More raw notes. Let me know if this gets dull.
99.99999% of what we know about the universe comes from understanding light. He’ll talk about the tools, because astronomy is a technology driven science. Theorists can come up with theories, but until we have the tool to test it, it’s just a theory.
Light and other Forms of Radiation
The Electromagnetic Spectrum. He’ll use “light” and “radiation” interchangeably. To the average person, light is what happens when you hit the switch on the wall. Radiation makes you sick and kill you. But to an astronomer, light could be electromagnetic.
Light as a Wave.
Waves λ (lambda) Lightwaves travel through space at the speed of light. c = 300,000 km/s = 3*108 m/s
Need to know your Greek letters if you’re going to do physics. (nu)
Light waves are characterized by a wavelength and a frequency f.
f and λ are related through f = c/λ
Wavelength is related to color. Different colors of visible light correspond to different wavelength. If you send in white light, a dispersive element like a prism, bends at different rates when it goes through glass obliquely. The speed of light changes slightly depending on its wave length. Wavelength is another way of saying the color. Ultraviolet has short wavelengths and infrared has long wavelengths.
Laura says: Imagine a car hitting a patch of sand at an angle, it will cause the car to skid and turn. The prism does much the same thing.
Biological issues regarding perception.
Light coming from an incandescent bulb is red. But there’s enough blue in it that our eyes perceive it as a full spectrum. If you crank down the intensity of the light, you’ll notice the red.
Dark Side of the Moon.
“There is no dark side really. It’s all dark.” — Pink Floyd.
Wrong but cool.
Looking at the album cover.
What is wrong with this picture?
Front: Not all primary colors (eg, pink magenta) also refraction angles are inconsistent.
Back: Spectrum is Convergent. If you run a rainbow through a prism, it will just refract more.
Light as a Wavelength
Wavelengths of light are measured in units of nanometers or angstrom
1 nm = 10-9 m
1A =
10-10
m = .01nm
Visible light has a wavelength between 4000 A and 7000
The Electromagnetic Spectrum
violet light 400nm
red 700 nm
There are animals that can see ultraviolet.
Gamma ray – X-rays – Ultraviolet – visible light – infrared – microwave – UHF – VHF – FM – AM
Shorter frequency wavelengths has more energy and can damage you.
Microwaves are like high energy radio waves.
Almost certainly evolved to see the visible spectrum because that’s what penetrates the Earth’s atmosphere. This is why you want to put telescopes on mountain tops because it puts you above the water vapor so you can see as much as possible. In order to see radio waves you have to get into space. Which is part of why astronomy has blossomed so much is that we can get telescopes up there now.
Light as Particles
Light can also appear as particles, called photons
A photon has a specific energy E, proportional to the frequency f: E = h* f
h=6.626×10-34
J* s ((Jule seconds)) is the Planck constant
The energy of a photon does not depend on the intensity of the light.
Frequency = how many times it happens per second
Wavelength = peak to peak
This is why there’s a relationship between speed, wavelength and frequency.
Why is energy per photon so important?
Real life example: Ultra-violet light hitting your skin
Threshold for chemical damage set by energy
Temperature and Heat
Thermal energy is “kinetic energy” of moving atoms and molecules
Hot material energy has more energy available which can be used for:
- chemical reactions
- nuclear reactions (at very high temperatures)
- escape of gasses from planetary atmosphere
- Creation of light
- Collision bumps electron up to higher energy orbit
- It emits extra energy as light when it drops back down to lower energy orbit
- Reverse can happen in absorption of light.
Temperature Scales
Want temperature scale with energy proportional to T
Celsius scale is “arbitrary” (Fahrenheit even more so)
By experiment, available energy = 0 at “Absolute Zero” = -273C (-459.7F)
Use Kelvin for most astronomy work
Available energy is proportional to T, making equation simple (really! OK simpler)
273K = freezing point water
373K = boiling point water
Plank “Black Body Radiation”
Hot objects glow (emit light) as seen in Predator, SSC Video, etc.
Heat and collisions in material causes electrons to jump to high energy orbits and as electrons drop back down, some of the energy is emitted as light.
Reason for name Black Body Radiation
In a solid body, the close packing of atoms means that the electron orbits are complicated.
Planck and other Formula
Plank formula gives intensity of light at each wavelength.
Wien’s law tells us what wavelength has maximum intensity
Stefan-Boltzmann law tells us total radiated energy per unit area
If you double the temperature of an object it will radiate black body spectrum with 16 times the energy.
If you had infrared eyes, a person with a fever would look brighter you.
What is wavelength at which you glow? – Room T = 300 K so This wavelength is about 20 times longer than what your eye can see. Thermal camera operates at 7-14 um.
Hot solids emit continuous spectra
Hot gasses try to do this, but can only emit discrete wavelengths. You need a lot of particles bumping up against each other so they don’t tend to emit black body radiation. Line emission instead.
Cold gasses try to absorb these same discrete wavelengths.
Hydrogen Lines.
If we have a hydrogen gas absorbing light, it will absorb at a particular ultraviolet, infrared wavelength. This is how we can tell that the sun is mostly hydrogen, because we can look at the wavelengths it emits.
Helium was first discovered in the solar spectrum, which is why it’s called Helium.
