Nebulas, Galaxies, Black Holes

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Nebulas, Galaxies, Black Holes and Other
Interesting Deep Space Stuff
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Vocabulary
Star Cluster a number of stars of common origin
held together as a group by gravitational
attraction. Includes globular clusters and open
clusters. Nebula a diffuse mass of
interstellar gas (mostly hydrogen) or dust. Types
of nebulae include dark, emission, planetary,
and reflection. Galaxy - a large system of
stars held together by mutual gravitation and
isolated from similar systems by vast regions of
space. Types of galaxies include elliptical,
spiral, barred, and irregular. Black Hole - a
theoretical massive object, formed at the
beginning of the universe or by the gravitational
collapse of a star exploding as a supernova,
whose gravitational field is so intense that no
electromagnetic radiation (such as light) can
escape.
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• Star Cluster Star clusters are groups of stars
  which are
• gravitationally bound.
• Globular Clusters are tight groups of hundreds of
  thousands of very old stars.
• Open Clusters generally contain less than a few
  hundred members, and are often very young stars
  loosely grouped.

Globular Cluster
Open Cluster
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A Planetary Nebula consists of a hot, blue-white,
central star surrounded by an envelope of
expanding gas.
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Planetary Nebulae are unrelated to planets. The
name originates from a similarity in appearance
to the gas giant planets. They are a short-lived
phenomenon, lasting a few tens of thousands of
years - that is, compared to a typical stellar
lifetime of several billion years. About 1,500
planetary nebula are known to exist in the Milky
Way Galaxy.
The first planetary nebula discovered was the
Dumbbell Nebula in the constellation of
Vulpecula, observed by Charles Messier in 1764
and listed as M27 in his catalog of nebulous
objects.
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An Emission Nebula is a cloud of ionized gas
(also known as a plasma) emitting light of
various colors. The most common source for
ionization are high-energy photons emitted from a
nearby hot star.
The North America Nebula
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The Horse Head Nebula
The Bubble Nebula
The Trifid Nebula
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A Dark Nebula is a type of interstellar cloud
that is so dense that it obscures the light from
the background emission or reflection nebula, or
that blocks out background stars.
The Horse Head Nebula
The Snake Nebula
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Reflection Nebulae are clouds of dust which are
simply reflecting the light of a nearby star or
stars. These nearby stars are not hot enough to
cause ionization in the gas of the nebula like in
emission nebulae but are bright enough to give
sufficient scattering to make the dust visible.
The Pleiades
The Witch Head Nebula
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A Galaxy is a massive, gravitationally bound
system that consists of stars, an interstellar
medium of gas and dust, and dark matter.
Astronomers classify galaxies based on their
overall shape (elliptical, spiral or barred
spiral). The system of galaxy classification is
called the Hubble "tuning fork" diagram.
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The Tuning Fork

• Spirals
• Sa - tightly-wound, smooth arms, and a bright
  central disc
• Sb - better defined spiral arms than Sa
• Sc - much more loosely wound spiral arms than Sb
• Sd - very loose arms, most of the luminosity is
  in the arms and not the disc
• Barred Spirals
• SBa - a bright center and tight spirals
• SBb - better defined arms than SBa galaxy and are
  more loosely wound
• SBc - even looser arms, and a much dimmer central
  portion of the galaxy

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• Spiral Galaxy
• Characteristics
• Composed of a central bulge surrounded by a disk
  
• The bulge resembles an elliptical galaxy,
  containing many old, so-called "Population II"
  stars, and usually a super-massive black hole at
  its center.
• The disk is a flat, rotating assembly consisting
  of interstellar matter, young "Population I"
  stars and open star clusters.

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• Elliptical Galaxies
• Characteristics
• The motion of stars is dominated by random
  motion, unlike spiral galaxies, which have very
  little random motion and are dominated by
  rotation.
• Very little interstellar matter, few young
  stars, and few open star clusters
• Consist of old, so-called Population II stars
• Larger elliptical galaxies typically have a
  system of globular clusters, indicating an old
  population.

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A Barred Spiral Galaxy is a spiral galaxy with a
band of bright stars emerging from the center and
running across the middle of the galaxy. Spiral
arms appear to emerge from the ends of the "bar"
in these galaxies, whereas they appear to emerge
directly from the core in ordinary spiral
galaxies.
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An Irregular Galaxy is a galaxy that does not
fall into the Hubble classification for galaxies.
These are galaxies that feature neither spiral
nor elliptical morphology. They are often chaotic
in appearance, with neither a nuclear bulge nor
any trace of spiral arm structure. Collectively
they are thought to make up about a quarter of
all galaxies. Most irregular galaxies were once
spiral or elliptical galaxies but were deformed
by gravitational action.
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Follow the M82 link to a great short video about
this irregular galaxy, called the Cigar Galaxy.
M82 M81
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http//amazing-space.stsci.edu/resources/explorati
ons/
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A Blackhole is defined to be a region of
space-time where escape to the outside universe
is impossible. The outer boundary of this region
is called the event horizon. Nothing can move
from inside the event horizon to the outside,
even briefly, due to the extreme gravitational
field existing within the region.
http//amazing-space.stsci.edu/resources/explorat
ions/
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