Characteristics of Stars

Astronomy Chapter 3.2Pages 103-109

• A star is a sphere of super-hot gases– Mostly Hydrogen and

Helium

• 1 to 2 percent of a stars mass may consist of heavier elements.

What is a Star?

• Our solar system’s star

• The Sun is 150 million km away from EarthLight from the Sun

takes 8 minutes to reach Earth

The Sun

• Billions of galaxies make up the universe

• All of space and everything in it make up the universe.

The Universe

Massive cluster of stars is called a galaxy• It would take you 25,000 years to travel to the

center (250 million billion km) of our galaxy at the speed of light.

Milky Way Galaxy

• Our neighboring galaxy

• Would take 2 million years to reach at the speed of light

• This galaxy is visible to the naked eye on a clear night.

Andromeda Galaxy

4) Helium

2) Nitrogen

3) Hydrogen

1) Oxygen

What is the most abundant element in most stars?

4) 25,000 years ago

2) 2 million years ago

3) 300,000 years ago

1) 10 million years ago

When did the light that you see actually leave the Andromeda galaxy?

Distance to Stars• Earth is about 150 million km from the Sun

(93million miles)• That is equal to 1 Astronomical Unit (AU)

Distance to Stars• Since Stars are so far

away:– A light year is used to

express great distances.– Light travels at 300,000

km/s– 1 light year = distance a

ray of light travels in 1 year (9.5 trillion km.)

Horsehead Nebula is 1,500 light years away)

• The next closest star is 40 trillion km from the Sun (more than 260,000AU)

• Light takes 4.3 years to reach Earth

• Most stars are much further away

Proxima Centauri

• Stars appear to be the same distance from Earth in the night sky

• How do we know that they are not?

Distance to Stars

Parallax• Parallax: the apparent

change in position of an object when you look at it from different positions.

• Only works for nearby stars (less than 1,000 light years away)

• Astronomers look at the star when Earth is on one side of the sun and again six months later.

Distance to Stars

• They measure how much the star appears to move against the background of stars• The less the star appears to move the

further away it is.

Distance to Stars

Distance to Stars

What is the speed of light?

4) 25,000 m/s

2) 2 million m/s

3) 300,000km/s

1) 10 km/s

Characteristics

• Size (Mass)

• Color & Temperature

• Brightness

Classifying Stars

Mass (size)• Mass is the total amount of

material in a body– How much “stuff” or matter

something contains

• Some stars are five, ten or more times the size of our Sun. Others are less massive.

Medium Size Star - Our Sun

Types of Stars

Giant Stars – are 10 to 100 times larger, having no more Hydrogen

Rigel

Types of Stars

Super Giant – 10 to 70 times more massive V838 Monocerotis

Types of Stars

White Dwarf - about the size of Earth, but with the mass of the Sun

Types of Stars

Neutron Star – stars that are super small, super massive, and have become pure neutrons

Types of Stars

4) Density

2) Gases present

3) Color

1) Temperature

If a star can differ in volume (size) and mass, what else will

differ as a result?

Density of a Star• One teaspoon full of material from Sirius

(White Dwarf) is so dense that on Earth it would weigh a ton.

4) Blue

2) Orange

3) Yellow

1) Red

What color would you expect a cool star to glow?

3) Yellow

2) Orange

4) Blue

1) Red

What color would you expect a hot star to glow?

Temperature

• How can the temperature of a star be determined?– By the color

• Red Stars = Cool• Blue Stars= Hot

4) 50,000˚C

2) 5,500˚C

3) 10,000˚C

1) 3,900˚C

The Sun is a yellow star, what temperature would you expect

the sun to be close to?

1. Red Stars ~ 3,000◦C (Betelgeuse)

2. Red-Orange Stars ~ 5,000 ◦C (Aldebaran)

3. Yellow Stars ~ 5,500◦C – 6,000 ◦C (Sun)

4. White Stars ~ 10,000◦C (Sirius)

5. Blue Stars ~ more than 15,000◦C (Rigel)

Temperatures & Colors

• Stars are far from Earth

• As a result, what problem does this cause?– Cannot accurately determine the actual

brightness of a star from earth

• How bright a star appears from Earth depends on far away the star is.

Brightness (Luminosity)

• Brightness depends on size and temperature

• Betelgeuse is large but cool. Its size makes it appear bright

• Rigel is smaller but is very hot so it shines brightly.

Brightness (Luminosity)

Apparent Magnitude• The measure of how bright a star appears

to be from earth.– Less bright objects that are closer to you

could be seem brighter than objects that are far and actually brighter

• The lower the magnitude the brighter the star.

• The brightness of a star if it was set at a standard distance from Earth.

– Astronomers calculate the star’s apparent magnitude and its distance from Earth.

– Then calculate the brightness if it were a standard distance from Earth.

Absolute Magnitude

4) Size and temperature of a star

2) Brightness of a star at a set distance3) Temperature of a star

1) How bright a star appears to be

Absolute Magnitude determines…

HR- Diagram

• Herzsprung and Russell

• A graph used to find out if temperature and brightness are related.

• Plots Absolute Magnitude vs. Surface Temperature.

HR-Diagram

• More than 90% of stars are Main Sequence, form a diagonal band.

HR- Diagram

What to Work On• Read Section 3.2, pages 103-109• Answer the section review questions on

page 109 (#’s 1-4)

DUE: Friday, April 13th