Chapter 27: Stars and Galaxies

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Chapter 27 Stars and Galaxies

• H. Thiele
• Earth Science

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Characteristics of Stars

• Size varies from 20km to 1billion km
• Color varies based on temperature
• Mass ranges between 50X less to 50X more than our
  sun
• Our sun is an average star

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Composition

• Star composition observed through a spectrometer
• Separates light into its individual colors
• Each color represents a different wavelength
• Three types of spectra
• Bright-line
• Dark-line
• Continuous
• Hydrogen is the most common element in the stars.
  Helium is the second most common

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Star Temperature and Composition
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Motion and Distance

• Motion
• Circumpolar stars that never go below the
  horizon
• Tracking circumpolar stars leaves a curved trail
• Distance
• Light Year 9.5x1012km
• Use parallax over a six-month period to determine
  distance
• Parsec Parallax second-A unit of distance
  derived from the measurement of stellar distances
  by parallax. One parsec is equivalent to roughly
  3¼ light years.
• Cepheid Variables Brighten and fade at regular
  intervals

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Circumpolar Stars
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Light Years
Because a light year is directly related to the
time light takes to travel through space, it
follows that we look out into the universe we
also look back in time. For example, in about the
year 5,350 BC, a star in the constellation of
Taurus exploded. That star was about 6,300 light
years from Earth, meaning that the light from the
explosion took 6,300 years to cross the
intervening space, and finally reached us in the
year AD 1054 that was the date Earthbound
observers finally saw the explosion that created
the Crab Nebula. This 'lag' is a consequence of
the immense distances between the stars - when we
look up at the Crab Nebula today, we see it not
as it is now, but as it was in about 4,300 BC.
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Stellar Magnitude

• Magnitude a way to measure the brightness of a
  body in the sky
• Absolute the true brightness of the object
• Would line up the stars all 10 parsecs from the
  earth
• Abbreviated M
• Apparent the brightness as seen from earth
• Difference between a 1 and a 5 is 100x
• Brightest star, Sirius, -1.46
• Brightest planet, Venus, -4.6
• Brightest in night sky, moon, -12.5
• Brightest in the sky, Sun, -26.8

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Magnitudes
Apparent Magnitude Scale
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Stellar Evolution
Nebula
Heavyweight
Lightweight
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Nebula
a cloud of interstellar gas and dust. Though they
can exist in many forms, nebulae tend to be
associated with the beginnings and endings of
stars. Nebulae form the raw material for the
formation of stars, but are also created by
material 'cast off' by stars in their final
stages of life.
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Protostars
The earliest stage in a star's development. A
protostar represents a region in which the
density of the interstellar medium is increasing
due to gravitational effects, and in the process
of collapsing to form a true star
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Fusion

• A star is born when fusion begins
• H H ? He
• Fusion is a reaction when two small nuclei come
  together to form larger nuclei
• The result is the release of a large amount of
  energy

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Main Sequence Star

• Composed mainly of Hydrogen
• The fusion reactions in its core releases
  enormous amounts of energy and forms the element
  helium
• A star in this phase of its life, when it belongs
  to the 'main sequence', runs on hydrogen 'fuel',
  which it will eventually convert entirely into
  helium.
• The lifetime of a main sequence star is heavily
  dependent on its mass.
• Our own Sun - a fairly typical main sequence star
  - has been burning hydrogen for about 5,000
  million years, and will probably continue to do
  so for another 5,000 million.
• More massive stars, though, have much shorter
  lifespans, often measured in mere hundreds of
  millions of years.

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Red Giant

• As the star's hydrogen supplies run out, its form
  changes significantly.
• Its core, now composed almost entirely of helium,
  begins to collapse upon itself, releasing further
  energy.
• This is sufficient to power an expansion of the
  matter around the decaying core, and the outer
  layers of the star swell to many times their
  original size.
• Meanwhile, the collapsing helium core reaches a
  point where fusion can proceed once again, this
  time fusing atoms of helium to produce carbon and
  oxygen.
• In this new phase, the temperature of the outer
  layers of the swollen star has cooled to give it
  a red light, and the resulting star type is known
  as a red giant.

