Transcript The Properties of Stars

The Properties of Stars
Stellar Variety
• Stars differ in:
• Brightness
• Colour
• Mass
The Brightness of Stars
• What factors influence a star’s
brightness?
The Brightness of Stars
• What factors influence a star’s
brightness?
1. Star’s Distance from Earth
2. Size
3. Luminosity
(The closest stars to Earth are not
necessarily the brightest)
Luminosity – a measure of the total amount
of energy a star radiates per second.
Star Colour
• Not all stars are the same
colour!
• Pale blues, greens, yellows,
orange-reds
What can colour indicate about a
star?
The Brightness of Stars
• What can colour indicate about a
star?
1. Temperature
2. Chemical Composition (what the
star is made of)
(Chemical composition determined by
spectroscopy)
The Electromagnetic
Spectrum
• There are many forms of electromagnetic radiation
(from short wavelength gamma rays to long
wavelength radio waves).
• The human eye can detect visible light
(wavelengths corresponding to the colours
ROYGBV).
Visible colour and Temperature
Red - Yellow - Blue
(In order of increasing temperature)
• Red = long wavelength = low energy
• Yellow = medium energy
• Blue = short wavelength = high energy
As wavelength increases energy
decreases and with that temperature
The Relationship between
Temperature and Colour
Astronomers use the colour of a star to infer its surface
temperature.
Colour
Temperature (K)
Blue
Above 31000
Blue-white
9750 – 31000
White
7100 – 9750
Yellow-white
5950 – 7100
Yellow
5250 – 5950
Orange
3800 – 5250
Red
2200 – 3800
Compositon
• Light from a star or galaxy can be
divided into its component
wavelengths (colours) using a
spectroscope
• Spectroscopy is the analysis of light
by breaking it down into its
component wavelengths (bands of
different colours)
Composition (cont’d)
Each element emits or absorbs some
wavelengths of light leaving a unique
pattern of bands in the spectrum (like a
fingerprint or a bar code)
From these spectral patterns scientists can
determine the composition of a star
Activity• Use a small spectroscope slide
to view the line spectrum of
white light and then several
electrified gases.
The lives of stars
Birth
Life-span
Death
Origin of the Solar
System
Our theory must explain the data
1. Large bodies in the Solar System revolve
and rotate in the same direction.
2. There are two types of planets.
– small, rocky terrestrial planets
– large, hydrogen-rich Jovian planets
3. Asteroids & comets exist in certain
regions of the Solar System
4. There are exceptions to these patterns.
The Solar Nebula Theory
• The nebular theory holds that
our Solar System formed out
of a nebula which collapsed
under its own gravity.
Solar Nebula - A cloud of gas
and dust
• observational evidence
– We observe stars in the
process of forming today.
– They are always found within
interstellar clouds of gas.
newly born stars in the Orion
Nebula
Gravitational Collapse: A
Scenario
1. The solar nebula was initially somewhat spherical
and a few light years in diameter.
– very cold
– rotating slightly
2. It was given a “push” by some event.
– perhaps the shock wave from a nearby supernova
3. As the nebula shrank, gravity increased, causing
collapse.
4. As the nebula “falls” inward, gravitational energy is
converted to heat.
5. As the nebula’s radius decreases, it rotates faster
Collapse and Flattening of
the Solar Nebula
• As the nebula collapses, it heats up,
spins faster and flattens.
• The spinning nebula assumes the
shape of a disk.
• As the nebula collapses, clumps of
gas collide & merge.
Orderly Motions in the
Solar System
• The Sun formed in the very center of the nebula.
– temperature & density were high enough for nuclear
fusion reactions to begin
• The planets formed in the rest of the disk.
• This would explain the following:
– all planets lie along one plane (in the disk)
– all planets orbit in one direction (the spin direction
of the disk)
– the Sun rotates in the same direction
– the planets would tend to rotate in this same
direction
– most moons orbit in this direction
– most planetary orbits are near circular
More Support for the
Nebular Theory
• We have observed disks around
other stars.
• These could be new planetary
systems in formation.
Building the Planets I:
Condensation
• Condensation – elements &
compounds began to condense
(i.e. solidify) out of the nebula….
depending on temperature!
Building the Planets II:
Frost Line
So only rocks & metals
condensed within 3.5 AU of the
Sun… the so-called frost line.
