Download Stars and Stellar Evolution

Stars and Stellar Evolution
Unit 6: Astronomy
What are stars?
Stars = spheres of very hot gas
 Nearest star to Earth is the sun
 Constellations = group of stars named
for a mythological characters

 88
Characteristics of Stars

Star color and
temperature

Color can give us a
clue to star’s
temperature
 Very hot (above
30,000 K) = blue
 Cooler = red
 In-between (50006000 K) = yellow
Characteristics of Stars

Binary stars and stellar
mass

Binary star = stars that
orbit each other (Pair)



Because of gravity
50% of all stars
Can calculate mass of
star

Equal mass --> center
of mass halfway
between stars
 Size of orbits known -->
masses can be
calculated
Distances to Stars


Light-year = distance
light travels in a year
(9.5 trillion kilometers)
Parallax = slight shifting
in the apparent position
of a nearby star due to
the orbital motion of the
Earth



Photographs
(comparisons) --> angle
Nearest = largest angles;
distant = too small to
measure
Only a few thousand
stars are known
How bright is that star?

Brightness =
magnitude
 Apparent magnitude
= a star’s brightness
as it appears from
Earth




How big it is
How hot it is
How far away it is
Larger number =
dimmer
-1.4
How bright is that star?

Absolute magnitude = how bright a star
actually is
 Magnitude
of star if I was a distance of
32.6 light-years
 Ex: Sun = apparent magnitude: -26.7,
absolute magnitude: 5
 More negative = brighter, more positive =
dimmer
Hertzsprung-Russell Diagram

H-R diagram shows the
relationship between
the absolute magnitude
and temperature of
stars



Can also help us infer
distance, life span mass
Stars are plotted
according to their
temperature and
absolute magnitude
Interpret stellar
evolution

Birth, age, death
H-R diagram

Bright stars are near
the top and dimmer
stars are near the
bottom
 About 90% are
main-sequence
stars


Hottest = brightest
Coolest = dimmest
H-R diagram

Brightness of mainsequence stars are
related to mass



Hottest blue stars are 50
times more massive than
the sun
Coolest red stars and
only 1/10 as massive
Main-sequence stars
appear in decreasing
order

Hotter, more massive
blue stars --> cooler,
less massive red stars
H-R diagram
Betelgeuse

Red giants



Above and to right of
main-sequence stars
Size --> compare
them with stars of
known size that have
same temperature
Supergiants =
bigger

Ex: Betelgeuse
H-R diagram

White dwarfs


Lower-central part
Fainter than mainsequence stars of
same temperature
Variable Stars

Stars might fluctuate
in brightness


Cepheid variables =
brighter and fainter in
regular pattern
Nova = sudden
brightening of a star



Outer layer ejected at
high speed
Returns to original
brightness
Binary systems
How does the H-R diagram
predict stellar evolution?

Illustrate changes
that take place in a
star in its lifetime
 Position on H-R
diagram

Represents color
and absolute
magnitude at various
stages of evolution
Stellar Evolution
How stars are born, age and die
 Study stars of different ages

Life of a Star
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
Star Birth

Born in nebula =
dark, cool,
interstellar clouds of
gas and dust


Milky Way = 92%
hydrogen, 7% helium
Dense --> contracts -> gravity squeezes
particles toward
center --> energy
converted into heat
energy
Protostar Stage

Protostar = a developing star not yet hot
enough to engage in nuclear fusion
 Contraction
continues --> collapse causes
the core to heat much more intensely than
the outer layer

When is a star born?
 Core
of protostar reaches about 10 million
K --> nuclear fusion of hydrogen starts
Balanced Forces

Hydrogen fusion



Gases increase motion -> increase in outward
gas pressure
Outward pressure from
fusion balances inward
force of gravity
Becomes main-sequence
star (stable)
Main-sequence stage

Balanced between forces of gravity (trying to
squeeze into smaller space) and gas
pressure (trying to expand it)
 Hydrogen fusion for few billion years




Hot, massive blue stars deplete fuel in only few
million years
Least massive main sequence remain stable for
hundreds of billions of years
Yellow star (sun) = 10 billion years
90% of life as main-sequence star

Runs out of hydrogen fuel in core --> dies
Red Giant Stage

Zone of hydrogen fusion moves outward -->
helium core

All hydrogen in core is used up (no fusion in core)
--> still taking place in outer shell




Not enough pressure to support itself against force of
gravity --> core contracts
Core gets hotter --> hydrogen fusion in outer shell
increases --> expands outer layer --> giant body
Surface cools --> red
Core keeps heating up and converts helium to carbon to
produce energy
Burnout and Death of Stars

Low-mass stars
 1/2
mass of sun
 Consume fuel slowly --> main sequence for
up to 100 billion years
 Consume all their hydrogen --> collapse
into white dwarfs
 Eventually ends up as a black dwarf
Burnout and Death of Stars

Medium-mass stars
 Similar
to sun
 Turn into red giants --> once fuel is gone,
collapse as white dwarfs --> eventually
black dwarf
Burnout and Death of Stars
Burnout and Death of Stars

Massive stars



Shorter life spans
End lives in supernovas
= becomes 1 million
times brighter (rare)
Consumes most of its
fuel -> gas pressure does
not balance gravitational
pull --> collapses -->
huge implosion --> shock
wave moves out and
destroys the star (outer
shell blasted into space)
Formation and Destruction of
Stars
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
Stellar Remnants

All stars collapse into one of the three: white
dwarf, neutron star, or black hole
 White dwarf = remains of low-mass and
medium-mass stars


Extremely small with high densities
Surface becomes very hot



Last stage of white dwarf = black dwarf (small,
cold body)
Smallest = most massive


No energy --> becomes cooler and dimmer
Collapse of larger stars
Largest = least massive

Collapse of less massive stars
Stellar Remnants

Neutron star =
smaller and more
massive than white
dwarfs


Remnants of
supernovas
Composed entirely of
neutrons
Stellar Remnants

Supernovae = outer
layer of star is
ejected --> collapse
into hot neutron star

Pulsar = emits short
bursts of radio
energy

Remains of
supernova
Stellar Remnants

Black hole = a massive
star that has collapsed
to such a small volume
that its gravity prevents
the escape of
everything



Cannot be seen
Evidence of matter being
rapidly swept into an
area
Animation
Where did the elements of the
universe come from?

After the universe became cool enough for
atoms to form, they began to clump together
into clouds of gas

First stars made up of mostly hydrogen with a
small amount of helium


Heavier elements like iron and silicon not yet made
inside stars
More and more stars formed, became mainsequence, grew old, and died

More and more matter was fused into heavier elements
and expelled back into interstellar space by supernovas
and dying red giant stars

Eventually our sun and its planets formed from this
interstellar gas and dust
Citations
TLC Elementary School: The Moon and
Beyond. Discovery Channel School.
2004.unitedstreaming.
 Science Investigations: Earth Science:
Investigating Astronomy. Discovery
Channel School. 2004. unitedstreaming.
