An Introduction to Astronomy Part XI: The Birth and Death of Stars

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An Introduction to AstronomyPart XI The Birth
and Death of Stars

• Lambert E. Murray, Ph.D.
• Professor of Physics

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Interstellar Gas and Dust

• In the late 1700s Henry Herschell discovered
  holes in the heavens where there appeared to be
  fewer stars than normal.
• In the late 1800s Edward Barnards photographs
  of these regions lead some to believe that they
  were clouds of material blocking out the
  starlight.

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Barnard 86 a Dark Nebula
Barnard 86 is a good example of one of
Herschells holes in the heavens.
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The Constellation Orion
Region of Horsehead Nebula
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The Horsehead Nebula A Dark Nebula
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Close-up of the Horsehead Nebula
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Evidence of Dark Nebula

• The Horsehead nebula is clearly a case where dark
  material is obscuring the brighter emission
  nebula behind it.
• The close-up actually reveals dim stars that are
  behind the dark nebula.
• This seems to be clear evidence for dark material
  in interstellar regions which can obscure the
  light from stars.
• Indeed, the reddish emission nebula behind the
  Horsehead is direct evidence of interstellar gas.

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CO Radar Mapping (2.6 mm)of Orion Monoceros
Region

• The following image is a radar map of the Orion
  Monoceros region of the sky taken at a wavelength
  of 2.6 mm.
• The 2.6 mm wavelength radar image maps CO
  concentrations. The concentrations of hydrogen
  are typically four orders of magnitude greater
  than CO in interstellar space, but this gives a
  measure of the amount of gas in interstellar
  space regions.
• You can tell that there are large concentrations
  of gas in the regions of the horsehead and Orion
  nebulae. These are believed to be rich star
  formation regions.

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M
More Dark Nebula
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Emission and Reflection Nebula

• The term nebula is used to denote a cloud of
  interstellar gas and dust.
• An emission nebula is one that glows because of
  the emission of specific spectral lines arising
  from the excitation of atoms within the
  interstellar gas cloud.
• A reflection nebula is one that glows because of
  scattered light from a star this scattered
  light is typically bluish in color (like the blue
  in our atmosphere).
• As white light passed through a dust cloud (much
  like smoke) blue light is scattered and, if the
  cloud is not too thick, the spectrum of the light
  passing through the cloud becomes more reddish
  (interstellar reddening).

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An Example of an Emission and Reflection Nebula
Reflection Nebula NGC 6589 NGC 6590
Emission Nebula IC 1283-4
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Extinction and Reddening

• Due to the interstellar nebula, light from
  distant stars does not appear as bright as they
  would the process is called interstellar
  extinction.
• Similarly, the interstellar nebula cause the
  light from distance stars to appear more reddish
  interstellar reddening.
• Measurements of interstellar extinction and
  reddening in various directions indicates that
  most of the interstellar nebula are confined to
  the regions of the Milky Way the faint band of
  hazy myriad stars which stretches across the
  night sky.

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The Milky Way is a Spiral GalaxyWe are Near the
Outer Edge
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Looking Toward the Nucleus of Our Galaxy Through
Dark Nebula
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The Formation of Protostars

• Astronomers believe that stars are born from the
  gravitational collapse of large, cold regions of
  dark nebular material. This collapse may be
  triggered by the explosion of nearby stars
  creating compression waves in the nebula.
• The picture on the next slide is a region of
  space where astronomers believe this process may
  be occurring.

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The Knobs on the Gas Clouds May be Regions of
Concentrated
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The Orion Nebula
Protostar Region?
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Pre-Main-Sequence Evolution

• As large regions of gas collapse under the
  influence of gravity, they heat up.
• Initially this heat is in the form of infrared
  radiation.
• Eventually the gas heats up enough to radiate in
  the visible. As the gas cloud shrinks greater
  internal energy is released, causing the
  temperature to rise more.

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A Stellar Nursery
H II region of the Swan in visible wavelengths
Same region in infrared wavelengths.
Notice the large number of cool stars, or
protostars, on the right-hand-side. These are
visible in the infrared because that wavelength
can penetrate the dust and gas.
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These Two Images are Lined up
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These Two Images are Lined up
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Pre-Main-Sequence Evolution Tracks of Protostars
Notice that this model gives results similar to
the mass-luminosity data plot.
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Variations in Luminosity with Stellar Mass

• The rising temperature coupled with the
  decreasing size causes protostars of mass greater
  than about 5 solar masses to maintain a
  relatively constant luminosity.
• Protostars less massive decrease in size more
  rapidly than the increase in surface temperature,
  and the luminosity decreases.

