Chapter 12 Lesson 1 p414-420

THE VIEW FROM EARTH

Vocabulary

Spectroscope – an instrument that spreads light into different wavelengths

Astronomical Unit – the average distance between Earth and the Sun, about 150 million km

Light-year – the distance light travels in 1 year

Apparent Magnitude – a measure of how bright it an object appears from Earth

Luminosity – the true brightness of an object

Looking at the Night Sky

If you look at the stars for a long time they seem to move. Why does this happen?

Polaris is a star almost directly above the north pole.

As Earth spins Polaris stays in place as stars near it seem to circle around it.

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

Naked-Eye Astronomy

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

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

Constellations

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

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

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

ORION

LEO

SCORPIO

Telescopes

Telescopes are able to collect much more light than the human eye

The electromagnetic spectrum is a continuous range of wavelengths Visible light is only one part of the spectrum Longer wavelengths have low energy Shorter wavelengths have higher energy

Different objects in space can emit different types of wavelengths.

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

Spectroscopes

Scientists study the spectra of a star using a spectroscope.

A spectroscope spreads light into different wavelengths.

This was scientists can study a stars’ characteristics, compositions, and energies. Newly formed stars emit mostly radio and

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

Measuring distance

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

object’s position caused by looking at it from 2 different points. For example: Look at your pencil with only your

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

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

Distance Within the Solar System

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

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

Distances Beyond the Solar System

Astronomers measure distances beyond the solar system using light-years. A light-year is the distance light travels in 1

year. 1 light-year equals about 10 trillion km. The nearest star to our Sun is 4.2 light-

years away. How far is that in km?

Looking Back in Time

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

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

Measuring Brightness

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

Apparent Magnitude

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

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

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

brightest star he saw in the night sky

Absolute Magnitude

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

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

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

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