Learning Goals Students will: 1) understand how spectra are
formed 2) understand how spectra are used to determine the
composition of the gases found within a star. 3) understand how
spectra are used to determine the distance a star.
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Success Criteria Students will show their understanding of the
learning goals by: 1) stating how spectra (especially absorption
spectra) are formed. 2) stating which types of information can be
determined by i nterpreting star spectra. 3) Understanding the
Herzsprung-Russell (H-R) Diagram. 4) Analyzing actual star
spectra.
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A modern tool of Astronomy
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Spectroscopy Remember Grade 10 Science we learned about how a
glass prism breaks light into a spectrum. We also learned that
electromagnetic radiation is produced at many wavelengths from
radio waves to gamma rays. The science in which the spectra of
light or any wavelength of radiation is studied is called
Spectroscopy.
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Spectroscopy There are two types of spectra emission and
absorption spectra. An electromagnetic spectrum is a pattern of
radiation that is either emitted or absorbed by matter depending on
the matters composition. Spectral lines are produced when electrons
are excited by radiation energy and jump to higher energy levels
and then fall back to their original energy levels. (Grade 12 Chem
students learn about this process) Each element and compound has a
unique emission/absorption pattern. Emission and absorption spectra
can be analyzed to determine the chemical composition of the gases
inside a star.
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Bohrs Model of the Atom
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The Hydrogen Spectra The 4 visible light lines of the Hydrogen
spectra come from the Balmer series. They form when electrons fall
back to the second orbital. Electrons falling back to the first
orbital form the Lyman series lines which are seen in the Infrared
spectrum. Electrons falling back to the third obital form the
Paschen series lines which are seen in the Ultraviolet
spectrum.
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Emission Spectrum A colour pattern emitted by an atom as its
electrons fall down energy levels. Each element has a unique
pattern.
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Absorption Spectrum The colour pattern emitted by gas when it
is illuminated from behind with white light. As the light passed
through the gas, the electrons in the atoms can absorb certain
frequencies of light (or a set amount of energy) and this causes
them to jump up energy levels. These patterns look like the
opposite of the emission spectrum.
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Comparing Spectra Absorption spectra are produced when light is
shone through a cooler gas. Emission spectra are produced when
light is emitted by the gas.
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Comparing Spectra Absorption spectra are produced when light is
shone through a cooler gas. Emission spectra are produced when
light is emitted by the gas.
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Sample Spectra Look at the absorption spectrum above. Note that
the detector has also printed out an intensity graph. The graphs
are far more commonly used than an actual spectrum by todays
astronomers. The spectra of common elements are shown at right note
that heavier elements have more lines The greater the percentage of
an element the sharper the lines. Higher temperatures tend to blur
lines and make them wider.
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The Relationship between Stars and Atomic Spectra The extreme
heat in the centre of a star produces a continuous spectrum (white
light). As the light goes through the outer layers of the star,
some frequencies are absorbed by the elements in stellar
atmosphere. Thus stars only produce an absorption pattern.
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By Interpreting Star Spectra much information from the Star can
be determined 1) Chemical Composition 2) Temperature 3) Colour The
above 3 properties are used to Classify Stars 4) Size and
Luminosity 5) The Relative Velocity of Stars 6) Red Shift (and
Binary Star Systems) 7) The Possibility of a Large Planet Orbiting
A Star
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Interpretation of Spectral Lines 1. Chemical Composition The
most important information that can be obtained from an absorption
spectrum.
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Interpretation of Spectral Lines 2. Temperature The temperature
of a star can be determined by the types and states of the elements
in the spectra. The more lines that are present, the cooler the
stars temperature.
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What is the sun made of? Based on these spectral lines, which
two elements make up most of the Sun? Calcium and iron Hydrogen and
sodium Hydrogen and helium Iron and sodium
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Analysing Spectra: Practice Analysing Chemical Composition and
Spectral Class http://www.learner.org/teacherslab/science/light/
color/spectra/spectra_2.html
http://www.learner.org/teacherslab/science/light/
color/spectra/spectra_2.html
http://www.pbs.org/wgbh/nova/space/decoding- cosmic-spectra.html
http://www.pbs.org/wgbh/nova/space/decoding- cosmic-spectra.html
Determining Red Shift
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Interpretation of Spectral Lines 3. Colour The colour of a star
is directly related to is absorption spectrum and temperature. Blue
stars are hot and red stars are cold.
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Star Classification The system works like this: From hottest to
coolest the major classes are O B A F G K M. Each class can be
broken down into tenths. For instance our sun is classified as
G2.
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Interpretation of Spectral Lines The spectra of about 30 stars
is shown above.
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Interpretation of Spectral Lines 4. Size and Luminosity Narrow
spectral lines indicate a large, bright star. This happens because
the stars density is so low that the hydrogen atoms are spread out
much further than a star on the main sequence.
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Remember the Doppler Effect The apparent change in frequency
and wavelength of a vibration as it is either moving towards you or
away from you.
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Interpretation of Spectral Lines 5. The Relative Velocity of
Stars Red and Blue Shifting of Light Spectra - this is the Doppler
Effect of Light Waves Blue-shift a colour in an absorption spectra
that is more towards the blue end of the spectrum than it should
be. The object is coming towards the observer.
