Bell Ringer 9/29

• What are stars made of?

29.2 - Stars

Star

• A star is a body of gases that gives off a tremendous amount of radiant energy in the form of light and heat

• Appear to be tiny specks of white light

• Most vary in color and are much larger than Earth

Motion

• Stars move through the night sky towards the west

• Stars rotate around the North Star, Polaris– Circumpolar = stars that circle around Polaris

• Because of the earth’s rotation, some stars are not visible during particular seasons

Constellations

• Pattern of stars• Location changes throughout the year due

to Earth’s orbit– Classified by season

• 3000 years ago, people observed and recorded shifting patterns– Knew when to plant, harvest, and celebrate

rituals based on location of constellations

Constellations

• Astronomers recognize 88 constellations

• Constellations are named for animals, ancient gods, & legendary heroes– Most do not look like what they are named for

Common Constellations

• Ursa Major (Big Dipper)

• Ursa Minor (Little Dipper)

• Draco the Dragon

• Orion

• Zodiac Animals

Constellation stars

• Astronomers use constellations to locate particular stars

• Stars within a constellation are named according to apparent magnitude– Brightest star is labeled alpha

– Next brightest beta and so on

• Some constellation stars have individual names– Polaris (North star) in the Little Dipper

Constellation Project

• Starting September 25, we will being a class project on Constellations

• You will pair up and pick a constellation

• Do some research on the constellation

• Make a visual aid about the constellation

Constellations to choose from• Aquarius

• Aries

• Cancer

• Capricornus

• Gemini

• Leo

• Ursa Major (Big Dipper)

• Ursa Minor (Little Dipper)

• Draco the Dragon

• Orion

• Cassiopeia

• Libra

• Pisces

• Sagittarius

• Scorpius

• Taurus

• Virgo

• Cygnus

• Hercules

• Pegasus

• Hydra

• Lyra

• Centaurus

Star Clusters

• Stars appear to be close to each other because the human eye cannot distinguish the distance a star is from earth

• Clusters are stars close to each other due to gravitational attraction

Binary Stars

• Multiple-star systems– Pairs of stars that

revolve around one another

– Gravitationally bound & orbiting a common center of mass

• Used to determine stellar mass

• Appear as a single star to the human eye

Distance

• Astronomers use different methods to measure the distance between stars and Earth

Distance to Stars

• Distance to stars from Earth is measured in Light-years– Light-year = distance light travels in one year– Light-year = 9.461 x 1015 m

Light

• Light travels 9.5 trillion km in one year– Speed of light = 300,000 km/s– Sun is 8 light-minutes away because it takes 8

minutes for the sunlight to reach Earth

• Parsec = 3.26 light-year = 3.086 x1016 m

Parallax

• Apparent shift in position caused by motion of observer

• Change in position of Earth as it orbits– Closer stars have larger change in parallax– Farther stars have smaller change in parallax

• Distance determined by angle of change

Parallax

Example of Parallax

• Extend your arm and hold up your left thumb

• Close your right eye and note location of thumb as compared to classroom wall

• Now close left eye and note location of thumb

Example of Parallax

• You will see your thumb appears to move

• This movement is because your eyes view everything at different angles, since they are a couple centimeters apart on your face

• If you move your thumb closer to your face you will see a bigger change in thumb location

In-Class Assignment/Homework

• WKT 29.2

29.2 B Notes

Properties of Stars

• Mass

• Diameter

• Luminosity (Magnitude)

• Temperature

Magnitude

• Apparent Magnitude = how bright a star appears

• System established by Greeks

• Brightest stars = +1

Magnitude

• Absolute = how bright a star would appear at 10 parsecs (30 light-years away)

• Allows for comparison based on how bright stars would appear at equal distances

Magnitude

• Luminosity = Energy output from surface of stars

• Measured in watts– Think light bulbs

Temperature

• Stars are assigned spectral type then further subdivided into numbers– O stars have a temperature of ~50,000 K

– M stars have a temperature of ~2000 K

• Based on temperature and pattern of spectra lines– Sun = G2, temperature = ~5800 K

• Temperature also related to luminosity and absolute magnitude

Composition

• All stars have nearly identical compositions

• ~73% Hydrogen

• ~25% Helium

• 2% - other elements

Classification of Stars

• H-R diagram = graph showing pattern between absolute magnitude and surface temperature of a star– Charts absolute magnitude,

temperature & spectral type

• Brightness of stars increases as surface temperature increases

H-R Diagram

H-R Diagram

• Main-sequence stars = band of stars running through middle of diagram– Sun and most stars in our sky are main-

sequence stars

• Band extends from cool, dim, red in the lower right corner to hot, bright, blue stars in upper left corner

Main Sequence

• Fusing Hydrogen in core

• As stars evolve they begin to fuse Helium and burn hydrogen– High mass stars burn Hydrogen faster than low

mass stars

H-R Diagram

• Other types of stars– Giants= very large, cool, bright star

– Supergiants = extremely large, giant star

– White Dwarfs = small, hot, dim star

In-Class Assignment/Homework