This document summarizes stellar evolution and the life cycles of stars. It describes how stars are born from nebulae and discusses the stages stars pass through, including their time as main sequence stars fueled by hydrogen fusion. As stars age and exhaust their hydrogen, they evolve into red giants and later planetary nebulae, leaving behind white dwarf cores. More massive stars explode as supernovae, forming neutron stars or black holes. Key concepts covered include nucleosynthesis, variable stars, and the end states of small and massive stars.
1. Astronomy
Ch. 05: Stellar Evolution
Sirius is a main sequence
star with a small, white
dwarf companion as
displayed in this HST
photo
2. Stellar Evolution
The changes that take place in stars as
they age
Life cycle of stars
Over millions-billions of years
3. Birthplaces
Stars form out of gigantic interstellar
clouds (nebulas)
Famous Orion Nebula located 1500 light-
years away, a region of intense star
formation
Orion Nebula, located in Orion’s Sword,
appears as a greenish-cloud in telescopes
5. A Star is Born
Protostar: Star in its earliest phase of
evolution; Baby star
Proplyd: “Protoplanetary Disk”, another
term for protostars and their nebular
clumps
6. Protostars
Protostar can be surrounded by rotating
disk that will form a solar system
Nuclear fusion when 10 million K internal
temp
Bipolar jets, material erupting into space
along the axes of rotation
8. 3 Steps in Birth of a Star
1. Gravitational contraction within a cloud of
gas and dust
2. Rise in interior temperature and pressure
3. Nuclear fusion begins once internal
temperature reaches 10 million Kelvin
10. Beta Pictoris Circumstellar Disk; Orion
Proplyd
Star Beta Pictoris is surrounded by
a disk of gas and dust, the nebula
from which the star formed
This HST image shows proplyds
located in the Orion Nebula
11. Lifetimes
A function of a star’s mass and chemical
composition
High mass stars evolve fastest, low mass
stars evolve slowest
Stars move throughout the HR Diagram
as they age; i.e., their temperatures and
luminosities change over time
Main Sequence stars are “adults”
13. Why Stars Shine
Fusion: 4 hydrogen nuclei are converted
into 1 helium nuclei, excess mass is given
off as energy (heat, light)
Energy released by fusion can be
calculated using Einstein’s famous E=mc2
(E=energy, m=mass difference, c=speed
of light)
14. Old Age of Stars
Main sequence stars shine until all
available hydrogen has been converted
into helium
Then the star begins to die
The sun has been shining for about 5
billion years. It is middle-aged
15. Massive Stars
Very massive, hot, bright stars die fastest
because they use up their hydrogen
rapidly;
Massive stars spend only a few million
years as main sequence stars. Ex: Rigel,
hot, blue star in Orion
Least massive, cool, dim stars such as
red dwarfs can last billions of years
16. Red Giants
Red giants are senior citizen stars
After hydrogen fuel in core runs out, star
swells into a giant
Red giants are cooler and redder, they
leave main sequence and enter upper
right corner of HR Diagram
Examples include Antares and
Betelgeuse
Our sun in the future
19. Nucleosynthesis
The creation of elements in stars
Main sequence hydrogen fusion
Helium Fusion
When red giant stars achieve 100 million K
internally, helium is converted into carbon
(helium flash)
20. Red Giant Nucleosynthesis
Red giant stars form internal shells that
produce progressively higher elements
Large red giants can create heavier
elements such as oxygen, aluminum, and
calcium
Stars can produce elements up to iron
before exploding
Elements higher than iron are produced in
the brief explosions of stars
23. Variable Stars
Stars that change brightness in regular or
irregular cycles
Pulsating Variable Stars
Move back and forth between the main
sequence and red giant region of the HR
diagram for unknown reasons
Such stars vary in light output, expand and
contract
Ex: Cepheid variables
25. Cepheids: Distance Markers
Period-Luminosity Relationship: For Cepheids,
the longer the period of brightness change, the
greater the luminosity
This relationship enables the calculation of
absolute magnitude.
