Download 10.2 Galaxies

Galaxies and Stars
The Milky Way
• The galaxy we live in is called The Milky Way. It is discshaped.
The Milky Way
• Looking into the night sky, in one direction we see many
sky objects since we are looking through the arms of our
galaxy. In other directions we see fewer sky objects since
we are not looking through our galaxy’s arms.
100 Billion : The Stars in The Milky Way
• The number of stars in the Milky Way is about the same
number as the number of sand grains in a full dump truck
of sand.
Astronomers Estimate 125 Billion Galaxies in Universe
• In 1925, Hubble identified the first galaxy besides the Milky
Way, the Andromeda galaxy, our nearest neighbor galaxy.
Since that time 125 billion galaxies have been identified.
An Empty Sky By the Big Dipper Handle?
• Earth telescopes showed darkness in the sky around the
Big Dipper handle. When the Hubble telecope in space
was pointed to the Big Dipper handle, it revealed a whole
universe of galaxies and sky objects where we only saw a
dark sky before.
Three Kinds of Galaxy
• Galaxies can be spiral, elliptical (egg-shaped) or irregular.
Stars
• A star is made up of extremely hot gases (millions of
degrees celsius) that give off light. The force of gravity
pulls the gases of a star together, raising their temperature
to the point where they begin the thermonuclear reaction
called fusion, the reaction where nuclei begin to join,
releasing huge quantities of energy.
More Stars than Sand Grains on Earth
• Astronomers estimate that the Observable Universe we are
a part of has about 9000 billion billion stars.
Interstellar Matter
• Space is filled with gases (mostly hydrogen gas) and dust
(only 1 % of the matter in space). A cloud of gas/dust is
called a nebula.
The Birth of a Star
• A star begins as a nebula, a matter cloud of gas/dust. Any
particle of matter (gas/dust) exerts an attraction force of
gravity on any other particle of matter so the cloud begins
to shrink as it is attracted together by forces of gravity
between all the matter particles in the cloud.
The Birth of a Star
• As gravity pulls a nebula into a smaller and smaller cloud,
temperatures rise within the nebula cloud. If the nebula is
massive enough (with enough gravity force), the
temperature rises to 10 000 000 celsius and at this
temperature, atomic fusion of atom nuclei begins,
releasing huge amounts of energy.
A Nebula with Too Little Mass?
• When a nebula has too little mass, it does not have enough
gravity to raise its temperature to 10 000 000 degrees
celsius. In this case, the nebula contracts into a giant gas
planet and never becomes a star – like jupiter, saturn,
neptune and uranus.
A Star is Born
• If the nebula cloud is massive enough, gravity within the
cloud will be large enough to shrink it and to raise its
temperature above 10 000 000 degrees celsius, a
temperature at which atomic fusion happens, releasing
huge amounts of energy in the form of all kinds of
electromagnetic radiation (gamma rays, x-rays, ultraviolet
rays, visible light, infrared rays, microwaves, radio waves)
Star Size
• Stars can be classified as low mass stars, intermediate
mass stars or high mass stars.
Low Mass Stars
• These small stars spend their long lives (100 billion years)
slowly burning their hydrogen fuel. They are called red
dwarfs which describes their lower temperatures.
Eventually they change into much smaller, hotter and
dimmer white dwarfs and burn themselves out.
Intermediate Mass Stars (Like Our Sun)
• Intermediate mass stars burn their hydrogen fuel faster,
lasting only 10 billion years. After a long stable period,
intermediate mass stars expand into red giants, expelling
much of themselves into space. Eventually after losing
much of its mass to space, the red giant collapses into a
small, dim white dwarf. The white dwarf then cools into a
black dwarf made up of mostly carbon and oxygen.
Intermediate Mass Stars (Like Our Sun)
• Our sun has about 5 billion years left before it will expand
into a red giant.
High Mass Stars
• Stars over 12 times the mass of the sun are classified as
high mass stars. These stars consume their hydrogen fuel
faster than any star, lasting only 7 billion years. They
become red giants, growing rapidly.
Stars as Factories Making New Elements
• As stars continue the process of fusion, they make heavier
and heavier elements which settle towards the centre of
the star.
High Mass Stars : Supernova Explosions
• As high mass stars continue to make heavier and heavier
elements that sink, they get layers of various elements.
Eventually they reach the stage where they make iron and
nickel, heavier elements that settle to their core. At this
stage high mass stars become unstable, collapse in on
themselves and form an explosion called a supernova.
Supernova Explosions of High Mass Stars
Supernovas: Suppliers of New Elements
• Some Supernovas are so bright that they can be seen in
the daytime sky. The extra energy of Supernova
explosions fuses the heavier elements past iron and
shoots them out into space. The elements of He to Fe on
earth are thought to have come from former stars and the
elements heavier than iron are thought to have come from
earlier supernova(s).
