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Night SkyNight Sky Lecture 5

Night Sky Lecture 5

by Pat Browne

Spring Astronomy Course Mississippi Valley NightSky Conservation

V Observing Star Clusters and Galaxies in the Night Sky Around Us

Night Sky for May 13 2013 - courtesy Earth Centered Universe

Night Sky for May 31 2013 – courtesy Earth Centered Universe

Because of the NightSky Conservation Program, we are fortunate to be able to observe our celestial ‘friends’: Galaxies, Globular Clusters and our own Milky Way Galaxy seen below.

Review of Lecture IV – Star Light Star Bright(ness), as artistic and scientific expressions

Starlight, in its myriad colors, spectral signatures and intensities revealed the secrets of the heavens first to Galileo and Newton…

What Can we glean from Starlight?
  • Color (representing peak wavelength)

  • Spectral lines in the Color Bands

  • Periodic Brightening of Special Stars (Variables)

What do stellar classifications tell us?

  • Mass

  • Temperature/Thermonuclear processes

  • Chemical Composition ‘metalicity’

  • Stage of life…Giants, Dwarfs, Stars on the Main Sequence

How far are the stars and star clusters?

  • The Calibrated Distance ladder – When do they start their hydrogen burning and pursue their unique evolutionary path?

  • Stellar Evolution: Stages of development in Stellar History

  • Spectral classifications tell stories of Red Giants, White Dwarfs, and supernovae as
    different stars pursue their unique evolutionary path

Without an understanding of stellar properties:

  • Stars with nebulousity around them
  • Star clusters
  • Entire galaxies

would not be distinguishable from clouds in the Night Sky.


This final session takes us on a journey to

  • Galactic (open ) Star Clusters found in the plane of the Milky Way
  • Globular Star Clusters found in a halo around the Milky Way
  • Galaxies beyond the Milky Way
  • Clusters of Galaxies in collections forming their own systems beyond our Local Group


  • Types of Star Clusters 
  • Types of Galaxies that host our Star Clusters

Within the Milky Way and in other galaxies we classify clusters into two types:

    • Open (or Galactic)
    • Globular (More sophisticated divisions are necessary within these classes but unless you are on an observing campaign to study masses, densities,etc, this division will suffice).
Open Star Clusters

are groupings of dozens or hundreds of stars that formed millions of years  ago from a single interstellar cloud of gas and dust. They are not very compact which gives them their name – Open. They are also called galactic clusters because they are generally located within the Galaxy’s disk. Stars in a cluster vary over a wide range of values for:

  • mass
  • luminosity
  • lifespan – that is stellar evolution

The brightest members live the shortest lives. They will explode as supernovae in a few millions to tens of millions of years. As the cluster ages the more massive bright stars evolve and become less visible At the same time, other low-mass stars are ejected after gravitational encounters with more massive stars within the cluster. Hence over time, the cluster becomes less prominent Here is the Beehive Cluster . After 4 weeks (starting at 3rd quarter moon ), this cluster is now located due West and will set before the night is up. In total, this open cluster of stars consists of about 400 stars in a loose open ‘swarm’. The cluster is located only about 500 light years. The bright orange stars are ones that have had time to evolve into red giants. From evidence like this, one can conclude it is a relatively old Open Cluster, about 400 million years old.




Contrast this with…




Globular Clusters

Although globular clusters may initially seem the same as open clusters, they are quite different. While open clusters may contain hundreds of stars, globular clusters may have up to millions of members. They formed when our Galaxy took shape about 12 to 13 billion years ago before our Galaxy had acquired its flattened disk. Therefore, their orbits are not constrained to the plane of the Galaxy. Their high total mass holds the stars of the cluster together against the Galaxy”s disruptive gravitational tides. Gases left over from the formation of the cluster’s stars have been stripped from the cluster by the galaxies gravity. Therefore, no new stars have formed for many billions of years. The most massive stars died billions of years ago. Only the longer lived intermediate and low mass stars remain.


How we see them depends on our viewpoint and their intrinsic geometry! Normally we describe them as face-on, edge-on or tilted when we observe them

Types of Galaxies


Edwin Hubble (our featured astronomer for this lecture) observed many, many galaxies, and developed the ‘tuning-fork’ diagram to help classify them according to their geometrical disposition.As with many early schemes, it is hard to include the right taxonomy, but this scheme is still used in whole or part today

        • Elliptical
          (E0..E7) – example M104

        • Spiral (Sa,Sb,Sc) – example M81

        • Barred Spiral – example M109 see this! http://apod.nasa.gov/apod/ap130523.html
        • Lenticular
          (S0). These galaxies consist of a bright central bulge,
          similar in appearance to an elliptical galaxy, surrounded by an extended, disk
          structure. Unlike spiral galaxies, the disks of lenticular galaxies have no visible
          spiral structure and are not actively forming stars in any significant quantity. The bulge component is often the dominant
          source of light in a lenticular galaxy. Example – M86

        • Irregular, no particular shape
          Peculiar, …Often these galaxies are interacting with nearby neighbours, or there are energetic and disruptive forces at work producing jets of gas and dust with high kinetic energy.


Implications of Classification Scheme (Spiral, Elliptical, Irregular…)

This classification scheme is not an evolutionary sequence.