Compare absolute to apparent magnitude to
estimate distance
Good to about 10 million light-years (closest
galaxies)
26. Delta Cephei Light Curve
Delta Cephei
has a roughly
5-day cycle of
brightness
27. Delta Cephei Star Map
Delta
Cephei Delta is a naked eye star in
Cepheus
28. RR Lyrae Variables
Named for star RR in Lyra
RR Lyrae stars are pulsating blue-white
giants with periods less than 1 day
Distance markers out to 600,000 ly
29. Long Period or Mira Variables
Mira in Cetus, pulsating red giants
Periods between 80-100 days from dim to
bright
Mira means the “Wonderful” star,
proclaimed after its recognition in 1638
Mira first variable star discovered
Mira brightest every 333 days
31. Mira Light
Curves
•The diagram shows the
changing brightness cycle
of Mira
•Each strip represents 15
years, and each dot
represents a magnitude
estimate
•Most of these estimates
were made by amateur
astronomers who do this
work as part of their hobby
33. Death of Stars
Depends on mass
Small stars, up to 1.4 times the sun’s
mass, go to planetary nebula stage, fade
away into dwarf stars
Larger stars (8 times the sun’s mass)
explode
34. Planetary Nebulas
Type of nebula ejected by dying stars
Size 0.5-1 ly in diameter
Leaves behind a white dwarf star in center
Famous examples: M57, the Ring Nebula
in Lyra; NGC6543, Cat’s Eye in Draco
Ring
Nebula
36. Cat’s Eye: Amateur & HST
•The Cat’s
Eye Nebula in
Draco
•Planetary
nebulas can
reveal bizarre
and complex
shapes
37. White Dwarfs
Remains after planetary nebula stage
Star can no longer resist inward pull of gravity,
squeezes down into an object about the size of
the earth
Very dense, you would weigh 35,000 times
greater if you could somehow stand on a white
dwarf
A teaspoon of white dwarf matter would weigh
over a ton
Can brighten suddenly as “novas”
39. Black Dwarfs
Gradually, the white dwarf cools, turns
dull red, and shines its last energy into
space
White dwarf becomes a black dwarf,
corpse of a star
Our sun’s ignominious end
40. Life Stages of a Sun-Like Star
1. Protostar, gravitational contraction of gas and dust
2. Stable, main sequence star shining by hydrogen fusion
3. Evolution to red giant when helium core forms
4. Red giant, shining by helium fusion
5. Variable star, formation of carbon core
6. Planetary nebula, outer atmosphere of star ejected into
space
7. White dwarf, mass packed into a star about the size of
the earth
8. Dead corpse, black dwarf in space
41. Exploding Stars
Stars 8 or more times greater than our
sun explode
Supernova: A gigantic stellar explosion
(exploding star)
Core of star begins fusing elements up to
iron
Star collapses and explodes violently
Supernovas can be seen in other
galaxies, sometimes even in small
telescopes
42. Supernovas
100 billion times the sun’s luminosity for a
brief moment
Brief instant fuse chemical elements
higher than iron on the periodic table
43. M51 Supernova (SN2005cs)
Where’s the supernova?
A supernova
appeared in
M51, a bright
galaxy in
Canes
Venatici, in
2005
This
supernova
was visible in
large amateur
telescopes
45. Supernova 1987A
•SN1987a appeared in the
Large Magellanic Cloud, a
small satellite galaxy of our
Milky Way that is visible
from the Southern
Hemisphere
•The supernova was
positioned near the
Tarantula Nebula, the large
red glow in left center of
the image to the right
Below: Large
Magellanic
Cloud; Right:
March ’97 Time
46. 1054 Supernova, Chaco Canyon, Crab
Nebula
This rock art in
New Mexico may
depict the 1054
supernova
The Crab Nebula (M1) is the
remnant of the 1054 supernova
47. M1 StarMap (Taurus)
The Crab Nebula is visible as a
glowing patch of light in small
telescopes, it is the first object
in Messier’s list (M1) http://www.eurekalert.org/images/release_gra
Ecliptic
48. Neutron Stars
From explosions of massive stars
Neutron star, a type of star more massive
than the sun but squeezed into a ball 10
miles across
Incredibly dense
49. Pulsars
Pulsars are rotating neutron stars
Pulsars can send sharp, strong signals
towards earth
Originally thought to be alien signals
(LGM) when first discovered in the 1960’s
Pulses range from milliseconds-4 second
Pulsar found at center of the Crab Nebula
50. Black Holes
Really massive stars can explode and
collapse into black holes
Black holes are denser than neutron stars
Represent the mass of entire star shrunk
into zero-radius object
Gravity is so immense, even light can’t
escape
51. Black Hole Terms
Event Horizon: Boundary of no return
where no light or matter will escape
Singularity: Center of black hole, a point
of infinite density where the pull of gravity
is infinitely strong
52. Anatomy of a Black Hole
Simulated black hole, the
intense gravity distorts
the light of stars in the
background
53. Black Hole Candidates
Cygnus X-1, intense X-ray source located
8000 ly away in Cygnus
Believed to be an eclipsing binary star
(two stars orbiting), period 5.6 days, with
unseen companion
Massive black holes may exist at the
center of the Milky Way and other galaxies
54. Cygnus X-1
•Cygnus X-1 is located in
Cygnus or the Northern Cross
•It is not visible in a telescope,
but you can identify its general
area using a star map
55. Center of Milky Way: Sgr A
Sagittarius A is a radio source at the
center of the Milky Way and likely marks
the location of a black hole
Sgr A
56. Stellar Evolution Summary
Sun-like stars
Protostar
Main sequence star
(yellow star)
Red giant
Planetary nebula
White dwarf
Black dwarf
Massive Stars
Protostar
Main sequence (blue
star)
Red supergiant
Supernova
Neutron star or black
hole (depending on
mass)