Stars%20and%20Their%20Characteristics

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Stars and Their Characteristics
Stars and Galaxies
VOCABULARY
constellation
Hydrogen and helium are the two most abundant
elements in stars.
apparent magnitude
astronomical unit
light-year
Stars can be grouped into constellations.
parsec
luminosity
absolute magnitude
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Stars and Their Characteristics
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Stars and Their Characteristics
Stars and Galaxies
Stars differ in mass, size, and surface
temperature. Surface temperature affects the
color of stars. Apparent magnitude, luminosity,
and absolute magnitude are used to describe the
brightness of stars.
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Stars and Their Characteristics
Stars that show variation in brightness are known
as variable stars.
Distances in space are measured in astronomical
units, light-years, and parsecs.

• Light-Year The distance that light travels in one
  year, about 9.5 trillion kilometers.
• Parsec A unit of measurement used to describe
  distances between celestial objects, equal to
  3.258 light-years.

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Spectral Types
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Apparent Magnitude
Some stars appear very bright but are actually
fainter stars that lie closer to us. Similarly,
we can see stars that appear to be faint, but are
intrinsically very bright ones lying far away
from Earth.
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Apparent Magnitude
Apparent Magnitude The measure of how bright a
star appears to be to an observer on Earth.
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Stars and Their Characteristics
Luminosity The brightness of a star or the power
radiated by the star.
The luminosity is a quantity that depends on the
star itself, not on how far away it is. For this
reason a star's luminosity tells you about the
internal physics of the star and is a more
important quantity than the apparent brightness.
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What does the luminosity of a star depend on?

• Temperature (proportional to T4)
• Size (proportional to R2)
• Full blown formula? L4pR2sT4

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A star can be luminous because it is hot or it is
large (or both!).
The luminosity of an object the amount of
energy every square meter produces multiplied by
its surface area.
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Caution!

• Do not confuse the size of an object with the
  mass of an object.
• Just because an object is large in dimension does
  not necessarily mean it is also large in mass.
• For example, you can have a forty foot tall by
  three foot across marshmallow that looks large,
  but that does not mass as much as that of a
  small football sized hunk of lead.

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Absolute Magnitude
Absolute Magnitude The measure of how bright a
star would be if it were located 10 parsecs from
Earth.
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On the left-hand map of Canis Major, dot sizes
indicate stars' apparent magnitudes the dots
match the brightnesses of the stars as we see
them.
The right-hand version indicates the same stars'
absolute magnitudes how bright they would
appear if they were all placed at the same
distance (32.6 light-years) from Earth. Absolute
magnitude is a measure of true stellar luminosity.
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Inverse Square Law

• As the light from a star goes into space it fills
  a larger and larger spheres.
• The area of a sphere is given by its radius A
  4 p d2
• d is the radius of the sphere

The amount of light we receive from a star
decreases with the square of our distance from
the star Amount of light L0 / d2
Fluxamount of light
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Measuring the Distance to Stars

• Measuring distances is difficult.

The best method for measuring distances of nearby
stars is called parallax. It relies on observing
a star from two different places.
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Measuring the Distance to Stars
Measuring the Parallax Angle The parallax angle
p is illustrated in the following figure.
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Measuring the Distance to Stars
Parallax, or more accurately motion parallax
(Greek pa?a??a?? (parallagé) alteration) is
the change of angular position of two stationary
points relative to each other as seen by an
observer, caused by the motion of an observer.
Simply put, it is the apparent shift of an object
against a background caused by a change in
observer position.
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The Distance to the Stars

• We obtain a different perspective on a star by
  observing it at different times of the year.
• In 6 months the Earth has moved 2 AU away.
• (2AU 300 million km)
• The parallax method lets us measure the distance
  to stars about 1000 light years away.

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Measuring Distances Parallax

• The larger the stars distance, d, the smaller
  its parallax p.
• So distance and parallax are inversely related.
• d 1 / p

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Measuring Distances Parallax

• Most stars have a parallax angle, p, which is
  very small.
• The angle of parallax, p, is usually measured in
  arc seconds
• 60 arc seconds 1 arc minute
• 60 arc minutes 1 degree.
• Distances to stars are measured in either light
  years, or parsecs.
• 1 parsec 3.2 light years
• If a stars parallax is 1 arc second, then its
  distance is 1 parsec.

