James Linzel's List: Assignment 2: Origins and Fate of Stars

Stars - Science Mission Directorate

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Lives and Deaths of Stars

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- Hydrogen and helium and some lithium, boron, and beryllium were created when
  the universe was created. All of the rest of the elements of the universe were
  produced by the stars in nuclear fusion reactions. These reactions created the
  heavier elements from fusing together lighter elements in the central regions
  of the stars. When the outer layers of a star are thrown back into space, the
  processed material
  can be incorporated into gas clouds that will later form stars and planets.
  The material that formed our solar system incorporated some of the remains of
  previous stars.
  All of the atoms on the Earth except hydrogen and most of the helium are
  recycled material---they were not created on the Earth. They were created in
  the stars.
- The atoms heavier than helium up to the iron and nickel atoms were made in the
  cores of stars (the process that creates iron also creates a smaller amount
  of nickel too). The lowest mass stars can only synthesize helium. Stars around
  the
  mass
  of our Sun can synthesize helium, carbon, and oxygen. Massive stars
  (M_* > 8 solar masses) can
  synthesize
  helium, carbon, oxygen, neon, magnesium, silicon, sulfur, argon, calcium,
  titanium, chromium, and iron (and nickel). Elements heavier than iron are made
  in
  supernova explosions from the rapid combination of the abundant neutrons with
  heavy
  nuclei.
  Massive red giants are also able to make small amounts of elements heavier than
  iron (up to mercury and lead) through a slower combination of neutrons with heavy
  nuclei, but supernova probably generate the majority of elements heavier than
  iron and nickel (and certainly those heavier than lead up to uranium). The
  synthesized elements are dispersed into the interstellar
  medium during the planetary nebula supernova stage (with supernova being the
  best way to distribute the heavy elements far and wide). These elements will
  be later incorporated into giant molecular clouds and eventually become part
  of future stars and
  planets (and life forms?)
Lives and Deaths of Stars

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stars stellar on 2008-02-24
- During the
  supernova outburst, elements heavier than iron are produced as free
  neutrons produced in the explosion rapidly combine with heavy nuclei to
  produce heavier and very rare nuclei like gold, platinum, uranium among
  others.
  This happens in about the first 15 minutes of the supernova.
  The most massive stars may also produce very powerful bursts of gamma-rays that
  stream out in jets at the poles of the stars at the moment their cores collapse
  to form a black hole.
  Add Sticky Note
- What remains after the outer layers are thrown off depends on the mass of
  the core. The core remnants are described fully in the next section. Here a
  brief description of each type of core remnant will be given.
  If the core has a mass less than 1.4 solar masses, it will shrink down to a
  white dwarf the size of the Earth. The electrons in the
  compressed gas bump
  right against each other to form a strange sort of gas called a
  degenerate gas. The electrons prevent
  further collapse of the core.
  
  If the core has a mass between 1.4 and 3 solar masses, the neutrons will
  bump
  up against each other to form a degenerate gas in a neutron star
  about
  the size of small city. The neutrons prevent further collapse of the core.
  Nothing can prevent the highest mass cores (greater than 3 solar
  masses) from collapsing to a point. On the way to total collapse, it may
  momentarily create a neutron star and
  the resulting supernova rebound explosion and powerful
  
  bursts of gamma-rays in bipolar jets (possibly the source of some of the ``gamma-ray
  burst'' objects). Gravity finally wins. Nothing
  holds
  it up. The gravity around the collapsed core becomes so great that Newton's
  law of gravity becomes inadequate and the gravity must be described by the
  more powerful theory of General Relativity developed by Albert Einstein. This
  
  will be discussed further below.
  
  The supercompact point mass is called a black hole because the
  escape velocity around the point mass is greater than the
  speed of light. Since the speed of light is the fastest that any radiation
  or any other information can travel, the region is totally black. The
  distance at which the escape velocity equals the speed of light is called
  the event
  horizon because no information of events occurring inside the
  event horizon can get to the outside. The radius of the event horizon in
  kilometers = 3 × core mass in solar masses.
  Add Sticky Note
Lives and Deaths of Stars

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- If the star is massive enough, gravity can compress the core enough to
  create high enough temperatures to start fusing helium (or heavier elements
  if it is
  repeating this stage).
- When the core fuel runs out again, the core resumes its collapse. If the
  star is
  massive enough, it will repeat stage 5. The number of times a star can
  cycle
  through stages 5 to 7 depends on the mass of the star. Each time through
  the cycle, the star creates new heavier elements from the ash of fusion
  reactions
  in the previous cycle. This creation of heavier elements from lighter
  elements
  is called stellar nucleosynthesis. For the most
  massive stars,
  this continues up to the production of iron in the core. Stars like our Sun
  will synthesize elements only up to carbon and oxygen in their cores.
  Each repeat of stages 5 to 7 occurs over a shorter time period than the previous
  repeat.
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Lives and Deaths of Stars

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- Eventually the star becomes stable because
  hydrostatic
  equilibrium has been
  established. The star settles down to spend about 90% of its life as a
  main sequence star. It is fusing hydrogen to form helium in
  the core.
  