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A Star is Born

• Once the protostar heats up to the point where
  thermonuclear fusion occurs, the radiation
  pressure will counteract the gravitational
  pressure and the star will become stable as a
  main-sequence star.
• Stars arising from larger mass clouds become very
  luminous stars, while stars arising from less
  massive clouds become less luminous.
• Gas clouds with a mass of less than about 0.08
  solar masses can never heat up sufficiently for
  nuclear fusion to occur, and the failed star
  becomes a hydrogen-rich brown dwarf (something
  like Jupiter).

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Young Stellar Disks and Jets

• From the discussion of our solar system, we
  postulated the formation of a solar system from
  the gravitational collapse of a dust and gas
  cloud, and the development of a disk of material
  rotating rapidly around the young star.
• Similar features have been recently observe with
  the Hubble Space Telescope.
• In the slide that follows, the knobby jets of
  material appears to be emitted along the polar
  axes of the star in what is called bi-polar
  outflow. These small nebula are known as
  Herbig-Haro objects.
• Such young, gas-ejecting stars are known as T
  Tauri stars, the first example having been
  discovered in the constellation Taurus.

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Young Star Jets and Disks
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Stellar Disks in the Orion Nebula

• The following images were taken by the HST of the
  Orion Nebula.
• In these pictures you will see evidence of
  stellar formation and of the presence of
  disk-like structures surrounding these new stars.
• The four massive stars that dominate this region
  are emitting radiation and gasses which are
  interacting with the smaller young stars being
  formed. These stellar winds may prevent the
  formation of possible solar systems.

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Main Sequence Stars

• Once a star reaches the main sequence, it will
  remain on the main sequence for about 90 of its
  lifetime. For our Sun (a moderately sized star),
  this means approximately 10 billion years.
• Larger, more luminous stars burn up their fuel
  much more rapidly, and remain on the main
  sequence for a shorter period of time.
• As we will see later, when a star leaves the main
  sequence, it expands and moves toward the region
  of giant and supergiant stars.

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Star Clusters Provide Evidence for our Model

• High-mass stars evolve more rapidly than low-mass
  stars.
• An association of hot, massive stars (an OB
  association) will emit vast quantities of uv
  radiation into the nebula from which it was born.
• This high-energy uv radiation actually ionizes
  the hydrogen gas of the nebula. These free
  protons then combine with free electrons and emit
  the characteristic red color associated with
  these emission nebula (so-called H II regions).
• We want to examine the HR diagram for the young
  stars associated with an H II region (NGC 2264).

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NGC 2264inMonoceros
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HR Diagram of the Young Star Cluster NGC 2264
Note that the more massive, luminous stars in the
cluster (at the upper left end) have already
reached the main sequence, while the less massive
protostars (at the lower end) have not yet joined
the main sequence.
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The Pleiades Star Cluster

• In contrast to the young star cluster NGC 2264,
  an HR diagram of the Pleiades star cluster shows
  that these stars have already reached the main
  sequence, and the older stars are actually
  beginning to leave.
• The Pleiades star cluster is a cluster of very
  bright, blue (hot) stars.
• Both star clusters we have examined are known as
  open clusters or galactic clusters, possessing
  barely enough mass to hold themselves together in
  a cluster.

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The Pleiades and Their HR Diagram
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Other Stellar Nurseries

• The radiation pressure from very bright stars may
  create compression waves in a surrounding nebula,
  as in the Rosette Nebula.
• Likewise, when a star dies, it may generate a
  massive explosion which can send large
  compression waves out into the interstellar
  medium, compressing the gasses that are there.
• This compression of interstellar gasses may be
  the breeding places for new stars.
• The following images illustrate some of these
  massive compressional waves generated by
  radiation pressure and by exploding stars.

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The Rosette Nebula Compression by Radiation
Pressure
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An Exploding Star
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Another Exploding Star
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Yet Another Exploding Star
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The Sun Expands in Old Age

• Once a star like our Sun becomes a main sequence
  star, it remains stable for about 10 billion
  years.
• At the end of that time, the hydrogen fuel in the
  center of the Sun will become depleted there is
  too much helium to efficiently continue the
  thermonuclear fusion process at the core.
• When that happens, the radiation pressure from
  the center of the Sun will be reduced and the
  core will collapse toward the center due to
  gravity.
• The region just outsider the core will heat up
  and begin to burn hydrogen and this will cause
  the Sun to expand. This process continues as the
  Sun expands out to the size of the Earths orbit,
  creating a red giant.

