Stars

What is a star?

• The objects that heat and light the planets in a system

• A star is a ball of plasma held together by its own gravityoNuclear reactions occur in stars (H

He)oEnergy from the nuclear reactions is

released as electromagnetic radiation

Characteristics of Stars

• DISTANCEoMeasured in light-years

The distance which a ray of light would travel in one year

About 6,000,000,000,000 (6 trillion) miles186,000 miles per second

Characteristics of Stars

• Magnitude (brightness)oA measure of brightness of celestial

objectsSmaller values represent brighter objects

than larger values oApparent magnitude

How bright a star appears to be from Earth oAbsolute magnitude (luminosity)

How bright a star actually is

Characteristics of Stars

• Temperature & ColoroThe color of a star indicates the T of the

staroStars are classified by T

Decreasing T (bright to dim)O, B, A, F, G, K, M [Oh Be A Fine Girl, Kiss Me ]

http://www.seasky.org/cosmic/sky7a01.html

Hertzsprung-Russell Diagram

http://www.dustbunny.com/afk/stars/lifecycle/hrdiagram.gif

Main Sequence Stars

• A major grouping of stars that forms a narrow band from the upper left to the lower right when plotted according to luminosity and surface temperature on the Hertzsprung-Russell diagram

Types of StarsClassification

Red2,000 – 3,500 KM

Orange3,500 – 5,000 KK

Yellow5,000 – 6,000 KG

Yellow-white6,000 – 7,500 KF

White 7,500 – 10,000 KA

Blue-white10,000 – 30,000 KB

Blue20,000- 60,000 KO

ColorTemperatureClass

http://www.answers.com/topic/stellar-classification

Life Cycle of Stars

http://hea-www.cfa.harvard.edu/CHAMP/EDUCATION/PUBLIC/ICONS/life_cycles.jpg

Life Cycle of Stars

• Begin their lives as clouds of dust and gas called nebulae

• Gravity may cause the nebula to contract• Matter in the gas cloud will begin to condense

into a dense region called a protostar• The protostar continues to condense, it heats up.

Eventually, it reaches a critical mass and nuclear fusion begins.

• Begins the main sequence phase of the star• Most of its life is n this phase

Life Cycle of Stars

Life span of a star depends on its size.• Very large, massive stars burn their fuel much

faster than smaller stars• Their main sequence may last only a few

hundred thousand years• Smaller stars will live on for billions of years

because they burn their fuel much more slowly• Eventually, the star's fuel will begin to run

out.

Life Cycle of Stars

• It will expand into what is known as a red giant

• Massive stars will become red supergiants• This phase will last until the star exhausts

its remaining fuel• At this point the star will collapse

Life Cycle of Stars

• Most average stars will blow away their outer atmospheres to form a planetary nebula

• Their cores will remain behind and burn as a white dwarf until they cool down

• What will be left is a dark ball of matter known as a black dwarf

Life Cycle of Stars

• If the star is massive enough, the collapse will trigger a violent explosion known as a supernova

• If the remaining mass of the star is about 1.4 times that of our Sun, the core is unable to support itself and it will collapse further to become a neutron star

Life Cycle of Stars

• The matter inside the star will be compressed so tightly that its atoms are compacted into a dense shell of neutrons. If the remaining mass of the star is more than about three times that of the Sun, it will collapse so completely that it will literally disappear from the universe. What is left behind is an intense region of gravity called a black hole

Life Cycle of Stars

http://www.seasky.org/cosmic/sky7a01.html