Lecture 4 – Night Sky May 1st 2015

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: Shawn McKay, Bob Hillier
  • Note: Donations can be receipted as charitable . Donations can be made online at canadaHelps – (choose Night Sky Conservation fund):
    Night Sky Conservation Fund

April 17 2015 – Quick Observing Report

We had 3 telescopes

• Stargazer Steve 6″ F8 Dobsonian Reflector
• Stargazer Steve 8″ F6 Ultimate (portable) Reflector
• Skywatcher 76mm F9.2 (700mm Focal Length) Newtonian Reflector

We aligned on the bright object – Planet Jupiter and observed the moons Io, Europa, Ganymede, Callisto

• Identified bright star Procyon beneath it and the Gemini Twins Castor and Pollux in the western sky
• Observed through the eyepiece: Castor as a mulitiple star system of White stars
• Close to Jupiter – observed Open Cluster M45: The Beehive cluster
• Observed rich open cluster M35 (distance 2800 lys) and attempted to find the much further distance cluster NGC 2158 in the periphery of the eyepice
• Next moved to Southern Sky, identified bright star Arcturus (“follow the arc (of the Big Dipper) to Arcturus”) and searched for Globular Cluster M3
  • Note that M3 is really 40% of the way from Arcturus toe Cor Corili (the bright star alpha CVn)

• Then moved to the South Eastern sky for galaxy hunt in Leo – M65 and M66
  • Using low power 30x (40mm eyepiece) in the 6″, we found the Leo Triplet but saw just the two of the three galaxies:  M65 and M66
• Attempts at M81, M82 in Ursa Major or M104 in Corvus (the “Sombrero Galaxy” needed to be deferred)
• We need to get away from the parking lot light!

See Early Spring Observing – Millstone News Night Sky”

May 1 2015 – What to observe when the Moon if Full?

When the moon is full, even the features on the moon are washed out. Shadows disappear, and everything is flooded with moonlight. But that’s no excuse to sit inside… Go out and …

• Observe and learn the constellations
• Observe the brightest stars and learn their names and starlore
• A special treat: Study their startlight by observing spectra using a spectroscope

Prisms – Sunlight – Seeing the different wavelengths of light

Rainbows of colour can be seen passing through water droplets or transparent media – see Rainbows and Rain – Millstone News article – October 2014
Here is an equilateral acrylic prism. If we set this up in front of a light source like our star, the Sun, (and even better if we arrange the light to squeeze through a slit…)

Here’s what we get:

Notice that the slit produces a well defined separation of colours, and the pinhole produces a more blurry image. Isaac Newton, the first to discover that light intrinsically contains all the wavelengths in the rainbow of colours, used a pinhole ‘camera’ to view the light streaming through the prism. However Newton produces a series of different colored images of his pinhole – each one overlapping the next, not a very clear image. With a narrow slit (discovered by Wollason) there is only a small amount of overlapping of neighbouring images of the slit, and we see a clear, well-defined spectrum. Later, Fraunhofer perfected the slit method and discovered vertical dark absorption lines when observing the solar spectrum.

See : https://millstonenews.com/2014/10/rainbows-and-rain-on-lunar-eclipse.html

What’s going on?

• Visible light, represented here by a white beam, is actually made out of light of several frequencies (colors) travelling together.
• These basic frequencies of visible light are part of what we call visible spectrum, and it is only tiny part of the entire spectrum.
• As light enters a the acrylic prism , each of its composing wavelengths will travel at a different speed in the new medium, and this change in speed is what bends the path in which light is travelling.
• This is the phenomenon we call refraction.
• The ratio between the speed of light in vacuum and the speed of light in a medium is what we call index of refraction, and this value is specific for a given wavelength and medium.

  

• Since light of different wavelengths will change direction by a different amount, we see the difference wavelengths of light, represented here by colored waves. This is what we call dispersion of wavelengths.
• In this animation, we can easily see the difference on their speeds.
• • Red, with a long wavelength, passes through almost without any change, whereas blue (with short wavelength) is left behind by all the other colors.
  • However, this difference in speed does not persist once the light exits the prism back into the air, and this can be seen on how all light exiting the prism will once again travel at the constant speed of light

Information – courtesy http://commons.wikimedia.org/wiki/File:Light_dispersion_conceptual_waves.gif

Diffraction gratings – how they work

When a beam of light is directed at a diffraction grating along its axis, a sef of coloured spectra are observed on both sides of the central white light as shown. The central white light will represent the un-diffracted image of the star. The different bands of colour are the result of wavefront bending as they get squeezed through the narrow apertures. As the waveforms exit, they interfere with each other.

