Learning Goals Students will: 1) understand how spectra are formed 2) understand how spectra are used to determine the composition of the gases found

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.

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.

A modern tool of Astronomy

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.

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.

Bohrs Model of the Atom

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.

Emission Spectrum A colour pattern emitted by an atom as its electrons fall down energy levels. Each element has a unique pattern.

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.

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.

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.

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.

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

Interpretation of Spectral Lines 1. Chemical Composition The most important information that can be obtained from an absorption spectrum.

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.

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

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

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.

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.

Interpretation of Spectral Lines The spectra of about 30 stars is shown above.

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.

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.

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.

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.

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.

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.

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.

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

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.

Decrease in apparent light intensity with distance Note that light (and sound) intensity decrease exponentially as distance increases.

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

Surface Temperatures of Stars Note how a stars classification is strongly related to its surface temperature.

Spectral Classes

Plotting Absolute Magnitude (Brightness) versus Temperature (Spectral Class) for all known stars

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.

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.

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.

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.

Super Giants Some stars, like Betelgeuse, are even brighter than normal giants. This indicates that they even more massive.

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.

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

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.

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.

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?

Documents

Learning Goals Students will: 1) understand how spectra are formed 2) understand how spectra are used to determine the composition of the gases found