Ohio University - Lancaster Campus slide 1 of 47 A star s color, temperature, size, brightness and distance are all related!
Ohio University - Lancaster Campus slide 2 of 47 The Beginnings Late 1800 s, early 1900 s how light is produced by atoms is being intensely studied by Gustav Kirchoff & Robert Bunsen Max Planck Josef Stefan... Ludwig Boltzmann Albert Einstein
Ohio University - Lancaster Campus slide 3 of 47 Black Bodies In 1862, Kirchoff coins the phrase black body to describe an imaginary object that would perfectly absorb any light (of any wavelength) that hit it. No light transmitted through, no light reflected off, just totally absorbed.
Ohio University - Lancaster Campus slide 4 of 47 a perfect absorber of light would also be a perfect emitter amount of light energy given off each second (its brightness or luminosity) and the color of its light are related to the object s temperature.
Ohio University - Lancaster Campus slide 5 of 47 Molten lava and hot iron are two good examples of black bodies, but a star is an excellent black body emitter.
Ohio University - Lancaster Campus slide 6 of 47 Max Planck, a German physicist, was able to make theoretical predictions of how much light of each color or wavelength would be given off by a perfect black body at any given temperature. These predictions or models are today called Planck Curves.
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Ohio University - Lancaster Campus slide 8 of 47 What 2 characteristics of the curves change as the temperature increases? (1) The size of the curve increases. (2) The peak of the curves shift to the left, to shorter wavelengths & higher energies.
Ohio University - Lancaster Campus slide 9 of 47 Can we draw some conclusions? Hotter stars should be brighter than cooler stars. Hotter stars should emit more of their light at shorter wavelengths (bluer light) Cooler stars should emit more of their light at longer wavelengths (redder light). All stars emit some energy at all wavelengths!
Ohio University - Lancaster Campus slide 10 of 47 In 1879, Josef Stefan discovered that the luminosity of a star was proportional to the temperature raised to the 4 th power. In 1884, Stefan s observations were confirmed when Ludwig Boltzmann derived Stefan s equation from simpler thermodynamic equations.
Ohio University - Lancaster Campus slide 11 of 47 Stefan-Boltzmann Law Today, we honor both scientists by naming the equation after them the Stefan- Boltzmann Law: At the surface of the star, the energy that s given off per square meter (Watts / m 2 ) called the luminous flux is... W / m 2 = 5.67 x 10-8 T 4
Ohio University - Lancaster Campus slide 12 of 47 At 100 K (cold enough to freeze you solid in just seconds), a black body would emit only 5.67 W/m 2. At 10x hotter, 1000 K, the same black body would emit 10 4 times as much light energy, or 56,700 W/m 2.
Ohio University - Lancaster Campus slide 13 of 47 If the temperature of a star were to suddenly double, how much brighter would the star become? If the temperature of a star somehow fell to 1/3 of what it was, how much fainter would the star become? 2 4 = 16 times brighter (1/3) 4 = 1/81, or 81 times dimmer
Ohio University - Lancaster Campus slide 14 of 47 In 1893, Wilhelm Wien (pronounce vine ) discovered by experiment the relationship between the main color of light given off by a hot object and its temperature. This main color is the peak wavelength, called λ max, at the top of the Planck Curve.
For each curve, the top of the curve is the peak wavelength.
Ohio University - Lancaster Campus slide 16 of 47 Wien s Law Wien s Law says that the peak wavelength is proportional to the inverse of the temperature: λmax = 2.9 x 10 6 T = 2.9 x 10 6 T T must be in Kelvin, and λ max in nanometers. λ max
Ohio University - Lancaster Campus slide 17 of 47 What is the peak wavelength of our sun, with a T = 5750 K? 2.9 x 10 6 = 504 nm (yellowish-green) 5750 K What is the peak wavelength of a star with a surface temperature of 3500 K? 2.9 x 10 6 = 829 nm (this star emits the 3500 K majority of its light as infrared, IR).
Ohio University - Lancaster Campus slide 18 of 47 A reddish star has a peak wavelength of 650 nm. What is the star s temperature? 2.9 x 10 6 = 4462 K (cooler than the sun) 650 nm A star has a peak wavelength in the ultraviolet of 300 nm. What is the star s temperature? 2.9 x 10 6 = 9667 K 300 nm
Ohio University - Lancaster Campus slide 19 of 47 We now have a color thermometer that we can use to determine the temperature of any astronomical object, just by examining the light the object gives off. We know that different classes of objects are at different temperatures and give off different peak wavelengths.
Clouds of cold hydrogen gas (nebulae) emit radio waves What kinds of objects? http://www.narrowbandimaging.com/images/vdb142_small.jpg
Warmer clouds of molecules where stars form emit microwaves and IR.
