Night Sky Session 5 – April 29 2016

MVNSC: Mississippi Valley Night Sky Conservation:

The Night Sky Around Us Lecture 5

Program developed by:

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

• Instructor: Pat Browne
  • Assistant: Shawn McKay
  • Note: Donations can be receipted as charitable . Donations can be made online at canadaHelps – (choose Night Sky Conservation fund):
    Night Sky Conservation Fund

Observations from this Week: More daylight hours northern hemisphere… This view is 8:30 pm April 29 2016

More daylight hours as the Sun’s declination is higher and stays above the horizon longer as we get closer to summer solstice.

Observations … about our observations:

• Western Sky  after Night Fall: Constellation Taurus has set as it will continue to be behind the glare of the sun for the summer months.

• Learning how to interpret the information in the charts:

• Lum: Luminousity – in units of Solar Luminousity (3.828×10²⁶ W – nuclear fusion output primarily)
• Ly – Fundamental unit of distance (~10 Trillion km)
• Mass – in units of Solar Masses
• ‘Double’ Stars: A , B etc – Primary, Secondary Magnitudes:
  • Example Algeiba: A=2.3 m B = 3.4 m where m stands for visual magnitude
  • Sep = 4″ – Separation in arcseconds of sky – Know your Magnification and Field of View!
• Eye Glasses – To don them or no? For trying to split binaries – your best bet is to leave them on when observing through the eyepiece. Sometimes one eye is noisier than the other and can pick up more detail. Dark adaptation is essential – regardless of your visual acuity. Sit like a mushroom in the dark before going out.

• Flat Horizons! In the open field at MoK I found it much easier to identify the stars in the constellations. Seeing the whole picture of the sky by standing on  a flat horizon always helps. OAFs have traditionally used an abandoned airstrip – affectionately known as “Nirvana”. Urbanites and cottagers get lost by not being able to see the sky through the ‘forest’ or burb…

KNOW what you will be looking at:

• This is a slight exaggeration of the low contrast field of view you could be getting…I reviewed getting to the Sombrero Galaxy and much of it had to do with remembering how tiny and delicate it appears in the eyepiece – the description “spectacular edge-on spiral” bears little resemblance to how the photons actually hit my brain ! However it took 30 Million light years to bounce off my tiny mirror and hit my retina.

Properties of Starlight for extracting Stellar Information

Now that we have seen how starlight spectra provide markers for

• Luminousity, Mass, Temperature and state of nuclear fusion processes
• Expressed as Spectral Classifications OBAFGKM  (first untangled by Annie Jump Canon)

We also discovered that appearances and strength of Absorption Lines (dark bands)  in the Spectra record atomic  energy states to classify the Stars on the Hertzprung Russel Diagram

• Spectral shifts in these lines are indicators of relative velocity – application: Spectroscopic Binaries, line of sight velocities to stars, clusters and even galaxies…

  Higher frequency waves towards us, lower frequency waves away from us (red shift)

• stellar classification in terms of O B A G K M

Starlight Luminousity – How bright is your star

At the Mill of Kintail we can gauge naked eye magnitude as roughly “mag 5”. In the city, only the brightest stars shine brighter than the light pollution mag 2 or 3.

Brightness of Distant Celestial Objects

The brightness or Luminousity of the star, star cluster or galaxy is the measure of the Source of radiation Energy: Luminosity is usually measured in watts or as a multiple of the Sun’s luminosity (Luminousity of the Sun(Joules/Sec) = 3.828×10²⁶ W)

As the radiant energy leaves the object it spreads outwards in a sphere of increasing radius. The energy passing through each square metre on the surface of the sphere per second is found by dividing the luminosity of the star, L, by the sphere’s surface area

Therefore The Light from the Star Falls Off as the Square of the Distance

Why stars are brighter or fainter – Distance or Luminousity?