  Stars initially begin their lives near other stars in a cluster. After a
  few orbits around the galactic center, gravitational tugs from other stars
  in the galaxy cause the stars in the cluster to wander
  away from their cluster and live their lives alone or with perhaps one or
  two companions.
- Eventually the star becomes stable because
  hydrostatic
  equilibrium has been
  established. The star settles down to spend about 90% of its life as a
  main sequence star. It is fusing hydrogen to form helium in
  the core.
  
  Stars initially begin their lives near other stars in a cluster. After a
  few orbits around the galactic center, gravitational tugs from other stars
  in the galaxy cause the stars in the cluster to wander
  away from their cluster and live their lives alone or with perhaps one or
  two companions.
Lives and Deaths of Stars

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starformation stars on 2008-02-24
- A giant molecular cloud is a large, dense gas cloud (with dust)
  that is cold enough for molecules to form. Thousands of giant molecular clouds
  exist in the disk part of our galaxy. Each giant molecular cloud has 100,000's
  to a few million solar masses of material.
- The Orion Complex is about 1500 light years away, several hundred
  light years across, and has enough material to form many tens of thousands
  of suns
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- - In addition to the most common molecule, molecular hydrogen,
    over 80 other molecules have been discovered in the clouds from simple ones
    like carbon monoxide to complex organic molecules such as methanol and acetone.
    Radio telescopes are used to observe these very dark, cold clouds. The clouds
    are dense relative to the rest of the gas between the stars but are still much
    less dense than the atmosphere of a planet.
  - These shock waves
    compress the
    gas clouds enough for them to gravitationally collapse. Gas clouds may
    start
    to collapse without any outside force if they are cool enough and massive
    enough to spontaneously collapse. Whatever the reason, the result is the
    same: gas clumps compress to become protostars.
  - As a gas clump collapses it heats up because the gas particles run into
    each other.
    The energy the gas particles had from falling under the force of gravity
    gets converted to heat energy. The gas clump becomes warm enough to produce a
    lot of
    infrared and microwave radiation. At this stage the warm clump is called a
    protostar. The gas clump forms a disk with the protostar in the
    center. Other material in the disk may coalesce to form another star or
    planets.
  - A protostar will reach a temperature of 2000 to 3000 K, hot enough to glow
    a dull red with most of its energy in the infrared.
  - Fusion starts in the core and the outward pressure from
    those reactions
    stops
    the core from collapsing any further. But material from the surrounding
    cloud
    continues to fall onto the protostar. Most of the energy produced by the
    protostar is from the gravitational collapse of the cloud material.
  - Stars are
    observed to be born in clusters.
  - Selecting the figure will bring up an expanded view of the
    Hubble Space Telescope image in another window (courtesy of
    Space Telescope Science Institute).
    Note that the tiniest fingers you see sticking out of the sides of the pillars
    are larger than our entire solar system!
Properties of Stars

The figure below illustrates the inter-dependence of measurable quantities with the derived values that have been discussed so far. In the left triangular relationship, the apparent brightness, distance, and luminosity are tied together such that if you know any two of the sides, you can derive the third side. For example, if you measure a glowing object's apparent brightness (how bright it appears from your location) and its distance (with trigonometric parallax), then you can derive the glowing object's luminosity. Or if you measure a glowing object's apparent brightness and you know the object's luminosity without knowing its distance, you can derive the distance (using the inverse square law). In the right triangular relationship, the luminosity, temperature, and size of the glowing object are tied together. If you measure the object's temperature and know its luminosity, you can derive the object's size. Or if you measure the glowing object's size and its temperature, you can derive the glowing object's luminosity---its electromagnetic energy output.\n\napparent brightness, distance, luminosity triangle + luminosity, temperature, size triangle\n\nFinally, note that a small, hot object can have the same luminosity as a large, cool object. So if the luminosity remains the same, an increase in the size (surface area) of the object must result in a DEcrease in the temperature to compensate.\n\n

