Stars
What is a star?
• A star is a large celestial body held
together by gravity and is composed
of gas and emits light.
• A star is born when the gases inside
a nebula contract together. Inside the
nebula you will find new starts.
Characteristics of Stars
• Size
• Stars vary in size
• The sun’s radius( half of diameter) is
approximately 695,000 km
• Astronomers use what is called solar radii
to compare other stars Average Radius of
sun=1 (See page 91 in textbook for
clarification)
Characteristics 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
Characteristics of Stars
Apparent Magnitude
• Is the measure of the brightness of a
celestial (star) object as seen from Earth
• The lower the apparent magnitude the
brighter the object
• Negative numbers indicated extreme
brightness
• Positive numbers indicate faint stars
Characteristics of Stars
Absolute Magnitude
•Is the measure of inherent brightness of a
celestial object really is, if all stars were the
same distance from Earth
•Lower the number the brighter the star,
negative numbers are extreme brightness
Characteristics of Stars
• Temperature & Color
– The color indicates the temperature of the star
– Stars are classified by temperature
– Seven main types of stars in order of
decreasing surface temperature
– O, B, A, F, G, K, M [Oh Be A Fine Girl, Kiss Me ]
http://www.seasky.org/cosmic/sky7a01.html
Hertzsprung-Russell Diagram
This diagram plots stars by color, temperature versus
absolute (luminosity) magnitude
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 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
Spectral Classes
Star
Typ
e
Color
Approximat
e Surface
Temperatur
e
Averag
e Mass
(The
Sun =
1)
Averag
e
Radius
(The
Sun =
1)
Average
Main
Luminosit
Characteristic
y (The Sun
s
= 1)
Examples
O
Blue
over 25,000
K
60
15
1,400,000
Singly ionized
helium lines (H
I) either in
emission or
absorption.
Strong UV
continuum.
B
Blue
11,000 25,000 K
18
7
20,000
Neutral helium
lines (H II) in
absorption.
Rigel
Spica
80
Hydrogen (H)
lines strongest
for A0 stars,
decreasing for
other A's.
Sirius, Vega
6
Ca II
absorption.
Metallic lines
become
noticeable.
Canopus, Procyon
Sun,Capella
7,500 11,000 K
10 Lacertra
A
Blue
F
Blue
to
White
G
White
to
Yello
w
5,000 - 6,000
K
1.1
1.1
1.2
Absorption
lines of neutral
metallic atoms
and ions (e.g.
once-ionized
calcium).
K
Orang
e to
Red
3,500 - 5,000
K
0.8
0.9
0.4
Metallic lines,
some blue
continuum.
Arcturus,
Aldebaran
Red
under 3,500
K
0.4
0.04
(very
faint)
Some
molecular
bands of
titanium oxide.
Betelgeuse,Antare
s
M
6,000 - 7,500
K
3.2
1.7
0.3
2.5
1.3
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