Transcript Structure of the solar system

E1: Intro to the Universe
Where are we???
• We are on planet Earth
• In the Solar System with 8 planets revolving around a
star called the Sun
• We are just one star in billions in the Milky Way Galaxy
• Our galaxy is one of a group of 40 called “The Local
Group”
• We are in a cluster of groups, called a Super cluster
• We are just one of billions of Galaxies in the universe
which is about 15 billion ly across (the age of the
universe is 15 billion years)
• Click here to get a better picture of where we are
Galaxy: a Star city
•A galaxy is a huge collection of stars held together by
gravity
•The largest galaxies contain thousands of billions of
stars
• Our own galaxy is called the Milky Way.
• 50 billion galaxies are visible to modern telescopes
•Most galaxies are thought to have a black hole in their
centre
•The nearest galaxies are 10^6 ly away and the
furthest are 10^10 ly away
Types of Galaxy
•There are 2 types of galaxy: disc
and elliptical (but also irregular)
•It is thought that the spiral arms
of galaxies arise as a result of
collisions between galaxies.
The Milky Way Galaxy
• The Milky Way is a disc galaxy with spiral arms
• It contains about 200 billion stars ( 2 x 109 stars)
•At the centre of our galaxy is a black hole with a mass of approx 100 million suns
•It is 2000 ly thick and 100,000 ly long
•The closest star in our galaxy, Proxima Centauri, is 4 ly away
M31 Andromeda
Galaxy
A disc galaxy
Distance:
about 700 kpc
The most
distant object
which can be
seen by the
naked eye
Diameter: 60
kpc (about
twice the
diameter of
the Milky
Way
Contains 400
billion stars
Whirlpool Galaxy
A disc Galaxy
with well
defined spiral
arms.
Distance
about 6
Mpc
(notice the
dwarf
galaxy
orbiting it)
M104 Sombrero Galaxy
M32 Elliptical Galaxy
Triangulum Galaxy
Whirlpool galaxy
Blackeye Galaxy
M82 Irregular Galaxy
Clusters and Superclusters
• Our Galaxy is just one of a group of galaxies
called the Local Group.
• The Local Group includes about 40
galaxies, including the Andromeda Galaxy,
The Magellanic Clouds and many dwarf
galaxies.
• The Local Group, the Virgo Cluster, and
Coma Cluster form a Supercluster about 300
million parsecs in radius.
• The distribution of galaxies in the universe is
not uniform (contrary to recent theories).
Virgo Cluster
A large cluster of
about 2500
galaxies.
Distance approx
17 Mpc
Globular Cluster
Nebulas( fuzzy regions)
•The word ‘Nebula’ comes from
the Latin word meaning ‘cloud’
•Many nebula are bright regions of
gas where star formation is taking
place
• Dark nebulae are large dusty
regions where light cannot pass
Triffid Nebula
The dark area (see
below) is illuminated
by bright stars hidden
behind the dust
Tarantula Nebula
Cone Nebula
Catseye Nebula
Horsehead Nebula
Coalsack dark Nebula
Cone Nebula
Structure of the solar system
•Planets revolve around the sun in a counterclockwise (viewed from above)
elliptical orbit.
•There is an asteroid belt (leftover debris from the sun’s formation) between
Mars and Jupiter.
•MVEM (inner/terrestrial planets) are composed of rock an metal
•JSUN (gas planets) are mostly hydrogen and helium.
Relative sizes of the planets
Relative distances of the planets http://www.youtube.com/watch?v=AUUkjWsNC9k&feature=related 2:20-2:40
Distances in terms of an Astronomical Unit (AU): M.4 V.7 E1 M1.5 J5.2 S9.5
U19.6 N30 P30-50 (sometimes Pluto is closer to the sun than
Neptune…but Pluto is no longer a planet, it is a “dwarf planet”)
• Stellar cluster: A group of stars so numerous they
appear to be a cloud. Example: Globular cluster
located in the constellation Hercules
• Constellation: A group of starts that appear to
form a pattern. Example: Orion
• Light Year (ly): The distance light travels in one
year = 10^13 km
• Parsec (pc): The distance to a point when the
parallax angle is 1” (one second of an arc (1/3600
degrees)). The distant stars appear stationary to
the Earth observer. Closer stars appear to move
throughout the year (parallax). The distance to
the imaginary star is one pc if the angle it makes
(parallax angle) is 1” (one arc second). 1pc =
3.26 ly Click on the stellar parralax animation for a good
example of this.
Pisces
Orion
North Pole
Leo
The time is mid-day. The
observer O at the equator looks
straight up and sees the sun
overhead.
O
Sun
Serpens
Pisces
Six hours later, the earth has
turned through 90o. The observer
has to look towards the horizon
(i.e. horizontally) to see the sun.
Direction of the sun
Orion
North Pole
Leo
Sun
Serpens
Pisces
From the observer’s point of view,
the sun has gone from overhead
down to the western horizon.
Direction of the sun
Orion
North Pole
Leo
Sun
Serpens
Pisces
Direction of Orion
Orion
Now, if he looks overhead, he
sees the constellation Pisces.
