StarsStars

What is a Star?

• Hot glowing masses of Gases

• Composition: Mostly Hydrogen and Helium

What is a star?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 gravity– Nuclear reactions occur in stars (H

He)

– Energy from the nuclear reactions is released as electromagnetic radiation

What makes a Star Shine?• The fusion of Atoms

Hydrogen atoms fusing to form Helium

releasing Heat, Light and energy• Astronomers use a Spectroscope to determine

the composition of a Star

Classifying StarsClassifying Stars

• Color

• Temperature

• Size

• Composition

• Brightness

Star DistanceStar Distance

• Measured in Light-Years

• Light-Year: The distance light travels in a year. (9.5 million million Kilometers)

5.87849981 × 1012 miles (6 trillion miles)

• Speed of light: 186,000 Miles/second 300,000 Kilometers/Second

Characteristics of StarsCharacteristics of Stars

• DISTANCE– Measured in light-years

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

• About 6,000,000,000,000 (6 trillion) miles• 186,000 miles per second

Finding the Distance to StarsFinding the Distance to Stars

• Parallax: The “apparent “ Shift that a star has over a period of Time

• The closer the object, the more it will shift position in the sky

*Background Star

Determination of DistanceDetermination of DistanceStellar Parallax Knowledge of the distance to the

stars is crucial for our determination of the luminosity of stars…

• Current technology allows us to determine the distance accurately to within a few hundred light-years.

• Hipparcos mission (European Space Agency) measured the stellar parallax of roughly 100,000 stars with precision of a few milli-arcseconds. So, it can measure distance of star up to 1,000 light-years away…

Simulation of Stellar Parallax…

Astronomical Distance UnitsAstronomical Distance Units• Light-year:

– The distance light travels (in vacuum) in one year.

– one light-year is 10 trillion (1013) km• Parsec: parallax & arcsecond

– One parsec: the distance to an object with a parallax angle of 1 arcsecond.

– One parsec equals to 3.26 light-year.– kiloparsecs: 1,000 parsecs.– megaparsecs: 1,000,000 parsec.

Characteristics of StarsCharacteristics of Stars

• Magnitude (brightness)– A measure of brightness of celestial

objects• Smaller values represent brighter objects

than larger values

– Apparent magnitude• How bright a star appears to be from Earth

– Absolute magnitude (luminosity)• How bright a star actually is

Star Brightness (magnitude)• Absolute Magnitude: the “Real”

brightness of the star. How much light it really gives off.(Need to know the distance to the Star)

• Apparent Magnitude: How bright the star appears to be.

Star Temperature

• Surface Temperature = Color

Blue-white White yellow orange red

Hottest *Sun coolest

Compare the sizes of StarsCompare the sizes of Stars• Our Sun is a medium sized Star

Characteristics of StarsCharacteristics of Stars

• Temperature & Color– The color of a star indicates the T of the

star

– Stars 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 DiagramHertzsprung-Russell Diagram

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

• Not all stars are the same color because different elements burn different colors. Some are red, some are blue, etc.

• Color tells the temperature of the star

• Hot stars are bluish/white and cooler stars are reddish/orange

• Astronomers call this a star’s spectral class. Spectral classes are O, B, A, F, G, K, and M

STARS ARE CLASSIFIED BY THEIR SIZE, COLOR AND TEMPERATURE

Flame Test Clip

O, B, A, F, G, K, and M... (Oh Be A Fine Guy Kiss Me) and for you guys: Oh Be A Fine Girl Kiss Me!!

Hertzsprung-Russell Diagram Hertzsprung-Russell Diagram (H-R Diagram)(H-R Diagram)

• Stars are ranked on the H-R Diagram based on their temperature and absolute magnitude

• Most stars fall in the middle of the diagram, called the main sequence

• Very few stars are white dwarfs or giants/supergiants

H-R Diagram---shows the H-R Diagram---shows the life cycle of starslife cycle of stars

temperature

Abs

olut

e m

agni

tude

Main sequence

supergiants

giants

White dwarfs

Main Sequence StarsMain 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 StarsTypes of StarsClassificationClassification

Class Temperature Color

O 20,000- 60,000 K Blue

B 10,000 – 30,000 K Blue-white

A 7,500 – 10,000 K White

F 6,000 – 7,500 K Yellow-white

G 5,000 – 6,000 K Yellow

K 3,500 – 5,000 K Orange

M 2,000 – 3,500 K Red

Type

Hydrogen Balmer LineStrength

ApproximateSurfaceTemperature

Main Characteristics Examples

O Weak > 25,000 K Singly ionized helium emission or absorption lines. Strong ultraviolet continuum.

10 Lacertra

B Medium 11,000 - 25,000 Neutral helium absorption lines .RigelSpica

A Strong 7,500 - 11,000Hydrogen lines at maximum strength for A0

stars, decreasing thereafter.SiriusVega

F Medium 6,000 - 7,500 Metallic lines become noticeable.CanopusProcyon

G Weak 5,000 - 6,000Solar-type spectra. Absorption lines of

neutral metallic atoms and ions (e.g. singly-ionized calcium) grow in strength.

SunCapella

K VeryWeak

3,500 - 5,000 Metallic lines dominate. Weak blue continuum.

ArcturusAldebaran

M VeryWeak < 3,500

Molecular bands of titanium oxide noticeable. Betelgeuse

-10

-5

0

5

10

15

-0.5 0 0.5 1 1.5 2 2.5

B-V Color Index

Abs

olut

e V

isua

l Mag

nitu

de

White Dwarfs

Main Sequence

Giants

Bright Supergiants

Supergiants

Bright Giants

Subgiants

Stars of the same spectral class can be distinguished from one another by using the luminosity effect to determine their size. It is found that, when we consider the data for a large number of stars, the points on the Hertzsprung-Russel diagram are approximately as shown below. There are very few stars that don’t have MV and B – V that plot near one of the lines in the graph. Based on this, we classify stars by luminosity as well as spectral class.

The luminosity classes areIa bright supergiantIb supergiantII bright giantIII giantIV subgiantV main sequence

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

Life Cycle of StarsLife Cycle of Stars

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

Life Cycle of StarsLife 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 StarsLife 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 StarsLife 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 StarsLife 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 StarsLife 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 StarsLife 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-a-star-discuss 1b.ppt

Life Cycle of Stars Life Cycle of Stars Videos\The Life Cycle of a Star - Animation.mp4Videos\The Life Cycle of a Star - Animation.mp4

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

VideosVideos