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Transcript
CHAPTER 10: THE STARS S&T P. 491 • 7. Why do astronomers need to know the apparent brightness and luminosity of stars? • The relationship between AB and luminosity is used to determine the distance to the star. S&T P. 491 • 8. Why doesn’t stellar parallax work to find distances to all stars? • As the stars get further away, the parallax angle gets smaller (and the apparent shift gets smaller). At distances beyond a few hundred light years, the angle and apparent shift are so small we can’t measure them from Earth. STELLAR PARALLAX Earth in January Earth in June Star A Star B Star C THE INVERSE SQUARE LAW • What is the relationship between Apparent Brightness and Luminosity (Power Output)? • It is an Inverse Square Relationship – as the distance increases, the AB decreases by the square of the distance. THE INVERSE SQUARE LAW • What is the equation that relates Apparent Brightness and Luminosity (Power Output) of a star? •𝐴𝐵 = 𝐿 4𝜋𝑑 2 S&T P. 497 • 2. If the distance from the source of energy increases 4 times, what happens to the apparent brighness, provided the luminosity remains the same? • 𝐴𝐵 = 𝐿 4𝜋𝑑 2 S&T P. 497 • 3. If the distance from the source of energy is cut in half, what happens to the apparent brightness, provided the luminosity remains the same? • 𝐴𝐵 = 𝐿 4𝜋𝑑 2 LUMINOSITY • What is a Cepheid variable star? • Stars that cycle from dim to bright in a very regular pattern (called a period) LUMINOSITY • Why are Cepheid variable stars important to astronomers? • Henrietta Swan Leavitt discovered that there is a relationship between a Cepheid’s period and its Luminosity. This is called the period-Luminosity Law. PUTTING IT ALL TOGETHER • How can we use the Inverse Square Law for stars? •𝐴𝐵 = 𝐿 4𝜋𝑑 2 FOR STARS THAT ARE CLOSE-BY • We can use the Inverse Square Law to calculate Luminosity •𝐴𝐵 = 𝐿 4𝜋𝑑 2 Calculate FOR STARS THAT ARE FAR AWAY • We can use the Inverse Square Law to calculate Distance •𝐴𝐵 = 𝐿 4𝜋𝑑 2 Calculate PRACTICE QUESTION • If the distance to Star A is 10 times greater than the distance to star B, and the 2 stars have the same luminosity, how would their apparent brightness compare? • 𝐴𝐵 = 𝐿 4𝜋𝑑 2 COLORFUL CHARACTERISTICS P. 501 • How can EM radiation be used determine important characteristics of stars (composition, temperature, color)? COLORFUL CHARACTERISTICS P. 501 • Recall in Chapter 3 we used a spectroscope to look at the emission spectra of various light sources. COLORFUL CHARACTERISTICS P. 501 • The emission spectrum is created because a gas is heated causing the electrons to gain a quantum of energy, then release the quantum of energy in the form of light • (spectral lines). COLORFUL CHARACTERISTICS P. 501 • Astronomers use absorption spectra to study starlight. • How are absorption and emission spectra related? COLORFUL CHARACTERISTICS P. 501 • Absorption spectra are created when light passes through a cold gas (like the gas in the cooler outer layer of a star) and that gas absorbs specific wavelengths of energy COLORFUL CHARACTERISTICS P. 501 • Scientists can determine which elements are present in the outer layers of a star by the wavelengths of light that are absorbed . COLORFUL CHARACTERISTICS P. 501 • Work with your team to complete P&P #1-8 p. 501-507 • Complete R&C #1-3 COLORFUL CHARACTERISTICS P. 501 • P&P #2a: What are some features of the spectra? • There are dark lines, and colors between them. The top star has hydrogen lines. There are a different number of lines in each spectrum. COLORFUL CHARACTERISTICS P. 501 • P&P #2b: What are the values in nm of the 5 main hydrogen lines? • 390 nm, 397 nm, 410 nm, 434 nm, 486 nm, 656 nm. COLORFUL CHARACTERISTICS P. 501 • P&P #2c: What are some similarities and differences among spectra? • All spectra have lines. All spectra have colors (red – violet), some have more lines than others, some lines are darker and/or wider. SPECTRAL INTENSITY GRAPH • Spectral Intensity Graph shows 2 main things: • The wavelength of peak intensity – corresponds to the color of the star • The absorption spikes – corresponds to the elements present. P&P #4 INTENSITY GRAPHS HANDOUT • Star A Star #2 • Star B Star #3 • Star C Star #1 • Star D Star #4 P&P #5 MYSTERY STAR INTENSITY GRAPHS HANDOUT • Mystery Star A Star #3 and intensity graph B • Similarities in the number of spikes • Similar peak intensities (450 nm) • Mystery Star B Star #1/ graph C • Very strong hydrogen lines • Peak intensities at 400 nm P&P #6 COLOR OF A STAR • Astronomers can determine the color of a star by the intensity graph. • The peak intensity wavelength determines the color of the star. P&P #7 COLOR OF A STAR • Star A (coolest) yellow color (600 nm peak) • Star B – bluish-white color (451 nm peak) • Star C – bluish color (402 nm peak) • Star D (hottest) blue color (385 nm peak) P&P #8 TEMPERATURE OF A STAR • To determine the temperature of a star we need to use Wien’s Law 2.9𝑥106 Units are in Kelvin 𝑇= λ max 𝑛𝑚 ( ) R&C #1 • What can you learn about stars when looking at their spectra? • Can identify the elements present (absorption spectra) • Surface temperature (from peak intensity wavelength) • Color (from peak intensity wavelength R&C #2 • The sun is a yellow-white star. What range of surface temperatures would you expect for the Sun? • Use Wien’s law and the shortest wavelength for a yellow-white star to find Tmax = 6000K • Use Wien’s law and the longest wavelength for a yellow-white star to find Tmin = 5000K • So the range of surface temp is 5000-6000K R&C #3 • Spectral analysis… COLORFUL CHARACTERISTICS P. 501 • Race to the Stars!