30.1 notes Characteristics of Stars

Std 1d: Know the evidence indicating that the planets are much closer to Earth than the stars are.

Std 2d: Know that stars differ in their life cycles and that visual, radio, and x-ray telescopes may be used to collect data that reveal those differences

Objective 1: Describe how astronomers determine the composition and temperature of stars

Objective 2: Describe one way astronomers measure the distance to stars

Objective 3: Explain the difference between absolute magnitude and apparent magnitude

Characteristics of Stars

• Star – a large celestial body that is composed of gas and that emits light – The energy comes from nuclear fusion – Stars vary in color (ex. Reddish, blue-white,

yellow)

Analyzing Starlight

• Astronomers learn about stars by analyzing the light they release

• Dark-line spectra shows the star’s composition and temperature – Used to determine the elements

• Dark-line spectra - bands of color crossed by dark lines where color is diminished

The Composition of stars

• Stars most common element is hydrogen. • Second is helium – Carbon, oxygen and nitrogen are in small amounts

The Temperature of Stars

• Temperature is indicated by color –Hottest are blue stars = 35,000 ⁰C –Coolest are red stars = 3,000 ⁰C –Yellow stars are about 5,500 ⁰C

Objective 1: Determining the composition and temperature of stars

• What are the 2 most common elements to make up stars?– Hydrogen and Helium

• What color are the hottest stars?– blue

• What color are the coolest stars?– Red

• What is used to determine a stars composition and temperature?– Dark-line spectra

The sizes and masses of stars

• Stars vary in size and mass – Dwarf stars can be as big as Earth – The sun is a medium-sized star – Giant stars have a diameters 1,000x the diameter

of the sun –Most stars visible from Earth are medium-sized

stars, similar to our sun

Stellar motion

a) Apparent motion is motion visible to the unaided eye. It is caused by the movement of Earth. –Earth’s rotation and

revolution causes the stars to move in the night’s sky

b) Actual motion- most stars have several types of actual motion 1) they rotate on their axis

2) they may revolve around another star

3) they either move away or towards our solar system

The Doppler Effect

• Doppler effect – an observed change in the frequency of a wave when the source or observer is moving

• Stars moving towards our solar system = blue shift • Star moving away = red shift – Distant galaxies have red-shifted spectra –What does galaxies having a red shift mean?• The universe is expanding (galaxies are moving

away)

Distances to stars

• Distance between Earth and stars are measured in light years

• Light year - the distance light travels in one year or 9.46 trillion km in one year

• The sun is 8 light minutes from Earth

• Nearest star to Earth – Proxima Centauri = 4.2 light years away

• Parallax - an apparent shift in the position of an object when viewed from different locations. The larger the shift the closer the object (sun) – Used to determine the distance of the star (if close)

Objective 2: Describe one way astronomers measure the distance to stars

• What color are galaxies that are moving away?– Red shift

• What does a red shift in galaxy color suggest?– The universe is expanding

• How doe astronomers measure the distance of close stars?– Parallax

Stellar brightness

• Visibility of a star depends on:

1. Brightness

2. Distance from Earth • Apparent magnitude- the brightness of a star as it

appears on Earth (lower # = brighter star)• Absolute magnitude- the brightness that a star

would have at a distance of 32.6 light-years from Earth

Objective 3:Difference between absolute and apparent magnitude

• What 2 things effect the visibility of a star?– Distance and brightness

• What is the brightness of the star 32.6 light-years from Earth?– Absolute magnitude

• What is the brightness of a star seen from Earth?– Apparent magnitude

• What does a lower apparent number mean?– The brighter the star