Photons, Spectra of Stars, and the Doppler Effect
How do we know what stars are made from? By the light they emit.
The Electromagnetic Spectrum
Any body heated above the absolute zero temperature emits electromagnetic radiation (0 K in the Kelvin scale; the Kelvin scale is similar to the Celsius scale, i.e. an interval of one Kelvin corresponds to an interval of 1 deg. C, but it is offset with respect to the Celsius scale. O K is -273 C, and 0 C is 273 K, or K = deg.C + 273). At any given temperature, electromagnetic radiation is emitted with a wide range of wavelengths, but with an intensity maximum closely related to the temperature of the body (Wien's law). Specifically,lmax = 3 107(in Angstrom) / T (in Kelvin).
Thus, the higher the temperature, the smaller the wavelength where the maximum in intensity occurs. For example, an object radiating at 1,000 K, has a lmax = 30,000 Angstrom. This wavelength is the wavelength at which there is a peak in intensity of emitted radiation; however, decreasing amounts of radiation are emitted at both higher and lower values of lmax. A 1,000 K object will emit a small amount of radiation in the visible, around 7,000 Angstrom of wavelength, and it will be seen as glowing with a dark orange color.
The Electromagnetic Spectrum
Celestial objects emit electromagnetic radiation over a wide range of wavelengths. There are objects that emit no visible light and yet they are powerful emitters of gamma rays. There are stars in the center of the galaxies that are not visible within the human range (red to violet), and yet infrared radiation from those stars can be detected since infrared light penetrates clouds of gas and dust, while visible light cannot.Radio waves (Km to meters to cm) give information on magnetic fields, free electrons, and molecules in the interstellar medium;
Microvawes (cm) come from atomic transitions;
Infrared radiation (100 to 0.7 mm) gives information about molecules and composition of dust particles;
Optical light (700 to 400 nm) gives information on presence of hydrogen;
Ultraviolet (UV) waves (400 - 100 nm) give information on cold electrons and excited states of atomic and molecular hydrogen (being, respectively, the most abundant atom and molecule in the universe);
X-rays (a few Angstrom) give information on hot electrons, i.e. electrons that signal the presence of interactions of particles at high energy;
Gamma-rays (sub Angstrom) signal the ocurence of high energy interactions.
Note: different units are often used for the wavelength of electromagnetic radiation depending on the range considered. There is no sharp delimitation between the ranges described above. 1 km = 1,000 m; 1 cm = 10-2 m; 1 mm = 10-6 m; 1 nm = 10-9 m; 1 Angstrom = 10-10 m)
The Spectra of Stars
Stars emit and adsorb electromagnetic radiation at certain wavelengths. Their spectra look like more the spectra shown in the figure above rather than the rainbow spectrum seen in the figure at the beginning. The reason is that atoms near the outer part of the stars emit (bright color lines) or absorb (dark lines) electromagnetic radiation. According to quantum mechanics, atoms can emit or absorb electromagnetic waves only if they have certain wavelengths. Each type of atom, i.e. hydrogen ,helium, carbon, etc., has a different mix of allowed emission/absorption wavelengths; by measuring the spectrum of a star, one can infer which elements are present there.
Example of absorption and emission of a photon by an atom
Resolution
When looking at images of celestial objects, one is concerned with spatial and spectral resolution.Spatial resolution is the ability to distinguish two nearby objects as distinct. It is the minimum angle two objects seen by the Earth can be separated and found distinct.
Spectral resolution is the capability of an optical instrument to differentiate two light signals that differ in wavelength or frequency.