Stars and Galaxies

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Stars and Galaxies
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Astronomers of Old

• Aristotle-(384-322 BC) the great philosopher,
  proved that the Earth is spherical, and believed
  that it was at the center of the universe. His
  reason for believing this was actually quite
  scientific he knew that if the Earth revolved
  around the Sun, then we should see the stars
  shift position throughout the year. Since he did
  not have the technology to detect this shift, as
  we do today, he concluded that Earth must rest at
  the center of the universe. According to him, the
  Sun, planets, and stars were located in spheres
  that revolved around the Earth.
• Aristarchus (310-230 B.C., Greek) was the first
  to publish the idea that the Sun was actually in
  the center of the universe. His theory was
  considered far too radical. Unfortunately,
  history tends to forget that he came to this
  conclusion about 1,750 years before Copernicus
  did! He also attempted to measure the relative
  distances between the Earth and the Sun and the
  Earth and the Moon. Even though he used a
  reasonable method, his results were not very
  accurate, because he lacked the technological
  equipment to make a precise measurement.
• Hipparchus (190-120 B.C., Greek) is widely
  considered to be the greatest astronomer of
  ancient times. He compiled the first known star
  catalog to organize astronomical objects, and
  also came up with a scale to define the
  brightness of stars. A version of this magnitude
  system is still used today. He measured the
  distance from the Earth to the Moon to be 29.5
  Earth diameters (we know today that the real
  value is 30 Earth diameters). Perhaps his
  greatest discovery was the precession, or wobble,
  of the Earth's axis, which is caused by the
  gravitational pull of the Sun and Moon.

Ptolemy- (about ad 100170), astronomer and
mathematician, whose synthesis of the geocentric
theorythat the earth is the center of the
universedominated astronomical thought until the
17th century. Copernicus -Nicholas Copernicus
(1473 -1543) was a Polish astronomer, well known
for his Copernican theory. His theory stated that
sun rested near the center of the universe, and
the earth, which spun daily on its axis, revolved
annually around the sun. Now, this process is
known as helocentric, or suncentered,
system Galileo -In 1609 he heard that in Holland
a spy glass had been invented, and he was
inspired to create the first telescope, which was
as powerful as a modern day field glass.  By
December of the same year, he had built another
telescope twenty times stronger than the first,
which he was able to discovery craters on the
moon with, stars in the milky way, and the four
largest satellites of Jupiter. Newton- Newton
made a huge impact on theoretical astronomy. He
defined the laws of motion and universal
gravitation which he used to predict precisely
the motions of stars, and the planets around the
sun. Using his discoveries in optics Newton
constructed the first reflecting telescope.
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Constellations

• Make believe pictures in the sky
• Named for mythological characters or animals
• Used to find locations for stars
• Ursa Major-Big Dipper
• Ursa Minor-Little Dipper
• Polaris Last star in Little Dippers handle
• Canis Major-Big Dog
• Sirius-Dogs shoulder
• Orion-Hunter with lion
• Betelgeuse-Orions shoulder
• Ursa Major as a Guide
• Orion as a Guide

Circumpolar constellations are visible all year
round because they circle around Polaris (the
north star. The are so close to Polaris that
they never set below the horizon. Ursa
Major Ursa Minor Casseopeia Cepheus Draco NORT
HERN CIRCUMPOLAR CONSTELLATIONS
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Magnitude

• Magnitude The brightness of a star
• Absolute Magnitude- the measure of the actual
  brightness of a star or the light it gives off
• Apparent Magnitude- The amount of light that
  reaches the earth
• A bright star may appear dim if it is far away
• The apparent magnitude scale is a reverse
  scale that is, the lower the number, the
  brighter the apparent magnitude
• The 26 Brightest Stars
• A relatively dim star might appear very bright if
  it is closer
• For instance, Sirius looks much brighter than
  Rigel in the sky. Is it really??? Sirius is
  REALLY much closer to Earth than Rigel. If they
  were the same distance from earth, Rigel would
  appear much brighter
• The 26 Nearest Stars
• Experience apparent magnitude when you are riding
  in a car at night. Observe the headlights as an
  approaching car gets closer.

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Parallax

• Parallax is the apparent shift of the position of
  an object when viewed from 2 different positions
• The distance to a fairly close star can be
  measured by measuring the parallax with respect
  to the background stars
• Parallax
• A light year is a unit of measure that is used
  for stars that are farther away.
• It is equal to 300,000 m/s or 9.5 trillion km/y
• The nearest star other than the sun is Proxima
  Centauri, at 4.2 light years away (about 40
  trillion km)

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Spectra

• The color of a star indicates its temperature.
  Blue white stars are hot. Red or orange stars
  are cooler
• The spectra of a star is its visible light broken
  down into colors (wavelengths)
• As light radiated from a star passes through its
  atmosphere, some of it is absorbed by elements
• Where these elements are present, there is an
  absence of color, or a black line in the spectra
• The black lines in the spectra indicate the
  elements in the stars atmosphere

