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Stars
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: blue
• Temperature: 28,00050,000 K
• Size: very large and
massive
• Spectra: ionized helium,
ultraviolet continuum
• Examples: 10 Lacertra,
Zeta Puppis, and
Iota Orionis A
The rock stars
of the
universe-they
live fast and
die young!
• Color: blue
• Temperature: 10,00028,000 K
• Size: large and
massive
• Spectra: neutral helium
lines,some hydrogen
• Examples: Beta
Centauri, Rigal,
Spica
B stars are
relatively
rare,
comprising
only 0.1% of
main
sequence
stars.
• Color: white
• Temperature: 75,00010,000 K
• Size: moderate sized,
very luminous
• Spectra: strong
hydrogen lines,
ionized metals
• Examples: Alpha Canis
Majoris (Sirius) and
Alpha Lyrae (Vega)
A stars are
amongst the
most
common
naked eye
stars.
• Color: white-yellow
• Temperature: 6,00075,000 K
• Size: 1.2 to 1.6 times
bigger than the Sun
• Spectra: weak hydrogen
lines, strong Calcium
and other ionized
metals
• Examples: Canopus,
Procyon
Often used as
targets for
extrasolar
planet searches
and SETI
programs.
• Color: yellow
• Temperature: 5,0006,000 K
• Size: 0.8 to 1.1 times
the mass of the Sun
• Spectra: weak hydrogen
lines, neutral and
ionized metals
• Examples: Alpha
Centauri A, Capella
The best
known
example of
a G star is
our SUN!
• Color: orange
• Temperature: 3,5005,000 K
• Size: smaller and cooler
than the Sun
• Spectra: faint hydrogen
lines, strong neutral
metallic lines
• Examples: Alpha Boötis
(Arcturus) and Alpha
Tauri (Aldebaran)
Also used as
targets for
extrasolar
planet
searches like
project Ozma
in 1960.
• Color: red
• Temperature: 2,5003,500 K
• Size: range from 0.5-25
(main sequencesupergiants) times
the mass of the Sun
• Spectra: strong metallic
lines and wide
titanium oxide
bands
• Examples: Antares and
Betelgeuse
Most
common
class by
number of
stars, since
90% of all
stars are red
dwarfs.
Types of Stars
Classification
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 Stars
http://hea-www.cfa.harvard.edu/CHAMP/EDUCATION/PUBLIC/ICONS/life_cycles.jpg
Life Cycle of Stars
1. Begin their lives as clouds of dust and gas
called nebulae
2. Gravity may cause the nebula to contract
3. Matter in the gas cloud will begin to condense
into a dense region called a protostar
4. 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 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 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 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
White Dwarf
Planetary Nebula
Nebulas
Life 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 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 Stars
http://www.seasky.org/cosmic/sky7a01.html