Protostars emit IR. http://www.antonine-education.co.uk/physics_gcse/unit_3/topic_10/protostar.jpg
Sun-like stars emit mostly visible light, while hotter stars peak in the UV. http://www.nasa.gov/images/content/138952main_whywe16full.jpg
Neutron stars and black holes peak in the X-ray.
Star cores emit gamma rays. http://aspire.cosmic-ray.org/labs/star_life/images/star_pic.jpg
Ohio University - Lancaster Campus slide 26 of 47 Where would the peak wavelength be for your body a lightning bolt the coals from a campfire
Ohio University - Lancaster Campus slide 27 of 47 A star s spectrum is also influenced by its temperature. In 1872, Henry Draper obtained the first spectrum of a star, Vega, in the constellation Lyra. Credit: Lick Observatory Archives photojournal.jpl.nasa.gov/jpeg/pia04204.jpg
Ohio University - Lancaster Campus slide 28 of 47 In 1885, Edward Pickering began a project at Harvard University to determine the spectra of many stars. Draper s widow funded the work. The first 10,000 spectra obtained were classified by Williamnia Fleming, using the letters A through Q.
Ohio University - Lancaster Campus slide 29 of 47 From 1901 to 1919, Pickering & his assistant Annie Jump Cannon classified and published the spectra of 225,000 stars (at the rate of about 5000 per month!) When Pickering died in 1919, Cannon continued the work, eventually classifying and publishing the spectra of 275,000 stars. Credit: amazing-space.stsci.edu
Ohio University - Lancaster Campus slide 30 of 47 Hotter stars have simpler spectra. Cooler stars have more complex spectra, since most atoms are not ionized.
Ohio University - Lancaster Campus slide 31 of 47 Class O >30,000 K bluish He lines in spectrum. (These stars are so hot that H is mostly ionized & doesn t shows lines.) Pleiades Class B 11,000-30,000 K He lines, weaker H lines Rigel, Regulus, Spica bluish Class A 8,000-11,000 K bluewhite H lines (Balmer Series) Sirius, Vega
Ohio University - Lancaster Campus slide 32 of 47 Class F 6,000-8,000 K white H, Ca lines, weaker H lines Procyon Class G 5,000-6,000 K yellow Ca, Na lines, + other metals Sun, Capella, α-centauri Class K 3,500-5,000 K Ca & other metals Arcturus, Aldebaran orange
Ohio University - Lancaster Campus slide 33 of 47 Class M <3,500 K red metal oxides (TiO2), molecules Betelgeuse, Antares Oh, Be A Fine Girl, Kiss Me!
Ohio University - Lancaster Campus slide 34 of 47 The stellar classes (OBAFGKM) are further subdivided with a number 0 to 9 following the letter. Our sun, a G2 star, is slightly cooler than the F range. A G9 star would be just a bit warmer than the K range.
Ohio University - Lancaster Campus slide 35 of 47 1910-1913, Henry Russell, a professor at Princeton, and Ejnar Hertzsprung, an astronomer at Leiden Observatory in the Netherlands, used the data from the Draper catalog to plot the temperature of the stars vs. their brightness or luminosity. What kind of result would you expect, a random scatter, or a pattern?
universe-review.ca/i08-01-hrdiagram.jpg
Ohio University - Lancaster Campus slide 37 of 47 Betelgeuse and Antares show on the diagram as being red stars, and red stars should be faint. Both stars are also hundreds of light years distant, so why do they appear so bright in our sky?
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Red Red Red Dwarfs
Ohio University - Lancaster Campus slide 42 of 47 The H-R Diagram makes a lot more sense when you realize that the different regions don t show different kinds of stars but stars at different stages of their lives.
Ohio University - Lancaster Campus slide 43 of 47 Determining distance using the HR Diagram From a star s color-temperature, determine its absolute magnitude (M). Observe the star s apparent magnitude (m) through a telescope. Use the distance modulus equation to calculate the distance.
Ohio University - Lancaster Campus slide 44 of 47 How far away is an F1 star that has a surface temperature of 8000 K, if its apparent magnitude is +9.6?
Ohio University - Lancaster Campus slide 46 of 47 distance in parsecs = 10^[(9.6-3.0 +5) 5] = 10^[11.6 5] = 10^2.32 = 209 parsecs (or 681 light years)
Where might this method run into trouble? Red & Orange star come in 2 varieties: giants & dwarfs. The spectrum of the star must be used to determine if the star is large or small. The presence of what element(s) in higher than normal percentages might indicate that the star is a giant, not a dwarf?