A star might be brighter than another star because:

• It is extremely energetic and its stellar properties produce intense visible radiation. In this case the objects intrinsic brightness or luminousity dominates. If we know the star’s intrinsic luminousity, we can determine its distance
• It is at a closer distance to our solar system

By comparing:

• this intrinsic brightness known as Absolute Magnitude M

…to…

• the visual (observed magnitude m ) we can determine the star’s distance.
• We arrive at the Distance modulus – Difference in M-m => Distance

These stars with known absolute Magnitudes are special stars used for calibration of the distance ladder. Typically these are stars on the Main Sequence:

Distances to the Stars, Star Clusters and Galaxies

Astronomers speak of a distance ladder. Each rung 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)

Here’s a description of how stellar parallax measurements are performed (from ground-based observations):

Khan Academy – stellar parallax distance

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.

Absolute Magnitude: Absolute magnitude is the measure of a celestial object’s intrinsic brightness. It is the the same as the visual magnitude of a star when it is placed at a distance of 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. This means that all stars at a distance of 10 parsecs or roughly 32.6 (like Arcturus) light years (measured using the direct method of trigonometric parallax !) have an absolute magnitude equal to their apparent magnitude.

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.

Examples:

Difference: M-m tells us the Distance Modulus

Sometimes we know the direct distances to stars based on trigonometric parallax
Therefore we can determine the absolute magnitude.

When m = M, we have a calibration of a star's intrinsic brightness.

Arcturus: Absolute Magnitude M:-0.30
          Apparent Magnitude m:-2.25  
M-m = 2  Distance is roughly 25 pc

Vega:    Absolute Magnitude:  0.58
         Apparent Magnitude:  0.03
M-m = 1   Distance from the table is < 10 pc

Distances derived from formula: starting with difference of magnitudes of 2.5 relating to the inverse square of distance:

Distance Modulus: M-m = 2.5 *log(d/10)^2
                      = 5 * (log(d) - log(10))
                      = 5 * (log(d) -1)
                      = 5 * log(d) -5
          log(d) = (M-m + 5)/5
              d  = 10^((M-m + 5)/5)

See Wikipedia article on Vega

Main Sequence Stars can be measured

Here is Mary Lou Whitehorne’s hand-made graph of the Brightest stars – mostly along the Main Sequence. She was curious to determine her own value for one of her favouate Variable Stars in Perseus.

Here are her calculations based on the M-m values that she picked off her graph

Main Sequence Fitting for Star Clusters – Open Cluster Example

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.

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

Average distance for the cluster:  m - M = 5.5
Average distance to the cluster =  10 ^((5.5 + 5)/5 = 10^2.1 = 126 pc

Because… Distance Modulus: m – M = 5*log(d/10)

                              (m-M) = 5 *(log(d) - log(10)
                              m - M = 5*log(d) - 5*1
                              ( m - M + 5 ) / 5 = log (d)

 Distance in parsecs:

                              d = 10 ^ (m - M + 5 )/5   (parsecs or pc)

Graphing Periodic Variation in Brightness – Cepheid Variable Stars in other clusters

Variable Stars Periodic Variations in their brightness lead to a calibration of values of Absolute Magnitude of special stars in the cluster .

Variable Stars – Henrietta Leavitt: Period-Luminosity Relationship

Watch how Henrietta discovered using Cepheid Variables the true, absolute brightness of these special ‘standard candles’

See: Henrietta Swan – Cepheid variables (courtesy Khan Academy)

• Calibrating the Main Sequence: Period-Luminousity Graph for Cepheid Variables in Star Clusters

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. Carefully studying the stars, finding that for Cepheid Variables, the longer the period, the higher the intrinsic luminousity provides stellar distance calibration. When we observe these objects, we can determine how far and therefore how long ago the light left that source.

Observing  –  From Pairs of Stars to Pairs of Galaxies.

• For understanding star clusters within our galaxy see: Millstone Night Sky Article – Star Clusters in our Galaxy
• Observations tonight take us beyond our own Galaxy: See Millstone Night Sky article – Galaxies

See: The Night Sky of Spring

References: References

To learn to be a good observer, we use the following guidelines: Observing Tips – Night Sky Course collective wisdom