Orion is just appearing at the
eastern horizon.
Direction of the sun
North Pole
Leo
Sun
Serpens
Pisces
Orion
O
North Pole
Leo
After another 6 hours it is
midnight. The observer sees
Orion straight overhead.
Sun
Serpens
Pisces
Pisces has reached the western
horizon and Leo is just
appearing at the eastern
horizon.
Direction of Pisces
Orion
O
North Pole
Direction of Leo
Leo
Sun
Serpens
Pisces
Orion has moved up from the
eastern horizon to the zenith
(overhead).
Direction of Pisces
Orion
O
North Pole
Direction of Leo
Leo
Sun
Serpens
Pisces
Orion
At dawn, the observer can see
Orion at the western horizon
and the sun is rising in the east.
Leo is now overhead.
North Pole
Direction of Orion
Direction of sunrise
Leo
Sun
Serpens
Pisces
As seen from “above” the north
pole, the earth rotates
counterclockwise. To an
observer on the earth, the sky
appears to move clockwise –
i.e. from east to west.
Apparent rotation of sky
Orion
North Pole
Rotation of earth
Leo
Sun
Serpens
During the course of a night, half of the entire
sky can be seen.
Which half depends on where the earth is in
its orbit.
The other half of the sky is still there, but it is
not bright enough to see during the daytime.
Something similar happens
during the year.
Pisces
Position of Earth
in September
Orion
Serpens
Sun
Position of Earth
in December
Position of Earth
in June
Position of Earth
in March
Leo
Pisces
In December, Orion is overhead
at midnight. Pisces is setting
on the western horizon and Leo
is rising in the east.
Orion
Sun
Serpens
Position of Earth
in December
Serpens cannot be
seen because of the
daylight.
Leo
Pisces
In March, Leo is overhead at
midnight. Orion is setting on
the western horizon and
Serpens is rising in the east.
Orion
Pisces is concealed
by the glare of the
sun.
Sun
Position of Earth
in March
Leo
Serpens
Pisces
In June, Serpens is overhead at
midnight. Leo is setting on the
western horizon and Pisces is
rising in the east.
Orion
Serpens
Sun
Position of Earth
in June
Orion is out of
sight.
Leo
Pisces
In September, Pisces is
overhead at midnight. Serpens
is setting on the western
horizon and Orion is rising in
the east.
Position of Earth
in September
Orion
Serpens
Sun
Position of Earth
in December
Position of Earth
in June
Position of Earth
in March
Leo cannot be seen.
Leo
During the entire year, the Earth rotates once around
the sun in a counterclockwise direction.
The entire sky appears to rotate round the Earth once in
that period of time.
E2: Stellar radiation and stellar
types
E2: Stellar radiation and stellar
types
•
•
•
•
•
•
The sun produces energy by fusion. Squeezing two hydrogen atoms (at
high T and high P) into a helium atom.
This produces what is called “radiation pressure” which tries to expand the
star (essentially blow it up).
The star is massive enough that it has a large “gravitional pressure” which
tries to compress and crush the star.
In a stable star there is an equilibrium between the gravitational and
radiation pressure.
Luminosity (L): The total power radiated by a star in watts (J/s)
Apparent brightness (b): The amount of radiation (in watts) that hits an area
on Earth perpendicular to the incoming radiation. Measured in Watts per
square meter. Note: A star could have a greater L but a smaller b as
compared to another star if it is further away from Earth.
» b=L/(4pid^2) note: inverse square law
» If b and L are known, then the star’s distance can be
determined
Stefan-Boltzmann Law
•
Stefan-Boltmann Law: Power radiated per unit area from the surface of a
body is proportional to the fourth power of it’s kelvin T.
» L=eσAT^4
» L=Luminosity (in watts)
» e=emissivity : a number (0 to 1) that is characteristic of the
material. Very black surfaces have an e=1 which means
they are very good absorbers and emitters of radiation
(blackbodies)
» σ=Stefan-Boltzmann consant 5.67E-8 W/m^2K^4
» A=Area of object (Sphere=4xpixradius^2)
» T=Temperature in Kelvin
Example: What is the Luminosity of the Sun? The surface T=5780K and the
radius=6.96E5 km. Assume the sun is a perfect blackbody (e=1)
L=(5.67E-8)(4x 3.14x6.96E8^2)(5780^4)=3.85E26 Watts
A star has half the Sun’s surface T and 400
times its L. How many times bigger is it?
L=eσAT^4 (e=1 since they are blackbodies, A=4piR^2)
L  AT 4
L  4  R 2T 4
L
4  R 2T 4

2
4
Ls
4  Rs Ts
L
R 2T 4

2
4
Ls
Rs Ts
4
LTs
R2

2
LsT 4
Rs
4
400 LsTs
R2

4
2
Ls Ts / 2 
Rs
R2
400 x16 
 6400
2
Rs
R

Rs
6400  80
The star has a radius that is 80
times that of the Sun.
