Stars

1
Stars

• Stellar Astronomy

2
Light

• Light transverse waves resulting from
  oscillations in the strength of electric and
  magnetic waves
• Period 1 / f
• Velocity ? / p
• Color of light depends on ?

3
More on Light

• The speed of light (c) does not depend on the
  speed of the source but the wavelength (?) does
• Doppler effect ? shifted toward RED going away
  (allows us to measure velocity)

4
Hubbles Law

• Red shift is directly proportional to distance
  away from us
• Red shift constant distance (where constant
  is 10-10 with distance in light years)
• Result billions of stars speeding through space
  at speeds up to 80 000 miles/second
• Galaxies moving at 20 000 miles/second

5
Light Curve Information

• Parallax cannot be used as a measurement tool for
  distances like those of a galaxy (traveling at c,
  how long would it take to cross the Milky Way?)
• Astronomers can use absolute brightness to
  measure distances to remote stars
• Absolute brightness is the total amount of energy
  given out as light in 1 second

6
Hipparchus

• Brightest stars 1st magnitude
• Faintest to naked eye 6th magnitude
• Photometry apparent magnitude judged by size of
  image it makes on photographic plate (brighter
  the star the larger the image on the plate)
• Photoelectric photometry small electrical
  current produced indicating the intensity of
  light falling on sensitive surface (0.01 of a
  magnitude)

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Magnitude

• Each step in magnitude scale is 2.5 times a
  bright as previous step
• Example star is 3 magnitudes brighter 16
  times as bright (2.5 x 2.5 x 2.5 16)
• Some of Hipparchus stars should be put in
  different categories scale extended to 0, -1,
  -2 for really bright stars

8
Problems

• Astronomers can only actually measure apparent
  brightness (the energy falling 1 second on a unit
  area of telescope that is perpendicular to the
  beam)
• Absolute brightness 4 p d2 apparent
  brightness
• Use photoelectric cells to record total radiation
  received

9
Distance calculation

• d v (absolute brightness/ 4 p apparent
  brightness)
• This is a great way to determine distances if
  both brightness values are known but observation
  only gives apparent brightness. The absolute
  brightness must be inferred in an indirect way.

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Cepheid variables

• Certain stars show regular changes in brightness
• Cepheid variables each one has a definite
  period of change (light curve repeats itself over
  time some have periods as short as 3 hours but
  most have about 7 days)

11
Miss H.S. Leavitt

• In 1912 studying Cepheids in Lesser Magellanic
  Cloud (neighboring galaxy)
• Noted that periods of fluctuation was longer for
  brighter stars ? generated smooth curve

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More on Cepheid variables

• These Cepheids can be assumed to all be about the
  same distance from the earth
• Effect of distance on the brightness must be the
  same and absolute brightness must lie on similar
  smooth curve
• Universal characteristic of Cepheids ? all
  Cepheids of the same period have the same
  absolute brightness.

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To calculate distance to star

• Observe star for several days
• Read off the graph its absolute brightness
• Observe apparent brightness
• Use distance formula
• d v (absolute brightness/ 4 p apparent
  brightness)

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Problems

• Problems absorption of light by gas and dust
  reduces apparent brightness thus making star more
  distant than they really are
• Strongest absorption lies in plane of Milky Way
• Cepheid variables only work in nearby galaxies

15
Edwin Hubble

• Found that the brightest star in most galaxies
  has about the same absolute brightness
• Extends observations out to a distance of about
  10 million light years
• Galaxies are different but nearby ones are quite
  near to average so we can use them
• Most astronomers use brightness instead of
  distance anyway

16
Variable Stars

• See handout here

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Spectral Types

• See handout here
• Whitest and hottest stars (O B spectral types)
  with surface temperatures
• gt to 20 000 K
• Reddest and coldest (K M spectral types) with
  surface temperatures near 3000 K

18
Mass-Luminosity Relationship

• Except for white dwarfs and red giants, this
  relationship makes it possible to calculate a
  stars mass based on absolute magnitude
• M m 5 5 log d M absolute mag.
• m apparent mag.
• d distance in pc
• Color roughly reflects surface temperature

19
Formation of Stars

• Age of our solar system 4.6 billion years with
  age of universe about 10-12 billion years
• Stars form from clouds of interstellar matter
  contracting inward on themselves
• Pre-stellar phase diameter about that of solar
  system opaque cloud contracts due to its mass
  gravitational energy from contraction converted
  into heat (100s K)

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More star formation

• Catastrophic event occurs contraction energy
  begins to ionize certain atoms and dissociate
  clouds molecules ? results is radiation of much
  more energy toward exterior of cloud (cloud
  becomes luminous and is now considered a star)

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Result

• Energy loss creates imbalance between
  gravitational force causing contraction and
  gaseous pressure that restrains it ? cloud
  collapses on itself and raises gaseous pressure
• After 6 months the two forces are in equilibrium
  (contraction force inward and gaseous pressure
  outward) ? diameter 100 times our Sun, surface
  temp. near 4000 K and it radiates 100 times more
  energy than the sun.

22
New Star

• Continues to contract more slowly
• Luminosity decreases (central temperature
  increases due to transfer of thermal energy from
  center of star to surface by convection)
• Central temp. reaches threshold for thermonuclear
  reactions (about 50 million years) hydrogen
  fusion begins (flux of energy produced balances
  gravitational force again

23
Lifespan

• A star of 10 solar masses will only take 100 000
  years to go through these phases while a star of
  0.5 solar mass will take 100 million years
• More massive stars will always be more luminous
  than a less star a more massive star is much
  hotter than a low mass star

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Fusion Reactions

• 99 of energy produced in core by proton-proton
  nuclear reaction
• 1H 1H ? 2H e ?
• positron
  neutrino
• 2H 1H ? 3He ? (radiation)
• 3He 3He ? 4He 4He 1H 1H
• CNO cycle in hotter stars

25
Life on Main Sequence

• Stable sun remains in normal state for about 10
  billion years as hydrogen is gradually being
  converted into helium
• As process continues -gt pressure drops and
  gravitational forces dominate compression of
  central region
• Density increases (as does temperature) till
  pressure forces balance it out

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Star Aging

• More massive stars exhaust their fuel faster
  (stays on Main Sequence less time)
• Less massive stars than our Sun remain on Main
  Sequence longer
• In about 5 more billion yearsSun will have
  burned up all its hydrogen at its core ? a shell
  forms around core

27
Life Cycle of Stars

• Shell expands and temperature increases
• Reaches 5000 K where convection is main mode of
  energy transfer between shell and surface
• Entire nucleus begins to heat up till temp.
  reaches 100 million K sun will now be a red
  giant of spectral type K3 to K5 with a surface
  temperature of 3500 K

28
New Sun

• Sun is radiating more energy about 400 times
  what it does now its diameter will be 55 times
  what it is now
• Helium burning begins to take place
• Sun becomes a variable star
• At high temperatures, protons and helium fuse to
  form atoms near magnesium on periodic table

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Last Stages

• Sun is now cold and large (300 times current
  diameter) will engulf Mercury, Venus, Earth,
  and possibly Mars
• Last stage is Nova radiates as much energy in a
  few months as our Sun does in 10 000 years
• Structure becomes so unstable that matter is
  ejected it contracts due to gravity

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Near the end

• The Sun is down to the size of the earth but hot
  (dim due to its size)
• Explosions cast out many heavy elements with
  radioactive elements as by-product
• Our atoms belong to 2nd generation star systems.