Astronomical Telescopes
He showed us the Northern Gemini Telescopes on Hawaii which is large. Mike has time on it later this year. Sometimes this means that he goes out there and does the work there, but it operates in Que mode, so he’ll send a program out which will tell them what to aim it at, with which filters etc.
Often these are very large to gather large amounts of light. In order to observe forms of radiation other than visible light, very different.
Refracting/Reflecting Telescopes
Refracting Telescope: Lens focuses light onto the focal plane
Reflecting Telescopes: Concave Mirror focuses light onto the focal plane
Astronomers don’t like lens because red and blue light enter at different speeds which cause the images to blur out. Plus it’s hard to get a clear lens that large. Prefer mirrors.
Size does matter! It’s the width, not the length. The collecting area of the telescope that matters.
Resolving power. The telescope aperture produces fringe rings that set a limit to the resolution of the telescope. Astronomers can’t eliminate these diffraction fringes, but the larger the image the more they can correct.
Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images.
Magnifying power= ability of telescope to make the image appear bigger. Amateurs tend to care about magnifying power. Pros tend to care about resolving power. The only time magnification is useful is if you are using your own eye and the object is just too small for your eye to perceive.
The Best Location for a Telescope – Far away from civilizations to avoid light pollution. Hawaii has some of the best seeing in the world.
On high mountain-tops to avoid atmospheric turbulence (seeing) and other weather effects.
Paranal Observatory (ESO) Chile is very good.
Adaptive Optics
Computer controlled mirror support adjust the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence. You can create an “artificial” star, by pointing a laser that vibrates particles in the upper atmosphere so you can sight the telescope on that.
Interferometry
Recall: Resolving power of a telescopes depends on diameter D. Combine the signals from several smaller telescopes to simulate one big mirror.
Don’t use photographic plates any more. Digital detectors. Data can be read directly into computer memory.
Imaging and Spectrometry are the bread and butter of astronomy. Grism instead of prism. It combines gratings and prisms.
Science of Radio Astronomy
Neutral hydrogen emits in the radio wave. As do a lot of molecules. Radio waves penetrate not only our atmosphere but a lot of objects in space as well.
Infrared Astronomy
Most infrared is absorbed by the upper atmosphere. So these telescopes need to be high altitude.
Ultraviolet astronomy has to be done from space as it’s absorbed by the ozone layer in the atmosphere.
NASA’s great Observatories in Space. Hubble is never obsolete, because it will can do things that we can do from the planet, such as seeing in the ultraviolet. Chandra X-ray observatory. If you want to observe millions of degrees hot gas, you need Chandra.
Chandra has a highly eccentric orbit that takes it 1/3 of the way to the moon.
Hubble is only used about half the time because the Earth is in the way. You can only look at an object for about 45 minutes before the Earth interferes.
Chandra can look for a significantly longer time. Chandra is the first x-ray telescope to have images as sharp as opticals. It has 4 nester hyperboloid mirrors and 4 nested paraboloid mirrors to focus the x-rays.
Spitzer telescope operates at mid-infrared (room temperature)
Kepler’s Supernova. There’s material there at MANY temperatures, so many wavelength’s are needed to understand it.
http://nasascience.nasa.gov/missions/mission_by_phase_list
The Future of Space-based Optical/infrared Astronomy.
James Webb Telescope goes up in 2013 or 2014 and will be at Lagrangian point. L2
Terrestrial Planet Finder. Will use both interferometry and coronagraphs ((This is like putting your hand up to block the sun so you can see an object in the sky)) to image Earth-like planets.
> If you had infrared eyes, a person with a fever would look brighter you.
Transgenics who’ve escaped from Manticore would also look brighter.
It’s been fun reading your astronomy notes. I’m just finishing up teaching astronomy at the University of Pittsburgh and this month I pick up classes at CCAC, Carnegie Mellon University and St. Vincent’s. Your posts have done a great job of refreshing me and making me confident that I can teach at five schools in one year (I also teach SF/F at Seton Hill University). Perhaps not the reason you posted, but I’m grateful for your efforts.
Five schools? Yoicks! Good luck with that.
The fact that I found the most interesting from this post is “Light coming from an incandescent bulb is red. But there’s enough blue in it that our eyes perceive it as a full spectrum. If you crank down the intensity of the light, you’ll notice the red.” Ah, damn, didn’t know I was fooled! This explained what I saw before the lights light up all the way, though. (“Ah… that’s why.”)
I have no idea why I knew about astronomers not liking lenses. I looked up that stuff at some point but I don’t remember why. Supposedly there are different ways of arranging the lenses and it affects the magnification and the accuracy of colors. There are, of course, terms for those things but I don’t remember.
See, I immediately thought of how black clothing will sometimes look red onstage in dim lighting.
Now that you mentioned it, oh yeah, that’s right. The next thing I thought of was the different white washes and how some may be warm and how some might be cool. Not sure if that’s related.
Hm. I’m not sure on that. I hear people talk about the warmth of a lamp, which relates to how it sits on the white to yellow scale. But it could also be the gels.
I believe we were talking about gels, yeah. I’m not sure how that worked, but basically you can have white, but you can have one white that feels warmer than the other. It has to do with which gels you use to add up to white.