ATOMS AND STARLIGHT

1. ATOMS AND STARLIGHT

2. TYPES OF SPECTRA KIRCHOFF'S LAWS • CONTINUOUS SPECTRUM • EMISSION (BRIGHT LINE) SPECTRUM • ABSORPTION (DARK LINE) SPECTRUM

3. CONTINUOUS SPECTRUM • Shows a blending of colors like the rainbow • Formed by a glowing solid, liquid, or dense gas • Glows at all wavelengths - visible and invisible

5. EMISSION SPECTRUM • Shows only discrete wavelengths (colors) of light • Formed by glowing diffuse (low density) gas

7. ABSORPTION SPECTRUM • Shows dark lines superimposed on a bright continuous spectrum background • Formed when a continuous radiation passes through a diffuse gas

9. THE BOHR MODEL OF THE ATOM • In order to explain the formation of spectra, the structure of the atom must be understood. • The model of the atom first conceived by Niels Bohr early in the 20th century does a good job of explaining how atoms radiate light.

10. PROPERTIES OF THE ATOM - 1 • The nucleus of the atom contains protons (+) and neutrons (0). • Electrons (-) "orbit" the nucleus somewhat like a miniature Solar System. • Nuclear force binds the protons and neutrons together. • Electromagnetic force binds the electrons to the nucleus. • Atoms are mostly empty space. • If the nucleus is the size of a marble, the electron would be 1 mile away.

13. PROPERTIES OF THE ATOM - 2 • Every atom (element) has its own unique number of protons in the nucleus. • A neutral atom has the same number of protons and electrons. • There are only certain allowable electron orbits (like rungs on a ladder). • An atom must absorb energy to move an electron to a higher (excited) energy orbit. • An atom must emit energy when an electron moves to a lower energy level.

17. ABSORPTION AND EMISSION

20. PROPERTIES OF THE ATOM - 3 • An atom can only emit as much energy as it has absorbed (conservation of energy). • The ground state is the lowest energy level in an atom. • An ionized atom has completely lost one or more electrons.

21. BLACK BODY (THERMAL) RADIATION • The actual distribution of light from a hot glowing solid, liquid, or dense gas can not be fully described unless the particle model of light is used. • This was first accomplished by Max Planck (1900), and these light curves are now called Planck curves. • Every temperature has its unique distribution of energy which determines the color that the glowing object appears.

22. The Planck radiation law assumes that the object observed is a perfect radiator and absorber of energy (black body). • Stars, although not perfect black bodies, are close enough so that Planck curves are useful descriptions of their radiation.

24. STAR TEMP (K) COLOR Betelgeuse 3,000 Red/Orange Capella 6,000 Yellow/White Sun 6,000 Yellow/White Sirius 12,000 White Rigel 18,000 Blue/White STAR COLORS

26. WIENS’S LAW • This law can be derived from Planck's Law. • It states that the radiation peak on the Planck curve varies inversely with the temperature. • Red stars are relatively cool, but blue stars are hot. • Maximum Peak Wavelength = constant / T

28. STEFAN-BOLTZMANN LAW • This law can also be derived from Planck's law. • It states that the total energy from a radiating object (like a star) at all wavelengths is directly proportional to the 4th power of the temperature. • Therefore, a small change in temperature results in a large change in the energy output. E = (constant) T4

30. FORMATION OF STELLAR SPECTRA • ABSORPTION SPECTRUM • EMISSION SPECTRUM • CONTINUOUS SPECTRUM

31. STELLAR ABSORPTION SPECTRUM • Electrons absorb energy and re-emit it. • Light is emitted in random directions. • Dark lines are formed against a continuous background. • Most stars have an absorption spectrum.

34. STELLAR CONTINUOUS SPECTRUM • Atoms are packed together so tightly that their outer electrons are influenced by neighbor atoms. • Orbit separations which determine how the electron can jump can no longer follow definite laws. • With no definite orbits, an atom is no longer confined to radiating a definite set of wavelengths. • It can radiate any one of a variety of wavelengths because a variety of orbits are possible. • At any given moment, billions of atoms in a solid are emitting billions of different wavelengths. • Hence the solid, liquid, or high pressure gas radiates a continuous spectrum.

36. STELLAR EMISSION SPRCTRUM • Electrons are excited into higher energy levels when the atom absorbs outside energy. • Electrons tend to drop back down to the ground state very rapidly. • They emit bursts of energy (light) when they drop back to lower energy levels. • The spectroscope uses a narrow slit to form these light emissions into "lines". • Each element's atom has its own unique set of spectral lines. • Gaseous nebulas and hot stellar atmospheres show bright line spectra.

38. SPECTRUM OF THE HYDROGEN ATOM

41. EMISSION NEBULAH – ALPHA LIGHT

42. STELLAR CLASSIFICATION • This pioneering work was done by Annie J. Cannon in the early part of this century. • The letter classification scheme actually expresses temperature classes. • Subclasses (0-9) further define very detailed spectral features. • The Sun is a G2 star.

50. INFORMATION FROM STELLAR SPECTRA • TEMPERATURE • CHEMICAL COMPOSITION • MOTION