LESSON 8: STARS What you will need: • Data projector and internet access. • The Infinity Series Part 2: Deep Space - The dance of Gravity video. • A copy of the handouts for each student Suggested Outline: • Hand out Skyways pg 61, 62 • Hand out and work through the following Ioncmaste: 9th Grade Astronomy Curriculum Resources webpage, Module 2 - #1 through 10. Use the applets provided on the website throughout. This works best on a data projector. If you do not have a data projector you may have to forgo using the applets and teach the lesson from the handouts alone. http://www.ioncmaste.ca/homepage/resources/web_resources/CSA_Astro9/files/h tml/module2/module2.html#6 • When discussing #3 “Types of Stars” o Hand out file “#8 - Types of Variable Stars” taken from the AAVSO website. (http://www.aavso.org/vstar/types.shtml) o Discuss the different types of variable stars (use applet) Intrinsic: Pulsating, Eruptive Extrinsic: Eclipsing Binary, Rotating o For more detailed variable star info and information on how to start observing them the students should go to the AAVSO website. http://www.aavso.org/ o Point out AAVSO Manual available from AAVSO website. http://www.aavso.org/publications/manual/ • When discussing #4 “Temperature and Colours of Stars” o Have students do Skyways HR Diagram activity (pg 72, 73) o Discuss stellar classifications (Oh, Be, A, Fine, Gal/ Guy, Kiss, Me) Show file “#6 – OBAFGKM chart” or use overhead provided. o Hand out Skyways pg 74, 75 • When discussing #6 “The Life Cycle of a Star” o use the “Balloon Stars” demo from Skyways pg. 63 to help illustrate a star in equilibrium • Watch The Infinity Series Part 2: Deep Space - The dance of Gravity video (First part only, about 40 min). The video discusses stellar evolution, quasars, pulsars, galaxies and a bit of cosmology. Between Class Assignments: o Read Beginner’s Observing Guide chapter 7 and 16 Information about the Brightest Stars Observing Variable Stars o Observe from ETUC Double & Multiple Stars and Variable Stars sections. The Brightest Stars The Nearest Stars Star Power Temperature Star Power Temperature log(L/Lsun) degrees Celsius log(L/Lsun) degrees Celsius Sun 0.00 5,840 Sun 0.00 5,840 Canopus 3.15 7,400 Alpha Centauri A 0.18 5,840 Procyon A 0.88 6,580 Alpha Centauri B -0.42 4,900 Achernar 2.84 20,500 Proxima Centauri -4.29 2,670 Altair 1.00 8,060 Barnard's Star -3.39 2,800 Fomalhaut 1.11 9,060 HD 93735 -2.30 3,200 Dened 4.76 9,340 UV Ceti (B) -4.48 2,670 Sirius A 1.34 9,620 Sirius B -2.58 14,800 Vega 1.72 9,900 Ross 248 -4.01 2,670 Betelgeuse 4.16 3,200 Ross 128 -3.49 2,800 Hadar 4.00 25,500 GX Andromedae -2.26 3,340 Antares 3.96 3,340 Epsilon Indi -0.90 4,130 Regulus 2.20 13,260 Wolf 359 -4.76 2,670 Adhara 3.96 23,000 L726-8 (A) -4.28 2,670 Bellatrix 3.60 23,000 Sirius A 1.34 9,620 Alnilam 4.38 26,950 Ross 154 -3.36 2,800 Alpha Crucis B 3.22 20,500 Epsilon Eridani -0.56 4,590 Al Na'ir 2.34 15,550 L789-6 -3.90 2,670 Elnath 2.54 12,400 GQ Andromedae -3.45 2,670 Alhena 1.90 9,900 61 Cygni A -1.12 4,130 Increasing Brightness Increasing Power Output Icreasing Stellar Mass POWER / BRIGHTNESS +5 +3 +6 +4 +2 +1 -5 -3 -6 -4 -2 -1 0 0 Red Stars Increasing 1 23456789 T emperature 10 TEMP 1 23456789 x1000 deg.C 20 1 23456789 Blue Stars 30 Student Handouts Module 2: The Sun and Stars Background Information 1. Introduction to Stars 6. The Life Cycle of a Star 2. Brightness of Stars 7. Composition of Stars 3. Types of Stars 8. The Surface of the Sun 4. Temperatures and Colours of Stars 9. Studying the Sun and Stars 5. Spectrum of Light 10. Summary 1. Introduction to Stars Our star is a ball of gas that produces energy in its core by means of nuclear reactions. These nuclear reactions process billions of kilograms of mass every second, producing enormous amounts of energy in the form of heat and light. The Sun, despite being a typical star, appears much brighter than all other stars simply because it is so close to us. Other stars are located so far away that their distances from the Earth can be very difficult to grasp. A helpful analogy for understanding this concept is to imagine the Earth as a grain of sand with the Sun situated a metre away, leaving the nearest star some 270 kilometres away. The farthest stars visible with the naked eye would be located almost half a million kilometres away from that small speck of sand. There are also millions of other stars even farther away. Light