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THE SUN

• 99 of all the matter in the solar system is in
  the sun. It sustains life on earth, but in
  space, it is just another star.
• The sun is a middle sized, middle aged star with
  average magnitude that shines with a yellow light
• Like other stars, it is a huge ball of gas which
  produces energy by fusing hydrogen into helium in
  its core
• Energy produced by the fusion in the core travels
  outward by radiation and convection.
• Layers of the sun
• Photosphere-lowest layer and layer that gives off
  light. Called surface of the sun. Temp 6000K
• Chromosphere-above photosphere. Extends 2000 km
  above photosphere. Above that is a transition
  zone (2000 km to 10,000 km)
• Corona-above transition zone. Largest layer of
  suns atmosphere. Extends millions of km into
  space. Temp 2 million K
• The sun is unusual in that it is NOT in a binary
  system. Most stars occur in pairs and orbit
• each other. Many times, stars are in groups,
  called star clusters. They are gravitationally
• attracted to each other.
• NASA/Marshall Solar Physics

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Sunspots

• Sunspots Areas of the sun that appear dark
  because they are cooler than surrounding areas.
• Galileo first identified sunspots. We learned
  from them that the sun rotates (sunspots appear
  to move)
• The sun rotates faster at the equator than at its
  poles. The sunspots at the equator take 27 days
  to make a rotation 31 days at the poles.
• Sunspot Maximums-times when there are more
  sunspots than at other times. These occur every
  11 years

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Solar Flares and Prominences

• Solar prominences- caused by the suns intense
  magnetic field
• huge arching columns of gas
• blast material from the sun into space at speeds
  of 60 km/s to 1000 km/s
• Chromospheric Features
• Solar Flares- when gases near a sunspot brighten
  up suddenly and shoot gas
• outward at high speed.
• -UV light and X-rays from flares reach earth and
  cause disruptions of radio signals. Radio and
  telephone communications can be difficult because
  of solar flares. High energy particles emitted
  by flares are captured by the Earths magnetic
  field and disrupt communications equipment
• -These particles also interact with the Earths
  atmosphere near the polar regions and create
  lights called aurora. In the north, it is
  called Aurora Borealis in the south, it is
  called Aurora Australis

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Flares, Spots and Prominences
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Evolution of Stars

• The H-R Diagram-Two scientists in the early
  1900s noticed a direct relationship between the
  magnitude of stars and their temperatures. They
  developed a graph to shop this relationship
• The temperatures of the stars in Kelvins go
  across the X axis and the increasing magnitude
  goes up the Y axis. Most stars fit in a diagonal
  band from upper left to lower right. The
  following links show a diagram, and how stars
  evolve.
• The Hertzsprung-Russell Diagram
• HR Diagram Simulator
• The diagonal band of stars is called the main
  sequence. It contains hot, blue stars in the
  upper left and cooler, red, dim stars in the
  lower right. The medium stars such as our sun
  are in the middle. (Yellow, medium-temperature,
  medium-brightness). About 90 of all stars are
  main sequence stars and most cluster in the lower
  right. Of the remaining 10, some are hot but not
  bright and located in the lower left (white
  dwarfs). Others are bright but not hot and are
  in the upper right. These are called giants or
  red giants or supergiants

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Suns Energy

• People have long wondered what could cause the
  sun (and other stars) to shine
• In 1920, a scientist hypothesized that
  temperatures within the sun must be hot (duh).
  Another scientist said that with these high
  temperatures, hydrogen could fuse into helium and
  tremendous amounts of energy would be released.
  Fusion occurs in the cores of stars because only
  there is the temperature and pressure high enough
  to cause atoms to fuse.

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Life of a StarCradle to the grave

• I. Stars begin as a large cloud of gas called a
  nebula(1)
• a. Particles of gas and dust exert a
  gravitational force of each other and contract.
• b. This causes the nebula to be unstable and it
  fragments into smaller pieces.
• c. Each of these collapses to form a
  protostar(2). This is the stage in the evolution
  of a young star after it has fragmented from a
  interstellar gas cloud but before it has
  collapsed sufficiently for nuclear fusion
  reactions to begin. It might last 100,000-10
  million years.
• d. Temperatures increase to 10 million K, fusion
  begins, energy radiates outward through the
  condensing ball of gas into space, and a star is
  born
• The new star becomes a main sequence star (3)
• a. The pressure of the heat from the fusion
  going on inside balances with the gravitational
  attraction. This stage may last for 10 billion
  years

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Newborn stars are forming in the Eagle Nebula.
This image, taken with the Hubble Space Telescope
in 1995, shows evaporating gaseous globules
(EGGs) emerging from pillars of molecular
hydrogen gas and dust. The giant pillars are
light years in length and are so dense that
interior gas contracts gravitationally to form
stars. At each pillars' end, the intense
radiation of bright young stars causes low
density material to boil away, leaving stellar
nurseries of dense EGGs exposed. The Eagle
Nebula, associated with the open star cluster
M16, lies about 7000 light years away.
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The Orion Nebula
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• III. Giants (4)
• a. After the star has burned up all its
  hydrogen, there is no longer a balance between
  the inside and outside, so the core contracts and
  heat inside increases.
• b. The core is hot enough for the helium to
  fuse to form carbon. The outer layers begin to
  expand, cool and shine less brightly. The
  expanding star is now called a Giant.
• c. In about 5 billion years, our sun will be a
  giant.
• d. When its helium supply is used up, the outer
  layers escape into space and all thats left is a
  hot, dense core which contracts under gravity.
  It is now