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luminosity properties stars on 2008-02-24
- The figure below illustrates the inter-dependence of measurable
  quantities
  with the derived values that have been discussed so far. In the left triangular
  relationship, the apparent brightness, distance, and luminosity are tied together
  such that if you know any two of the sides, you can derive the third side.
  For example, if you measure a glowing object's apparent brightness (how bright
  it appears from your location) and its distance (with trigonometric parallax),
  then you can derive the glowing object's luminosity. Or if you measure a glowing
  object's apparent brightness and you know the object's luminosity without knowing
  its distance, you can derive the distance (using the inverse square law). In
  the right triangular relationship, the luminosity, temperature, and size of
  the glowing object are tied together. If you measure the object's temperature
  and know its luminosity, you can derive the object's size. Or if you measure
  the glowing object's size and its temperature, you can derive the glowing object's
  luminosity---its electromagnetic energy output.
- 2 more annotations...
- - Finally, note that a small, hot object can have the same luminosity
    as a large, cool object. So if the luminosity remains the same,
    an increase in the size (surface area) of the object must result in a DEcrease
    in the temperature to compensate.

The Sun and Stellar Structure

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on 2008-02-23

Stars are held together by gravity. Gravity tries to compress everything to the
center. What holds an ordinary star up and prevents total collapse is thermal and
radiation pressure. The thermal and radiation
pressure tries to expand the star layers outward to infinity.

In any given layer of a star,
there is a balance between the thermal pressure (outward) and the weight of the
material above pressing downward (inward). This balance is called
hydrostatic equilibrium. A star is like a balloon. In a balloon the gas
inside the balloon pushes outward and the elastic material supplies just
enough inward compression to balance the gas pressure. In a star the star's internal
gravity supplies the inward
compression. Gravity compresses the star into the most compact shape possible: a
sphere. Stars are round because gravity attracts everything in an object to the
center. Hydrostatic equilibrium also explains why the Earth's atmosphere does not
collapse to a very thin layer on the ground and how the tires on your car or
bicyle are able to support the weight of your vehicle.

The Sun and Stellar Structure

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Core

The core is the innermost 10% of the Sun's mass. It is where
the energy from nuclear fusion is generated. Because of the enormous amount
of gravity compression from all of the layers above it, the core is very
hot and dense. Nuclear fusion requires extremely
high temperatures and densities. The Sun's core is about 16 million K
and has a density around 160 times the density of water. This is over 20 times
denser than the dense metal iron which has a density of ``only''
7 times that of water. However, the Sun's interior is still gaseous all the
way to the very center because of the extreme temperatures. There is no molten rock
like that found in the interior of the Earth.
Convective Zone

Energy in the outer 15% of the Sun's radius is
transported by the bulk motions of gas in a process called convection. At
cooler temperatures, more ions are able to block the outward flow of photon
radiation more effectively, so nature kicks in convection to help the
transport of energy from the very hot interior to the cold space. This part
of the Sun just below the surface is called the convection zone.

WMAP Cosmology 101: Life Cycle of Stars

Astronomers believe that molecular clouds, dense clouds of gas located primarily in the spiral arms of galaxies are the birthplace of stars. Dense regions in the clouds collapse and form "protostars". Initially, the gravitational energy of the collapsing star is the source of its energy. Once the star contracts enough that its central core can burn hydrogen to helium, it becomes a "main sequence" star.

