Ohio University - Lancaster Campus slide 1 of 47 Spring 2009 PSC 100 A star’s color, temperature, size, brightness and distance are all related!

Molten lava and hot iron are two good examples of black bodies, but… Ohio University - Lancaster Campus slide 5 of 47 Spring 2009 PSC 100 Molten lava and hot iron are two good examples of black bodies, but… a star is an excellent black body emitter.

These predictions or models are today called Planck Curves. Ohio University - Lancaster Campus slide 6 of 47 Spring 2009 PSC 100 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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The size of the curve increases. Ohio University - Lancaster Campus slide 8 of 47 Spring 2009 PSC 100 What 2 characteristics of the curves change as the temperature increases? The size of the curve increases. (2) The peak of the curves shift to the left, to shorter wavelengths & higher energies.

Can we draw some conclusions? Ohio University - Lancaster Campus slide 9 of 47 Spring 2009 PSC 100 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 14 of 47 Spring 2009 PSC 100 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 λmax , 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 Spring 2009 PSC 100 Wien’s Law Wien’s Law says that the peak wavelength is proportional to the inverse of the temperature: λmax = 2.9 x 106 T = 2.9 x 106 T λmax T must be in Kelvin, and λmax in nanometers.

Ohio University - Lancaster Campus slide 19 of 47 Spring 2009 PSC 100 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.

What kinds of objects? Clouds of cold hydrogen gas (nebulae) emit radio waves 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

Where would the peak wavelength be for Ohio University - Lancaster Campus slide 26 of 47 Spring 2009 PSC 100 Where would the peak wavelength be for your body a lightning bolt the coals from a campfire

A star’s spectrum is also influenced by its temperature. Ohio University - Lancaster Campus slide 27 of 47 Spring 2009 PSC 100 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. photojournal.jpl.nasa.gov/jpeg/PIA04204.jpg Credit: Lick Observatory Archives

spectra of many stars. Draper’s widow funded the work. Ohio University - Lancaster Campus slide 28 of 47 Spring 2009 PSC 100 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.

From 1901 to 1919, Pickering & his assistant Ohio University - Lancaster Campus slide 29 of 47 Spring 2009 PSC 100 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

Hotter stars have simpler spectra. Cooler stars have more complex Ohio University - Lancaster Campus slide 30 of 47 Spring 2009 PSC 100 Hotter stars have simpler spectra. Cooler stars have more complex spectra, since most atoms are not ionized.

Class A 8,000-11,000 K blue-white H lines (Balmer Series) Sirius, Vega Ohio University - Lancaster Campus slide 31 of 47 Spring 2009 PSC 100 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 bluish He lines, weaker H lines Rigel, Regulus, Spica Class A 8,000-11,000 K blue-white H lines (Balmer Series) Sirius, Vega

H, Ca lines, weaker H lines Procyon Class G 5,000-6,000 K yellow Ohio University - Lancaster Campus slide 32 of 47 Spring 2009 PSC 100 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, -Centauri Class K 3,500-5,000 K orange Ca & other metals Arcturus, Aldebaran

metal oxides (TiO2), molecules Betelgeuse, Antares Ohio University - Lancaster Campus slide 33 of 47 Spring 2009 PSC 100 Class M <3,500 K red metal oxides (TiO2), molecules Betelgeuse, Antares Oh, Be A Fine Girl, Kiss Me!

The stellar classes (OBAFGKM) are further Ohio University - Lancaster Campus slide 34 of 47 Spring 2009 PSC 100 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.

1910-1913, Henry Russell, a professor at Ohio University - Lancaster Campus slide 35 of 47 Spring 2009 PSC 100 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

Betelgeuse and Antares show on the diagram Ohio University - Lancaster Campus slide 37 of 47 Spring 2009 PSC 100 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

The H-R Diagram makes a lot more sense when you realize that the Ohio University - Lancaster Campus slide 42 of 47 Spring 2009 PSC 100 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.

Determining distance using the HR Diagram Ohio University - Lancaster Campus slide 43 of 47 Spring 2009 PSC 100 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.