The Deep Sky and Telescope Observing

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The Deep Sky and Telescope Observing
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Deep Sky Objects

• ? Deep sky objects faint objects which are
  not visible to the
• unaided eye.
• ? The most popular lists of deep sky targets
• - "Messier catalogue (Charles Messier)
• - "New General Catalogue" and the "Index
  Catalogue (John Dreyer) .
• ? Deep sky objects provide some of the most
  spectacular and well-
• known astronomical images

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• Unfortunately, after seeing these photographs,
  many people who view some of these deep sky
  objects directly through the eyepiece of a small
  telescope are disappointed to find a view that is
  less visually stimulating. The difference between
  naked eye and photographic views is because
• - Many of the photographs are taken at the focus
  of larger telescopes than are available to
  amateur astronomers.
• The photographs are time exposures that collect
  photons from the source for long periods of time,
  many hours in some cases.
• In contrast, the "integration time" (or exposure
  time) of the human eye is about 1/30 of a second.
  
• - At low illumination levels, the human eye loses
  the ability to perceive color.

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Messier Catalog

• ? The Messier catalog was assembled from 1758 to
  1782 by French astronomer Charles Messier.
• ? Messier was devoted to finding comets.
• - survey diffuse source looking like comets
• - record uninteresting objects for comet
  searchers
• ? 110 objects galaxy, star cluster, nebular
• ? Messier objects are listed by an "M" and their
  number
• - "M31" -- the Andromeda galaxy
• - "M42" -- the "Orion Nebula".

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• Images of the Messier objects in approximate
  numerical order

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New General Catalog and Index Catalog

• ? John Louis Emil Dreyer
• - New General Catalog, 1988
• - Index Catalog, 1895 Second Index
  Catalog, 1908
• ? These catalogues contain many more objects
  found after Messier by a number of observers. The
  original source of objects came from a "General
  Catalogue" of nebulae and star clusters published
  by Sir John Herschel in 1864.
• ? The Royal Astronomical Society of England asked
  J.L.E. Dreyer, a Dane working in Ireland, to make
  sense and order out of the numerous lists coming
  in from observers around the world. Dreyer set
  about to make a "new" General Catalogue.

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New General Catalog and Index Catalog

• ? John Louis Emil Dreyer
• - New General Catalog, 1988
• - Index Catalog, 1895 Second Index
  Catalog, 1908
• ? These catalogues contain many more objects
  found after Messier by a number of observers. The
  original source of objects came from a "General
  Catalogue" of nebulae and star clusters published
  by Sir John Herschel in 1864.
• ? The Royal Astronomical Society of England asked
  J.L.E. Dreyer, a Dane working in Ireland, to make
  sense and order out of the numerous lists coming
  in from observers around the world. Dreyer set
  about to make a "new" General Catalogue.

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• ? Dreyer worked at the observatory of the third
  Earl of Rosse in Parsontown, Ireland, where the
  Earl had built the largest telescope in the world
  -- a 72-inch reflector called The Leviathan,

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• ? There are 7840 objects in the present NGC and
  5386 in the two ICs.

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Deep Sky Objects Globular Clusters
The globular clusters M13, M72, M15, M55, and M22

• ? Group of 104 106 or more stars
  gravitationally bound to one another in a
  spherical cluster.
• ? Stars in globular clusters are among the
  oldest stars in the Galaxy (and universe) -
  typically more than 12 Gyr old.
• ? Stars in a globular cluster were born together
  at the same time, from the same cloud of gas.
  Thus, all stars in the cluster have the same age
  and chemical composition, but a range of masses.

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• ? The globular clusters are typically
  associated with the older parts of the Milky Way
  galaxy - e.g., the halo and the bulge.

THOUGHT PROBLEM TIME OUT 1. Why would the
brightest stars in the cluster appear
yellowish-red (as in the color image of M22
above)? 2. In the early part of the 20th
century, an astronomer named Harlow Shapley used
the observed distribution of globular clusters to
estimate the distance of the Sun from the center
of our galaxy. Can you guess how this worked?
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Deep Sky Objects Open Clusters
The open clusters M44, M37, M52, and M6

• ? Open clusters are not as centrally
  concentrated as globular clusters, and there is
  more "open space" between the stars.
• ? Contain about 103-105 stars, all born at the
  same time from the same gas cloud
• ? Open clusters are younger than globular
  clusters, with ages ranging from millions of
  years to 10 billion years of age.
• ? They are associated with younger populations of
  the Milky Way, like the star forming disk.

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• THOUGHT PROBLEM TIME OUT
• Why are open clusters typically found near the
  path of the Milky Way around the sky, whereas
  globular clusters can appear in any direction in
  the sky?

