Grade 8 Integrated Science Chapter 12 Lesson 1 on the view of space from Earth. This lesson gives a short introduction on constellations, relative brightness, luminosity, and the apparent size of a star.

1. The View from EarthThe View from Earth
Chapter 12 Lesson 1 p414-420Chapter 12 Lesson 1 p414-420

2. VocabularyVocabulary
• Spectroscope (417) – an instrument that spreadsSpectroscope (417) – an instrument that spreads
light into different wavelengthslight into different wavelengths
• Astronomical Unit (418) – the average distanceAstronomical 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 1Light-year (418) – the distance light travels in 1
yearyear
• Apparent Magnitude (419) – a measure of howApparent 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 anLuminosity (419) – the true brightness of an
objectobject

3. Looking at the Night SkyLooking at the Night Sky
• If you look at the stars for a long time theyIf 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 thePolaris is a star almost directly above the
north pole.north pole.
• As Earth spins Polaris stays in place as starsAs Earth spins Polaris stays in place as stars
near it seem to circle around it.near it seem to circle around it.

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

5. Naked-Eye AstronomyNaked-Eye Astronomy
• Naked-eye astronomy means gazing at the skyNaked-eye astronomy means gazing at the sky
using just your eyes; no binoculars orusing just your eyes; no binoculars or
telescopes.telescopes.
• Before telescopes, people used the stars toBefore telescopes, people used the stars to
tell time, seasons, and finding directions.tell time, seasons, and finding directions.

6. 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 identifiedThe 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 ancientPresent-day astronomers use many ancient
constellations to divide the sky into 88 regionsconstellations to divide the sky into 88 regions
also called constellationsalso called constellations

8. TelescopesTelescopes
• Telescopes are able to collect much more lightTelescopes are able to collect much more light
than the human eyethan the human eye
• The electromagnetic spectrum is a continuousThe 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 differentDifferent objects in space can emit different
types of wavelengths.types of wavelengths.
• The range of wavelengths a star emits is called itsThe range of wavelengths a star emits is called its
spectrum.spectrum.

10. SpectroscopesSpectroscopes
• Scientists study the spectra of a star using aScientists study the spectra of a star using a
spectroscope.spectroscope.
• A spectroscope spreads light into differentA 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 infraredNewly formed stars emit mostly radio and infrared
waves. While exploding stars emit high energywaves. While exploding stars emit high energy
ultraviolet waves.ultraviolet waves.

13. Measuring distanceMeasuring distance
• Astronomers use angles created by parallax toAstronomers 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’sParallax is the apparent change in an object’s
position caused by looking at it from 2 differentposition caused by looking at it from 2 different
points.points.
• For example: Look at your pencil with only your leftFor example: Look at your pencil with only your left
eye. Then, without moving the pencil, look at it witheye. Then, without moving the pencil, look at it with
only your right eye.only your right eye.
• Astronomers create a parallax by using 2Astronomers create a parallax by using 2
points in Earth’s orbit around the Sunpoints in Earth’s orbit around the Sun

14. Distance Within the Solar SystemDistance Within the Solar System
• Distances within the solar system areDistances within the solar system are
measured using astronomical units or AU.measured using astronomical units or AU.
– An astronomical unit is the average distanceAn astronomical unit is the average distance
between Earth and the Sun, about 150 million km.between Earth and the Sun, about 150 million km.

15. Distances Beyond the Solar SystemDistances Beyond the Solar System
• Astronomers measure distances beyond theAstronomers 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?

17. Looking Back in TimeLooking Back in Time
• Because it takes time for light to travel, youBecause it takes time for light to travel, you
see a star not as it is today, but as it was whensee 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 CentauriAt 4.2 light-years away, Proxima Centauri
appears as it was 4.2 years ago.appears as it was 4.2 years ago.

18. Measuring BrightnessMeasuring Brightness
• Astronomers measure the brightness of starsAstronomers 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

19. Apparent MagnitudeApparent Magnitude
• Scientists measure how bright stars appear fromScientists measure how bright stars appear from
Earth using a scale developed by the ancientEarth using a scale developed by the ancient
Greek astronomer HipparchusGreek astronomer Hipparchus
– Hipparchus assigned a number to every star he saw inHipparchus 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 aThe 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 brightestHipparchus assigned the number 1 to the brightest
star he saw in the night skystar he saw in the night sky

21. Absolute MagnitudeAbsolute Magnitude
• Stars can appear bright or dim depending onStars can appear bright or dim depending on
their distances from Earth, but stars also havetheir 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 sizeThis 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 apparentA star’s luminosity, distance, and apparent
magnitude are all related. Thus, if a scientistmagnitude are all related. Thus, if a scientist
knows two of these factor, he canknows two of these factor, he can
mathematically determine the third.mathematically determine the third.