Measuring Temperature

1. Measuring Temperature • It is useful to think of temperature in a slightly different way than we are accustomed to • Temperature is a measure of the motion of atoms in an object • Objects with low temperatures have atoms that are not moving much • Objects with high temperatures have atoms that are moving around very rapidly • The Kelvin temperature scale was designed to reflect this • 0  K is absolute zero –the atoms in an object are not moving at all!

2. Blackbodies • A body that absorbs all energy incident on it and emits energy of all wavelengths is called a blackbody • The Sun, a stovetop element, or a piece of charcoal approximate a blackbody

3. Results of More Collisions • Additional collisions mean that more photons are emitted, so the object gets brighter • Additional hard collisions means that more photons of higher energy are emitted, so the object appears to shift in color from red, to orange, to yellow, and so on. • Of course we have a Law to describe this…

4. Wien’s Law: Hotter bodies emit more strongly at shorter wavelengths SB Law: The luminosity of a hot body rises rapidly with temperature Wien’s Law and the Stefan-Boltzmann Law

5. Taking the Temperature of Astronomical Objects • Wien’s Law lets us estimate the temperatures of stars easily and fairly accurately • We just need to measure the wavelength (max) at which the star emits the most photons • Then,

6. The Stefan-Boltzmann Law • If we know an object’s temperature (T), we can calculate how much energy the object is emitting using the SB law •  is the Stefan-Boltzmann constant, and is equal to 5.6710-8 Watts/m2/K4 • The Sun puts out 64 million watts per square meter – lots of energy!

7. The atom has a nucleus at its center containing protons and neutrons Outside of the nucleus, electrons whiz around in clouds called orbitals Electrons can also be described using wave or particle models Electron orbitals are quantized – that is, they exist only at very particular energies The lowest energy orbital is called the ground state, one electron wave long To move an electron from one orbital to the next higher one, a specific amount of energy must be added. Likewise, a specific amount of energy must be released for an electron to move to a lower orbital These are called electronic transitions The Nature of Matter

8. The Chemical Elements • The number of protons (atomic number) in a nucleus determines what element a substance is. • Each element has a number of electrons equal to the number of protons • The electron orbitals are different for each element, and the energy differences between the orbitals are unique as well. • This means that if we can detect the energy emitted or absorbed by an atom during an electronic transition, we can tell what element the atom belongs to, even from millions of light years away!

9. If a photon of exactly the right energy (corresponding to the energy difference between orbitals) strikes an electron, that electron will absorb the photon and move into the next higher orbital The atom is now in an excited state If the photon is of higher or lower energies, it will not be absorbed – it will pass through as if the atom were not there. This process is called absorption If the electron gains enough energy to leave the atom entirely, we say the atom is now ionized, or is an ion. Absorption

10. Emission • If an atom drops from one orbital to the next lower one, it must first emit a photon with the same amount of energy as the orbital energy difference. • This is called emission.

11. Seeing Spectra • Seeing the Sun’s spectrum requires a few special tools, but it is not difficult • A narrow slit only lets a little light into the experiment • Either a grating or a prism splits the light into its component colors • If we look closely at the spectrum, we can see lines, corresponding to wavelengths of light that were absorbed.

12. Imagine that we have a hot hydrogen gas. Collisions among the hydrogen atoms cause electrons to jump up to higher orbitals, or energy levels Collisions can also cause the electrons to jump back to lower levels, and emit a photon of energy hc/ If the electron falls from orbital 3 to orbital 2, the emitted photon will have a wavelength of 656 nm If the electron falls from orbital 3 to orbital 2, the emitted photon will have a wavelength of 486 nm We can monitor the gas, and count how many photons of each wavelength we see. If we graph this data, we’ll see an emission spectrum! Emission Spectra

13. Emission spectrum of hydrogen • This spectrum is unique to hydrogen • Like a barcode! • If we were looking at a hot cloud of interstellar gas in space, and saw these lines, we would know the cloud was made of hydrogen!

14. Different atom, different spectrum! • Every element has its own spectrum. Note the differences between hydrogen and helium spectra below.

15. Absorption Spectra • What if, instead of hot hydrogen gas, we had a cloud of cool hydrogen gas between us and a star? • Photons of an energy that corresponds to the electronics transitions in hydrogen will be absorbed by electrons in the gas • The light from those photons is effectively removed from the spectrum • The spectrum will have dark lines where the missing light would be • This is an absorption spectrum! • Also like a barcode!

16. Types of Spectra • Kirchoff’s Laws: • If the source emits light that is continuous, and all colors are present, we say that this is a continuous spectrum. • If the molecules in the gas are well-separated and moving rapidly (have a high temperature), the atoms will emit characteristic frequencies of light. This is an emission-line spectrum. • If the molecules of gas are well-separated, but cool, they will absorb light of a characteristic frequency as it passes through. This is an absorption line spectrum.

17. Spectra of Astronomical Objects

18. You have probably already experienced the Doppler shift in sound! Standing on the sidewalk, watching cars go past As a car approaches, the sound from the car seems to have a rising pitch – moving to shorter wavelengths As the car passes, the sound shifts over to longer wavelengths, getting lower in pitch Police radar guns work on the same principle. The waves reflected off the car will be shifted by an amount that corresponds to the car’s speed. Doppler Shift in Sound

19. Doppler Shift in Light • If an object is emitting light and is moving directly toward you, the light you see will be shifted to slightly shorter wavelengths – toward the blue end of the spectrum, or blue-shifted • Likewise, if the object is moving away from you, the light will be red-shifted. • If we detect a wavelength shift of  away from the expected wavelength , the radial (line-of-sight) velocity of the object is:

20. Telescopes • Telescopes have been used for hundreds of years to collect light from the sky and focus it into an eyepiece. An astronomer would then look through this eyepiece at planets, nebulae, etc. • The human eye is not very sensitive to dim light, and was replaced in astronomy by the film camera. • Film is sensitive to only around 10% of the impinging light, and is usually replaced by a…

21. The CCD, similar to those found in commercial digital cameras and phones, utilizes the photoelectric effect to collect around 75% of the visible light that is focused on it! It has revolutionized astronomy – images can be recorded and downloaded to a computer anywhere in the world for analysis The science of developing new methods for sensing, focusing and imaging light in astronomy is called instrumentation The Charge-Coupled Device (CCD)

22. Many objects of astronomical interest are visible only in wavelengths other than the visible! Much can be learned from studying a star, planet or nebula in multiple wavelengths. Radio telescopes can be used from the ground to image pulsars and other bodies Observations in other wavelengths require instrumentation to be lifted above the Earth’s atmosphere. X-ray, Gamma ray and infrared wavelength telescopes are currently in orbit! Outside the visible spectrum