemy identified The Greek astronomer Ptolemy identified dozens of star patterns nearly 2000 years ago.dozens of star patterns nearly 2000 years ago.– These are called ancient constellations todayThese are called ancient constellations today

• Present-day astronomers use many ancient Present-day astronomers use many ancient constellations to divide the sky into 88 regions constellations to divide the sky into 88 regions also called constellationsalso called constellations

TelescopesTelescopes• Telescopes are able to collect much more light Telescopes are able to collect much more light

than the human eyethan the human eye• The electromagnetic spectrum is a continuous The electromagnetic spectrum is a continuous

range of wavelengthsrange of wavelengths– Visible light is only one part of the spectrumVisible light is only one part of the spectrum– Longer wavelengths have low energyLonger wavelengths have low energy– Shorter wavelengths have higher energyShorter wavelengths have higher energy

• Different objects in space can emit different Different objects in space can emit different types of wavelengths.types of wavelengths.

• The range of wavelengths a star emits is called its The range of wavelengths a star emits is called its spectrum.spectrum.

SpectroscopesSpectroscopes

• Scientists study the spectra of a star using a Scientists study the spectra of a star using a spectroscope.spectroscope.

• A spectroscope spreads light into different A spectroscope spreads light into different wavelengths.wavelengths.

• This way scientists can study a stars’ This way scientists can study a stars’ characteristics, compositions, and energies.characteristics, compositions, and energies.– Newly formed stars emit mostly radio and infrared Newly formed stars emit mostly radio and infrared

waves. While exploding stars emit high energy waves. While exploding stars emit high energy ultraviolet waves.ultraviolet waves.

Measuring distanceMeasuring distance

• Astronomers use angles created by parallax to Astronomers use angles created by parallax to measure how far objects are from Earth.measure how far objects are from Earth.– Parallax is the apparent change in an object’s Parallax is the apparent change in an object’s

position caused by looking at it from 2 different position caused by looking at it from 2 different points.points.• For example: Look at your pencil with only your left For example: Look at your pencil with only your left

eye. Then, without moving the pencil, look at it with eye. Then, without moving the pencil, look at it with only your right eye.only your right eye.

• Astronomers create a parallax by using 2 Astronomers create a parallax by using 2 points in Earth’s orbit around the Sunpoints in Earth’s orbit around the Sun

Distance Within the Solar SystemDistance Within the Solar System

• Distances within the solar system are Distances within the solar system are measured using astronomical units or AU.measured using astronomical units or AU.– An astronomical unit is the average distance An astronomical unit is the average distance

between Earth and the Sun, about 150 million km.between Earth and the Sun, about 150 million km.

Distances Beyond the Solar SystemDistances Beyond the Solar System

• Astronomers measure distances beyond the Astronomers measure distances beyond the solar system using light-years.solar system using light-years.– A light-year is the distance light travels in 1 year.A light-year is the distance light travels in 1 year.– 1 light-year equals about 10 trillion km.1 light-year equals about 10 trillion km.– The nearest star to our Sun is 4.2 light-years away. The nearest star to our Sun is 4.2 light-years away.

How far is that in km?How far is that in km?

Looking Back in TimeLooking Back in Time

• Because it takes time for light to travel, you Because it takes time for light to travel, you see a star not as it is today, but as it was when see a star not as it is today, but as it was when light left it.light left it.

• At 4.2 light-years away, Proxima Centauri At 4.2 light-years away, Proxima Centauri appears as it was 4.2 years ago.appears as it was 4.2 years ago.

Measuring BrightnessMeasuring Brightness

• Astronomers measure the brightness of stars Astronomers measure the brightness of stars in two ways:in two ways:– By how bright they are from EarthBy how bright they are from Earth– By how bright they actually areBy how bright they actually are

Apparent MagnitudeApparent Magnitude

• Scientists measure how bright stars appear from Scientists measure how bright stars appear from Earth using a scale developed by the ancient Earth using a scale developed by the ancient Greek astronomer HipparchusGreek astronomer Hipparchus– Hipparchus assigned a number to every star he saw in Hipparchus assigned a number to every star he saw in

the night sky based on its brightness.the night sky based on its brightness.• Today these are called number magnitudesToday these are called number magnitudes• The apparent magnitude of an object is a The apparent magnitude of an object is a

measure of how bright it appears from Earthmeasure of how bright it appears from Earth– Hipparchus assigned the number 1 to the brightest Hipparchus assigned the number 1 to the brightest

star he saw in the night skystar he saw in the night sky

Absolute MagnitudeAbsolute Magnitude

• Stars can appear bright or dim depending on Stars can appear bright or dim depending on their distances from Earth, but stars also have their distances from Earth, but stars also have actual, or absolute, magnitudesactual, or absolute, magnitudes

• Luminosity is the true brightness of an objectLuminosity is the true brightness of an object• This depends on the star’s temperature and size This depends on the star’s temperature and size

rather then its distance from Earthrather then its distance from Earth• A star’s luminosity, distance, and apparent A star’s luminosity, distance, and apparent

magnitude are all related. Thus, if a scientist magnitude are all related. Thus, if a scientist knows two of these factor, he can knows two of these factor, he can mathematically determine the third.mathematically determine the third.