Stars
•Section
8.3
Introduction

Star - Luminous sphere of gas held
together by gravity

There is only one star in our
solar system: The Sun

Binary or Multiple-Star systems:
two or more stars bound together by gravity
 orbit a common centre of mass
 may appear as one star

Albireo, a binary star system that can be
easily distinguished with a regular telescope
Distances Outside Our Solar System

distance inside our solar system:
the astronomical unit (AU)
 150 000 000 km


outside: the light-year (ly)
 equals the distance travelled by light in one year
speed of light = 300 000 km/s
 1 ly = 9.5 x 1012 km (9 500 000 000 000 km)!!

Some Famous Stars
Polaris
 the North Star
 430 ly away
 actually a trinary star system
 used as a navigational tool
Sirius
 the Dog Star
 brightest star in the night sky
 binary system
 6.8 ly away
 significance: signal of changing seasons

ancient Egyptians, Greeks, Polynesians
Alpha Centauri
 4.3 ly away - closest star system to Earth!
 3rd brightest star in the night sky
 trinary star system

Alpha Centauri A, B, and Proxima Centauri
Arcturus
 Alpha Bootis
 37 ly away
its light was
used to start
the Chicago
World’s Fair in
1933!
Betelgeuse
 650 light years away
 one of the brightest stars in the sky
 waiting for a supernova…
Vega
 25 light years away
 first star other than the Sun
to be photographed
 was the North Star
ca. 12 000 BCE

will be again ca. 13 727 CE!
Outline: Part One
1.
Properties of stars




2.
Brightness
Temperature
Composition
Size and mass
Hertzsprung Russell diagrams

plotting star properties + defining classes
Property #1: Brightness aka “Magnitude”
STAR
BRIGHTNESS
Luminosity
Distance from
Earth
The amount of energy
produced by a star
each second (J/s)
Temperature
hotter = more
energy
closer = brighter
Size
bigger = more
energy
What combination of
temperature, size, and
distance will make a star
appear the brightest?
Two ways of expressing magnitude
Apparent magnitude vs. Absolute magnitude
Apparent Magnitude = measure
of a star’s brightness, as
observed from Earth.
Absolute magnitude = The
apparent magnitude a star would
have, if it were located 33 ly from
Earth.
Lower number = Brighter star
Distance is no longer a factor 
More accurate measure of a star’s
luminosity.
Lower number = More luminous
star
Two ways of expressing magnitude
describes how
much light a
star produces
describes how
much energy a
star produces
Apparent Magnitude
measure of a star’s
brightness, as seen
from Earth
Absolute Magnitude
The brightness a star
would have, if seen
from a distance of
32.6 ly
Whatever the measure,
lower number = brighter star
Which star will appear brighter from Earth?

Star A (apparent magnitude = 4.8) or
Star B (apparent magnitude = -0.86)?
Star B
Which star produces more energy/second?

Star C (absolute magnitude = -2.5) or
Star D (absolute magnitude = 4.0)?
Star C
Which star will appear brighter from Earth?

Star E (apparent magnitude = 1.5) or
Star F (absolute magnitude = 3.0)?
Can’t tell!
Property #2: Temperature
 The
colour of a star is used to infer its
temperature
 Red
is relatively cool,
Yellow is hot, and
Blue is extremely hot
Property #3: Composition (what it’s made of)

Starlight can be analyzed using a spectroscope to
infer which gases make up the star.

complete visible spectrum (all colours):

light emitted by different elements:
If a star contains hydrogen and helium, what
would its spectrum look like?
Hydrogen + Helium
Property #4: Size and Mass

The size of a star can be inferred indirectly from its
luminosity and temperature.
e.g. “Red Giant” star
 luminosity: very bright
 temperature: very cool