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Deep Sky Objects Diffuse Nebulae
The Reflection nebulosity around the Pleiades
(M45), excited and reflected emission around the
Trifid Nebula (M20), the famous
emission/reflection nebula in Orion (M42).

• ? Collection of interstellar gas and dust.
• ? Typically collecting through self-gravity and
  on the way to forming stars and star clusters, or
  leftover from such a process.
• ? A dust cloud illuminated by the reflection of
  light from nearby stars is called a reflection
  nebulae .
• - Typically found near bright, high mass stars
  in associations that had formed out of the cloud.
  
• - The blue wisps around the stars in the
  Pleiades (shown above) are part of a famous
  reflection nebula.

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Deep Sky Objects Diffuse Nebulae
The Reflection nebulosity around the Pleiades
(M45), excited and reflected emission around the
Trifid Nebula (M20), the famous
emission/reflection nebula in Orion (M42).

• ? Other clouds of gas glow by their own light,
  particularly when they are very hot. These emit
  an emission line spectrum.
• - For example, the Trifid Nebula. Some of these
  regions that are hot enough to have both excited
  and ionized hydrogen are called HII regions .

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Deep Sky Objects Supernova Remnants
The Crab Nebula (M1) and Veil Nebula
(NGC6992/6995/6996) supernova remnants.

• ? Represent the violent explosion of a star,
  either a high mass stars (Supernovae Type II) or
  a white dwarf star in a binary system in which
  gas from the other star falls onto the white
  dwarf and detonates the explosion (Supernovae
  Type I).
• ? The famous Crab Nebula (M1) is the remains of a
  star that was recorded by Chinese astronomers to
  have exploded in the year 1054.
• ? The nebulae glows with an emission like
  spectrum.

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Deep Sky Objects Galaxies
The spiral galaxy M51 with a dwarf companion, a
barred spiral galaxy M91, an irregular galaxy
M82, and the elliptical galaxy M60 with
companion. The latter three are shown at a bit
higher quality than you might expect to see them
in your telescope.
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Deep Sky Objects Galaxies
The Large (left) and Small (right) Magellanic
Clouds.

• ? Irregular galaxies that are satellites of the
  Milky Way.
• ? The LMC is about 1/10 the mass of the Milky Way
  (about 1011 solar masses), the SMC about 1/30.
• ? The LMC and SMC are important, and obvious,
  naked eye features of the southern hemisphere
  sky.
• There are about 20 other known satellite galaxies
  of the Milky Way (a half dozen or so just
  discovered in this LAST YEAR!), but these are
  typically too faint and diffuse to see with a
  small telescope, and, indeed, have only been
  discovered in the past few decades in most cases.
  
• These smallish systems are from 106 to some 109
  solar masses (i.e. a million to a billion times
  smaller than the Milky Way) and are called dwarf
  spheroidals.
• In some cases, these satellites are being
  cannibalized by the MW.

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Deep Sky Objects Galaxies
A dwarf spheroidal satellite galaxy of our Milky
Way in Leo. Note how wimpy this magnitude 11
object looks compared to the bright star in the
picture, which is Regulus. This picture took
almost eight hours of exposure with a 4-inch
telescope and a digital camera.

• ? There are about 20 other known satellite
  galaxies of the Milky Way, but these are
  typically too faint and diffuse to see with a
  small telescope, and, indeed, have only been
  discovered in the past few decades in most cases.
  
• ? From 106 to some 109 solar masses (i.e. a
  million to a billion times smaller than the Milky
  Way) and are called dwarf spheroidals.
• ? In some cases, these satellites are being
  cannibalized by the MW.

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Deep Sky Objects Galaxies
M31 (left) and M33 (right)

• ? The Andromeda Galaxy (M31) is a near twin of
  the Milky Way, and is the dominant member of the
  so-called Local Group of galaxies.

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Deep Sky Objects Cluster of Galaxies
The Virgo cluster has 2000 galaxies. The most
obvious seen in this image are 16 objects that
are in the Messier catalogue. This image is about
4 degrees across.

• ? Many of the other galaxies in Messier's list
  are concentrated near the constellation Virgo.
• - This is the direction to the Virgo Cluster ,
  the nearest large collection of galaxies to the
  Local Group.

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• THOUGHT PROBLEM TIME OUT
• The highest concentration of Messier objects is
  near the constellation Sagittarius. This is also
  the direction toward the center of the Milky Way
  galaxy. Is this a coincidence?
• 2. What kinds of Messier objects should, or
  should not, be concentrated in this direction?