Wien’s Law
•
Wien’s Law: The surface T of a star depends on the peak wavelength (λp)
emitted by the star. T=2.90x10^-3/λp
– A blackbody (perfect emitter and absorber of radiation) emits a characteristic
radiation spectrum
The spectrum
contains a continuous
range of frequencies.
As T increases, the
peak wavelength
becomes shorter
(higher frequency)
and the total radiation
(area under curve)
increases. Notice the
Sun (6000K) peaks in
the visible part of the
spectrum. Lower T
appear reddish,
higher T appear blue.
Example: What is the surface T of a star whose λp is 966 nm?
T = 2.90E-3/966E-9 = 3000K
Stellar Spectra
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•
•
•
•
Photons emitted from a star passes through its own atmosphere.
The photons are of a continuous wavelength (see the spectrum on the
previous slide).
As the photons pass through the atmosphere of the star some of them are
absorbed. Certain wavelengths are absorbed depending on the
composition of the star’s atmosphere.
Click here to see the absorption spectrum of different gases.
If the star or galaxy is moving away from Earth, the spectrum will be shifted
toward the longer wavelengths (same as the Doppler effect for sound). This
is called “red-shift” as the light is shifted toward the red wavelengths.
If the star or galaxy is moving towards Earth, the spectrum is shifted toward
the shorter wavelengths called “blue-shift”.
Spectral classification
Spectral Classes: O, B, A, F, G, K, M. You can remember this by using “Oh
Be A Fine Girl/Guy Kiss Me”. The systems is based on the surface
temperature of the star.
O ≥ 30,000 K blue (most massive)
B 10,000–30,000 K blue to blue white
A 7,500–10,000 K white
F 6,000–7,500 K yellowish white
G 5,200–6,000 K yellow
K 3,700–5,200 K orange
M ≤ 3,700 K red (least massive)
Our star, The Sun, is a class G star and is yellowish.
Types of Stars
Single star: our Sun
Binary star: two stars that orbit each other around its common center of mass.
If you can visually see two stars they are called “visual binaries”, if not they
are “spectroscopic binary” (more on that later) and they can also be
“eclipsing binary” if one eclipses the other during its orbit.
Cepheids: stars which brighten and dim periodically (more later)
Red Giants: A star in a late stage of evolution. The outter atmosphere has
inflated and its surface T is low. It is very luminous due to its large size.
Supergiant: A more massive and larger Red Giant. More luminous than Red
Giants due to their size.
White Dwarfs: A star with a small mass (such as our Sun) in the final stage of
evolution. It is the size of Earth but with the mass of the Sun. Its surface T
is very high but its luminosity is low due to its size.
Hertzsprung-Russell (HR) Diagram
Most stars are in the region called “The Main Sequence”. Supergiants and Giants are
above The Main Sequence stars and White Dwarfs are below The Main Sequence
stars on the diagram. Notice that Supergiants are at a relatively low T but are very
luminous due to their large size. White Dwarfs are very hot but not luminous due to
their small size. This is not a linear scale. Mass of Main Sequence star depends on
the position in the diagram. O is most massive, M is least massive.
Spectroscopic Binary
When two stars can not be seen but can be inferred due to the shift in their
spectral lines. As one star (B) is moving away from Earth, its spectral lines
(or absorption lines) will be red-shifted. As the other star is moving towards
Earth (A), its lines will be blue-shifted. As they two stars are moving
horizontally with respect to the Earth, the spectral lines are normal. Then B
moves towards Earth and A moves away. The spectral lines move apart
then come together twice per revolution. See the simplified animation
below.
Eclipsing Binary Stars
•
•
When in a binary star system, one of the stars passes in front of the other,
during their orbit, relative to Earth.
The intensity of the system decreases relative to Earth when an eclipse
occurs. If the stars are of different intensity the dip in the intensity during
the eclipse depends on which star is being seen.
E3: Stellar Distances
How to determine stellar distances
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•
•
Stellar Parallax Method
Absolute and Apparent Magnitude Method
Spectroscopic Parallax Method
Cepheid Variables as “Standard Candles”
Stellar Parallax Method
•
Only can be used for stars within 100 pc because
as the star is further away the parallax angle
becomes too small to be able to measure
accurately.
• Distance 1-2 is 2 AU.
• Θ = parallax angle
• D= distance of the star
• Click here for animation of this method
Example: How far is the closest star to us, Alpha
Centauri, if the largest parallax angle measured is
0.76” (0.76 arc seconds)? Use your formula
booklet.
Θ=1AU/D (small angle approx only works with
radians)
.76” = 2.1E-4 degrees = 3.7E-6 radians
D=1AU/3.7E-6 = 271,400 AU = 4.1E16 m
=4.3 ly
Simplified formula…
d(parsec)=1/p(arc-secpond)
Apparent magnitude (m) scale
Greece 300 B.C.: Astronomers classified all the stars in the
sky according to how bright they appeared. 1 was the
brightest and 6 was just visible by eye.
Magnitude 1 is 100x brighter than a magnitude 6 star.
A difference in one magnitude is 2.51 times difference in
brightness. (The fifth root of 100 = 2.51)
Problem: What magnitude is the Sun or the full Moon? What
about stars that we can only see with telescopes?
