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# 10.1 Observing the Universe

Astronomers learn about the universe by deciphering messages carried by the radiation from extraterrestrial bodies. Electromagnetic radiation travels across the vacuum of space at 300,000 kilometers per second. The light we see with our eyes is just one example of this radiation. Light spread out in order of wavelength is a visible spectrum of the object. However, there are also wavelengths far too short (too blue) to see, and wavelengths far too long (too red) to see. The entire electromagnetic spectrum covers a tremendous wavelength range, from gamma rays the size of an atomic nucleus to radio waves many meters long.

Electromagnetic radiation can be described as a wave or a particle, depending on the kind of interaction it has with matter. Waves have a wavelength, or typical distance between peaks. Their frequency is a measure of the number of peaks that pass per second. Wavelength and frequency are inversely related. Radiation can be considered either as a particle, or a photon, whose energy is proportional to its frequency. Atoms absorb and release energy in the form of photons — these discrete units of matter and energy mean that the microscopic world is "grainy." Most of what we know about the universe depends on the ways that matter and radiation

interact with each other.

Objects in space reveal their nature by the radiation they emit. The smooth thermal spectrum has a peak wavelength that indicates the temperature. In addition to a smooth spectrum, any hot gas has sharp spectral features. These sharp lines appear because the electrons around an atomic nucleus can only inhabit certain fixed energy levels. When electrons change their energy level, they emit a spectral line of a specific wavelength. Each element and compound has its own lines and bands. The spectral lines act like a fingerprint that helps us figure out what a hot object is made of.

Even though scientists make very accurate measurements, the microscopic world is fundamentally "fuzzy." There is a limit to the precision with which scientists can measure microscopic quantities, a concept expressed in the Heisenberg uncertainty principle. This means that subatomic particles cannot have their positions and motions measured with certainty, leading to a situation that requires us to recognize that both our data and our knowledge of the physical universe may be limited. This limitation is not apparent when large objects like people or planets or stars.

Telescopes are devices that improve on the light-gathering power of the eye and allow astronomers to resolve finer details of an astronomical target. Thus we can see objects much fainter than those visible to the unaided eye. The light-gathering power and the resolution of a telescope increase with increasing aperture. Most modern telescopes are reflectors. There is currently a surge in the construction of large telescopes on mountaintop sites around the world. We have a dozen telescopes with apertures of 8 meters and larger, and several in the range 20-30 meters are planned. Since optical detectors are almost perfectly efficient, astronomers need a larger collecting area to see deeper into the universe. The technique of interferometry gives astronomers far better resolution than can be achieved with a single telescope. Other telescopes have been placed in orbit to give ultra-sharp optical images and to detect long and short wavelengths that cannot penetrate the Earth's atmosphere. Perhaps the most exciting revolution is the peeling back of the electromagnetic spectrum, revealing for the first time the invisible universe.