PHYS 420: Astrophysics & Cosmology

Transcription

1 PHYS 420: Astrophysics & Cosmology

2 Dr Richard H. Cyburt Assistant Professor of Physics My office: 402c in the Science Building My phone: (304) My My webpage: In person or is the best way to get a hold of me.

3 My Office Hours TR 5:30-17:00am W 5:00-5:00pm Meetings may also be arranged at other times, by appointment

4 Douglas Adams Hitchhiker s Guide to the Galaxy

5 Cosmology: Progress Report after Newton Recall: Cosmology is a mystery story Based on evidence we can observe, want to understand: structure of Universe: how big? map of contents across space? contents of Universe: what ingredients is it made of? origin & evolution of Universe: birth? past? future? rules of the game: what makes the Universe this way? Cosmological progress so far: Copernicus Sun-centered Universe Kepler precisely described planetary motion Newton explained planetary motion, agrees w/ observations

6 Lessons from Newtonian Triumph Lesson from Newtonian success: insight into rules of the game powerful laws exist explaining all motion due to forces planetary motion is due to force of gravity gravity determines behavior of the cosmos! Also important implications for structure of Universe

7 Lessons from Newtonian Triumph Recall: to naked eye, stellar parallax is undetectable stars don t seem to wiggle annually on sky so if parallax exists, wiggle must be small stars must be very far away! But if stars hugely far away, and can still be seen they must emit a lot of light perhaps stars are like the Sun!

8 Where do we go from here?

9 Light Light deeply connected to electric charge and electric and magnetic force fields Experiments show: Changing E force generates M force Changing M force generates E force Maxwell s Equations 1. E&M linked: electromagnetic force 2. EM disturbances can travel through space: each regenerates the other: E M E M... Electromagnetic waves= EM radiation

10 Light Light deeply connected to electric charge and electric and magnetic force fields Experiments show: Changing E force generates M force Changing M force generates E force Maxwell s Equations 1. E&M linked: electromagnetic force 2. EM disturbances can travel through space: each regenerates the other: E M E M... Electromagnetic waves= EM radiation

11 Light as Electromagnetic Waves light is an electromagnetic phenomenon: created by motions (in fact: acceleration) of electric charges that is: light must be created by charged particles e.g., in light bulb, hot filament drives electron motion but light itself is made of electromagnetic fields which exert forces on charged particles that is: light interacts with charged paricles e.g., in antenna, electrons driven back and forth note: if an elementary particle has zero electric charge (neutral): it cannot emit light it cannot absorb or scatter light

12 Light as Electromagnetic Waves light is a type of wave: a wave is oscillating disturbance in a medium wave can travel, medium does not Demo: the wave! snapshot in space, taken at one instant of time: diagram wavelength λ size of one cycle wave ID number intensity I strength of wave = height of peaks Demo: slinky: same wavelength, diff t intensities Q: Sound waves: how do we experience λ? I? Q: Light waves: how do we experience λ? I?

13 Light and Sound Waves SOUND λ pitch high pitch (treble): small λ low pitch (bass): large λ intensity = loudness LIGHT λ color visible light: larger λ: more red smaller λ: more blue intensity = apparent brightness Note for experts: in more detail, we distinguish flux = apparent brightness of unresolved (point-like) source e.g., stars, distant quasars intensity = surface brightness of resolved (extended) source e.g., Moon, planets, nearby galaxies

14 The Speed of Light Light is very fast! So fast that it was a feat to measure the speed in lab now known quite well c = constant = 299,792,458 m/s = m/s = 186,000 miles/s = miles/hr enormous but not infinite! finite speed of light hugely important for astronomy telescopes are time machines Q: how? note: light speed c is same for all λ Q: what would happen if this were not true?

15 The Electromagnetic Spectrum EM waves can have λ outside of visible range Generally, light is combination of pure waves with different λ distribution of intensities: different brightness at different λ: spectrum diagram: sketch spectrum axes Q: spectrum of laser pointer? Q: spectrum of white light?

16 The Electromagnetic Spectrum

17 Matter, Temperature and Light hot matter glows (think stove burner) temperature radiation connection microscopic picture: temperature atom motion but atoms made of charged particles motion changing EM forces light thermal body (at some temp T ) emits EM radiation: which λ emitted?

18 Matter, Temperature and Light spectrum of light; depends on T perfect absorber of light: blackbody absorbed energy heats up re-emits according to T blackbody radiation = thermal radiation

19 Thermal Spectrum: Light as a Thermometer peak λ is color seen: λ peak 1/T where T is absolute temperature in Kelvin recall: T Kelvin = T Celsius hotter more blue shorter λ Wien s Law Turn the equation around: T 1/λ peak, namely T = 3000 K 10 (6 m λ peak so: can find T just from light! spectrum as thermometer color measures temperature Intensity (arbitrary units) K Visible spectrum Infrared Ultraviolet 4000 K 300 K 7000 K Frequency (Hz) Wavelength (nm)

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21 Solar Spectrum Sun s spectrum peaks in middle visible wavelengths Q: but the Sun is not all at one temperature what has this T? Q: does Wien s Law apply to people?

22 Thermal Spectrum note: sunlight comes from Sun surface ( photosphere ) we have found T Sun,surfac even hotter inside! Humans and thermal radiation: we are not at zero temperature can and do radiate! but infrared invisible to naked eye, very visible to IR sensors But note: T-Shirt color thermal radiation (good thing!) reflected light, not glow from heat!

23 Separating out the spectrum of white light

24 Separating out the spectrum of the Sun

25 What do you think?

26 Temperature, Atoms, and States of Matter atoms always in random motion collide with each other! imagine starting cold, and turning up T at lowest T: cold matter low speeds gentle collisions if dense enough: atoms locked together solid! random jiggle too weak to dislodge atoms if warmer: atoms freed to roll past each other melt liquid! atoms move ( flow ) but still touch if warmer still: atoms can fly free evaporate gas! and atoms bounce back when collide if really hot: gas atoms collide at high speed atoms torn apart = ionized free e and nuclei: plasma hot gas in galaxy clusters: ionized plasma, no bound atoms cold gas clouds: all atoms are bound, none ionized

27 The Quantom Atom at small distances (size of atoms) Newton s laws fail! atoms, light obey new & different rules: quantum mechanics electron orbits nucleus + e: like solar system? No! QM e not like planet in atom, acts like wave!?! most orbits forbidden! only special orbit distances allowed quantized in steps allowed orbits energy levels lowest energy stable orbit, closest to nucleus ground state

28 Photons just as matter (like e) can sometimes act like waves light can sometimes act like particles... on small length-scales or low intensities light acts like particle: photon, symbol γ discrete lump or packet of energy different colors different energies smaller λ higher E: E photon 1 λ

29 The Electromagnetic Spectrum

30 Light-Atom Interaction If light hits atom and photon energy = atom energy level 1. atom absorbs photon 2. e jumps to higher level 3. atom in excited state but excited = unstable and after some time, 1. e jumps back to ground state 2. emits photon whose energy = excited ground state difference

31 Light-Atom Interaction Atoms absorb/emit light atom structure sets energies, and λ 1/E...which is different for different atoms so energy level spacings different for different atoms Intensity 650 Hydrogen Sodium Helium Neon Mercury Wavelength (nm) light spectrum gives atom fingerprint or barcode spectrum composition Wavelength (nm)

32 Measuring the Composition of the Cosmos Example: The Sun Sun, stars hotter, denser in center cooler, less dense at surface so: sunlight/starlight shows Q: what kind of spectrum? amount absorbed in each line amount of atoms composition of Sun; works for other stars too!