Unfortunately we have kept this system.
m Sun = -26.73
m full Moon = -12.6
m faintest objects = 31.5
Absolute magnitude (M) scale
An apparent magnitude of 1 appears brighter than the that of an apparent
magnitude of 2 for two reasons:
It is actually a brighter star (more luminous)
It is much closer than the m=2 star (but may actually be less luminous)
Therefore if the apparent magnitude is less (appears brighter) it may not
actually be more luminous.
To correct this problem we use absolute magnitude.
Absolute magnitude of a star is the apparent magnitude of the star if it was 10
pc from us. So it is as if we have put all the stars the same distance away
from us so we can accurately compare them to each other.
The M scale is the same as the m scale (1 step is a change of 2.51 times the
brightness)
m-M=5lg(d/10) (d is in parsecs) Examples on next slide
m vs. M
apparent magnitude
m
Absolute magnitude
M
Distance Modulus
m-M
Sirius
-1.44
1.41
-2.85
Betelgeuse
0.45
-5.14
5.59
GJ 75
5.63
5.63
0.00
Star
Sirius has a smaller apparent magnitude than Betelgeuse but a larger absolute magnitude. What
does that mean?
It appears brighter from Earth but it is actually less luminous.
Sirius has a smaller apparent magnitude than and absolute magnitude. What does that mean?
That it is closer than 10 pc. As Sirius is moved to 10 pc it becomes dimmer, therefore it
must be closer than 10 pc.
Betelgeuse has as smaller absolute magnitude, what does this mean?
That it is further than 10 pc, as it it moved to 10 pc it become brighter, therefore it must
have moved closer.
GJ 75 has equal magnitudes, why?
It is already at 10 pc from Earth
Find the distance of Sirius
m-M=5lg(d/10) (d is in parsecs)
-1.44-1.41=5lg(d/10)
-2.85/5=lg(d/10)=-0.57
(antilog) 10^-.57=d/10=.27
d=2.7 pc
Find the distance of Betelgeuse.
Answer= 131 pc
Spectroscopic parallax
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•
Actually does not have anything to do with parallax
For distances too far for stellar parallax (trigonometry)
Can measure distances up to about 10Mpc
The star’s apparent magnitude (m) is measured
From its spectrum, the star’s class and type can be determined
From the HR diagram, its absolute magnitude (M) can be determined
Using the equation m-M=5lg(d/10),
the d (in parsecs) can be calculated
Gives an estimation of the d
You can also use the formula
b=L/(4pid^2)
Apparent brightness and luminosity
• Apparent brightness (b): The amount of radiation (in
watts) that hits an area on Earth perpendicular to the
incoming radiation. Measured in Watts per square
meter. Note: A star could have a greater L but a smaller
b as compared to another star if it is further away from
Earth
• Luminosity can also be determined from the HR diagram
» b=L/(4pid^2) note: inverse square law
» If b and L are known, then the star’s
distance can be determined
Example: A main sequence star has a peak wavelength at 2.4E-7 m. It’s apparent
brightness is measured to be 4.3E-9 Wm^-2. How far away is the star? (Hint: you
have to use Wien’s law to find the T and the HR diagram to find the Luminosity, Ls =
3.9E26).
T=2.90x10^-3/λp
T=12,000K
L=10^2Ls=100x3.9E26 =3.9E28
b=L/(4pid^2)
d=
L
4b
d=8.5E17m = 90ly = 28 pc
Cepheid variables
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•
•
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The outer surface of the star undergoes contraction and expansion the
gravitation and radiation pressure not in equilibrium
They are very bright and massive stars
This produces a periodic variation in its luminosity
The period (usually somewhere between one and 50 days) can easily be
determined
Cepheid variables
•
Once the period is known, its luminosity (average L) can be determined by a
known relationship (notice it is a log scale on the y-axis)
•
Once you know the luminosity, you can measure its apparent brightness
and use b=L/(4pid^2) to determine the distance
•
Cepheid variables are called “standard candles” because their luminosity is
known (like a candle’s would be) so if you can spot a Cepheid variable in a
galaxy you can determine its distance (can be used for objects very far
away)
Example 1: LMC has a period of 4.76 days
days and mean apparent magnitude of
15.56. Find its absolute magnitude and
distance.
M = -3.57
M-M=5logd-5
antilog(15.56- -3.57+5)/5=d
d = 67,000 parsecs
Example 2: Zeta Gem has a period of 10 days
and an apparent magnitude of 4.0. What is the
absolute magnitude and the distance?
M = -4.3
d = 460 pc
E4: Cosmology (the study of the
universe: its origin and future)
(includes parts of E6)
Olber’s Paradox
Why is the night sky dark?
Night Sky
If you are in a big forest and keep
walking you will bump into a tree
• If you are in a big
(infinite) universe and
keep going in a straight
line you will bump into a
star.