• (parsec PARallax of one arcSEC)

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Parallax Examples

• If a stars parallax is 1 arc second its distance
  is 1 parsec
• Question If a star has a parallax of 0.1 arc
  seconds what is its distance in parsecs?
• Answer d 1 / p
• d 1/ (0.1) 10 parsecs 3.2 light
  years

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Constellations A group of stars that appear to
form a pattern in the sky.
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Constellations

• Constellations are easily recognizable patterns
  that help people orient themselves using the
  night sky. There are 88 official
  constellations.
• Hundreds of the brightest stars, those visible
  with the unaided eye, were given names in ancient
  times.
• Today stars are named by their coordinates on the
  celestial sphere. This is an imaginary sphere
  surrounding Earth.

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Constellations
All stars and objects in space, can be mapped
relative to the poles and equator of the
celestial sphere. Their position north or south
of the celestial equator essentially their
latitude is called declination. Their
position east or west essentially is their
longitude, or right ascension, measured in hours,
minutes, and seconds.
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Constellations
The stars are distant objects. Their distances
vary, but they are all very far away. Excluding
our Sun, the nearest star, Proxima Centauri, is
more than 4 light years away. As Earth spins, the
stars appear to move across our night sky from
east to west, for the same reason that our Sun
appears to rise in the east and set in the
west.
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Constellations

• If observed through the year, the constellations
  shift gradually to the west. This is caused by
  Earths orbit around our Sun. In the summer,
  viewers are looking in a different direction in
  space at night than they are during the winter.

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Constellations

• Stars close to the celestial poles, the imaginary
  points where Earths north and south axes point
  in space, have a very small circle of spin.
  Polaris, Earths north pole star, will appear
  to move very little in the night sky. The farther
  from Polaris, the wider the circle the stars
  trace.

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Constellations

• Stars that make a full circle around a celestial
  pole, like those in the Big and Little Dippers in
  the northern hemisphere, are called circumpolar
  stars. They stay in the night sky and do not
  set. At the equator, there are no circumpolar
  stars because the celestial poles are located at
  the horizon. All stars observed at the equator
  rise in the east and set in the west.

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Constellations
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Life Cycles of Stars
Stars and Galaxies
VOCABULARY
main sequence
giant star
supergiants
white dwarf
nebula
planetary nebula
supernova
neutron star
pulsar
black hole
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Hertzsprung-Russell Diagram

• Ejmar Hertzsprung (1873-1967) Copenhagen
  Began his career as a Chemical Engineer. While
  working and independently at the same time
• Henry Norris Russell (1877-1957) Princeton
  Student then professor.
• A graph that separates the effects of temperature
  and surface area on stellar luminosities.
• The HR Diagram is much like the same thing as
  producing a graph of peoples height vs. weight.

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Life Cycles of Stars
Stars and Galaxies
A stars fate depends on its mass.
A star with a mass similar to the suns will
become a white dwarf.
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Life Cycles of Stars
Stars and Galaxies
A star with a mass eight or more times greater
than the suns will either become a black hole or
a neutron star.
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Properties of stars

• Color and temperature
• Hot star
• Temperature above 30,000 K
• Emits short-wavelength light
• Appears blue
• Cool star
• Temperature less than 3000 K
• Emits longer-wavelength light
• Appears red

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Properties of stars

• Color and temperature
• Between 5000 and 6000 K
• Stars appear yellow
• e.g., Sun
• Binary stars and stellar mass
• Binary stars
• Two stars orbiting one another
• Stars are held together by mutual gravitation
• Both orbit around a common center of mass

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Hertzsprung-Russell diagram

• Shows the relation between stellar
• Brightness (absolute magnitude) and
• Temperature
• Diagram is made by plotting (graphing) each
  star's
• Luminosity (brightness) and
• Temperature

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Hertzsprung-Russell diagram

• Parts of an H-R diagram
• Main-sequence stars
• 90 of all stars
• Band through the center of the H-R diagram
• Sun is in the main-sequence
• Giants (or red giants)
• Very luminous
• Large
• Upper-right on the H-R diagram

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Hertzsprung-Russell diagram

• Parts of an H-R diagram
• Giants (or red giants)
• Very large giants are called supergiants
• Only a few percent of all stars
• White dwarfs
• Fainter than main-sequence stars
• Small (approximate the size of Earth)
• Lower-central area on the H-R diagram
• Not all are white in color
• Perhaps 10 of all stars

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Hertzsprung-Russell diagram
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Birth of a Star Nebula

• Stars are born in a glowing cloud of interstellar
  gas and dust (mostly hydrogen), called a nebula.
• Gravity causes every atom and every bit of dust
  to pull on every other one and all move to the
  center, causing the protostar to collapse.
• Because the atoms move faster and faster as they
  fall toward the center, friction is created as
  they rub together and the temperature rises.

