StarsStarsStarsStars

What is a star?What is a star?

• Objects that heat and light the planets in a system

• A ball of plasma held together by its own gravity– Nuclear reactions (fusion) occur in stars

(H He)

– Energy from the nuclear reactions is released as electromagnetic radiation

• Stars are born with a specific mass

• Mass is the main factor in determining the star’s brightness, temperature, expected lifetime, type of death, and spectra.

• Classified according to their spectra

Astronomers realized that large numbers of stars exhibit a small number of distinct patterns in their spectral features.

• Spectral lines are caused by different elements present in a star

• Overall spectrum is strongly related to the surface temperature of the star

• Stars are categorized into classes by: 1. Strength of the spectral lines

2. Shape

• The 7 classes of stars are:

O B A F G K M

• From O M:

1. Stars decrease in temperature

2. Stars generally become less massive (smaller)

O B A F G K M Oh Be A Fine Guy/Girl, Kiss Me!

One Bug Ate Five Green Killer Moths

• The Hertsprung-Russell or H-R diagram reveals that about 90% of all stars lie along a smooth diagonal curve called the main sequence with hot, luminous stars in the upper left and cool, dim stars in the lower right.

• Not all stars fall on the main sequence. Stars below the main sequence are called white dwarfs and those above it are called giants.

Since hotter stars are bluer, and cooler stars are redder, a White Dwarf is hotter than a Red Giant.

• Color:Color: blueblue• Temperature:Temperature: 28,000- 28,000-

50,000 K50,000 K• Size:Size: very large and very large and

massivemassive• Spectra:Spectra: ionized helium, ionized helium,

ultraviolet ultraviolet continuumcontinuum

• Examples:Examples: 10 Lacertra, 10 Lacertra, Zeta Puppis, and Zeta Puppis, and Iota Orionis AIota Orionis A

The rock stars of the

universe-they live fast and die young!

• Color:Color: blueblue• Temperature:Temperature: 10,000- 10,000-

28,000 K28,000 K• Size:Size: large and large and massivemassive• Spectra:Spectra: neutral helium neutral helium

lines,lines,some some hydrogenhydrogen

• Examples:Examples: Beta Beta Centauri, Rigal, Centauri, Rigal, SpicaSpica

B stars are relatively

rare, comprising only 0.1% of

main sequence

stars.

• Color:Color: whitewhite• Temperature:Temperature: 75,000- 75,000-

10,000 K10,000 K• Size:Size: moderate sized, moderate sized,

very luminousvery luminous• Spectra:Spectra: strong strong

hydrogen lines, hydrogen lines, ionized ionized metalsmetals

• Examples:Examples: Alpha Canis Alpha Canis Majoris (Sirius) Majoris (Sirius)

and and Alpha Lyrae (Vega)Alpha Lyrae (Vega)

A stars are amongst the

most common

naked eye stars.

• Color:Color: white-yellow white-yellow• Temperature:Temperature: 6,000- 6,000-

75,000 K75,000 K• Size:Size: 1.2 to 1.6 times 1.2 to 1.6 times

bigger than the Sunbigger than the Sun• Spectra:Spectra: weak hydrogen weak hydrogen

lines, strong lines, strong Calcium Calcium and other and other ionized ionized metalsmetals

• Examples:Examples: Canopus, Canopus,ProcyonProcyon

Often used as targets for extrasolar

planet searches and SETI

programs.

• Color:Color: yellow• Temperature:Temperature: 5,000- 5,000-

6,000 K6,000 K• Size:Size: 0.8 to 1.10.8 to 1.1 times times the the

mass of the Sunmass of the Sun• Spectra:Spectra: weak hydrogen weak hydrogen

lines, neutral and lines, neutral and ionized metalsionized metals

• Examples:Examples: Alpha Alpha Centauri A, CapellaCentauri A, Capella

The best known

example of a G star is our SUN!

• Color:Color: orange orange• Temperature:Temperature: 3,500- 3,500-

5,000 K5,000 K• Size:Size: smaller and cooler smaller and cooler

than the Sunthan the Sun• Spectra:Spectra: faint hydrogen faint hydrogen

lines, strong neutral lines, strong neutral metallic linesmetallic lines

• Examples:Examples: Alpha Boötis Alpha Boötis (Arcturus) and Alpha (Arcturus) and Alpha Tauri (Aldebaran)Tauri (Aldebaran)

Also used as targets for extrasolar

planet searches like project Ozma

in 1960.

• Color:Color: red red• Temperature:Temperature: 2,500- 2,500-

3,500 K3,500 K• Size:Size: range from 0.5-25 range from 0.5-25

(main sequence-(main sequence-supergiants) times supergiants) times the mass of the the mass of the

SunSun• Spectra:Spectra: strong metallic strong metallic

lines and wide lines and wide titanium oxide titanium oxide bandsbands

• Examples:Examples: Antares and Antares and BetelgeuseBetelgeuse

Most common class by

number of stars, since 90% of all

stars are red dwarfs.

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

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

1. Begin their lives as clouds of dust and gas called nebulae

2. Gravity may cause the nebula to contract3. Matter in the gas cloud will begin to condense

into a dense region called a protostar4. Protostar continues to condense, it heats up

– Eventually reaches a critical mass and nuclear fusion begins.

5. Begins the main sequence phase of the star– Most of its life is in 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– 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

• 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 (ionized gas emission)

• 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

Planetary Nebula

White Dwarf

Nebulas Nebulas

Life Cycle of StarsLife Cycle of Stars• If the star is massive enough, the collapse will

trigger a violent explosion known as a supernova

• Supernova = stellar explosion that briefly outshines an entire galaxy, gives off as much energy as the Sun over its entire life span, before fading from view over several weeks or months

Cassiopeia Supernova Remnant

Life Cycle of StarsLife Cycle of Stars• 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

• Neutron star = star remnant that results from collapse of a massive star after a supernova– Densest and smallest stars known – Can have a mass of about two times that of the

Sun

Life Cycle of StarsLife Cycle of Stars

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