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Astronomers believe that molecular clouds, dense clouds of gas located primarily in the spiral arms of galaxies are the birthplace of stars. Dense regions in the clouds collapse and form "protostars". Initially, the gravitational energy of the collapsing star is the source of its energy. Once the star contracts enough that its central core can burn hydrogen to helium, it becomes a "main sequence" star.
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Main sequence stars are stars, like our Sun, that fuse hydrogen atoms together to make helium atoms in their cores. For a given chemical composition and stellar age, a stars' luminosity, the total energy radiated by the star per unit time, depends only on its mass. Stars that are ten times more massive than the Sun are over a thousand times more luminous than the Sun. However, we should not be too embarrassed by the Sun's low luminosity: it is ten times brighter than a star half its mass. The more massive a main sequence star, the brighter and bluer it is. For example, Sirius, the dog star, located to the lower left of the constellation Orion, is more massive than the Sun, and is noticeably bluer. On the other hand, Proxima Centauri, our nearest neighbor, is less massive than the Sun, and is thus redder and less luminous.
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- Thus, it is fortunate that our Sun is not more massive than it is since high mass stars rapidly exhaust their core hydrogen supply. Once a star exhausts its core hydrogen supply, the star becomes redder, larger, and more luminous: it becomes a red giant star. This relationship between mass and lifetime enables astronomers to put a lower limit on the age of the universe
- Meanwhile, the core of the star collapses under gravity's pull until it reaches a high enough density to start burning helium to carbon. The helium burning phase will last about 100 million years, until the helium is exhausted in the core and the star becomes a red supergiant. At this stage, the Sun will have an outer envelope extending out towards Jupiter. During this brief phase of its existence, which lasts only a few tens of thousands of years, the Sun will lose mass in a powerful wind. Eventually, the Sun will lose all of the mass in its envelope and leave behind a hot core of carbon embedded in a nebula of expelled gas. Radiation from this hot core will ionize the nebula, producing a striking "planetary nebula", much like the nebulae seen around the remnants of other stars. The carbon core will eventually cool and become a white dwarf, the dense dim remnant of a once bright star.
- Massive stars burn brighter and perish more dramatically than most. When a star ten times more massive than Sun exhaust the helium in the core, the nuclear burning cycle continues. The carbon core contracts further and reaches high enough temperature to burn carbon to oxygen, neon, silicon, sulfur and finally to iron. Iron is the most stable form of nuclear matter and there is no energy to be gained by burning it to any heavier element.

Star Formation

Despite what you might think, space is not a perfect vacuum. The space between the stars is filled with a tenuous range of material that provides the building blocks of stars. This material is gas and dust and collectively is known as the interstellar medium (ISM). The ISM gas is predominantly hydrogen whilst the dust is about 1% by mass and includes carbon compounds and silicates. Dust is responsible for the interstellar reddening and extinction of starlight. The more of the ISM a star's light travels through on its way to an observer on Earth the more it gets scattered and absorbed, decreasing the star's apparent brightness and reddening its appearance.

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Despite what you might think, space is not a perfect vacuum. The space between
the stars is filled with a tenuous range of material that provides the building
blocks of stars. This material is gas and dust and collectively is known as
the interstellar medium (ISM). The ISM gas is predominantly
hydrogen whilst the dust is about 1% by mass and includes carbon compounds and
silicates. Dust is responsible for the interstellar reddening and extinction
of starlight. The more of the ISM a star's light travels through on its way
to an observer on Earth the more it gets scattered and absorbed, decreasing
the star's apparent brightness and reddening its appearance.
Stars form in regions of the ISM where there is sufficient material available.
These are the giant molecular clouds or GMCs
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- Their main
  constituent is molecular hydrogen gas but other molecules such as water, carbon
  monoxide, CO, ammonia, NH₃ and methanol, CH₃OH are also
  present.
- An emission nebula is a cloud of dust and gas that shines due to the emission
  of its own light. Gas is these nebulae is hot enough to be ionised and the emitted
  light is due to interactions between the free electrons and the ions and to
  electron transitions within ions
- In astronomy, HI refers to neutral hydrogen whilst HII is ionised hydrogen.
  HII regions generally display distinctive red and green colours.The red is caused
  by hydrogen-alpha emission and forbidden transitions of singly-ionised nitrogen.
  OIII, doubly-ionised oxygen ions in which both the outer electrons have been
  lost, produces a green emission due to forbidden transitions. This process only
  takes place if photons have high enough energy such as the UV photons emitted
  from hot O and B stars within or close to the nebula.
- Reflection nebulae are much less dense than emission nebulae. They appear bluish
  due to the dust grains preferentially scattering blue light from nearby hot
  stars. Our sky appears blue for much the same reason, blue light from the Sun
  is scattered in all directions
- Protostars form when sections of giant molecular clouds start to collapse.
  Clouds are initially diffuse enough that they do not contract unless something
  triggers an increase in the density of some regions within a cloud.
- The expanding shock front from a nearby supernovae sweeps up the local
  ISM and compresses it. As the front passes through a cloud it causes irregularities
  and local regions of higher density that may then collapse.
  Spiral galaxies rotate without their arms "winding up". This is
  due to a spiral density wave that sweeps around the plane of the galaxy, compressing
  the material in its path including clouds of dust and gas. This helps account
  for the observed star formation in spiral arms.
  Stars and clouds are not fixed in place, as they move they may pass near
  another cloud, triggering gravitational collapse due to variations in
  density.
  Regions with a number of young, hot O and B-class stars (called OB Associations)
  produce large amounts of visible and ultraviolet radiation that compress
  the ISM due to radiation pressure.
- The gravitational collapse of a GMC does not result in a single, massive star.
  Instead the cloud tends to fragment into smaller denser regions that each collapse
  to form star systems. Up to a few thousand stars may typically form in a collapsing
  GMC resulting in an open (or galactic) cluster.
  