There is a huge range
of star sizes!
These stars must be very large to produce
the amount of light that they do
Star masses are compared to the mass of the Sun.
 Sun’s mass= 1 solar mass = 2 x 1030 kg
Example: A star with a solar mass of 0.50 has a
mass that is smaller than the Sun
(specifically 50% the mass of the Sun)
Hertzprung-Russell diagram:
plot of luminosity vs. temperature
“Main Sequence”
Use the H-R diagram …
a)
Is the Sun hotter or cooler than most main
sequence stars?
cooler
b)
Arcturus is about the same temperature as the Sun, but
it is much more luminous – why?
Arcturus is bigger than the Sun
c)
What type of stars are the brightest stars in the sky?
Red Supergiants
HOMEWORK: Properties of Stars
Pg. 342 #1-4
 Worksheet: Hertzsprung-Russell diagram

forms within
a nebula
The Evolution of
a Star:
Birth to Death
Low
Mass
Intermediate
Mass
High
Mass
Birth of a Star
 All stars begin
(singular: nebula)

as nebulae
cloud of dust and gas
 When
a nebula acquires enough density,
gravity can begin to act on it.

Matter is pulled inwards, forming a protostar.
The protostar continues gathering matter.
 The temperature and pressure in the core begins to rise.
 Once critical temperature (10 million °C) is reached:

nuclear fusion begins.

nuclear fusion provides the energy for the star throughout its life
2
1
3
1
H
H
http://www.youtube.com/watch?v=mzE7VZMT1z8
(8:57)
4
2
He
Early and Middle Life

star achieves a balance of
outward and inward forces
balance results in a stable state
 main-sequence stars are in this state
 size is determined by this balance


star spends most of its life
in stable state
Hot gas
pushes outer
layers away
from core
Gravity pulls
inner layers
towards core
Entering Old Age…

eventually stars begin to run out of hydrogen
to fuse:


this is the beginning of old age!
amount of time it takes, and ultimate fate, depend
on the star’s mass
The Death of a Star (8:17)
Old Age: Low-Mass Stars

consumes hydrogen slowly

lives a long time:
as long as 100 billion years
The end of a low mass star’s
life is not very dramatic!
 loses mass over lifetime,
and fades  white dwarf star



very hot
takes billions of years to cool down
theorized: eventual fate = black dwarf
Old Age: Intermediate-Mass Stars
(like our Sun)

As hydrogen starts to run out, fusion slows:

inner layers contract due to gravity, and heat up

outer layers expand and cool  Red
Giant star
 Outer
layers eject gas and
dust into space.

The Red Giant gradually
loses its outer layers, and
becomes a white dwarf.
matter ejected
from outer
layers
core: will
become a
white dwarf
Helix planetary nebula
(700 ly away)
Old Age: High-Mass Stars

Hydrogen is consumed rapidly.
 heavier stars burn out more quickly

star swells into a Red

helium gets fused into heavier elements

Supergiant
eventually Iron is produced
in the core
26
Fe
iron
55.85
Death: Red Supergiant  Supernova

iron can’t undergo
nuclear fusion  fusion stops

star collapses due to gravity

outer layers explode outwards, in
what is called a supernova.


produces a series of shock waves,
and a nebula of gas and dust
a core is left behind
Crab Nebula
remnants of 1054
supernova
Two possible fates: Neutron Star vs. Black Hole
Supernova
Neutron Star
Black Hole
From a relatively
small supernova core.
From a relatively
large supernova core.
Very dense concentration of
neutrons
Extremely dense quantity of
matter in space from which no
light or matter can escape.
Doesn’t emit visible light, but emits
radio waves
Evidence of black holes?

No light can escape, therefore can’t be seen.


Evidence comes from indirect observations of the
interaction between matter and black holes
Evidence:
X-rays emitted by matter as
they get pulled into black holes
 motions of stars located near
suspected black holes

Artistic rendering of the interaction between a
black hole and its companion star, v404 Cygni
forms within
a nebula
Low
Mass
Intermediate
Mass
High
Mass
HOMEWORK: Stellar Evolution
Pg. 347 #5, 6
 Pg. 349 #1, 3, 4
