Night Sky Course Spring 2014 Lecture 4

MVNSC: Mississippi Valley Night Sky Conservation: The Night Sky Around Us Lecture 4

Program developed by:

Mississippi Valley Conservation
Authority Royal Astronomical Society of Canada
Ottawa Astronomy Friends

Instructor: Pat Browne
Assistants: Andrew Lindstrom, Bob Hillier

Course runs each Friday during the month of April Course time: 19:45 – 22:00 formally with priority given to observing when clear sky .

Note: Donations to Night Sky Conservation can be receipted as charitable .

Star Clusters In Around and Beyond Our Galaxy


The Sky around us is different this week from last week:

Celestial backdrop has shifted a bit …

Constellation Leo has advanced westward, rising 4 minutes earlier each day . What causes this shift is the difference between the star time taken from one day to the next, and solar time taken from one day to the next. Star time is called Sidereal time and can be understood as follows:

Sidereal Time = A Day On Earth using Star Time

Sidereal Time = Hour and Day with respect to the Stars..(not the Sun)

  • 1 Day = 1/365th of a circle about one degree around the Sun.
  • Earth rotates on its axis as well as rotating around the sun.
  • So, the time for a star to return to the same place in our sky the following evening is only 23 hours, 56 minutes and 4 seconds (not 24 hours)This is called a Sidereal Day. It measures both Earth Rotation and Orbit with respect to the stars.




We have to rotate 4 more minutes of time to point to the sun again. So the stars advance in our night sky – night after night.

Here is the planisphere setting for this evening. Sidereal time is not so obvious from one week to the next, but over the course of a month will show the changing of the constellations.


For understanding what we are looking at:

Examples of Stellar Systems in our Milky Way :

Distances to the Stars, Star Clusters and Galaxies

By comparing this intrinsic brightness to how bright it appears to be, one can determine the star’s distance if and only if the star is on the Main Sequence

Astronomers speak of a distance ladder. Each run gives a ‘leg-up’ to the next method of distance determination. The first method of measurement is direct; the rest use indirect methods calibrated using a ‘standard candle’, something that you know is a certain absolute luminosity (magnitude) and hence determine its distance by measuring its absolute magnitude and comparing it to its apparent magnitude

Direct Measurement to Nearby Stars using Stellar Parallax (Satellite Hipparcos data)

The KEY to going from Direct Measurements such as trigonometric parallax for nearby stars, to indirect measurements, is we calibrate the indirect measure of Absolute Magnitude using parallax. We can calibrate the relative magnitude observed (even visually) to the Absolute magnitude derived scientifically and get the true brightness and distance modulus for that star.

Recall our visual magnitude scale:




Distance Modulus

Absolute Magnitude: Absolute magnitude is the measure of a celestial object’s intrinsic brightness. It is the hypothetical magnitude of a star at a standard luminosity distance of exactly 10.0 parsecs or about 32.6 light years from the observer

So according to this definition we compare magnitudes between stars that we know at a given distance to stars of unknown magnitude. …
The absolute magnitude of a star, M is the magnitude the star would have if it was placed at a distance of 10 parsecs from Earth. By considering stars at a fixed distance, astronomers can compare the real (intrinsic) brightness of different stars

To convert the observed brightness of a star (the apparent magnitude, m) to an absolute magnitude, we need to know the distance, d, to the star. Alternatively, if we know the distance and the apparent magnitude (m) of a star, we can calculate its absolute magnitude M.

m – M = 5/log(d/10)

Sometimes we know the distances to stars based on trigonometric parallax … and ..

Sometimes we know the average absolutes overall of a star cluster based on the stellar classifications and its placement on the Hertzprung Russell diagram .

We use the known absolute magnitude of a well-known star cluster to determine that of the unknown cluster. Astronomers take images of the cluster through special Blue and Visual filters to determine the apparent magnitudes very accurately. The difference between the Apparent and the Absolute magnitudes gives the distance to the cluster. This formula is for measurements in parsecs:

In this example, we measure a difference in the two clusters of 5.5…

m – M = 5.5

m – M = 5 * log(d) – 5

Therefore D = antilog((m – M + 5)/5)

D = 10^2.1 = 126 pc


Stellar Magnitude and Periodic Brightness Observations

Variable Stars – Periodic Variations in their brightness lead to a calibration of values of Absolute Magnitude which help to determine true stellar distances.


Variable Stars – Henrietta Leavitt: Period-Luminosity Relationship

Variable Stars – Henrietta Leavitt: Period-Luminosity Relationship . Watch how Henrietta discovered using Cepheid Variables the true, absolute brightness of these special ‘standard candles’; thereby calibrating the Main Sequence:

Cepheid Variables 1-1

For Cepheid Variables , Henrietta determined the Period-Luminosity Relationship which calibrates the Absolute Magnitude Scale:





Re-Searching the Stars…

We have seen…observations and recording techniques

      • Star Gas and Dust (nebulosity)
      • Star Light Colour(peak wavelength) Spectral lines (Classes OBAFGKM)
      • Star Brightness – Variable Period

…Lead to

      • Stellar Classification, (temperature), stellar atmosphere, gaseous chemical composition
      • Absolute Magnitudes which leads to distances in light years
      • Mass determination relative to our Solar Mass
      • thermonuclear processes and stellar evolution


Observing Plan – Early Spring within and Beyond the Galaxy From Pairs of Stars to Pairs of Galaxies.

To learn to be a good observer, we use the following guidelines:

For our Observing Exercise, we will use