STARSPage KeeleyWhere do Stars Go?

Learning goal:Explain the physical properties of stars.

Think About It When you look at the night time sky, how many

stars do you see? Hundreds? Thousands? Millions?

• Although there might be too many stars to count, all stars share similar characteristics? Can you come up with four different groups to characterize the stars?

How are stars classified?

• Stars are classified according to their physical properties.

• Color (Temperature)• Size• Distance from the Earth• Brightness – Apparent magnitude - Absolute magnitude

Color and Temperature

• Color reveals a star’s temperature• Hot stars appear blue (short wavelength)• Cool stars appear red (long wavelength)• Other colors in between like orange, yellow

and white

Size of Stars (Mass)

• Many stars come in pairs called “Binary Stars”• Binary stars rotate around each other• Scientists use this property to calculate mass

of stars

Brightness of a star

Apparent magnitude: How bright a star looks/appears from Earth depends on

1. Its size2. Its temperature3. Its distance from EarthAbsolute magnitude : how bright a star actually

is.

Distances of Stars from Earth

• Measured in:1. Light years2. Use Parallax angles (apparent change in

position of an object when you look at it from opposite sides of Earth’s orbit).

3. The nearest stars have a larger parallax angle

Apparent and Absolute Star’s Magnitude Lab

Star light, Star bright Lab

How is a star born?

• A star is born when the contracting gas and dust from a nebula become so dense and hot that nuclear fusion starts

• Nuclear fusion takes place when hydrogen atoms fuse forming helium.

• What is a nebula?

What determines how long a star will live?

• A star’s lifespan depends on its mass.• Small mass stars live longer because they use their fuel

more slowly• Large mass stars live shorter• A young star has more hydrogen• An old star has more helium• When a star begins to run out of hydrogen it becomes a

red giant or a supergiant.• When a star runs out of hydrogen it becomes a white

dwarf; a neutron star or a blackhole

Life Cycle of a Star

Life Cycle of a Star

Prentice Hall page 710

H-R Diagram

• A graph that shows relationship between absolute magnitude and temperature of stars.

• About 90% of the stars are in the main sequence stage

• Hottest main sequence stars are the brightest and the coolest stars are the dimmest

• Red Giants and Super Giants brightness is because of massive size, not temperature.

• The Sun is in the main sequence stage

H-R diagram

Galaxies

• A huge group of stars, dust and gas bound together by gravity.

What are the main types of galaxies?• Spiral galaxies (Solar System is in a spiral

galaxy called The Milky Way)• Elliptical galaxies• Irregular galaxies

Constellations

• A group of stars that form a picture

How can scientists determine whether a star is moving toward or away from the Earth?

• They use Doppler shifts• The light from a star that is moving away from

Earth appears red (its waves become longer)• The light from a star that is moving towards

Earth appears blue (its waves become shorter)

The Expanding Universe

• Most galaxies have Doppler Shifts towards the red end of the spectrum. This shows that the Earth and the source are moving away from each other.

• The red shift of distant galaxies indicate that the universe is expanding.

The Big Bang Theory

• States that :-• At one time the entire universe was confined

to a dense, hot, super massive ball.• Then, about 13.7 billion years ago, a violent

explosion occurred, hurling this material in all directions