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Birth of a Star Nebula

• Heat causes the protostar to glow in with its own
  light, giving off even more light than our Sun
  even though it is not nearly as hot.
• When a temperature of about 27,000,000F is
  reached, nuclear fusion begins. This is the
  nuclear reaction in which hydrogen atoms are
  converted to helium atoms plus energy. This
  energy (radiation) production prevents further
  contraction of the star.
• The protostar is now a stable main sequence star
  which will remain in this state for about 10
  billion years. After that, the hydrogen fuel is
  depleted and the star begins to die.

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Birth of a Star Nebula
Black Widow Nebula
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Birth of a Star Nebula
Crab Nebula
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Birth of a Star Nebula
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Main Sequence Stars

• Main Sequence A star that is at the point in
  its life cycle in which it is actively fusing
  hydrogen nuclei into helium nuclei

Our sun is a main sequence star.
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Giant Stars

• A Giant Star is large star with great luminosity
  and a diameter 10 to 100 times greater than that
  of the sun.

A giant star is one of two kinds very large stars
the other being a Red giant or Supergiant
Red giants are stars of 1000 times the volume of
the Sun which have exhausted the supply of
hydrogen in their cores and switched to fusing
hydrogen in a shell outside the core.
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Supergiants

• Supergiants are the most luminous, most massive
  stars, with diameters greater than 100 times the
  diameter of the sun.

The best known example is Rigel, the brightest
star in the constellation of Orion. It has a mass
of around 20 times that of the Sun and gives out
more light than 60,000 suns added together.
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White Dwarf The remnant of a giant star that
has lost its outer atmosphere the glowing
stellar core.

• A white dwarf is what stars like our Sun become
  after they have exhausted their nuclear fuel.
  Near the end of its nuclear burning stage, such a
  star expels most of its outer material, creating
  a planetary nebula.

White Dwarf SiriusB, this white dwarf is very
hot due to high density and rapid spin.
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Neutron star The superdense remains of a massive
star that collapsed with enough force to push all
of its electrons into the nuclei they orbit,
resulting in a mass of neutrons.

• A neutron star is formed from the collapsed
  remnant a Type II, Type Ib, or Type Ic supernova.

• Pulsar general term for neutron stars that emit
  directed pulses of radiation towards us at
  regular intervals due to their strong magnetic
  fields.

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Supernova The brilliant burst of light that
follows the collapse of the iron core of a
massive star.

• Supernovae are the main source of all the
  elements heavier than oxygen, and they are the
  only source of many important elements.

X-ray image of the remnant of Kepler's Supernova
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Supernova Remanent
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Black hole The final life stage of an extremely
massive star, with a gravitational field so
intense that not even light can escape.
Black holes are areas in space where there is a
huge amount of mass in a very small space. The
gravity of this mass is so great that everything
in the area is pulled toward the mass. Even
light, with its tiny mass, is pulled into the
center of the hole. No object can escape the
gravitational pull of a black hole.
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Black hole The final life stage of an extremely
massive star, with a gravitational field so
intense that not even light can escape.

• We can't see a black hole because no light
  escapes the event. Astronomers use other ways to
  look for black holes. Since they have large
  masses and gravities, they affect the surrounding
  stars and systems. They have found evidence of
  black holes in the dark centers of galaxies and
  systems that emit large amounts of x-rays.

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How does a black hole form?

• A black hole forms when any object reaches a
  certain critical density, and its gravity causes
  it to collapse to an almost infinitely small
  pinpoint. Stellar-mass black holes form when a
  massive star can no longer produce energy in its
  core. With the radiation from its nuclear
  reactions to keep the star "puffed up," gravity
  causes the core to collapse. The star's outer
  layers may blast away into space, or they may
  fall into the black hole to make it heavier.

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Black hole The final life stage of an extremely
massive star, with a gravitational field so
intense that not even light can escape.

• Supermassive black holes containing millions to
  billions of times the mass of the sun are
  believed to exist in the center of most galaxies,
  including our own Milky Way.
• Intermediate-mass black holes, whose size is
  measured in thousands of solar masses, may exist.
  Intermediate-mass black holes have been proposed
  as a possible power source for ultra-luminous X
  ray sources.

