1446 Introductory Astronomy II

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1446 Introductory Astronomy II

• Chapter 16A
• Galaxies, Clusters Cosmic Rays
• R. S. Rubins
  Fall, 2009

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Spiral and Barred Spiral Galaxies 1

• Spiral galaxies (Sa, Sb, Sc) are characterized by
• i. a nuclear bulge
• ii. a flattened disk containing spiral arms
• iii. a spherical halo, in which most globular
  clusters are found.
• Barred spirals (SBa, SBb, SBc) contain, in
  addition, a bar of stars passing through the
  nuclear bulge.
• In moving from Sa to Sc (or from SBa to SBc), the
  following changes occur
• i. the spiral arms become more loosely
  wound
• ii. the nuclear bulge decreases in size
• iii. the percentages of gas and dust
  increase.

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Spiral and Barred Spiral Galaxies 2

• As the disk rotates about the galactic center,
  the outer parts of the arms rotate more slowly,
  giving trailing spiral arms.
• The most commonly occurring number of spiral arms
  is 2.
• Spiral galaxies have diameters in the range
  50,000 200,000 ly, containing about a billion
  (109) to a trillion (1012) stars.
• The Milky Way galaxy is near the middle of this
  range, with a diameter of 100,000 ly, and
  containing about 200 billion stars

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NGC 1357 an Sa Galaxy
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M51 an Sc Galaxy

• M51 is popularly known as the Whirlpool
  Galaxy.

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NGC 4321 an Sc Galaxy
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M104 an Sa Galaxy
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Flocculent and Grand Design Spirals

• Flocculent spiral galaxies have fuzzy,
  poorly-defined arms.
• Grand design Sc spiral galaxies have
  clearly-defined arms.

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M74 an Sc Galaxy

• M74 is possibly the most photogenic of grand
  design galaxies.

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M58 an SBa Galaxy
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NGC 1365 an SBc Galaxy
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Gas Rich and Poor Regions

• New star formation occurs in the gas and dust
  associated with the interstellar medium.
• In a spiral or barred spiral galaxy, such regions
  occur in the galactic nucleus and spiral arms,
  which are termed gas-rich.
• Conversely, the halo is gas-poor, in general,
  even within globular clusters.
• While stars in the disk orbit the galactic center
  in the same rotational direction, those in
  globular clusters and the nuclear bulge orbit
  with random orientations and directions.

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Density-Wave Theory I

• In the 1960s, Lin and Shu at MIT developed
  density wave theory, in which the spiral arms are
  regions of new star creation in high gas-density
  regions caused by their passage through a shock
  wave.
• In this theory, waves of enhanced density, which
  trigger new star formation, orbit more slowly
  than the stars and gas.
• A flaw later found in density wave theory is that
  the shock waves would lose their energy in the
  interstellar gas.
• In the 1980s and 1990s, Francoise Coombe rescued
  the density wave model by showing that
  interstellar gas collisions produce new shock
  waves.

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Density-Wave Theory 2

• This proposal has been likened to the crowding of
  autos on a portion of a highway where they have
  to overtake a large truck.

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The Changing Shapes of Spiral Galaxies 1

• Spiral and barred spiral galaxies appear to us as
  permanent static objects, but in fact have
  continuously changing shapes, since the inner
  stars orbit much faster.
• In the five billion years of the Milky Ways
  existence, the innermost stars have orbited the
  galactic center thousands of times, which may be
  compared to the roughly twenty orbits of the Sun.
• The difference in orbital rates is the reason why
  spirals cannot be fixed structures, since this
  difference would cause them to coil up tightly
  around the galactic nucleus, and disappear within
  about a billion years.

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The Changing Shapes of Spiral Galaxies 2

• This computer simulation shows how a particular
  galaxy switches between a barred spiral (at about
  5 and 14 billion years) and a regular spiral (at
  about 11 and 20 billion years).

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Elliptical Galaxies 1

• As viewed from the Earth, the shapes of
  elliptical galaxies vary from spherical (E0) to a
  very elongated ellipsoid (E7).
• Note that a galaxy denoted E0, as viewed from
  Earth, may in fact be elongated when viewed from
  a different direction.

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Elliptical Galaxies 2

• Elliptical galaxies have neither spiral arms nor
  a disk.
• Giant ellipticals are appreciably larger than
  spirals, containing trillions of stars in regions
  of up to about ten million ly across.
• Dwarf ellipticals are appreciably smaller than
  spirals, containing typically 10 to 100 million
  stars in regions about 1 to 10 thousand ly
  across.
• Dwarf ellipticals outnumber giant ellipticals by
  about 101.
• Stellar orbits in elliptical galaxies are
  oriented randomly.
• Most elliptical galaxies are old, containing
  little interstellar dust, and consisting mainly
  of low-mass stars, although collisions with other
  galaxies could produce appreciable quantities of
  younger or middle-aged stars.

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Dwarf Elliptical E4

• Known as Leo I, E4, a satellite of the Milky
  Way, is just 3000 ly across, and contains so few
  stars, that one can see through its center.