from the Sun takes about eight minutes to reach the Earth, about 4.3 years from the nearest star, and hundreds of years from the most distant visible stars. Stars are extremely bright and massive objects, but they are so incredibly distant that they appear as mere points of light in our sky. 2. Brightness of Stars While the thousands of stars in the night sky appear to be very similar, they are more distinct from one another than their appearance would suggest. Stars have various sizes, masses, temperatures, colours, luminosities, compositions and lifecycles. The largest stars are several hundred times the diameter of the Sun, and in our solar system, would easily engulf the Earth’s orbit, while the smallest stars are smaller than the Earth itself. The most noticeable distinction between stars, however, is the difference in their brightness. The apparent brightness of an object in the sky is denoted by its magnitude, a numeric scale established by Hipparchus c. 160 BC. The brightest stars in the sky, Hipparchus stated, were of first magnitude, and the dimmest were of sixth magnitude, making smaller numbers correspond to brighter objects. The scale has been expanded and can now be applied to any object in the sky. The full moon has a magnitude of -12.7, Venus at its brightest is -4.1, the brightest star is -1.46 and the faintest objects detectable through the largest telescopes are about +29. Magnitude values are logarithmic, where a difference of five magnitudes is defined as a brightness differential of 100 times. The Sun and the dimmest objects detectable through telescopes are about 56 magnitudes apart, but this corresponds to an actual difference in brightness of over 25 x 1021. Load Applet Stellar Magnitudes 3. Types of Stars Binary Star System courtesy Brian Martin Most of the stars in the sky are double stars, which are pairs of stars located in nearly the same position in the sky. The two stars that make up a double star may not actually be close to each other in space, but simply lie in the same line of sight from the Earth. They usually appear as a single point of light because they are so closely aligned, so cannot be seen as individual stars unless viewed through a telescope. Systems of double stars that are gravitationally bound and are in orbit around each other are called binary stars. Binary stars are often so close together they are only perceivable as two stars by analyzing their combined light. Binary stars are very common, the Sun being rare in that it is not part of a binary system. There are also a few individual stars that vary in their apparent brightness as seen from Earth, called intrinsic variable stars. The time period of the change in intensity of variable stars can be erratic or can be very regular, ranging from days to years. Data Sheet Load Applet Variable Stars 4. Temperatures and Colours of Stars Sunrise over the Pacific Ocean courtesy David Vandervelde Stars are classified by their temperature, which will affect their colour. The stars range in colour from red through yellow and white to blue. The Sun’s yellow surface is about 5800K (Kelvin), while some red stars are 3000K and blue stars can have surface temperatures of over 30 000K. When we look up at the stars, they all appear to dazzle bright white, but many stars actually do have colour imperceptible to our eyes. The human eye is not sensitive to colours at low light intensities like those of the stars. When we use a telescope to look at the stars, they appear brighter and the colours become noticeable. Spectroscopy allows for a more detailed classification system using the chemical composition of stars. The light emitted by stars can be broken down into the component colours at various wavelengths. The spectrum will depend primarily on the star’s temperature, but it will also contain absorption lines which characterize the elements present in the star. Dark bands appearing along the spectrum will indicate the presence of specific elements, and their abundance will affect the width of the band. A spectrum is like a fingerprint and will reveal the chemical abundance within the star. 5. Spectrum of Light White light is composed of a mixture of colours, but appears white because our eyes are unable to perceive the individual colours of the spectrum. It is possible to prove this phenomenon by separating a beam of white light into its component colours using a prism.