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

4. Orion’s Sword

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

7. Hydrostatic Equilibrium
Hydrostatic Equilibrium:
Internal balance
Gravity balances pressure of hot gases in star
Holds star together
Stars spend their lives fighting the inward
crush of gravity

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

9. Protostar Diagram
This artist’s view of a
protostar displays
bipolar jets

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”

12. Life Cycles of Stars (HR Diagram)

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

17. 5,000,000,000 AD
Talk about
global
warming!

18. Red Giant Stars are HUGE! Ex:
Betelgeuse

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

21. Red Giant Nucleosynthesis
Each shell in the
red giant
produces
progressively
heavier elements
with depth

22. Betelgeuse
http://malyszp.tripod.com/stars/betelgeuse.jpg
Beetle Juice
(1989) was
inspired by the
star in Orion

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

24. Cepheid Variables
Luminous, yellow
Brightness varies from 1-70 days
Famous example, Delta Cephei
Period-luminosity relationship, used to
calculate distances

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

30. The Wonderful Star

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

32. Mira
-Feb
2007
•In late
winter,
Cetus and
Mira
appear to
be setting
in the west
after
sunset
•This
photo was
taken in
Stuttgart,
Germany

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

35. M57
Ring
Nebula:
HST
Note the
central star,
a white
dwarf

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”

38. Ziggy

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

44. Historic Supernovae
1054, Crab Nebula
1597, Tycho’s Star
1604, Kepler’s Star
Supernova 1987A
Tycho
(top) and
Kepler

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)

57. Summary