Hydrogen compounds (ices)
condensed beyond the frost
line.
Building the Planets III:
Accretion
Accretion -- small grains stick to
one another via electromagnetic
force (imagine “static electricity”)
until they are massive enough to
attract via gravity to form
planetesimals.
Building the Planets IV:
Planetesimals
Planetesimals then will:
• combine near the Sun to form
rocky planets
• combine beyond the frostline to
form icy planetesimals
which…
• gravitationally capture H/He far
from Sun to form gas planets
Building the Planets V: Jovian
planets
and their moons
• Each gas (Jovian) planet formed
its own “miniature” solar
nebula.
• Moons formed out of the disk.
The Birth of a Star
(summary)
• All stars begin as a
nebula, a large cloud of
gas (mostly hydrogen)
and dust.
• This cloud collapses
inward under its own
gravity.
• The heat and
compression leads to a
protostar and
eventually hydrogen
fusion ignition.
Activity• Complete your worksheet
detailing the “birth” of a star.
A Newborn Star
• A star is “born” when hydrogen
fusion begins.
• Fusion creates a huge outward,
expanding pressure. This is
countered by the inward force
of gravity. Thus, a star remains
a constant size on the main
sequence of a H-R diagram.
Hertzsprung-Russell Diagram
• A graph of star
properties that
charts luminosity
against colour
(temperature).
• Working
separately, Russell
and Hertzsprung
found that when
they plotted
luminosity against
colour, the stars
fell into distinct
groups.
How to interpret the diagram.
• A star enters the diagram
somewhere in the main
sequence and then moves off
the sequence when it runs out
of fuel.
• How long it stays on the main
sequence and where it moves to
depends on size.
A Star’s Lifespan
• Depends on initial size.
• Is it a…
• Low mass star
• Medium mass star, or
• High mass star
Low mass star
• Fuses hydrogen
into helium for
hundreds of billions
of years.
• After running out of
fuel, these stars
contract due to
gravity and heat up
becoming white
dwarfs.
• They will
eventually cool to
black dwarfs.
Mid mass Stars (yellow
dwarfs/ Our Sun)
•
•
•
•
Mid-mass stars spend their mainsequence lives fusing hydrogen
into helium in their cores. (50
billion years)
When the core runs out of
hydrogen, the push outward due
to fusion decreases and gravity
contracts the star causing fusion
to begin in a shell of hydrogen
surrounding the core.
Shell-hydrogen burning takes
place at a higher rate than
hydrogen fusion did during the
stars main-sequence life.
As shell-hydrogen burning
proceeds, the core and the
burning shell of hydrogen
continue to contract, while the
outer layers of the star expand
producing a red giant.
Red giant to white dwarf
•
•
•
•
When the core of a low-mass
star reaches 100 million K,
helium fusion begins in the
core.
The burning helium core
pushes the shell of burning
hydrogen outward, lowering
its temperature and its
burning rate, and the star
contracts.
Shell helium burning later
starts and the star expands
again.
Eventually, the star sluffs off
its outer layers forming a
planetary nebula, and the
star contracts to become a
white dwarf.
High Mass Stars
• Hot, bright, blue stars live a
relatively short life (30 million
years)
• Cores complete many fusion
reactions:
• Hydrogen →helium → carbon → neon → silicon →
iron
• when fusion stops, gravity takes over and
the star collapses, recoiling into a
supernova explosion.
Supernova
• Only occurs in large
stars when they use
up their fuel.
• The star first
collapses on itself
(due to gravity), and
then explodes
outward with great
force.
• During this time, it
shines so brightly that
it can be seen during
the day.
• A supernova star will
either become a
neutron star or black
hole.
Neutron star / pulsar
• Occurs when the
star is 5X bigger
than our Sun.
• A small but
extremely dense
core spinning fast.
• The spinning
generates a
magnetic field and
the star spews out
radiation like a
lighthouse beacon.
Black hole
• Occurs when the star
is 10X bigger than our
Sun.
• Created when a
massive star collapses
due to gravity into a
single point.
• At this point, called the
singularity, pressure
and density are
infinite.
• When anything gets too
close to the event
horizon, it gets pulled
in and cannot escape.
Lifespan summary
Activity• Complete the worksheet on the
lifespan of stars.