27.1 STARS

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Chapter 25.1 "Properties of Stars"
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It's all about the Stars...

• STARS radiate energy into space in all
  directions.
• The key to understanding the universe is to
  collect this radiation and unravel the secrets it
  holds.
• Properties of stars can be divided into three
  categories
• Color
• Temperature
• Mass

Shown here The Wolff-Rayet Star WR124 Courtesy
of the Hubble Space Telescope.
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• A stars color is directly related to its
  temperature.

Star Color
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Star Color Temperature

• Blue hottest - 35,0000C and up - 10 Lacetae
• Blue white - 10,000-30,0000C - Rigel, Spica
• White - 7,500-10,0000C - Vega, Sirius
• Yellow white-6,000 - 7,5000C - Canopus, Procyon
• Yellow - 5,000-6,0000C - Sun
• Orange - 3,500-5,0000C - Aldebaran
• Red - coolest less than
• 3,5000C - Betelgeuse, Antares

Betelgeuse one of the stars in the
constellation Orion, is red in color.
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Star Color Temperature

• Each star is assigned a letter based on its
  temperature
• ( O) Blue
• ( B ) Blue white
• ( A ) White
• ( F ) Yellow white
• ( G ) Yellow
• ( K ) Orange
• ( M ) Red
• Remember it with this anagram
• Oh Be A Fine Girl/Guy Kiss Me.

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Our Sun

• Our sun is an average star which is actually a
  good thing for us!
• More massive stars increased temperature, which
  means increased brightness, which means it would
  burn fuel faster and therefore burn out quicker.

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Binary Stars Stellar Mass
A binary star is one of two stars revolving
around a common center of mass under their mutual
gravitational attraction.
Binary stars are used to determine the star
property most difficult to calculate - its mass.
More than 50 of the stars in the universe may
occur in pairs or multiples.
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Common Center of Mass
Note For stars of equal mass, the center of mass
lies in the middle Note A star twice as
massive as its partner is twice as close to the
center of mass. It therefore has a smaller orbit
than its less massive partner.
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Measuring Stellar Distance

• Parallax is used to determine distance between
  the stars and the Earth
• As Earth circles the sun, observers are able to
  study the stars from slightly different angles.-
  during a six month period a nearby star will
  appear to shift slightly.
• The closer the star, the greater the shift.
• Calculations to any star within 1,000 light year
  can be made.

• Thumb Demo - Hold a hand up and thumb and close
  one eye and then the next

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Parallax

• The nearest stars have the largest parallax
  angles, while those of distant stars are too
  small to measure.

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Distance to the Stars

• Because space is so vast, everything is measured
  in light years distance light travels in 1 year
• Speed of light is 300,000 km/s
• Light travels 9.5 trillion km per year
• Suns light takes about 8 min to reach us
• Next closest star to earth I Proxima Centauri.
  It is 4.3 ly, nearly 300,000 times the distance
  from earth to the sun.
• Sirius, brightest from earth is 9 lya
• Polaris is 700 lya

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Stellar Magnitude

• Visibility depends on stars brightness and
  distance form earth
• 2 different scales can be used
• Based on how bright stars appear from earth
• Based on how bright the star would be if all
  stars were the exact same distance from earth

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Stellar Magnitude

• APPARENT MAGNITUDE - Brightness of a star as it
  appears from the earth
• Brightest stars lowest number
• Dimmest stars higher numbers see page 553
  figure 27.5
• Sun is brightest at -26.8
• Moon is second brightest object at -12.5
• Sirius brightest far star is -1.46
• Proxima Centauri is 11
• The faintest star that can be seen by the unaided
  eyes has an apparent mag of 6 6th mag star
• 1st magnitude star is the brightest

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Stellar Magnitude

• Three factors control the apparent brightness of
  a star as seen from Earth
• The size of the star
• The temperature of the star
• The distance of the star

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Stellar magnitudes
Stellar Magnitude

• ABSOLUTE MAGNITUDE brightness of a star as it
  would appear if located 32.6 light years from the
  earth
• Depends on how much light it gives off and how
  far it is from earth.
• If sun was 32.6 ly away 5th magnitude star
  which puts it in the middle of the range.

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Stellar Magnitude
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Classification of Stars

• Plotting the surface temperature absolute
  magnitude reveals a pattern on the
  Hertzsprung-Russell (HR) Diagram
• Majority fall in middle (like our sun-diameter is
  1,300,000km) main sequence stars
• Large, cool bright stars in upper right corner
  red giants.
• Some are so large they are called supergiants
  (diameter is greater than 2.7 million km)
• Lower left White dwarfs are hot but dim because
  of small size (about size of earth)

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HR Diagram
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Cepheid Variable
A Cepheid variable is a star whose brightness
varies periodically because it expands and
contracts it is a type of pulsating star.
Light period interval between 2 successive
occurrences of max. brightness.
Longer light period greater absolute
magnitude is in general.
Once the absolute magnitude is known, it can be
compared to the apparent magnitude of the
Cepheid. This allows astronomers to estimate the
stars distance to Earth.
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Images of a Nova Taken Two Months Apart

• A nova is a star that explosively increases in
  brightness.
• The outer layer of the star is ejected at high
  speed.
• It reaches max. brightness in a few days, remains
  bright for a few weeks then slowly returns in a
  year or so to its original brightness.

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Interstellar Matter

• We study interstellar matter because we believe
  it eventually forms into planets and moons
• A nebula is a cloud of gas and/or dust in space.
• If this interstellar matter known as a nebula is
  close to a very hot star, it will glow and is
  called a bright nebula.

• Dark Nebula- nebula that is not close enough to a
  bright star to be lit up like the Horse Head
  Nebula.

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Interstellar Matter

• There are 2 types of bright nebula
• 1. Emission Nebula (largely Hydrogen)
• Absorb UV radiation emitted by nearby hot star.
• Emit this energy as visible light.
• Conversion of UV to visible light is known as
  fluorescence.
• 2. Reflection Nebula reflects the light of nearby
  stars