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The Sun as a Red Giant
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Helium Core Burning

• Once a star becomes a red giant, it will remain a
  red giant as its outer regions continue to burn
  the available hydrogen.
• During this time, helium ash continues to
  accumulate in the center of the star as gravity
  pulls this heavier material to the center,
  heating up the core.
• Eventually, the stars core will re-ignite when
  the temperature of the core gets hot enough for
  helium to burn.
• The ash of helium burning is both oxygen and
  carbon.
• For low-mass stars (less than 3 solar masses) the
  onset of helium burning produces an explosive
  helium flash.
• After the helium flash the star settles down to
  burning helium and becomes a smaller, hotter star.

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Post-Main-Sequence Evolution
Core helium burning begins where the evolutionary
tracks make a sharp downward turn in the red
giant region of the diagram.
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HR Diagrams for Globular Clusters

• Many globular clusters are associated with our
  galaxy (and with others).
• These star clusters can be easily analyzed using
  an HR diagram, because all the stars are
  essentially the same distance from us. We need
  plot only their apparent magnitude vs. their
  temperature.
• An HR diagram of M13 is shown on the next slide.
• This diagram indicates a group of stars in which
  the hottest stars have already moved off the main
  sequence. This star cluster, therefore, must be
  relatively old.

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HR Diagram for M13
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The Horizontal Branch

• The horizontal branch stars in this last HR
  diagram are believed to be stars that have
  already experienced the helium flash.
• Note the gap in the horizontal branch. This is
  the region occupied by the variable stars.

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Data from Hipparcos
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Composite HR Diagrams for Several Star Clusters
The turn-off point for each cluster gives us an
estimate of the age of each star cluster. Notice
the gap in the central part of this diagram.
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Population I and Population II Stars

• The younger star clusters in this last HR diagram
  correspond to open, or galactic, clusters found
  in the arms of our galaxy.
• The older star clusters correspond to globular
  clusters which are found in a halo about the
  central nucleus of our galaxy.
• These older clusters contain stars which are
  metal-poor, believed to have arisen at the
  earliest stages of our galaxy, while the younger
  clusters contain stars which are metal-rich,
  deriving the heavier metals from previously
  existing stars. Metal-rich stars are also called
  population I stars, while metal-poor stars are
  called population II stars.

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The Instability Strip (or Gap)

• The apparent gap in the previous HR diagram for
  various star clusters seems to correspond to the
  region where we often find variable intensity
  stars.
• Stars moving across the upper region of this
  strip correspond to Cepheid variables, while
  stars moving across the lower part of the strip
  correspond to RR Lyrae variables.

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Variable Stars

• Variable stars are stars that periodically vary
  in brightness.
• There are two types of variable stars
• RR Lyrae variables correspond to low-mass, post
  helium-flash stars. Their periods are all
  shorter than one day.
• Cepheid variables correspond to high-mass stars
  and appear to pass back and forth through the
  instability strip. These stars are particularly
  important because astronomers have found that
  their period is directly related to their average
  luminosity.

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Mira (omicron Ceti),the First Variable

• This pulsating variable was discovered in 1595 by
  a Dutch minister and amateur astronomer David
  Fabricius.
• He noted the Omicron Ceti varied in apparent
  brightness sometimes being bright enough to see
  with the naked eye, and sometimes fading
  completely from view.
• By 1660, astronomers realized that the stars
  brightness varied with a period of 332 days.
• Mira is an example of long period variables
  cool red giants that vary in brightness by a
  factor of 100 or more over a period of months or
  years.

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Mira A Long-Period Variable
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RR Lyra Variables

• These are typically low-mass, metal poor stars
  often associated with globular clusters.
• They all have periods of less than a day.

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Cepheid Variables

• Cepheids have a characteristic light curve
  showing an abrupt brightening, followed by a
  slower dimming.
• They are large mass, highly luminous stars which
  can be seen over great distances.
• Delta-Cephei, the first discovered Cepheid
  variable was discovered in 1784, and was found to
  vary in brightness by a factor of 2.3 with a
  period of about 5.4 days.

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The Period-Luminosity Plot for Cepheid Variable
Stars
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Cepheid Variables and Distance

• Variations in the brightness of Cepheid variables
  corresponds to variations in the size of the star
  (as determined from Doppler shift measurements).
• Type I Cepheid variables are metal-rich, and are
  brighter than type II Cepheid variables.
• Cepheid variables can act as standard candles
  to determine the distance to stars, since the
  period luminosity curve provides a means of
  calibration of the Cepheids.
• Knowing the luminosity, and the apparent
  magnitude of a Cepheid variable enables
  astronomers to determine the distance to the star.

