The HERTZSPRUNG RUSSEL DIAGRAM

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The HERTZSPRUNG RUSSEL DIAGRAM
A classification system for stars. It organizes
stellar information by making a correlation
between a stars luminosity (absolute magnitudes)
and a its temperature. Named after Ejnar
Hertzsprung (Danish) and Henry Norris Russell
(United States) Developed in the 1920s
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What do you think color has to do with this
diagram? Hint Which color has the most energy
and how does that energy relate to temperature?
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• Hotter (bluer) stars lie to the left
• Cooler (redder) stars lie to the right
• Luminosity (brightness) goes up the y-axis
• Notice that the vertical scale is luminosity as
  fractions of a stars absolute luminosity compared
  to the Sun. A star that is similar in brightness
  to the Sun has a L value of 1. A star that is
  10,000 times brighter than the Sun has a L value
  of 104. One that is 100 times fainter than the
  Sun has a L value of 10-2. Likewise the
  horizontal axis is plotted in terms of
  temperature as measured in Kelvins.
• Temperature goes across the x-axis. Note that
  the higher temperatures are to the left and
  cooler stars are to the right.

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Spectral Class

• The spectral classification types were more
  accurate then attempts to measure the temperature
  of a star by its color. So often the temperature
  scale on the horizontal axis is replaced by
  spectral types, OBAFGKM. Spectral types refers to
  the state of the atoms on the star. This had the
  advantage of being more linear than temperature
  (nicely space letters) and contained more
  information about the star than just its
  temperature (the state of its atoms).
• It uses the letters O, B, A, F, G, K, M
• Oh Be A Fine Girl/Guy, Kiss Me
• Even though it is a scale based on stellar
  spectra it does contain correlations between the
  stellar spectra and the surface temperature of
  the star..

O 30,000 K G 6,000 K B 20,000 K K
4,000 K A 10,000 K M 3,000 K F 7,000 K
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Nebulas

• A nebula is the first stage in a stars life
  cycle.
• When the center of a nebula hits 10 million
  degrees that creates fusion.
• Fusion is a process that combines two hydrogen
  atoms and they create a new atom.
• A nebula is a ball of gases that will make a new
  main sequence star. We will discuss this later
  when we talk about the births and deaths of stars.

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Main Sequence Stars
O B A F G K M

• Sun

30,000 10,000 5,000 3,000
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Main Sequence Stars
O B A F G K M

• After plotting many stars, patterns begin to
  emerge. One of the most noticeable is the main
  sequence, which is a band stretching diagonally
  across the H-R diagram, from top left (high
  temps, high luminosity, large size) to bottom
  right (low temps, low luminosity, small size).
• This shows a trend that cold stars tend to be
  faint, and hot stars tend to be bright.

30,000 10,000 5,000 3,000
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• As mentioned, there are trends on the main
  sequence.
• The large, hot, and bright stars are found in the
  upper left of the diagram.
• Because of their color and size they are referred
  to as blue giants.
• The Very largest of these stars are called blue
  supergiants

O B A F G K M
30,000 10,000 5,000 3,000
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O B A F G K M

• At the other end, stars are small, cool, and
  faint.
• For this reason these stars are called red
  dwarfs. These stars are likely to remain on the
  Main Sequence for the longest time.
• Our sun is in the middle of these two groups.

30,000 10,000 5,000 3,000
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• The white-dwarf region is characterized by a
  group of stars with very high temps and small
  luminosities. These stars, which are the most
  dense, make up about 9 of all stars.
• The red-giant region is characterized by a group
  of stars with low temperatures and high
  luminosities. These make up about 1 of all stars
  and they are the largest stars by volume.

White Dwarfs and Blue Giants
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So where do stars come from anyway?

• Stars form inside relatively dense
  concentrations of interstellar gas and dust known
  as molecular clouds. These regions are extremely
  cold (temperature about 10 to 20K).
• Normally, molecules can not form in space
  because photons of starlight (energy) will break
  them apart. But in the centers of dark clouds,
  molecules like CO and H2 (the most common
  molecules in interstellar gas clouds) can form
  because of the low temperatures.
• The deep cold also causes the gas to clump in to
  high density clouds. When the density reaches a
  certain point, stars form.
• Since the regions are dense, they are opaque to
  visible light and are known as dark nebula.
• - Since they don't shine by optical light, we
  must use IR and radio telescopes to investigate
  them.