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• A White Dwarf (5)(and when it stops shining a
  Black Dwarf
• Supergiants(5b) and Supernovas(6b)
• a. Massive stars evolve in a similar way as
  small stars except the stages happen in a more
  quick and dramatic way.
• b. Heavier elements form in the core and the
  star expands into a Supergiant
• c. Iron starts to form in the core and when
  this happens fusion can no longer occur.
• d. The core collapses and violently sends a
  shockwave outward, the outer portion explodes and
  a Supernova forms which can be billions of times
  brighter than the original star

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SN2004dj supernova has the brightness of 200
million suns
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Neutron Stars and Black Holes

• The collapsed core of a supernova shrinks to
  10-15 km in diameter and becomes a neutron star.
  The protons and electrons combine to form
  neutrons (neutron star)
• Neutron Stars are so dense that a teaspoonful of
  the matter in one would weigh TONS.
• Neutron Stars and Pulsars - Introduction
• If the remaining core is more than 2 X more
  massive than the sun, it evolves into a black
  hole, an object so dense that NOTHING can escape
  its gravity field!!
• Black Holes-If you shined a flashlight on a black
  hole, the light wouldnt shineit would just
  disappear.
• Scientists locate black holes by looking for
  X-rays around them. The matter pulled into black
  holes collides with other material and generates
  X-rays

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The Big Bang

• Big Bang Theory states that about 15 billion
  years ago, the universe began with a huge
  explosion.
• The Big Bang Theory
• We can see photos taken with satellites
  telescopes like Hubble that are light years in
  the past (since they are light years away) and
  see how the universe looked back then. These
  galaxies are in various stages of development and
  may be from when the universe was no more than 1
  billion years old.
• Explosion about 15 billion years ago
• Within fractions of a second the universe grew
  from the size of a pen point to 2000 times the
  size of the sun
• When it was a second old, it was a dense opaque
  mass of elementary particles
• After about 300,000 years, matter collected and
  formed into galaxies. As matter cooled, hydrogen
  and helium gas formed.
• More than 1 billion years after the initial bang,
  the first stars were born and light shone from
  them

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GALAXIES

• A galaxy is a large group of stars, gas and dust
  held together by gravity. The Milky Way contains
  about 200 billion stars including the sun.
  Galaxies
• Galaxies are group together into clusters.
• The Milky Way is part of the Local Group which
  contains about 30 galaxies of various types and
  sizes.
• The three types of galaxies are Elliptical,
  Spiral and Irregular. The Milky Way is a normal
  spiral galaxy and its spiral arms are composed of
  stars and gas and radiate out from an area of
  densely packed stars called the nucleus. It is
  about 100,000 light years across and the sun is
  about 30,000 light years from its center. The
  sun orbits around the center of the Milky Way
  once every 240,000 million years
• The Andromeda Galaxy is in our Local Group and is
  about 2.2 million light years away. The Andromeda
  Galaxy appears as a fuzzy blur in the
  constellation.
• Elliptical galaxies are the most common type.
  They are shaped like large three-D
  ellipsesfootball-shaped or round. Irregular
  galaxies are irregular shapes. Two of these are
  the Clouds of Magellan.

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Doppler Shift

• We know the Universe is expanding because we can
  see evidence of stars moving away from each
  other. This is known as the Doppler shift. We
  see the wavelengths of light stretched apart when
  stars move away from us. This is called a red
  shift because the dark lines in the spectrum
  shift toward the red end. A blue shift happens
  when a star moves away from us and the black
  lines move toward the blue end of the spectrum

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The schematic diagram below shows a galactic star
at the bottom left with its spectrum on the
bottom right. The spectrum shows the dark
absorption lines first seen by Fraunhofer. These
lines can be used to identify the chemical
elements in distant stars, but they also tell us
the radial velocity. The other three spectra and
pictures from bottom to top show a nearby galaxy,
a medium distance galaxy, and a distant galaxy.
The pictures on the left are negatives, of
course, so the brightest parts of the galaxies
are black. Notice how the pattern of absorption
lines shifts to the red as the galaxies get
fainter. The numbers above and below the spectra
are the measured wavelengths in nm
(nanometers). In the star which is at rest with
respect to us, or in a laboratory standard, the
line wavelengths are 393 397 nm from Ca II
ionized calcium 410, 434, 486 656 nm from H
I atomic hydrogen 518 nm from Mg I neutral
magnesium and 589 nm from Na I neutral
sodium. By measuring the amount of the shift to
the red, we can determine that the bright galaxy
is moving away at 3,000 km/sec, which is 1
percent of the speed of light, because its lines
are shifted in wavelength by 1 percent to the
red.