Newton thought the Universe was
infinitely old and static
Centre of
mass
• Gravity makes the stars
collapse towards their
centre of mass
• There must be more stars
pulling out to stop this
happening
• And more outside these
to stop them collapsing
• So the universe must be
infinitely big or it would
have collapsed
Remember Inverse Square Law
d
2d
• As light travels it spreads
out
• At a distance of 2d the
light will have spread out
over 4 times the area
• So the object will have
quarter the brightness
• Apparent brightness is
proportional to 1/d2
Look in any direction
Imagine spherical shells
surrounding Earth
d
2d
Shell at 2d has 4 times
the area, hence 4 times
the volume. It contains 4
times as many starts so
emits 4 times as much
light
3d
9 times as many
stars
Each shell has same brightness
• Shell at 2d has 4 time as
many stars
• Remember inverse
square law
• They have ¼ brightness
• So apparent brightness
of each shell is the same
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•
•
If the universe is infinite
Then there are an infinite number of shells.
Each shell has the same brightness
If you add the light from each you get an
infinitely bright sky
Here’s the Paradox
• So if the universe is infinitely big then the
sky should be bright
• But the sky is dark
• So the universe is not infinitely big
• So it should have collapsed (if it was
infinitely old)
What assumptions did Newton
make?
• 1. The universe is
infinitely old.
• 2. The universe is
static
Note that clouds of
dust blocking light
does not solve the
problem because the
clouds would heat
up and re-radiate the
energy
• 1. Hubble constant
estimates age of
universe about 15 billion
years. Light from stars
beyond 15 billion light
years hasn’t reached us
yet
• 2. The universe is
expanding. Gravity
slows the expansion and
may eventually reverse
it. (This depends on
how much matter is in
the universe.)
Confused with the IDEA?
Isaac
Newton
• discovery of the law of
gravity
• realized that gravity is
always attractive.
• Every object in the sky
attracts each other If the
universe were finite, the
attractive forces of all the
objects in the universe
should have caused the
entire universe to collapse
on itself.
Albert Einstein
• theory of gravity in the general
theory of relativity
• encountered same as Newton’s
problem
• His equations said the universe
was either expanding or collapsing.
 Created the cosmological
constant : BAD!
• “the greatest blunder of my
life”
My name is Albert!
But why didn’t the universe collapse like
Newton and Einstein’s equation suggested ?
•Because the universe had
been expanding from the
moment of its creation
and it is in a constant
state of change.
Heinrich
Olbers
• In the early 1800s, German
astronomer, argued that our
universe were finite.
• He said, “If the universe was infinite
and contained starts throughout, then
if you looked in any particular
direction, you line of sight would
eventually fall on the surface of a star.
Although the apparent size of a star in
the sky becomes smaller as the
distance to the star increases, the
brightness of the smaller surface
remains a constant.”
• Therefore, if the universe was infinite, then
the whole surface of the night sky should
be as bright as a star. However, as we know,
the sky has dark areas, proving that it is
finite.
Expanding Universe
And the Big Bang
`
Doppler Effect –
ReD ShIfTs
• Doppler effect is the result of sound increasing or decreasing in
pitch as an object moves towards or away form you. (APPLIES
TO ANY WAVES)
• If a light-emitting object is moving away from a person,
each wave of light leaves the object from a point slightly
farther away from the person than the previous wave did.
Therefore, the distance between waves, or wavelength,
that the person sees is longer than it would be if the
object were motionless.
• In visible light, the longest wavelength belongs to red
light, and shortest to the violet light
• most often used by astronomers to measure the velocity
of galaxies.
Vesto Slipher
•
•
First to encounter redshifting galaxies
• Discovered a new cosmic
riddle for astronomers of his
time.
In 1916, he observed about fifty nearby
galaxies in Lowell Observatory in
Arizona, spreading his light out using
a prism, and recorded the results onto
film
• Almost every object he observed had its
light stretched to redder colors,
indicating essentially everything in the
universe was moving away from earth.
Hi, I’m Vesto
Edwin Hubble
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•
•
•
•
discovered the existent of other
distant galaxies
In 1929, while working at the Carnegie
Observatories in Pasadena, California, he
measured the red shifts of a number of distant
galaxies and their relative distances
When he plotted the numbers, he discovered,
that the red shift of distant galaxies increased
as a linear function of their distance =
universe expands
Also discovered that the galaxies were
receding from us at a velocity proportional to
their distance.
The more distant the galaxy, the greater its
red shift, and therefore the higher the velocity;
this relationship was known as Hubble’s law.
Big Bang model
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Light from distant galaxies is red shifted (more later)
This means that the universe is expanding (the space between the galaxies
is expanding, not matter itself)
This means that the universe must have emerged from a tiny dense dot of
matter (Big Bang) 10-20 billion years ago
Time and space originated at the Big Bang
Time and space is created as the universe expands
You can not ask these questions…
“What existed before the Big Bang?”
“What is the universe expanding into?”
The answer is……………………….NOTHING
“But what is nothing?”
………………………………………..NOTHING
Any questions?
The Cosmic Background Radiation
1.
2.
