Tools of the Trade Lecture Astro 203. Sept 2, 2003 Lecturer: Rigby Topic: What is light, and how can light teach us about the Universe? Major Points: * Almost everthing we have learned from the universe, we've learned from light. * Light is a weird quantum mechanical thing. Wave or particles, okay. * A piece of light is called a "photon". * Any mass with a temperature gives off light. * More energetic objects give off more energetic photons. * Most light we can't see! (radio, infrared, UV, x-ray, gamma-ray) * Different stars emit at different energies of light. * Different energies of light tell you diferent things. * Want to understand stars, need to gather light of many energies. * Need telescopes to gather more light than your eye can --> see fainter. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% As Dr. Olszewski explained earlier, the few things in the universe can we directly examine (touch, prod, examine under microscope, pour acid on, stain with dyes) are nearby - in our solar system! From homework, travelling to the nearset star would take 100,000s yrs. Travelling to nearby galaxies takes longer than current age of universe. We may never travel to distant suns and galaxies. (no Star Trek yet) BUT WE CAN SEE those distant galaxies and stars! They're faint, but we can see them, even if the light's been travelling since just after the universe began. Since we can't explore the universe with spaceships, we need messengers. Fast messengers! (ideally as fast as the cosmic speed limit == speed of light) Messengers have to travel FROM the distant stars TO our telescopes. What are some messengers? 1 Neutrinos. Made in stars, but hard to detect. Only seen from 2 stars. If you're curious, read the footnote1 for more info. 2 Gravity waves. 2 objects orbiting each other create ripples in space, called "gravitational radiation". These move at the speed of light. We can see the effects of gravity waves in inary pulsars (see star #72 in Kaler), but we've never directly detected them. Under construction is an experiment called LIGO (http://www.ligo.caltech.edu/LIGO_web/PR/scripts/facts.html) which aims to directly detect them. But they haven't yet. (So far, we've detected 2 sources of neutrinos, and zero sources of gravity waves. Pretty dull sky so far.) 3 LIGHT!!!! A WONDERFUL MESSENGER! THE MAIN WAY WE'VE LEARNED ABOUT THE UNIVERSE! ALMOST EVERYTHING WE KNOW ABOUT ASTRONOMY, WE KNOW FROM STUDYING LIGHT! %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% What is light? Well, we all sortof know what light is. It's bright stuff that the sun and lamps make. But what is light, really? Light is the best example of quantum mechanics in your daily life. (What the heck's quantum mechanics? See footnote2). Light can be interpreted as a wave, or as a bunch of particles. particles: light is bunch of particles streaming through space. particles of light = pieces of light are called "photons" wave: light is travelling waves of electric and magnetic fields. waves described by a wavelength. More energy --> shorter wavelength EITHER INTERPRETATION, particles or wave, is equally correct. Weird, huh? QM is weird! Regardless of wave-or-particle question, these things are true: * Light moves at the cosmic speed limit, the speed of light. * Light carries energy (the sun is losing energy because it's making light). * Light pushes on stuff (physics-speak = "they carry momentum".) - example. the sun pushes dust grains out of the solar system - ex. sunlight pushes me, but it's about 10^10 times weaker than air pressure) How does light get made? Several different ways. A few important ones: a) accelerating (speeding up or slowing down) an electric charge. This is now FM radio waves are generated: electrons run up and down the antenna. In astronomy, there are lots of ways that accelerating electric charges make light. b) related method -- when electrons change energy states in atom, they give off light. These are called emission lines. We'll talk about them in the spectroscopy lecture/lab. c) ANY MASS WITH A TEMPERATURE GIVES OFF LIGHT. (this is "thermal radiation") %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Digression: we need to understand what temperature is Stuff is made up of atoms. atoms move about with an average speed. more energy in stuff <--> atoms move faster <--> temperature higher <-> pressure higher example: Check your tire pressure, then drive around. Now check the tire pressure -- it's higher. The friction of the tire on the road heats up the tire's air (raises the temperature.) Air molecules whizz around faster, so they smack against the tire wall harder. We measure this as increased tire pressure. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Okay, enough digression. Point is: a) everything with a temperature gives off light, b) the higher the object's temperature, the more energy its photons have. For example, heat steel in a hot fire. First it's black, but as you heat it, it starts glowing dull red, then orange, then yellow, then white-hot, then blue-white. Cooler=redder. Hotter=bluer. What about before you stuck the steel into the fire? Was it glowing? YES, but with light invisible to you! It was "infrared light". ("Infrared" means "redder than red".) MOST LIGHT WE CAN'T SEE! Our eyes only see the visible part of the spectrum: red orange yellow green blue purple cats can see redder than us; bees can see bluer than us light exists across a huge "spectrum" of energies (see full EM spectrum at http://www.phy.ilstu.edu/~aremijan/Phys102/spectrum.jpg) ("spectrum" = light broken down into different energies = rainbow) important regions of light spectrum: radio, sub-mm, infrared, optical, ultraviolet, x-ray, gamma-ray ----higher energy--> ---shorter wavelengths--->h light currently used by astronomers has wavelengths of 30 to 10^-18 meters -- factor of 10^20! if light is a piano, visible light is one octave, and there are ~66 octaves on the piano!! higher energy photon = shorter wavelength = bluer "redder" is lower energy photon; "bluer" is higher energy hotter things emit bluer light, cooler things emit redder light %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% So there are lots of kinds of light. In everyday usage we often consider them to be very different phenomena ("radio waves", "UV rays", "gamma radiation"), but it's all light. What are examples of the different energies of light, in daily life and in astronomy? radio daily: FM/AM/VHF, broadcast TV, cell & cordless phones; broadband internet; RC cars stars: pulsars; supernova remnants infrared daily: motion detectors; cops' IR laser speed traps; TV remote; people stars: brown dwarfs; newly-forming stars; planets; dust visible daily: light from sun or lamps that you use to see; LEDs; squeezing crystals stars: most stars ultraviolet daily: sun's UV rays that tan/burn/cause skin cancer; tanning beds; sanitizing stars: white dwarfs eating their companions x-rays: daily: dental and bone x-rays images; airport security stars: neutron stars (thermal & pulsating); black hole accretion disks gamma: daily: nuke-sniffing radiation detectors; food irradiation to kill bacteria stars: gamma ray bursts; magnetars We see that different kinds of stars emit different energies of light. To learn as much as possible, need to collect light at many energies! %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% But there's a problem. The Earth's atmosphere doesn't let all the Universe's light through. The atmosphere is transparent to radio, some infrared, and optical light. The universe blocks UV, x-ray and gamma-ray light, as well as some infrared. Good for our skin and mutation rate, but bad for astronomy! Q: How do we look at the light that doesn't reach the ground? A: Launch telescopes into space! It's much cheaper to build telescopes on the ground, but if light doesn't reach the ground, we must go into space. A few famous space telescopes you may hear about, by wavelength: infrared: SIRTF (will be renamed in 90 days) optical: Hubble Space Telescope UV: Hubble Space Telescope; x-ray: Chandra; XMM-Newton gamma-ray: Compton-GRO (There have been many more space telescopes.) Also, should mention famous radio telescopes: Arecibo (in Puerto Rico) and the Very Large Array (VLA) in New Mexico Soooo..... (tying things together) What kinds of stars can the various telescopes see? Arecibo (radio): radio pulsars; supernova remnants SIRTF: dusty galaxies; cool stars; just-forming stars Hubble (optical): normal stars and galaxies (big groups of stars) Hubble (UV): really hot stars; hot white dwarfs and CVs Chandra: neutron stars and x-ray pulsars; active stars Compton: GRBs See! To understand the formation, lives, and deaths of stars, we need all these kinds of light! %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Why do astronomers need telescopes, anyway? (1) (optional) See energies of light that human eye can't. 2. Gather more light!!! "Giant eyeballs" or "light buckets" --> see faint stars. 3. Resolution = fine detail = magnification. See closely-separated things. #3 is less important than you might think. For most of astronomy, we really want #2 == more light. What product do astronomers get from telescopes? What is their "data", anyway? We generally use telescopes in 2 modes: "imaging" -- take pictures. In optical, we use fancy digital cameras. But we image in other wavelengths, too: take radio and x-ray pictures. "spectroscopy" -- spread the light out by energy, into a rainbow. Count up how much light at each energy. As we'll see later, spectroscopy tells you what stars are made of. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% INFRARED (10 MICRON) CAMERA DEMONSTRATION Definition: 1 "micron" = 1 micro-meter = 10^-6 meter = 1/100 human hair Humans see light at 0.3 - 0.7 microns. Objects at ~100F, like people, emit most of their light at ~10 microns. We brought to class an infrared camera, which detects light at 10 microns. Humans are quite bright at this energy (a person emits ~100 watts, same power as light-bulb.) We imaged Dr. Olszewski. We noticed: His skin is bright at 10 um -- hotter than his shirt. His glasses are black because glass isn't clear at 10um. Ice water he's holding is darker than his skin because it's cooler. If he ices his skin, it doesn't emit as much --> darker. person: too cool to glow in optical, but glows at 10 micron hot plate: barely hot enough to glow in optical, but glows at 10 micron light bulb: when on, hot enough to glow in optical and at 10 micron. when just switched off, too cool to glow in optical, but still glows at 10 micron. ice: too cool to glow in optical, but glows faintly at 10 micron liquid N2: too cool to glow much in optical or @ 10 micron. Brightest at ~40 micron. So optically-faint things that are bright at 10 micron.... are heat sources! (but not too hot, or they'd glow in optical, too.) We used a piece of aluminium to reflect light. See, infrared light behaves like light. You can bounce it off mirrors, spread it out into its constituent energies, etc. DIFFERENT ENERGIES OF LIGHT TELL YOU DIFFERENT THINGS. INFRARED TELLS YOU THINGS OPTICAL CAN'T. * We saw heat-footprints in the carpet where someone had just walked. * From a distance, we could easily tell which cups held hot water, and which cold. THIS IS WHY, IN ASTRONOMY, WE TRY TO GATHER AS MANY ENERGIES OF LIGHT AS POSSIBLE. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Last, we played with liquid nitrogen. Nitrogen is: a boring, non-reactive gas which makes up 80% of the air you breathe. You don't use it; your body wants the 20% oxygen, and the nitrogen just comes in & back out. Liquid nitrogen is: Nitrogen from ordinary air, cooled so cold that it condenses into a liquid. When exposed to the hot lecture room, the liquid nitrogen boils (at -320°F!). The "steam" you see is actually nitrogen evaporating away. The room's already full of gaseous nitrogen, so it doesn't really change much. This seems less strange when you make the analogy with water. At every-day temperatures and pressures, water is a liquid. Start with frozen water (ice). Heat it to 0°C (=273K =212°F) and it will melt. Heat it to 100°C (=373K =212°F) and it boils. Well, nitrogen is a gas at ordinary temperatures. If you start with frozen nitrogen and heat it, it melts at -210°C (=63K =-346°F.) If you heat it some more, it boils at -200°C (=77K =-320°F). How cold is the liquid nitrogen we were using? -320F = 77K = -200C Why do astronomers use it? Detectors of infrared light must be very cold, or else the detector will give off more light than the distant star! So we use liquid nitrogen and helium to cool our detectors. (We also play with liquid nitrogen on cloudy nights at the telescope.) In the IR camera, the liquid nitrogen (and things dropped in it) looked very black. The liquid nitrogen is emitting light (mostly at 40 micron), but not much. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% [This section is optional. Your lecturer has omitted an important effect for brevity, and this section explains how things really work.] Here I'll explain why skin *really* looks brighter than ice in the 10 micron camera. As a human-temperature object gets colder, it gets dimmer in the IR camera for 2 reasons. Reason 1 (talked about in class) is that the spectrum shifts to lower energies, energies below the camera's range. That's true, but there's an important second reason that we ignored, which is this: The total brightness (in any energy) of an object depends on the object's temperature to the fourth power. So as an object cools, its spectrum shifts to lower energies proportional to T, but the intrinsic brightness drops as T^4. Which of these effects (spectral shifting or T^4 brightness) dominates depends on wavelength in question and the temperatures of the sources. Here are two examples. At 10 microns, skin is 1.6 times brighter than ice. A few percent of this occurs because ice's spectrum peaks at 11 microns rather than skin's 10 micron. But most of the reason skin is brighter than ice is the T^4 dependence. Liquid nitrogen is a million times fainter than skin at 10 micron. The spectral shift and the total power (T^4) have comparable influence in dropping the flux to really, really low levels. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% FOOTNOTES (extra stuff for the curious): footnote1: more info on neutrinos for the curious: [As chapter 1 of Wheeler explains, neutrinos don't interact via the electromagnetic force or strong force. They only obey gravity and the weak nuclear force. As we saw in lecture 1, you're mostly made of empty space, interacting via electric forces. Since neutrinos don't feel electric forces, they see you as mostly empty space, so they course right through. That's GOOD because they're generated in the core of the sun and course right through the rest of the sun (so we can directly probe the sun's heart!) but it's BAD because it's darned difficult to actually catch a neutrino! They pass right through our traps! Neutrino detection is described in CH 1 of Wheeler -- basically, it's darned hard. As a result, only 2 objects in the sky have ever been seen giving off neutrinos: 1) the sun; 2) one star blowing up, called a "supernova". We'll talk about this in chapters 6&7 of Wheeler).] footnote2: (Quantum Mechanics is physics that describes very small stuff. It's based on probability- nothing is certain or exact, but you can talk about how likely an event is to happen, or how likely a particle is to be near a certain point at near a certain time.)