  The dense regions collapse due to gravitational attraction between the particles.
  Individual gas or dust particles move in towards the centre of the collapsing
  region, losing gravitational potential energy. As the total energy of the system
  is conserved the loss of gravitational energy is balanced by an increase in
  the kinetic energy of the particles. These particles then undergo more collisions
  which in turn raises the temperature of the gas. At this stage further collapse
  is only possible if the cloud can radiate away the thermal energy so that the
  radiation pressure outwards remains lower than the inward gravitational pull.
  This is achieved via convection cycling warm material upwards within the cloud,
  making the collapsing cloud visible in the infrared region.
  
  Credit: NASA,
  John Trauger (Jet Propulsion Laboratory) and James Westphal (California Institute
  of Technology), Nolan Walborn (Space Telescope Science Institute) and Rodolfo
  Barba' (La Plata Observatory)
  
  HST visible and infrared images of star forming region,
  30 Doradus in the Large Magellanic Cloud. The arrows point to protostars that
  are obscured in the visible but visible at infrared wavebands. More details
  are available from the NASA press
  release.
  
  Continued collapse leads to higher densities so that eventually the cloud becomes
  opaque, trapping the thermal energy within the cloud. This then causes both
  the temperature and pressure to rise rapidly - the collapsing cloud is now a
  protostar

Chandra :: Field Guide to X-ray Astronomy :: Stellar Evolution

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Stellar Evolution

If the star is about the same mass as the Sun, it will turn into a white dwarf star.
If it is somewhat more massive, it may undergo a supernova explosion and leave
behind a neutron star. But
if the collapsing core of the star is very great—at least three times the mass of the Sun—nothing can stop the collapse. The
star implodes to form an infinite gravitational warp in space—a black hole.

The brightest X-ray sources in our galaxy are the remnants of massive stars that
have undergone a catastrophic collapse—neutron stars and black holes. Other
powerful sources of X-rays are giant bubbles of hot gas produced by exploding
stars. White dwarf stars and the hot, rarified outer layers, or coronas, of normal stars are less intense X-ray
sources.

Chandra :: Field Guide to X-ray Sources :: White Dwarfs & Planetary Nebulas

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BBC - Science & Nature - Space - Star Birth

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it took 300,000 years for stable hydrogen and helium atoms to form. Gradually these atoms began to clump together into gas clouds called nebulae.

The dying core eventually forms a white dwarf - a spherical diamond the size of the Earth, made of carbon and oxygen. From this point on the Sun will gradually fade away, becoming dimmer and dimmer until its light is finally snuffed out.

BBC - Star Types

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hey have collapsed so much that their atoms have been crushed, squashing the protons and electrons together until they merge to leave only neutrons.

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James Linzel

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These pages help describe the formation of, the aging and fate of stars found in our universe.

Updated on 2009-05-19
Created on 2007-09-21

Category: Schools & Education

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James Linzel

on 2008-09-22

( Oakville , Ontario , Canada )

pressure outward = gravity compression inward

Hydrostatic
equilibrium: gravity compression is balanced by pressure outward.

more compression creates more outward pressure

Greater gravity
compresses the gas, making it denser and hotter, so the outward pressure increases.

James Linzel's List: Assignment 2: Origins and Fate of Stars

Stars - Science Mission Directorate

Lives and Deaths of Stars

Lives and Deaths of Stars

Lives and Deaths of Stars

Lives and Deaths of Stars

Lives and Deaths of Stars

Properties of Stars

The Sun and Stellar Structure

Gravity Holds a Star Together

The Sun and Stellar Structure

Core

Convective Zone

WMAP Cosmology 101: Life Cycle of Stars

Star Formation

Chandra :: Field Guide to X-ray Astronomy :: Stellar Evolution

Stellar Evolution

Chandra :: Field Guide to X-ray Sources :: White Dwarfs & Planetary Nebulas

BBC - Science & Nature - Space - Star Birth

BBC - Science & Nature - Space - Why Stars Twinkle

BBC - Science & Nature - Space - Inside Stars

BBC - Science & Nature - Space - Star Death

BBC - Star Types

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James Linzel's Public Lists (13)

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