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Galaxies and the Universe
VOCABULARY
galaxy
quasar
Galaxies contain millions or billions of stars.
There are three major types of galaxies spiral,
elliptical, and irregular.
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Galaxies and the Universe

• Normal galaxies emit as much radiation as
  that given off by their stars. Active galaxies
  emit much more radiation than that given off by
  their stars, possibly due to supermassive black
  holes at their center.
• Quasar A very distant, extremely luminous
  celestial object that scientists consider to be a
  type of active galactic nuclei.

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Galaxies and the Universe

• Galaxies are defined as large groupings of stars,
  dust, and gas held together by gravity. They vary
  greatly in size and shape. Most of the objects we
  know of in space are contained within galaxies.
  They contain stars, planets, moons, comets,
  asteroids, nebulae, dust, neutron stars, and
  black holes. Many probably even contain large
  amounts of unseen dark matter. Since most of the
  space between galaxies is thought to be empty, a
  galaxy is essentially an oasis in space.

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Galaxies and the Universe Types of Galaxies

• Spiral Galaxy - Spiral galaxies are characterized
  by a distinct flattened spiral disk with a bright
  center called the nucleus. Our own Milky Way is a
  spiral galaxy. Spiral galaxies are represented by
  the letter S and are divided into subgroups.

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Galaxies and the Universe Types of Galaxies
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Galaxies and the Universe Types of Galaxies

• Barred Spiral Galaxy - A barred spiral galaxy is
  very similar to a spiral with one important
  difference. The arms spiral out from a straight
  bar of stars instead of from the center. About
  one third of all spiral galaxies are barred
  spiral in shape.

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Galaxies and the Universe Types of Galaxies
NGC 1365 is one of the most prominent barred
galaxies in the sky. It is a supergiant galaxy
with a diameter of about 200 000 light years.
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Galaxies and the Universe Types of Galaxies

• Elliptical Galaxy - Elliptical galaxies vary in
  shape from completely round to extremely
  elongated ovals. Unlike spiral galaxies, they
  have no bright nucleus at their center.
  Elliptical galaxies are represented by the letter
  E and are divided into seven subgroups according
  to their shape.

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Galaxies and the Universe Types of Galaxies
NASA's Chandra X-ray Observatory shows hot gas in
nine different elliptical galaxies.
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Galaxies and the Universe Types of Galaxies

• Irregular Galaxy - A fourth type of galaxy is
  known as the irregular galaxy. These galaxies
  have no discernable shape or structure. Irregular
  galaxies are divided into two classes, Im and IO.
  Im class galaxies are the most common and show
  just a hint of structure. Sometimes the faint
  remnants of spiral arms can be seen. IO class
  galaxies are completely chaotic in form.

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Galaxies and the Universe Types of Galaxies

• Irregular galaxy, as observed by the Hubble Space
  Telescope.

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Galaxies and the Universe Types of Galaxies
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Paired Galaxies
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Life Cycle of a Star
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Colliding Galaxies
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Galaxy Cluster
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Galactic Research

• Edwin Hubble was the first person to figure out
  how to tell the distance of a galaxy. He used a
  type of pulsating star known as a Cepheid
  variable as a kind of galactic yardstick.

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Galactic Research

• Hubble noticed a correlation between the period
  required to complete one pulsation brightness and
  the amount of energy the star gives off. This was
  the first major breakthrough in galactic
  research. Hubble also discovered that there was a
  correlation between the red shift of a galaxy and
  its distance. This is known today as the Hubble
  constant.

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Galactic Research

• Today astronomers are able to measure the speed
  and distance of a galaxy by measuring the amount
  of redshift in its spectrum. We know that all
  galaxies are moving away from each other. The
  farther a galaxy is from us, the faster it is
  moving.

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A. Early Observations 1. Constellations
2. Apparent MagnitudeB. Distances to
StarsC. Elements in StarsD. Mass, Size, and
Temperature of Stars 1. Temperature and Color
of StarsE. Luminosity and Absolute MagnitudeF.
Variable Stars
Stars and Galaxies
Stars and Their Characteristics
VOCABULARY
constellation
apparent magnitude
astronomical unit
light-year
parsec
luminosity
absolute magnitude
Cepheid variable
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Stars and Galaxies
Life Cycles of Stars
VOCABULARY
main sequence
giant star
supergiants
white dwarf
A. The Hertzspring-Russell Diagram
nebula
B. Birth of a Star
planetary nebula
supernova
C. Death of a Star Like the Sun
neutron star
D. Death of a Massive Star
pulsar
black hole
E. Remnants of Massive Stars
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Galaxies and the Universe
Stars and Galaxies
VOCABULARY
galaxy
quasar
A. What are Galaxies?
B. Types of Galaxies
C. Active Galaxies