Neutron Stars and Black Holes
• A high mass star (after a supernova explosion) can
collapse into either a neutron star or a black hole. Stars 12
to 15 times as massive as the sun collapse from a diameter
of a million km to a diameter of just 10 km. The core of
neutron star is as hot as 100 000 000 degrees celsius and
takes trillions of years to cool.
How Can High Mass Stars Collapse?
• Stars are made up of atoms and atoms are mostly empty
space. If the large force of gravity in high mass stars pulls
the electrons, protons and neutrons of atoms together, the
atoms collapse into a space over 99.99% smaller.
Black Holes
• High mass stars over 25 times as massive as the sun
become black holes after a supernova explosion. A black
hole is even more massive than a neutron star and has so
much gravity force that it pulls light back into itself which
is why it is black (no light can escape from it).
Extreme Mass of Black Holes
• If the sun were to collapse into a black hole, its diameter of
1.39 x 106 km would collapse to a diameter of 3 km. This 3
km diameter black hole would have a mass of 1.989 x 1030
kg, the same mass as its full size. 1 cm3 of this black hole
(the size of a sugar cube) would have a mass of 1.41 x 1014
kg. A piece of this black hole the size of a grain of sand
would have a mass of 17, 580 kg (Like fitting the mass of 4
elephants into a grain of sand).
How do Astronomers Know Black Holes Exist?
• 1. Material pulled towards black holes (because of their
extreme gravity pull) emits electromagnetic radiation that
can be measured.
How do Astronomers Know Black Holes Exist?
• 2. The extreme gravity of black holes has effects on stars
and galaxies that pass close to the black holes. These
effects have been seen even though the black hole itself is
not visible.
How do Astronomers Know Black Holes Exist?
• 3. Computer models show that super-dense black holes
should distort light coming from distant stars and these
predicted distortions have actually been observed.
Two High Mass Stars in the Constellation, Orion
• The constellation, Orion (The Hunter), found in the
southern sky during winter has two high mass stars.
Betelgeuse and Rigel in Orion
• Betelgeuse and Rigel are two high mass stars found in the
constellation, Orion.
Colour and Temperature
• As an object is heated more and more, it first glows red,
then with more heat it glows yellow, then white and finally
blue. Celsius temperature + 273 = Kelvin temperature
Star Patterns for Brightness and Colour
• In 1910, a Dutch astronomer, Ejnar Hertzsprung and an
American astronomer, Henry Norris Russell, both
independently plotted star luminosity (brightness) versus
star temperature (colour) for thousands of stars. They
both observed important patterns in their diagram, a
diagram that came to be called the Hertzsprung-Russell
Diagram.
The Hertzsprung-Russell Diagram
• There are 4 regions in the H-R diagram. 90% of all stars
(including our own sun) are found along the curved line
called the main sequence. The other types of stars fit into
the white dwarf region, red giant region or super giant
region. Super giants are cool and bright while white
dwarfs are hot and dim.
The Hertzsprung-Russell Diagram
• The H-R diagram was used to determine the life cycles of
the various stars. Stars begin as nebuae, ignite due to
fusion and most settle into the main sequence. Later in life
they become red giants which then become white dwarfs.
Another
Form of
the H-R
Diagram
The Colors of Stars
• The colours of stars relate to their temperatures and the
stage of their life cycle that they are in.
How Can Astronomers Tell What Stars Are Made Of?
• Astronomers look at the spectra of stars to determine what
they are composed of.
Light Waves and Wavelength
• White light is a mixture of many colours of light : ROYGBIV (red,
orange, yellow, green, blue, indigo, violet).
• The different colours of light have different wavelengths.
• Violet light has shorter wavelength while red light has longer
wavelength.
Light Waves and Energy
• Light towards the red end of
the spectrum has longer
wavelengths and lower
energies.
• Light towards the violet end
of the spectrum has shorter
wavelengths and higher
energies.
Bright and Dark Line Spectra
• When an element is
energized (by
heating it or passing
electricity through it),
it gives off a
spectrum which is
made up of just
specific energies of
light.
• When white light
(made up of all light
energies) is passed
through a cool gas of
an element, a
spectrum with dark
lines is produced.
Element Spectra : Unique Like Fingerprints
• Each element has its own unique bright line and
absorption spectrum which identify it like different
fingerprints identify different people.
Hydrogen’s Visible Light Spectrum: Balmer Lines
• A Swiss teacher and mathematician, Johann Balmer discovered a
mathematical pattern in bright lines of hydrogen’s visible light
spectrum. The visible light lines in hydrogen’s spectrum are called the
Balmer series.
What Are Stars Made Of ?
• Spectra of stars are
used to determine
what they are made
of and what gases
surround them.
A
• A