Possible sources of information on the Universe 1) In situ (on site) measurements and samples (e.g. Moon rocks): possible only for the Solar System 2) Meteorites, and interplanetary or sometimes interstellar dust (collected by high-flying airplanes) 3) Cosmic rays: About as much energy as is in starlight is delivered to Earth by cosmic rays, or high-energy subatomic particles (above Earth's atmosphere, mostly electrons and protons; near sea level, mostly muons and alpha particles [or helium nuclei]). This has yielded interesting information, for example that the Universe is probably mostly made of matter and not half matter/half antimatter, since cosmic rays are almost all matter. The information content of cosmic rays is limited, however, since their directions get randomized in the magnetic field of the Galaxy. 4) Other particle radiation, from the Sun (called "The Solar Wind", although the really energetic ones are called "Solar cosmic rays"), or trapped by Earth's magnetic field (called the Van Allen belts). As with cosmic rays, this includes protons, electrons, and alpha particles. 5) Neutrinos: these nearly massless, neutral particles have been detected from the Sun and from Supernova 1987A. They are the direct products of astrophysical nuclear processes; they're also very difficult to observe. 6) Gravitational waves: there have been no direct detections yet, but there may be soon, with the new LIGO detectors, under construction. 7) Tachyons: faster-than-light particles may or may not exist. Many physicists are skeptical that they can exist, since they would have to have very strange properties, such as traveling backwards in time, and having imaginary rest masses. (What does either mean, anyway?) 8) LIGHT, including radio waves, X-rays, etc. Since "light" is often taken to mean "visible light", when talking about light at all wavelengths, both visible and invisible, we often use the term "electromagnetic radiation." The electromagnetic spectrum ---------------------------- Gamma Rays X-rays Ultraviolet Optical Infrared Radio (hard) (soft) From From From From nuclei inner outer molecules -> (Energetic!) electrons electrons -> Wavelength: 1 A 10 A 100 A 1000 A 4000-7800 A 1 micron 1mm 1cm 1m where 1 A = 1 Angstrom = 10^-8 cm = 10^-10 m Temperatures (K): 3e6 3e5 30,000 ~10^4 3,000 3 K Photon energies: > 1 MeV 12.4 1.24 0.124 keV 1 eV = 1 electron volt = 1.602 x 10^-19 Joules = 1.602 x 10^-12 ergs, so 1 keV = 10^3 eV and 1 MeV = 10^6 eV. Recall E = hc/lambda for each photon, where lambda = wavelength, h = Planck's constant, and c = the speed of light. Most e/m radiation, including ultraviolet (UV), X-rays, most gamma rays, and most infrared (IR) are absorbed by Earth's atmosphere, so spacecraft are needed to observe them. Spacecraft are expensive, though: much worthwhile observing, especially in the optical, near-IR, and radio, can be done from ground-based telescopes.