Proof for the Big Bang
The light from distant galaxies is red shifted which means they are all
getting further away from us which means the universe is expanding
The Cosmic Microwave Background Radiation (CMB):
•
If the universe was a tiny dense speck it would have been very hot
•
High energy radiation would have been produced in the early stages
of the bang
•
This radiation should still be “out there”
•
As the universe expands, the radiation would be red shifted
(stretched out) or “cooled” and would now be long wave radiation
•
Penzias and Wilson discovered this background radiation
emanating from all points in the sky
•
It was “cooled” to 3 K (in the microwave region of the EM spectrum)
•
We are constantly being bombarded by microwaves left over from
the big bang, we are in a Universal Microwave Oven!
Watch this video about the discovery (chapters 1 and 2)
Models of our universe
• Closed (left) – [high mass] a universe that would
eventually stop expanding and re-collapses on itself,
possibly leading to another big bang.
• Flat (center) – [critical mass] a universe which would also
expand forever, but the expansion rate would slow to close
zero after an infinite amount of time .
• Open (right) – [low mass] a universe that will continue
its expansion forever
C
L
O
S
E
D
O
P
E
n
The fate of the universe
Notice that the age of universe depends on which type it is….
The fate of the universe depends on its density
Critical density (pc) = the density of the universe for it to be
flat.
If p> pc than the universe is open (current evidence suggests
the universe is open)
If p= pc than the universe is flat
If p< pc than the universe is closed
The problem with finding the density is that there is a lot of
“dark matter” out there, matter we can not see and measure
Dark matter can be brown dwarfs, WIMPS (weakly interacting
massive particles), neutrinos, and MACHOS (massive
compact halo objects). Basically we have no idea what it
is!
E5: Stellar Processes and Stellar
Evolution
The evolution of a star…
The evolution of a star…
Click here for a step by step guide, then follow the summary below.
The life cycle depends on the mass of the star
NEBULA
•
Gas cloud (nebula) exists with at least 8% the mass of our Sun.
•
The gas cloud collapses under it own gravitational pressure
MAIN SEQUENCE
•
Fusion start to occur in the core if there is enough mass to produce high enough
pressure and temperature (two hydrogen atoms are squeezed together to
produce a helium atom– this is commonly referred to “hydrogen burning” although
it is not burning as in combustion)
•
The star is in equilibrium (gravitational P=radiation P)
•
Fusion can continue for up to 50 billion years, this is referred to as the “Main
Sequence”, this is what our star, the Sun, is doing right now
•
More massive stars burn hydrogen quicker and live shorter but hotter lives
•
L  mass 3 4 The exact value is not known
GIANT PHASE
•
Hydrogen runs out in the core and the core contracts
•
Core starts to fuse Helium
•
T in core increases, surface expands due to increased T, surface can also burn
left over hydrogen heating it up further
•
Surface expands and cools
•
This creates a Red or Super Giant, they are very luminous but are red due to a
low surface T
Death of star
How a star dies depends on its mass
•
•
•
•
•
•
Small Mass
Outer shell rips off,
called “Planetary Nebula”
(it has nothing to do with
planets!)
Leaves hot core behind
made of carbon
Core glows white
Core can not contract
anymore due to “electron
degeneracy pressure”
Becomes a white dwarf
Ejected shell becomes
another nebula
•
•
•
•
•
•
•
•
•
Medium Mass
There is enough gravitational
pressure to fuse carbon
Fusion continues until iron is
produced
Surface collapses onto the dense
core and bounces off it and is
ejected - called a supernova –
heavier elements are formed
The core collapses and electrons
are squeezed into the protons
which produces neutrons
Core can not contract anymore
due to “neutron degeneracy
pressure”.
Becomes a neutron star
•
•
Large mass
If at the point the
neutron star is
massive enough
the gravitational
pressure
overcomes the
neutron
degeneracy
pressure and it
continues
Forms a black
hole
Chandrasekhar limit – the upper limit of a white dwarf. If the star has a mass of over 1.4
solar masses than it will continue to collapse into a neutron star and will not become a
white dwarf. Remember this is the mass AFTER ejecting its surface in the supernova
Oppenheimer-Volkoff limit – the upper limit of a neutron star. If the star is greater than
2-3 solar masses (after the supernova) it will continue to contract into a black hole
Pulsar – a rapidly spinning neutron star that emits pulses of radiation on the order of
seconds
Nucleosynthesis
Elements above iron will not be formed (binding energy per nucleon peaks
with iron…for elements above iron more energy is used than produced
when they are formed by fusion)
Stellar evolution on an HR diagram
Draw the path of a small star’s evolution once it leaves the main sequence
(hydrogen burning stops)
The star leaves the main sequence and become a giant, then a white dwarf.
E6: Galaxies and the Expanding
Universe
(and parts of E4)
Remember Edwin Hubble?
•
•
•
•
•
discovered the existent of other
distant galaxies
In 1929, while working at the Carnegie
Observatories in Pasadena, California, he
measured the red shifts of a number of distant
galaxies and their relative distances
When he plotted the numbers, he discovered,
that the red shift of distant galaxies increased
as a linear function of their distance =
universe expands
Also discovered that the galaxies were
receding from us at a velocity proportional to
their distance.
The more distant the galaxy, the greater its
red shift, and therefore the higher the velocity;
this relationship was known as Hubble’s law.