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The Value of the Red Shift Red-shift The colours from the
absorption pattern are closer to the red end of the pattern than
they should be. A red shift indicates that the star is moving away
from the observer. The amount of red shift can be used to determine
the distance of an object. It turns out that the light reaching us
is predominantly red-shifted. This means that most stars are moving
away from us. The great American astronomer Edwin Hubble noticed
that the greater the distance of a star, the greater its velocity
away from us.
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Star Spectra: Red & Blue Shifts Note the spectra produced
by a galaxy there is an absorption line in the green region of the
spectrum. As the galaxy moves away from Earth, the spectral line
shifts (towards the red side of the spectrum) into the yellow
region. As the galaxy moves away from Earth, the spectral line
shifts (towards the blue side of the spectrum) into the blue-green
region.
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Interpretation of Spectral Lines 6. Binary star systems In this
case two spectra are present. In a binary star system one of the
spectra will be blue-shifted and one will be red- shifted. As the
stars revolve around each other the patterns will alternate between
red and blue shifting.
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Interpretation of Spectral Lines 7. Possibility of a large
Planet The possibility of a large planet orbiting a star is evident
when a single spectra alternates between a red and blue shift.
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Properties of Stars 1) Apparent magnitude of luminosity A
measure of how bright a star appears to the naked eye. There are
two problems with this measurement a) Things that are farther away
appear dimmer than they actually are. b) An object could be close
by, but be dimmer by nature
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Properties of Stars 2) Absolute magnitude of luminosity This
measures the intrinsic luminosity of an object at a standard
distance. (10 parsecs) Thus if a star is farther than 10 parsecs
away, its absolute magnitude is greater than its apparent magnitude
of luminosity This patterns follows an inverse square law. Example
If a star is 2 times farther away, it would be 4 times
fainter.
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Decrease in apparent light intensity with distance Note that
light (and sound) intensity decrease exponentially as distance
increases.
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Properties of Stars 3) Colour Index A comparison between the
amount of blue light (B) a star emits and amount of visible light
(V) a star emits. A very hot star emits more blue light than
visible light. A B-V colour of zero (both lights are emitted at the
same amount) indicates the star has a temperature of 10 000 K or 10
273 o C
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Surface Temperatures of Stars Note how a stars classification
is strongly related to its surface temperature.
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Spectral Classes
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Plotting Absolute Magnitude (Brightness) versus Temperature
(Spectral Class) for all known stars
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Putting This all together Hertzsprung-Russell Diagrams (H-R
Diagrams) A plot of the temperature versus its brightness for all
known stars. The temperature increases from right to left. Thus the
hottest stars are on the left. A colour index or colour magnitude
diagram is used to show the temperature. The brightness goes from
top to bottom. Thus the brightest stars are at the top.
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H-R Diagrams In 1910, when Ejnar Herzsprung and Henry Norris
Russell plotted all of the known stars on this graph - a
distinctive pattern is seen. Most stars are found on the diagonal
line that goes from the top left to the bottom right. Thus the
hotter stars are normally brighter than the cooler ones. This band
across the diagram is called the main sequence.
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The Main Sequence Note that the vast majority of stars fall on
a line that extends from the top left and extends to the bottom
right. What can also be determined from this graph, is that as you
move from left to right stars are getting older. This suggested to
astronomers that most stars follow a pattern during their
lifetimes. Our own sun is in the main sequence. Only
giant/supergiant stars and white dwarfs fall outside of the main
sequence.
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Giants These stars are intrinsically brighter (higher
luminosity) than a main- sequence star. BUT their surface
temperatures are cooler than main sequence stars of similar
luminosity. These stars are generally much more massive than main
sequence stars.
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Super Giants Some stars, like Betelgeuse, are even brighter
than normal giants. This indicates that they even more
massive.
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White Dwarfs The stars that are located below the main
sequence. They are smaller and fainter than the stars of the same
spectral type. We know that white dwarves are the remnants of
supernovae.
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Stellar Highlights The Sun lies in the middle of the diagram
There are many more faint, cold stars then bright ones. The sun is
actually larger than the average sized star. Brighter stars live
short lives and burn bright. Red giant stars are in their final
phase of life. Only supermassive stars produce supernovas or black
holes. Only these stars produce heavy elements in their cores
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Comparison of Stars Sirius (the Dog Star* from the
constellation Canis Major) is one of the brightest stars in the
sky. (*remember Sirius Black from Harry Potter, he changes into a
dog (werewolf)). The Sun and Sirius are both found on the main
sequence, and are similar in size. Since Sirius is found farther to
the left, it is likely a much younger star than the Sun but will
eventually be much like the sun in a few billion years.
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Comparison of Stars Rigel, Deneb, Sirius and Procyon B are all
hot, white stars. They are all found in the same spectral class A
However, despite their heat they shine with much different absolute
magnitudes. The reason Rigel and Deneb are so bright is the fact
that they are so large both are White Supergiants.
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Homework Describe two types of stars that are not on the main
sequence. Explain the difference between apparent magnitude and
absolute magnitude. What are the main properties of stars? What are
the two fundamental Properties that are being plotted on the H-R
diagram? What is the significance of the main sequence?