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Dark Nebulas
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Star Formation Continued

• Star formation begins when the denser parts of
  the cloud core collapse under their own
  weight/gravity.
• The cores are denser than the outer cloud, so
  they collapse first.
• As the cores collapse they fragment into clumps
  around 0.1 parsecs in size and 10 to 50 solar
  masses in mass. These clumps then form into
  protostars and the whole process takes about 10
  millions years.

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OKLets ask the obvious question

• How do we know this is happening if it takes so
  long and is hidden from view in dark clouds?
• 1. Most of these cloud cores have IR sources
• 2. Evidence of energy from collapsing protostars
  (potential energy converted to kinetic energy).
• 3. Also, where researchers do find young stars,
  they find them surrounded by clouds of gas, the
  leftover dark molecular cloud.
• 4. And they occur in clusters, groups of stars
  that form from the same cloud core.

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Protowhater????

• Protostar
• Once a clump of dense matter has broken free from
  the other parts of the cloud core, it has its own
  unique gravity and identity and it is then called
  a protostar.
• As the protostar forms, loose gas falls into its
  center. The infilling gas releases kinetic energy
  in the form of heat and the temperature and
  pressure in the center of the protostar goes up.
• As its temperature approaches thousands of
  degrees, it becomes a IR source.

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Brown Dwarfs

• If a protostar forms with a mass less than 0.08
  solar masses, its internal temperature never
  reaches a value high enough for thermonuclear
  fusion to begin. This failed star is called a
  brown dwarf, halfway between a planet (like
  Jupiter) and a star.

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Becoming a Main Sequence Star

• Once a protostar starts burning hydrogen in its
  core, it quickly (in a few million years) becomes
  a main sequence star.
• There are three divisions in a stellar interior.
  They are are the nuclear burning core, convective
  zone and radiative zone.
• Energy, in the form of gamma-rays, is generated
  solely in the nuclear burning core.
• Energy is transferred towards the surface in the
  convective zone.
• The radiative zone is where energy is radiated
  out and away from the star.

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The size of the zone depends on the mass of the
star.
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So what happens after the Main Sequence?

• As the supply of hydrogen in the core begins to
  decreases, the fusion rate goes down, and the
  amount of energy generated drops.
• From there thermal equilibrium takes over and as
  the temperature begins to drop the pressure in
  the fusion core will also decrease.
• The rest is a tug of war
• Because of hydrostatic equilibrium, there will be
  a drop in pressure, which means that the core
  region of the star will contract slightly. This
  will cause the temperature to go up again, and
  the fusion rate, for the remaining hydrogen in
  the core, jumps up (even though the core hydrogen
  is almost gone (a last gasp).

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The Hydrogen Burning Shell

• The sole source of energy in a dying star.
• Once the hydrogen burning shell is created, the
  star makes a small jump off the main sequence in
  the HR diagram. It becomes a little brighter and
  a little cooler.
• The drop in surface temperature is because the
  envelope of the star expands a small amount,
  which increases the surface area. This increased
  surface area also increases the luminosity of the
  star.
• Once the last of the hydrogen is used up in the
  core of an aging main sequence star, fusion stops
  in the core and the temperature drops and the
  core collapses.
• The collapsing core converts gravitational energy
  (potential energy) into thermal energy (kinetic
  energy). This energy is directed into the
  hydrogen burning shell, which expands to consume
  more fuel in the star's interior.

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The hydrogen burning shell generates more energy
than the core did (it has access to a much larger
volume of the star's mass) and the star increases
sharply in luminosity and expands in size to
become a red giant. Even though the star is
brighter, produces more energy, its pressure has
increased such that its surface area has become
very large, and the surface temperature of the
star drops.
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• This whole process takes several million years
  but, in the end, the main sequence star becomes
  either a red supergiant or a red giant, depending
  on its initial mass.
• Notice that where and how fast a star evolves is
  determined by its main sequence mass. Hot,
  massive stars age quickly and become red
  supergiants. Cooler, less massive stars live for
  10 billion years, then evolve into red giants.

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From Red Giant to Planetary Nebula

• A planetary nebula forms when a star can no
  longer support itself by fusion reactions in its
  center. The gravity from the material in the
  outer part of the star takes its inevitable toll
  on the structure of the star, and forces the
  inner parts to condense and heat up. The high
  temperature central regions drive the outer half
  of the star away in a brisk stellar wind, lasting
  a few thousand years. When the process is
  complete, the remaining core remnant is uncovered
  and heats the now distant gases and causes them
  to glow.

red supergiants