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Giant Ellipticals in the Virgo Cluster
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Comparison of Galactic Types
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Hubble Diagram
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Lenticular Galaxies

• Lenticular galaxies, like spirals, have disks and
  central bulges, but unlike spirals have no spiral
  arms.
• Their designations are S0 or SB0

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Irregular Galaxies

• Irregular galaxies are misshapen spirals
  caused by galactic collisions.
• The Large Magellanic Cloud (shown below) is close
  enough to be seen in the southern hemisphere with
  the unaided eye.

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Rotation Curves of Four Spiral Galaxies

• Just as found for the Milky Way, the rotation
  curves for the galaxies shown below indicate that
  the total mass in each galaxy consists of 90
  dark matter.

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On Dark Matter

• The results obtained for the Milky Way appear to
  be a general rule for spiral and barred spiral
  galaxies.
• Many other indirect observations made in the last
  few years have been explained by the presence of
  dark matter.
• One of the primary quests of the present time is
  to determine the particles which constitute dark
  matter.
• In an alternative theory known as Modified
  Newtonian Dynamics (MOND), Milgrom (1983) showed
  that the failure of orbital speeds to decrease
  with distance from the galactic center can be
  explained by modifying Newtons Law of
  Gravitation.
• A similar approach, which is modification of
  Einsteins General Relativity, was used by Moffat
  (2005).

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Clusters of Galaxies

• A cluster is a grouping of galaxies, which are
  bound to each other by gravity, and so orbit a
  common center.
• Clusters may be rich or poor, and regular or
  irregular.
• A rich cluster may contain many thousands of
  galaxies, a poor cluster less than a hundred.
• A regular cluster is a spherical distribution
  with an increasing concentration as the center of
  the sphere is approached.
• The Milky Way is a member of a poor, irregular
  group, consisting of about 40 galaxies, which is
  known as the Local Group, and extends about 3
  million ly from the Sun.

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The Local Group 1
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The Local Group 2

• There are just three spiral galaxies in the Local
  Group the Milky Way, M33, and Andromeda (M31).
• While Andromeda was previously thought to be the
  largest galaxy in the Local Group, a 2008 study,
  has indicated that our galaxy is about 50 percent
  more massive than previously thought, putting us
  roughly equal in mass to Andromeda.
• Infrared measurements on M33, the 3rd largest
  galaxy in the Local Group, made in 2009, have
  indicated that it is appreciably larger than
  previously deduced from optical studies.
• The other galaxies are irregulars, such as the
  Large and Small Magellanic Clouds, and dwarf
  ellipticals, such as the recently-discovered
  Sagittarius, Antlia and Canis Major (the nearest
  at 25,000 ly from the Milky Ways galactic
  nucleus).

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Andromeda M31
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Nearby Clusters

• The Virgo Cluster, about 50 million ly away is
  our nearest neighboring cluster, while the Coma
  Cluster is a rich regular cluster about 300
  million ly away, containing an estimated 10,000
  galaxies.

The Fornax Cluster (opposite) is about 60
million ly away.
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Gallactic Collisions 1
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Gallactic Collisions 2

• The larger galaxy (NGC 2207) is expected to
  eventually incorporate the smaller one (IC 2163).
• In about 2 billion years time, Andromeda and the
  Milky Way are expected to begin a similar
  collision, billions of years later combining to
  form a large elliptical galaxy.

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Superclusters and Hubbles Law

• The largest scale of groupings of gravitationally
  attracted objects appears to be clusters of
  clusters, or superclusters, which are typically
  over 100 million ly across.
• We belong to the Local (or Virgo) Supercluster,
  which includes the Virgo Cluster, and has a total
  mass of about a million billion (1015) solar
  masses.
• Galaxies beyond the Local Supercluster all recede
  from us, with speeds that appear to be
  proportional to their distances from us.
• This rule is known as Hubbles Law, which applies
  to galaxies beyond distances of about 10 Mpc (or
  33 Mly).

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Our Address
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Galactic Distributions 1
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Galactic Distributions 2
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Cosmic Rays 1

• Cosmic rays are charged particles, high energy
  protons together with nuclei, mainly up to the
  size of nickel. These bombard the Earth from all
  directions, traveling at close to the speed of
  light.
• Cosmic rays were discovered in 1912 by an
  Austrian, Victor Hess (the 1936 Nobel Prize
  winner), who made balloon flights to 17,000 ft.
• Possible sources of cosmic rays, such as the very
  high energy particles emitted from the material
  falling into supermassive black holes, are still
  under investigation.
• The major experiments today are being run by the
  NASA Balloon Facility, which is sending balloons
  to heights of about 130,000 ft in Antarctica.

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Cosmic Rays 2

• When cosmic rays collide with the much denser gas
  in the Earths atmosphere, typically about 15 km
  (50,000 ft) above the Earths surface, a cascade
  of lower-energy particles is produced, which
  reaches the surface as a cosmic ray shower.
• These particles are known as secondary cosmic
  rays.
• In 1 hour, about 100,000 such particles strike a
  person.
• Muons are unstable elementary particles produced
  when cosmic rays strike atoms in the upper
  atmosphere. They have average lifetimes of
  several microseconds, and should travel less than
  1000 ft before decaying.
• The fact that many muons reach the ground is an
  experimental verification of the time dilation
  and length contraction predictions of Einsteins
  special theory of relativity.

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Cosmic Rays 3
Cosmic ray shower