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.

8. Why doesn’t stellar parallax work to find distances to all stars? 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 Star A Star B Star C Earth in June

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 4𝜋 𝑑 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 4𝜋 𝑑 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 4𝜋 𝑑 2

What is a Cepheid variable star? Luminosity What is a Cepheid variable star? Stars that cycle from dim to bright in a very regular pattern (called a period)

Why are Cepheid variable stars important to astronomers? 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 4𝜋 𝑑 2

For stars that are close-by We can use the Inverse Square Law to calculate Luminosity 𝐴𝐵= 𝐿 4𝜋 𝑑 2 Calculate Light meter Stellar Parallax

For stars that are Far AWAY We can use the Inverse Square Law to calculate Distance 𝐴𝐵= 𝐿 4𝜋 𝑑 2 Period-Luminosity Law Light meter 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 4𝜋 𝑑 2

Colorful Characteristics p. 501 How can EM radiation be used to 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 6 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

Astronomers can determine the color of a star by the intensity graph. 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.

Star A (coolest) yellow color (600 nm peak) 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 𝑇= 2.9𝑥10 λ max (𝑛𝑚) to the 6th Units are in Kelvin

What can you learn about stars when looking at their spectra? 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

So the range of surface temp is 5000-6000K 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…