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The Death of Stars

• The manner in which a star burns out depends
  strongly on the mass of the star.
• Low-mass stars (with masses less than about 2 3
  solar masses) end relatively quietly, producing a
  planetary nebula and a white dwarf.
• High-mass stars, however, end much more
  violently, producing a supernova and either a
  neutron star, or a black hole depending upon
  the original mass of the star.

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The Death of a Low-Mass Star
Expansion upon hydrogen burn-out.
Expansion upon helium burn-out.
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Planetary Nebula Arise when the Outer Shell of
the Star is Blown Away
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The shapes of the planetary nebula are quite
varied, depending upon the magnetic fields
associated with the star and upon previous nebula
surrounding the star.
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White Dwarfs

• The remaining core of the low-mass star is small
  and hot and is called a white dwarf.
• This white dwarf slowly burns out, with no
  further excitement, eventually becoming a black
  dwarf unless it has a companion star which can
  feed it more stellar material!

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Companion Stars
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White Dwarfs as Companion Stars

• Sometimes a white dwarf may be one of a binary
  star system.
• As material (hydrogen) from its companion leaks
  onto the surface of the white dwarf, the pressure
  and temperature build up until the outer hydrogen
  shell ignites and causes an explosion producing
  one class of nova.
• This type of nova may occur several times, being
  somewhat periodic as material leaks from the
  companion star and subsequent nova occur these
  are called recurrent nova.

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Type Ia Supernova

• When the companion star of a binary system
  expands greatly and looses a large amount of its
  outer gas shell very rapidly, the white-dwarf
  companion may be compressed to the point where
  the carbon-oxygen core begins to burn.
• This gives rise to a type Ia supernova a very
  large explosion which destroys the white dwarf.

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The Death of High-Mass Stars

• Unlike the low-mass stars, the death of high-mass
  stars is much more dramatic.
• These stars end their existence explosively in
  what is known as a type II supernova event, where
  the outer layers of the star are blown away.
• During the supernova explosion, the stars
  luminosity may increase as much as 100 million
  time.
• The inner part of the star is compressed in the
  supernova explosion and produces either a neutron
  star, or a black hole.

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Pulsars

• In November of 1967 Jocelyn Bell, working with a
  newly developed radio telescope at Cambridge
  University detected a strange periodic signal
  with an extremely regular period of 1.3373011
  seconds.
• This was initial taken as an indication of
  intelligent life, until other similar objects
  were detected in other parts of the sky.
• The regular pulsing radio sources soon came to be
  known as pulsars.

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Radio Emissions from one of the First Pulsars to
be discovered, PSR 032954, with a period of
0.714 seconds.
But how can it pulse so rapidly?
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What is the source of these Pulsars?

• Many different explanations were initially
  proposed, which were later discarded.
• Some proposed that the stars surfaces pulsated
  this rapidly, but pulsations this rapid would
  cause the star to explode.
• Some astronomers proposed that the pulsating
  signal arose from the rotation of a star.
• Although most astronomers at that time believed
  that the majority of dead stars were white
  dwarfs, the discovery of very rapid pulsars, like
  the one in the Crab Nebula (period of 0.033
  seconds), indicated that a new type of star
  much smaller and much more dense must be the
  source of these pulsations.
• These stars must be similar to the neutron
  stars proposed by Fritz Zwicky and Walter Baade.

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Neutron Stars and Pulsars
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The Crab Nebula Remnants of a Supernova
X-ray image
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The Crab Pulsar

• Shortly after the discovery of the first pulsars,
  strong bursts of radio energy were observed
  coming from the Crab Nebula with a frequency
  almost 10 times greater than previous pulsars.
  This was additional proof that the source must be
  neutron stars.
• Further evidence for a rotating neutron star was
  the discovery that the frequency of the Crab
  pulsar was slowly decreasing as would be the case
  of a source constantly emitting energy.
• Later, both an optical pulsar and an x-ray pulsar
  with the same frequency were observed in the Crab
  Nebula.

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The Spin-Down of a Pulsar

• As a pulsars continuously emits radiation, it
  slowly decreases its rotational velocity. This
  is the so-called spin down of a pulsar.
• Adjustments in the surface of a neutron star
  (similar to earthquakes) cause sudden jumps in
  the stars angular momentum, introducing
  glitches in a pulsars spin-down plot.

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Long and Short Pulsars

• In 1982, scientists were surprised to find a
  pulsar with a frequency of 642 Hz. Since then
  many millisecond pulsars have been discovered.
• The longest period pulsar has a period of over 8
  seconds.

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End of Part XI
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The Eagle Nebula
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Blow-Up of the Eagle NebulaShowing Star
Formation Regions
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