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What is a star?
A cloud of gas and plasma, mainly
hydrogen and helium
 The core is so hot and dense that
nuclear fusion can occur.
 The fusion converts light elements into
heavier ones
Relative Size of Planets and Stars
Every star is different
 Tells us how much energy is being produced in the
 Can be calculated using apparent magnitude and
 Tells us the surface temperature of the star
 Determined by analyzing the spectrum of starlight
 Determines the life cycle of a star and how long it
will last
 Given relative to our sun’s mass
(ex: 0.8 solar masses)
Units of luminosity
We measure the luminosity of every day
objects in Watts.
 How bright is a light bulb?
By comparison, the Sun outputs:
 380,000,000,000,000,000,000,000,000 Watts
 This is easier to write as 3.8 x 1026 Watts
To make things easier we measure the
luminosity of stars relative to the Sun.
Units of temperature
Temperature is measured in Kelvin
 The Kelvin temperature scale is the same as
the Celsius scale, but starts from -273o.
 This temperature is known as “absolute zero”
-273 oC
-173 oC
0 oC
100 oC
1000 oC
100 K
273 K
373 K
1273 K
Kelvin = Celsius + 273
Measuring the temperature
The temperature of a star is indicated by
its color
 Blue stars are hot, and red stars are
Red star
Yellow star
Blue star
3,000 K
5,000 K
10,000 K
Colors of Stars
Stars appear
different colors
depending on the
peak wavelength of
light they emit.
 The sun, whose
data is depicted in
this graph, appears
yellow-orange to
our eyes.
Spectral Class
Boy, A Failing Grade Kills Me)
Determined by analyzing a star’s spectra
 O stars are the hottest and most massive
 M stars are the coolest and least massive
 Our Sun is a G star
Spectral Classes
The Hertzsprung Russell Diagram
We can also compare stars by showing
a graph of their temperature and
Hertzprung-Russell Diagram
What information is plotted on the H-R
 Temperature and luminosity
What are the main stages of stars?
 Main sequence, giant, supergiant, dwarf,
Do stars always stay in the same stage?
 No, they change throughout their “lifetimes”
“Birth” of Stars…
Stars (and their solar systems) are created in giant
molecular clouds of cosmic dust and gas
When gravity causes intense heat and pressure in
the core of the proto-star, it triggers fusion and a star
is “born”
 The planets and other solar system objects are formed from
the left-over materials in the proto-planetary disk surrounding
this new star
The Carina Nebula (HST photo)
Artist rendering
Mass and Stellar Evolution
The life cycle of a star is determined by its
 More massive stars have greater gravity,
and this speeds up the rate of fusion
 O and B stars can consume all of their core
hydrogen in a few million years, while very
low mass stars can take hundreds of billions
of years.
Brown Dwarf– a “Failed Star”
If a proto-star does not have enough
mass, gravity will not be strong enough
to compress and heat its core to the
temperatures that trigger fusion
 If the mass is less than 0.08 x solar
mass, it will form a Brown Dwarf
 Brown Dwarfs are not true stars, but
they do give off small amounts of light
as they cool
The Main Sequence
Longest life stage of a star
 Energy radiating away from star balances
gravitational pull inward (hydrostatic
 Main-sequence stars fuse hydrogen into
helium at a constant rate
 Star maintains a stable size as long as there
is ample supply of hydrogen atoms
 The Sun will spend a total of ~10 billion years
on the main sequence
SDO image
When hydrogen in the core starts to
run low…
In stars with masses more than 0.4 x solar
mass, fusion slows down
 Outer layers of the star begin to swell and
surface temperatures fall
 The shell surrounding the core begins to
fuse hydrogen
 Stars move out of the Main Sequence
Giants and Supergiants
“Old” stars
Helium produced through shell fusion
becomes part of the core
Star’s core temperature increases as
the more massive core contracts
The increased core temperature
causes the helium left to fuse into
carbon atoms (triple-alpha process)
A red
nearing the
end of it's
“Death” of Stars
Depends on MASS
 “Low mass stars” are less than 8 solar
 “High mass stars” are greater than 8
solar masses
The End of Low-Mass stars
All stars spend most of their “lives” on
the main sequence
 Near the end of their lives, low mass
stars (0.4 – 8.0 x solar mass) leave the
main sequence and become red giants
once they run out of hydrogen and begin
to fuse helium
Low-Mass Giants
In low mass stars (0.4 – 8.0 x solar mass)
strong solar winds and energy bursts from
helium fusion shed much of their mass
 The ejected material expands and cools,
becoming a planetary nebula (which
actually has nothing to do with planets, but
we didn’t know that in the 18th century
when Herschel coined the term)
 The core collapses to form a White Dwarf
Helix Nebula (HST photo)
White Dwarf Stars
The burned-out core of a star less than
8 x solar mass becomes a white dwarf
 The carbon-oxygen core that remains is
about the size of earth, but much more
 Theoretically, after all of the stored
energy radiates out into space, these
dead stars will become giant crystals of
carbon and oxygen (Black Dwarfs)
main sequence
star Sirius A
White dwarf Sirius B
White Dwarf
the image of
Sirius A so that
the dim Sirius
B could be
HST photo
Massive stars continue fusion
Massive stars (> 8 x solar mass) have
more gravity than low-mass stars
 When helium fusion ends, gravity collapses
the core and the temperature rises beyond
600 million K
 Fusion of the atoms from heavier elements
begins, and the star becomes a luminous
 These stars produce neon, magnesium,
oxygen, sulfur, silicon, phosphorous, and
Supernova explosions
The iron-rich core signals the impending
violent death of the massive star
 The core collapses in seconds, and the
resulting temp. exceeds 5 billion K
 Intense heat breaks apart the atomic
nuclei in the core, causing a shock wave
 After a few hours, the shockwave
reaches the star’s surface, blasting
away the outer layers in a supernova
Artist’s rendering of a Supernova
Crab Nebula (HST image)
Remnants of
a Supernova
recorded in
11 ly across
remnants are
sources of Xrays and
radio waves
Supernova 1987A
This HST picture
shows three rings
of glowing gas
encircling the site
of supernova in
February 1987.
 The supernova is
169,000 ly away
in the dwarf
galaxy called the
Large Magellanic
Neutron Stars
The cores left over after Supernovae
can become Neutron Stars-- very
small, dense balls of NEUTRONS
1 teaspoon of this would be
approximately 1 billion tons on Earth
Due to the great density it rotates very
rapidly, and some become PULSARS
Rapidly-spinning neutron
stars with very strong
magnetic fields.
 Jets of charged particles
are ejected from the
magnetic poles of the
 This material is
accelerated, producing
beams of light in all
wavelengths from the
magnetic poles.
 We can see this
“lighthouse effect” many
times per second
Computer model
Chandra X-Ray
image shows a
pulsar at the
center of the
Crab Nebula
Black Holes
Supermassive stars (>25 x solar mass)
collapse into neutron stars too massive
to be stable
 They collapse in on themselves, forming
a region of infinite density and zero
volume– a SINGULARITY at the center
of a Black Hole
 Space “curves inward” and traps all
matter and electromagnetic radiation
Stellar life cycles video