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Red Giants
Antares
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Planetary Nebula

• When a red giant reaches the end of its life, it
  casts off its outer shell of matter while its
  core collapses into a white dwarf.
• The shell expands outwards at incredible speeds
  forming a distinctive type of nebula the
  Planetary Nebula.
• Planetary Nebulae can take the form of simple
  rings or 'bubble' shapes (like the Ring Nebula in
  Lyra), or more complex spiralling forms, such as
  that shown by the Cat's Eye Nebula in Draco.

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Planetary Nebula
Ring Nebula
Cats Eye Nebula
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Dwarfs

• The decaying remnant of a star that has exhausted
  all available sources of nuclear fusion. They
  burn very hot.
• White dwarfs are typically just a few times more
  massive than the Earth
• A white dwarf has no internal energy source, and
  will eventually lose what energy it has, becoming
  completely a dead star known as a brown dwarf.

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White Dwarfs
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H-R Diagrams
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The Path Our Sun Will Follow
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Type 1 Supernova

• Happens in binary star systems
• Need a brown dwarf and a red giant
• The brown dwarf takes gases from the red giant
  and undergoes fusion once again

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1987A
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Stellar Evolution of a Massive Star
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Super Red Giants (Super Giants)

• Work the same way as red giants, but more
  massive.
• They grow larger, cooler, and brighter
• Burn out quicker

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Supernova

• Stars which are 5 times or more massive than our
  Sun end their lives in a most spectacular way
  they go supernova.
• A supernova explosion will occur when there is no
  longer enough fuel for the fusion process in the
  core of the star to create an outward pressure
  which combats the inward gravitational pull of
  the star's great mass.
• In less than a second, the star begins the final
  phase of gravitational collapse.
• The core temperature rises to over 100 billion
  degrees as the iron atoms are crushed together.
• The repulsive force between the nuclei overcomes
  the force of gravity, and the core recoils out
  from the heart of the star in an explosive shock
  wave.
• As the shock encounters material in the star's
  outer layers, the material is heated, fusing to
  form new elements and radioactive isotopes.
• The shock then propels the matter out into space.
  The material that is exploded away from the star
  is now known as a supernova remnant.

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Neutron Stars

• Neutron stars are born during supernova
• A neutron star has roughly the mass of our Sun
  crammed in a ball ten kilometers in radius.
• Its density is therefore a hundred trillion times
  the density of water at that density, all the
  people on Earth could be fit into a teaspoon!
• Because of its small size and high density, a
  neutron star possesses a surface gravitational
  field about 300,000 times that of Earth.

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Size of a Neutron Star
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Pulsars
Crab Nebula from Supernova of 1054AD

• Radio pulsars, the first observed in the Crab
  nebula in the late 1960s
• Believed to be neutron stars that spin at
  velocities of up to 600 revolutions a second
• Send a beacon of radio waves whirling across
  space.

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Black Holes

• Black holes are formed when an extremely massive
  star dies in a supernova
• black hole is a region of space in which the
  matter is so compact that nothing can escape from
  it, not even light
• The "surface" of a black hole, inside of which
  nothing can escape, is called an event horizon.
• The matter that forms a black hole is crushed out
  of existence

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Black Hole Images
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The Constellations
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What are Constellations

• They are not real
• They are images created by people in the past
• They represent everyday objects or mythical
  people from ancient times
• They break up the sky into parts that allow us to
  find our way around

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Is the Big Dipper a Constellation?

• NO
• The Big Dipper is an asterisim.
• Asterisim
• Familiar groupings in the sky
• Can be parts of one or many constellations
• Examples big dipper, little dipper, summer
  triangle, northern cross

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What Constellations Should I Know?

• Cassiopeia
• Cepheus
• Cygnus
• Orion
• Ursa Major
• Ursa Minor

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The End