Young Galaxies are detected now as a result of merger events. In recent years, a great deal of focus has been put on understanding
merger events in the evolution of galaxies. Our own Milky Way galaxy has a tiny satellite galaxy, the Sagittarius Dwarf Elliptical Galaxy
which is (currently) gradually being ripped up and “eaten” by the Milky Way. It is thought these kinds of events may be quite common in the evolution of large galaxies. http://en.wikipedia.org/wiki/Galaxy_formation_and_evolution


Do clusters form in terms of their parent galaxies?

Do galaxies form, and are they mergers or the result of matter accretion (top down) or mergers (bottom up)

“All galaxies began forming at about the same time approximately 13 billion years ago. The origin of galaxies and how they changed over billions of years is an active field of research in astronomy today. Models for galaxy formation have been of two basic types: “top-down” and “bottom-up”. The “top-down” model on the origin of the galaxies says that they formed from huge gas clouds larger than the resulting galaxy. The clouds began collapsing because their internal gravity was strong enough to overcome the pressure in the cloud. If the gas cloud was slowly rotating, then the collapsing gas cloud formed most of its stars before the cloud could flatten into a disk. The result was an elliptical galaxy. If the gas cloud was rotating faster, then the collapsing gas cloud formed a disk before most of the stars were made. The result was a spiral galaxy. The rate of star formation may be the determining factor in what type of galaxy will form. But, perhaps the situation is reversed: the type of galaxy determines the rate of star formation. Which is the “cause” and which is the “effect”?”


This excerpt was copied from Nick Stobel’s Excellent chapter on Galaxy Origins
Nick Strobel’s Astronomy Notes.
Go to his site at www.astronomynotes.com
for the updated and corrected version.


The research continues…


The phenomena of red shift of the wavelength in spectral recordings of objects beyond our galaxy, as correlated with known spectral lines led to a revolution in our understanding of the distance (and hence the time) to these objects

        • Edwin Hubble – Discovery of Galaxies – early 20th century
        • Maarten Schmidt – Discovery of Quasars (Quasi-Stellar (Active Galaxy) Objects – 1963


Cosmological Redshift

Edwin Hubble determined that the recessional velocity of galaxies increased for fainter galaxies. He determined this by comparing the redshift in the spectral signal of their galaxies and their luminosities.
The redshift quantity (the percent by which the wavelength shifted) x C the speed of light is proportional to the distance x the Hubble constant: In other words

z*c = d*H.

Where z*c represents the recession velocity…


This animation was copied from
Nick Strobel’s Astronomy Notes.
Go to his site at www.astronomynotes.com
for the updated and corrected version.

Hubble’s law was determined for galaxies like the Virgo …Cluster of Galaxies which we can see in the Spring.


Extreme Cosmic Distances: Quasars Now a ½ century old

Astronomers have observed Quasi-Stellar objects which are at redshifts > 1 .For this to hold, the value of the object would be mathematically receding faster than the speed of
light. Therefore the relativistic transformation must apply. For a redshift (Z) of 2, the quasar velocity gets up to 0.8C

– calculations courtesy Zelik,Introductory Astronomy and Astrophysics, section on Quasars

Richard Preston, describing Maarten Schmidt’s First Light, p56 and 176

In March 16, 1963, Caltech astronomer
Maarten Schmidt wrote in the journal Nature that he had solved a puzzle about the quasi-stellar radio source 3C273. This object appeared starlike, like a point of light, with a mysterious jet. But itsspectrum – the range of wavelengths of its light – looked odd. Astronomers routinely use spectra to learn the composition of distant objects. But, in 1963, emission lines in the spectrum of 3C273 didn’t appear to match any known chemical elements. Schmidt had a sudden realization that 3C273 contained the very ordinary element hydrogen. He realized that the spectral lines of hydrogen appeared strange because they were highly shifted toward the red end of the spectrum. Such a large red shift could occur if 3C273 were very distant, about three billion light-years away.

To be so far away and still visible, 3C273 must be intrinsically very bright. It’s now thought to shine with the light of two trillion stars like our sun. That’s hundreds of times the light of our entire Milky Way galaxy. Yet 3C273 appears to be less than a light-year across, in contrast to 100,000 light-years for our Milky Way.

So 3C273 is not only distant. It is also exceedingly luminous, implying powerful energy-producing processes unknown in 1963.

Observing Session – WHERE

  • Open Star Clusters, we are looking into the Milky Way between 500 – 1000 light years distance. M44, the Beehive is a good example
  • Globular Clusters we are looking 10x more deeply out of the disk of the galaxy in a halo around it – M3, at a distance of 39,000 light years is one example

  • A Galaxy, we are looking outside of our own galaxy =~11 Million lightyears … BEYOND the Milky Way, M81 is a good example

  • Clusters of Galaxies – Leo 1 for example

In the Spring the NigthtSky of our earth orbit points outward towards the NGP (North Galactic Pole, located in Coma Berenices)

Before Astronomical Twilight – Let’s do Spectral Hues in Bootes – see Observing Section of Lecture IV

Here’s our List for a Spring Sky Evening

  • Stellar Nebula – M57 (The Ring Nebula)
  • Open Clusters – M44 (The Beehive Cluster), M67 (fainter, twice the distance)
  • Globular Clusters – M3, M13
  • Galaxies – Spiral – M57
  • Edge-on – M104 (the Sombrero Galaxy),
  • M82 Peculiar (emanating jet)
  • Elliptical – M84
  • Lenticular – M86
  • Clusters of Galaxies – Leo 1 – M95, M96, M105
  • The Virgo Cluster
  • Quasars anyone? 3C273




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