Starlight & Atoms

1. Starlight & Atoms What we can know from a star’s light.

2. Essential Questions • What is an atom? • How do atoms interact with light? • What kind of spectra do you see when you look at celestial objects? • What can you learn from a star’s spectrum?

3. Atoms • An atom consists of a nucleus surrounded by a cloud of negatively charged electrons. • The nucleus is composed of positively charged protons and neutral neutrons.

4. Types of Atoms • The number of protons in an atom determines which element it is. • Hydrogen—1 proton, Helium—2 protons, Carbon—6 protons, Oxygen—8 protons • Atoms of the same element (that is, having the same number of protons) with different numbers of neutrons are called isotopes. • Hydrogen—1p, Hydrogen2, 1p and 1n, Hydrogen3, 1p and 2n.

5. Atoms vs. Ions • A neutral atom is surrounded by a number of electrons equal to the number of protons. • An atom that has lost or gained electrons is called an ion.

6. Quantized Electron Orbits • The electrons in an atom may occupy various permitted orbits around the nucleus but not orbits in between. • They make a “quantum leap” when going from one orbit to the next.

7. Light and Matter • The Amazing Story of Light… (see handout)

8. Light and Matter

9. Light and Matter • The size of the electron’s orbit depends on the energy stored in the motion of the electron. • An electron may change energy through collisions with other atoms or by absorbing or emitting a photon of proper energy.

10. Light and Matter • The motion of particles in a solid, liquid or dense gas causes the emission of black body radiation. • Blackbody radiation produces a continuous spectrum—a continuous rainbow of colors without bright or dark lines.

11. Blackbody Radiation

12. A Celestial Thermometer • The hotter an object, the more it radiates, and the shorter the wavelength of maximum intensity, λmax. • This allows us to estimate temperatures of stars from their colors.

13. Wien’s Law • λmax = 3,000,000 / T • The wavelength of maximum intensity (in nanometers) is given by the equation above where T is the temperature in Kelvins. • That reminds me…a bit about the Kelvin temperature scale…

14. Temperature Scales

15. Stellar Spectra • Because orbits of only certain energies are permitted in an atom, photons of only certain wavelengths can be absorbed and emitted. • Each kind of atom has its own characteristic set of spectral lines. (↓ Na)

16. Elemental Emissions

17. Bright—Line Spectrum • If you look at a low density gas that is excited to emit photons, you see bright lines in the spectrum. • Such a spectrum is called an emission spectrum. (bottom image)

18. Dark—Line Spectrum • If light passes through a low-density gas on its way to your telescope, the gas absorbs selected wavelengths and you see dark lines in the spectrum. • Such a spectrum is called an absorption spectrum.

19. Elemental ID • Go forth and make your spectroscopes!

20. A Star’s Spectrum • Nearly all spectra are absorption spectra. • This is the spectrum of the star Arcturus in the constellation Böotes. • The next spectrum is that of our own star the Sun.

21. Solar Spectrum

22. Composition • A Spectrum can tell you the chemical composition of a star. • The presence of spectral lines of a certain element shows that element must be present. • Be careful though, if the star is too cool or hot, the lines of a certain element may be weak or absent.

23. O,B,A,F,G,K,M • The strength of the spectral lines can tell you the star’s temperature. • Oh, Be AFine Girl, Kiss Me • Long after this sequence was created, L and T stars were found at lower temperatures.

24. The Doppler Effect

25. The Doppler Effect • When a star is approaching, you observe slightly shorter wavelengths (a blueshift). • When it is receding, you observe slightly longer wavelengths (a redshift). • This Doppler effect reveals a star’s radial velocity, that part of its velocity toward or away from Earth.

26. Hertzsprung-Russell Diagram • Temperature increases to the left! • Luminosity (brightness) increases toward the top • The curve running from top left to bottom right is called the “Main Sequence” • Stars spend most of their lives on the Main Sequence

27. HR-Diagram of selected stars