For a diffraction grating, the condition for the nth order maximum is given by

Note here – m represents an integer number of wavelengths where maximum brightness occurs
Since
increases with wavelength :

• red light which has the longest wavelength is diffracted through the largest angle
• Violet light has the shortest wavelength and is diffracted the least.

Thus, white light is split into its component colours from violet to red light. The spectrum is repeated in the different orders of diffraction. Only the zeroth order spectrum is pure white.

-information courtesy: http://h2physics.org/?cat=49

“No laboratory jar on Earth holds a sample labelled ‘star stuff’ and no instrument has probed inside a star. The stars are beyond our reach, and only information we can obtain about them comes to us hidden in light” – Michael A. Seeds , Horizons – Exploring the Universe

Star Colours Determine Star Temperature

Stars show different colours because their of their temperature; some are hotter than others. Deep in their interior all stars are enormously hot (measured in millions of degrees), but their temperature lessens towards their outer layers, and the coolest star pours out most of their visible radiation in the red part of the spectrum. Hotter stars like the Sun appear yellow, still hotter stars appear white, and the hottest appear blue. Stars radiate light a little like glowing coals in a campfire. Just as a glowing red-hot coal is cooler than a white-hot coal, for example, so a red star is cooler than a white star, and a white star is cooler than a blue star.

Star Color and Temperatures

Heat is kinetic energy at the level of atoms. The vibrating atoms collide with the electrons in the material, and each time the motion of the electrons gets disturbed in at a particular quantum level, a photon is emitted. Heated objects emit electromagnetic radiation. The hotter the object, the more radiation it emits. The energy is most intense at a specific peak wavelength .

Courtesy Michael A. Seeds, Horizons

.

Stars have color variations which we can perceive even without optical aids, and this is related to the characteristic wavelength recorded in the stars.

From the graph, we can see that the hotter the object, the shorter the peak wavelength. We know the relationship as Wien’s Law:

The wavelength of the peak (like Red) the lower the temperature like 5000K. The short wavelength of the peak, for high energy blue light records a hight temperature.

Peak radiation for Shorter(bluer) Wavelengh = high temperature ~ Blue Stars
Peak radiation for Longer (redder) Wavelenth = low temperature ~ Redstars

Max = 3,000,000/T

This physical law gives us the key to estimate a star’s temperature from the starlight. Knowing the temperature (thousands of degrees), we can apply our models of the energetic nature of stars and their physical processes to graph them and classify them.

Star Color and Spectra

But that’s not all – When we use a grating on our telescope, we find that stars show spectra with dark and bright lines mixed in at special intervals …

Why are Stellar Spectra Important?

To those who can read its meaning, the spectral code tells at a glance just what kind of object the star really is: its color, size, and luminosity compared to the Sun and stars of all other types; its peculiarities, its history, and its future. …

See :  Sky and Telescope – The spectral types of stars

• A special treat: Study their starlight by observing spectra using a spectroscope
• A spectroscope is a device we put on the telescope that splits the starlight into spectral components and reveals the special signature of dark (or bright lines) at various places along the ‘rainbow’ of wavelengths.
  Rainbow Optics Spectroscope – What you will see!

Starlight – The Signature of Stars “hidden in light”

Starlight Analysis in terms of Spectra

When we record starlight passing through a prism and diffraction grating (a spectroscope) unto special cameras, we split the components of light into a rainbow of colors interspersed with spectral lines. The position of the lines is compared to a standard ‘plate’.

Each dark line that appears in a certain colour band (frequency) represents the detection of the gaseous state of a chemical element. when plotted as a light curve the dark lines reveal reduced intensity (dips) how much hydrogen or helium gas is absorbing light radiation.

Each line indicates ion of a certain chemical chemical element with the line strength indicating the abundance of chemical components such as hydrogen and helium in a gaseous ionic form (with electrons removed from the atom).