Hubble’s Law
• is a statement of a direct link between the
distance to a galaxy and its recessional
velocity as determined by the red shift. It
states that the velocity (v) of a galaxy
moving away from Earth is proportional
to the galaxy's distance (d) from Earth.
As distance increases, velocity also
increases. The constant value that
relates velocity and distance is called
Hubble's constant and is usually written
as H0.
Or mathematically like this:
•
H0 is the proportionality between recession velocity and distance of a
galaxy = the universes’ rate of expansion
• H0 identified in 1929, but its numeric value is still uncertain.
• Astronomers know that it falls between 50 and 80
kilometers per second-megaparsec. A megaparsec
is a unit of length equal to 1 million parsecs. A
parsec is 30.86 trillion km (19.18 trillion mi).
• These units are used to make redshift calculations
easier.
•
Could be represented as 1.5 × 10-15 to 3.1 × 10-15 1/s or
• How do you find the the velocity of moving
galaxies?
Astronomers know the speed of which the
galaxies are moving by their color. As I stated
when something moves toward you very, very
fast, it looks a little bit bluer, and when it
moves away it looks redder. So astronomers
can measure speeds by how red a galaxy
looks.
• How do you find the distance of the moving
galaxy?
You tell me because I already told you the answer.
So what’s my point…
• Hubble’s law could determine the age of our universe, about 12-15 billion
years old because Ho offers the needed information.
• When we know the definite value of the Hubble constant, the
future of the universe could be determined.
• Hubble’s law has an apparent linearity  our universe is
uniformly expanding  no matter which galaxy we are in,
all of the other galaxies are moving away from us  earth is
not the center of the universe and everything else is receding
from us.
• Suggests that the galaxies are not moving away through space, they
are moving away with space.
And again…….
Edwin Hubble 1889-1953
HUBBLE’S LAW
Hubble’s Law
“the farther away a galaxy is from its observer, the faster it appears to be
moving away from the observer”
v = Hod
V = redshift
Ho = Hubble’s parameter at observer
d = current distance of galaxy from observer
By knowing Hubble’s constant, physicists can determine the rate of expansion
of the universe and suggest scenarios for its ultimate fate
If v=d/t…then t=d/v….so v/d=Ho…so d/v=Ho^-1…..so t=Ho^-1. t is the age of
the universe!!!
A value for the Hubble constant is 100 km s–1
Mpc–1. Use this value to estimate the age of the
Universe in years. (1 Mpc ≈ 3 E19 km, 1 year ≈
3 E7 s)
v = Hod
t= Ho^-1
Ho= 100 km s–1 Mpc–1=100000ms1/(3E22m) = 3.33E-17
t=3E16s=1E10years or 10 billion years old
Expansion of the
universe?? : The evidence
• Edwin Hubble observed that the
wavelengths of light from distant galaxies
change and are longer than they were at
their source i.e. that the spectra of most
galaxies are red shifted (display a longer
wavelength of light)
Redshift : a change in the wavelength of light
where the wavelength observed is greater
than the wavelength emitted.
possible causes of redshift:
The Movement of the Source; the
Doppler effect
Stationary
galaxy
y
x
Galaxy
moving
y>x
The Expansion of Space
• Light will have a longer wavelength, (i.e. will
experience red shift) when it passes through
space which is expanding: expanding space will
“stretch” the light to longer wavelengths
• NB: The stretching of space is different to the
movement of the source
Space-time is stretching
therefore wavelength of
light increases
Gravitationally
bound galaxy
(not stretching)
Gravity
• Gravitational effects of large masses cause the
redshift or blueshift of light
• This is usually a very small effect and therefore
is unlikely to be the main reason for the redshift
of light in the universe
• BLACK HOLES are an exception: as light
approaches the event horizon, redshift becomes
infinite.
Measuring redshift
• Measured by comparing the absorption and
emission spectra of atoms to those light from of
the same atoms from galaxies.
• Light shifts from higher to lower frequencies
when its wavelength increases and this is
apparent when the spectra are compared.
The Cause of Redshift in the
Universe is accepted as….
X
The Movement of the Source;
the
Doppler effect (insufficient movement)
X
Gravity (insufficient strength)
The Expansion of Space
So: The Universe is Expanding!
•
Extrapolating back in time suggests that the universe was at “the
beginning”, a gravitational singularity.
present
This led to the
formulation of the
UNIVERSE
Singularity=when
universe has no
size
Big Bang Theory
The further the way the galaxy,
The more space has expanded since the time
the light left the galaxy,
The more ‘stretched’ the light is,
The greater the redshift,
The faster the light appears to be moving away
from us.
The Expansion of the Universe
A galaxies motion through space time is ignored when
measuring red shift. Distant galaxies motions are
insignificant when measuring the expansion of the
universe.
(rephrased, one component of a galaxies red shift may
be due to the galaxies motion in space-time but for
the most part the red shift is caused by the expansion
of space-time itself)
The universe is expanding equally in all directions as far
as we can tell.
