Hubble

1
Hubble's Tuning Fork
By Tali Yahalom March 28, 2006
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How It All Began

• Scientists today know about the existence of
  galaxies, but a couple of hundred years ago, the
  only known celestial objects were point-like
  sources, including planets, comets, and
  relatively steady, distant stars.
• But curiosity spread and knowing about point-like
  sources was not enough.
• While working in an observatory in Paris in 1758,
  Charles Messier was looking for Halleys comet
  when he discovered faint-looking objects in the
  sky.

Charles Messier 1730 - 1817
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• Messier called these objects NEBULAE, and
  started to catalog them in order to aid his
  observations of comets.
• The first entry in his catalogwhich he noted as
  M1is what we today call the CRAB NEBULA.
• When Messier published
• his catalog in 1781,
• he listed 103 nebulae.
• 7 more objects were
• added later on.

4 Different Views of the Crab Nebula
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BUT
Harlow Shapley ? ? ?
Heber Curtis ? ? ?

• The nature of the nebulae was constantly
  questioned because the telescope resolution was
  not good enough to see the objects details
  clearly.
• So, in 1920, the National Academy of Sciences in
  Washington organized a debate for astronomers
  Harlow Shapley and Heber Curtis concerning the
  nature of the nebulae.
• The 2 opposing theories were that
• 1) The Universe is rather small, containing
  nebulae that are nearby gas-clouds (Shapley)
• 2) The nebulae are very distant island
  universes (Curtis)
• This Great Debate incorporated important
  issues, like how to measure distance to nebulae,
  how to determine the material composition (gas or
  many stars), and why there is a zone of
  avoidance, i.e., a region of the sky where
  nebulae are not found.

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Uh Oh!!!

• A few years
• later, Edwin Hubble
• settled the issues

How can I classify the Universe?
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Spiral Galaxies

• Hubble isolated Cepheid variable stars in the
  Andromeda galaxy (M31) and measured the distance
  to these stars.
• Remember A Cepheid variable star is a member of
  a particular class of variable stars (varying
  luminosity), notable for a fairly tight
  correlation between their period of variability
  and absolute luminosity. The greater the period,
  the greater the luminosity.)
• The distance turned out to be much bigger than
  the possible size of the Milky Way!
• Hubble, therefore, concluded that M31 must be a
  separate and distant island Universe, i.e., a
  spiral galaxy similar to our own.

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Hubbles Pictures

• Hubble started to take pictures of many galaxies.
• He soon realized that they come in many shapes
  and sizes.
• In order to make sense of his findings, he
  implemented a system of classification based on
  the appearance of these systems

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Discovery ? Classification

• The two types of galaxies he discovered were
  Elliptical and Spiral.
• Hubble divided spiral galaxies into two groups
  Normal and Barred.
• Normal Spiral bulge in the center, spiral arms
  extending out from the central regions.
• Barred Spiral spiral arms originate from the
  tips of a straight bar and go through the center
  of the galaxy.

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Normal Spiral vs. Barred Spiral
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Theres More

• Hubble also introduced the classes of lenticular
  and irregular galaxies.
• Lenticular between ellipticals and spirals,
  consist of a large bulge with a small flattened
  disk around them without visible spiral
  structure.
• Irregular account for 5 of the galaxies we
  observe nearby, are forming, colliding, and do
  not fit in with any of the other 3 categories.

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Lenticular and Irregular Galaxies
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ORGANIZATION?!?!

• Hubble constructed a tuning-fork diagram to
  illustrate his classification system.
• Elliptical galaxies (E) are on the stem and are
  subdivided into classes 0 7. The higher the
  number, the more elongated the galaxy.
• Lenticulars are denoted S0 (or SB0, with a bar)
  and are subdivided into classes 1, 2, and 3
  depending on dust absorption within the disk.
• Spirals are denoted S (normal) and SB (barred)
  and are subdivided into classes a, b, and c. a
  signifies a large bulge and relatively weak,
  tightly wound spiral arms. As the letters
  continue, these traits change to their opposite.

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Visualization
Normal Spiral galaxies that begin with large
bulges and small arms that change
Elliptical galaxy with a round looking elliptical
Elliptical galaxy with an elongated elliptical
Barred Spiral galaxies that begin with large
bulges and small arms that change
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How Does Hubble Apply Today?

• Since Hubbles time, tens of thousands of
  galaxies have been imagined using different kinds
  of telescopes and instruments.
• Many of these galaxies do not fit into Hubbles
  relatively simple classification system.
• Many more extensions to the tuning fork diagram
  came about after Hubbles discovery.
• De Vaucouleurs, for example, distinguished
  between s-shapes and rings, thus making the
  tuning fork 3-D.
• While Hubbles diagram is neither finite nor as
  simple as it may seem, it is nevertheless the
  backbone for all observations and classifications
  that still go on today.

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A Diagram that Includes Lenticular and Irregular
Galaxies
A CLOSER LOOK
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Bibliography

• http//www.astro.princeton.edu/frei/Gcat_htm/cat_
  ug_1.htm
• Foundations of Modern Cosmology, John Hawley and
  Katherine Holcomb
• www.wikipedia.org (Cepheid Variable Stars)