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Night SkyNight Sky - Galaxies

Night Sky – Galaxies

Night Sky around us … and beyond our Galaxy

  • Galaxies beyond the Milky Way
  • Clusters of Galaxies in collections forming their own systems beyond our Local Group



Galaxy Definition: A Galaxy is a collection of billions of stars. At the early stages of the creation of the universe, all the matter in the universe fragmented into huge pieces which broke into billions of bits. These huge pieces became galaxies, and the individual bits became billions of stars which make up each galaxy. Ours is one such galaxy – the Milky Way (a barred spiral). The universe is made up of billions of galaxies which tend to be grouped into superclusters. We can see these things only through telescopes and binoculars, – except for the very closest – the Andromeda Galaxy, and the Large and Small Magellenic Clouds.

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 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!
  • 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…)

NOTE: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.


Question: When 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. See

The research continues…


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

Andromeda Galaxy shown below – First measured period-luminousity of Cepheid Variables to determine distance and discover it is beyond our Milky Way.




Relative Velocities of Stars, Stellar Clusters and Galaxies and Space itself… Doppler Shifts

The change in wavelength of  light gets  recorded  by spectral line shifts. These shifts represent a velocity of the object moving towards or away from us. We can measure advancing (blue shifts of wavelength) or receding (red shifts of wavelength) for individual stars, clusters or galaxies. When objects move away from us the emitted electromagnetic radiation appears to have a longer wavelength. This is called ‘red-shift’ and the same phenomena can be ‘heard’ when an object is emitting soundwaves but traveling away from us. The phenomena is called the “Doppler Shift”dopplerDwgWaveWavelength of the source is lengthened as the total distance traveled in 1 unit of time is extended by the amount vt.

  •              Length of Wave of receding source (red-shift for light):
    λ = (ct + vt)           longer wave

(Where O is the observed wavelength and E is the actual emitted wavelength )

  •               Length of Wave of approaching source: (blue-shift for light):
    λ = (ct – vt)              shorter wave


The fractional difference in wavelength…

corresponds to the fractional relative velocity


Cosmological Red-shift

The discovery of characteristic red shifts in spectral lines  of very objects at some distance beyond our galaxy,  led to a revolution in our understanding of the recessional velocity and hence the cosmological expansion of distance to these objects.

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 red-shift 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 for the updated and corrected version.

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

Markarian Chain – part of the Virgo Cluster of Galaxies 60 Million light years away

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

Example: Since z = Δλ/λ = v*c ,
z cannot exceed 1 otherwise the
recessional velocity of the quasar
would exceed c.

If Z == 2 we must apply the
relativistic transformation
Zrelative1Zrelative2So to find how fast this thing is receding (as a fraction of the speed of light)

(1 + v/c) / (1 – v/c) = 3^2 = 9
i.e. v/c = 8/10 for Z = 2

So the distance to that object, assuming a Hubble constant of 70km/s per Mpc :

d = (0.8c)/70 = (0.8 * 300,000 km/s) / 70 km/s per Mpc =
3428 Mpc or 11182.74 Mly i.e 11.1 Billion ly.

We can measure redshifts and know that we are dealing with relativistic recessions in space-time …

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 its spectrum – the range of wavelengths of its light – looked odd. Astronomers routinely use spectra to learn the composition of distant objects.

  • But, the 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.
  • Distance measured as z*c = Hd (H = 70km/s/Mpc

Z measured was .138

.138 * 300,000km/s = 70km/s/Mpc = 591,428Mpc = 1.8 or 2Billion light years!



Postscript – Supernovae in other galaxies – Standard Candles for the Cosmos

Now that we have discovered optical sources at the far end of the universe, we need to establish standard candles to make accurate measurements – to probe the cosmos (the extragalactic neighbourhoods).

The Supernova Cosmology Project has done that using Supernovae Type 1a observations in other galaxies . They studied high Z red shifted spectra of the supernova – writing code to pick out the spectra and light curve of the supernovae from the host galaxy. Much of this work was done in the 1990s culminating in the discovery of the observational evidence of dark (invisible but having an effect) energy .

From the wikipedia article: “The similarity in the absolute luminosity profiles of nearly all known Type Ia supernovae has led to their use as a secondary standard candle in extragalactic astronomy. Improved calibrations of the Cepheid variable distance scale and direct geometric distance measurements to NGC 4258… Type Ia supernova distances have led to an improved value of the Hubble constant.In 1998, observations of distant Type Ia supernovae indicated the unexpected result that the Universe seems to undergo an accelerating expansion.”


  • Type 1a supernovae are BRIGHT so they can be detected at great distances… in other host galaxies. An astronomy class found one this year, and there was a chance for amateurs to image it! – see
  • High-redshift type 1a supernovae are much fainter than the models predict. Their distance therefore is further because their velocity is increasing. Hence the expansion of the universe is accelerating. The Hubble constant now measured to a high degree of accuracy is a measurement of the current state of the universe’s expansion.



Summary for the Night Sky Around Us

Lectures 1 through 5







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