In this illustration the

• light from the source must pass through the hydrogen gas (representing a stellar atmosphere) before it can reach the telescope.
• These photons pass through the gas because they are absorbed by the first photon they meet.
• The atom is excited, goes into higher energy level, and then drops down to a lower energy level when a new photon is emitted
• The original photon was travelling towards us, but the re-emitted photon goes off in some random direction, “scattered” – so the result is a black line in the spectrum at that location.
• These dark lines are called absorption lines because the atoms absorb the photons
• These are the same lines that Fraunhofer saw in the solar spectrum
• Because only photons of certain wavelengths can only be absorbed or emitted in packets (quanta) of energy (quanta state in the electron orbits), each kind of atom has its own set of spectral lines
• Hence we can see not only hydrogen absorption lines, but sodium, calcium and other chemicals indicating the chemical composition of the stellar atmosphere

Forum for Amateur AstroSpectroscopy

Characteristic Lines by Spectral Class

Below is a basic list containing a few elemental lines in stellar spectra. Some of the prominent, or characteristic, lines are shown that might be used to broadly identify the types. Observing all of them will depend upon equipment, the seeing conditions as well as your eye acuteness. Wavelengths given are in Angstroms.

• Type O: Hottest blue stars (>55,000 °K to 30,000 °K). Ionized Helium (4541 and 4687) predominates. Occasionally the Hydrogen Balmer lines are seen weakly (see Balmer Series below under Type A) as well as very weak neutral Helium (3888, 4472 and 5877). Other very faint ionized Oxygen, Nitrogen, Silicon and Carbon lines may be present.
• Type B: Hot blue stars (30,000 °K to 10,000 °K) .Neutral Helium (3888, 4121, 4472 and 5877) lines dominate and max at B2. Hydrogen Balmer lines become progressively stronger through this type. Other very faint ionized Magnesium, Silicon and Carbon lines may be present.
• Type A: Blue stars (10,000 °K to 7,400 °K). Hydrogen Balmer lines dominate and max at A0. The H and K lines of ionized Calcium (3968, and 3934) become strong as neutral metals may appear weak. The Hydrogen Balmer lines: Ha – 6563; Hb – 4861; Hg – 4340; Hd – 4102; He – 3770.
• Type F: White stars (7,400 °K to 6,100 °K). Hydrogen lines are weakening. H and K lines of ionized Calcium strengthen as well as many fine lines due to other metals.
• Type G: Yellow stars (6,100 °K to 5,300 °K). Many fine lines appear due to many neutral metal elements such as Iron, Manganese and Calcium. The broad molecular CH G-band (4314) appear.
• Type K: Orange-red stars (5,300 °K to 3,800 °K). Hydrogen lines are gone. Broad TiO bands (4661, and 4955) begin to appear. The ionized Calcium lines are strong. CH band is very strong.
• Type M: Red stars (3,800 °K to 2,200 °K). Broad TiO bands (4661, and 4955) dominate the spectrum. Neutral metal lines throughout the spectrum.

Notes and Suggestions

1. Everyone’s eyes respond differently to light. To “Calibrate” your eye response to the grating or prism work sheet spectrum, observe a bright A0 star, such as Sirius or Vega, which contains the very strong Balmer absorption lines of Hydrogen.
   

• Compare the positions of these Hydrogen lines with respect to where you perceive the location of the color boundary lines (i.e. between violet and blue, blue and green, etc.).

Determine if they match the standard dashed color boundary lines shown on the blank work spectrum as illustrated below.

• If they do not, you may wish to make more personalized boundaries on your work sheet copies for better accuracy when recording the more complicated lines of the cooler (F, G, K and M) stars.
• When observing spectra for line details, just like observing visual objects such as planets, galaxies, etc., you should take your time on each subject. The middle and cooler stars usually have a few strong lines with many fine ones that may take some practice to detect visually.

-courtesy “Forum for Amateur AstroSpectroscopy”

This map is good for any MOONY night:

Good for learning

• Location of the Brightest stars
• Constellations

Colour and Spectra (if we use our Rainbow Optics Diffraction Grating)

For understanding Stars and Starlight in our Milky Way: https://millstonenews.com/2014/03/stars-in-our-milky-way-galaxy.html

Examples of Stellar Systems in our Milky Way : https://millstonenews.com/2014/04/night-sky-course-stars-and-star-clusters-within-the-milky-way.html

To learn how variations in starlight provide fundamental measurements of

• distances to the stars
• mass. stellar atmosphere, radial velocity of the stars
• evolutionary state of the star

See: https://millstonenews.com/2014/04/night-sky-course-spring-2014-lecture-4.html