The Expansion of the Universe
IMPORTANT! The expansion of the universe is the
expansion of space-time - matter itself is not ‘moving’
Planets and galaxies are gravitationally bound. Things
which are gravitationally bound do not expand,
however the space between these things does. There
for the space between galaxies is increasing and this
is what causes the redshift. (Balloon does not
account for this)
Red shift and blue shift
• So: spectra of most galaxies are red shifted
• The universe is expanding equally in all
directions
• Some galaxies are actually blue shifted,
these are nearby galaxies - their motion in
space-time is towards us (they are close
enough to feel the gravitational effects of our
galaxy the Milky Way), and this overrides the
expansion of space-time itself
Using Hubble’s Law
• Becomes more complex for very distant
galaxies.
• This is because the light we receive from
the galaxies is from the distant past (as
light has a finite speed of 3x108 m/s)
The centre of the universe
• There is no centre of the universe. Anyone
at any point in space-time will think they
are at the centre of the universe according
to their observations.
• This is because – and don’t forget! -motion
is relative
•
Each observer regards their galaxy as stationary.
Good explanation:
• “If we see a galaxy B moving away from us at 10,000 km/s, an
alien in galaxy B will see our galaxy A moving away from it at
10,000 km/s in the opposite direction. If there is another galaxy
C twice us far away in the same direction as B we will see it
moving at 20,000 km/s and the alien will see it moving at 10,000
km/s.
• A B C from A 0km/s 10,000km/s 20,000km/s from B 10,000km/s 0km/s 10,000km/s So, from the point of view of the
alien at B everything is expanding away from it, which ever
direction it looks in, just the same as it does for us”
- Usenet Physics FAQ
Distance
• For very distant galaxies, it is very difficult to
measure distance. A method called the Distance
ladder is used.
• This involves using a series of methods in a
stepwise manner to deduce the distance of the
galaxy from the Earth, “narrowing down” on the
real distance value with each step.
Is it or not?
NB. There some scientists who believe that it is
not the expansion of space-time which is the
cause of redshift, but rather something else. As
a result, some of these scientists do not accept
the expansion of the universe. Although it is
widely accepted: maybe the universe isn’t
expanding!??
Hubble’s constant
• Hubble’s constant is not actually a constant at
all, as it was recently discovered that its value
varies with the age of the universe
• It is now known as ‘Hubble’s Parameter’
• Used to determine the age of the universe 10-20
billion years old
EINSTEIN
• Einstein’s Theory of General Relativity, 10 years
before Hubble discovered Hubble’s Law,
predicted that the universe was either expanding
or contracting.
• Einstein didn’t like this idea
EINSTEIN
• So he added the cosmological constant so the
universe was static in his theory.
• After Hubble’s observations that galaxies appear
to be receding away from us, Einstein described
his inclusion of the cosmological constant as his
“Biggest Blunder!”
The cosmological constant remains a
controversial matter. Now there is increasing
talk about the existence of some
cosmological constant so maybe Einstein was
right after all! This is not yet well understood.
There is also increasing support that the
universes expansion is not slowing down but
rather, accelerating.
The Fate of the Universe
Acceleration : the continual expansion of the
universe – possible if gravitational pull of matter
can be overcome
Deceleration: if gravity is greater than expansion
the expansion of the universe will eventually
come to a halt and a reverse big bang will occur
- a big crunch
The early universe…
Temperature was 10^32 K at the Big Bang, no nuclei could exist at this
T.
As the universe expanded and cooled, light nuclei stabilized
10^-43 s: Forces are unified
10^-35 s: Strong nuclear force separates, 10^27 K
Inflation Epoch Begins, it lasts 10^-32 s, the size of the universe increases
by a factor of 10^50
10^-12 s: Forces separate, 10^16 K
10^-2 s: Nucleon form, 10^10 K
3 minutes: Nuclei of light atoms form (nucleosynthesis), 10^9 K
3x10^5 years: Atoms form
5x10^5 years: First stars and galaxies form
Matter vs Antimatter
Click here for film on antimatter, then read the summary below
Matter: The stuff we can see
Antimatter: We can not see it…so how do we know it is there?
In classic mechanics, there is not enough stuff (gravity) to hold
galaxies together in clusters, so there has to be matter out there we can’t
see holding them together
In the early universe there was a slight asymmetry to particles and antiparticles
There were slightly more particles, one in 10^9 particle-antiparticle pair
(otherwise we wouldn’t be here)
When the universe was very hot (over 10^13K) pair annihilation and pair
production occurred at the same rate (see particle physics option)
As the T dropped below 10^13 K, pair production stopped but annihilation
continued
What is left is the one extra particle per 10^9 pairs
This is the matter we see today
Resources
• www.wikipedia.com
• www.hubblesite.org
• http://math.ucr.edu/home/baez/physics/index.ht
ml (Usenet Physics FAQ)
• http://cif.rochester.edu/~shera/expansion.htm
• www.physicsforums.com
• www.hyperspace.phyastr.gsu.edu/hbase/astro/hubble.html
• Giancoli – Fifth Edition
• http://highered.mcgrawhill.com/sites/0072482621/student_view0/i
nteractives.html# animations