SUMMARY OF KEY IDEAS
By studying the wavelengths of electromagnetic radiation emitted and abosrbed by an astronomical object, astronomers can learn about the object's temperature, chemical compostition, companion objects, and movement through space.
A blackbody is a hypothetical onject that perfectly absors electromganetic radiation at all wavelenghts. The radiation that it emits depends only on its temperature. Stars closely approximate black bodies.
Wien's law states that the dominant wavelength of radiation emitted by a blackbody is inversely proportional to its temperature. The intensities of r adiation emitted at various wavelengths by a blckbody at a given temperature are shown as a blackbody curve.
The Stefan-Boltzmann law relates the temperature of a blackbody to the rate at which it radiates energy.
Spectroscopy- the study of electromagnetic spectra- provides imortnat information about the chemical composition of remote astronomical objects.
Kirchhoff's three laws of spectral analysis describe the conditions under which absorption lines, emisssion lines, and a continuous spectrum can be observed. Spectral lines serve as distinctive "fingerprints" for the chemcial elements and chemical compounds comprising a light source.
ATOMS AND SPECTRA
An atom consists of of a small, dense nucleus (composed of protons and neutrons) surrounded by electrons. Different elements have different numbers of protons, different isotopes have different numbers of neutrons.
Quantum mechanics describes the behavior of particles and shows that electrons can only be in certain allowed orbits around the nucleus.
The spectral lines of a particular element correspond to the various electron transition between allowed orbits of that element with different energy levels. When an electron shifts from one energy level to anojther, a pohoton of the appriopriate energy (and hence a psecific wavelength) is absorbed or emitted by the atonm.
The spectrum of hydrogen at visible wavelengths consists of the Balmer series, which arises from electron transitions between the second energy level of the hydrogen atom and higher levels.
Every different element, isotope, and molecule has a different set of spectral lines.
When an atom loses or gains one or more electrons it is said to be charged. One way for it to lose an electron is for the electron to abosrb an energetic photon and thereby fly free from its atom.
The equation describing the Doppler efect states that the size of a wavelngth shift is propoprotional to the radial velcoity between the light source and the observer.
VISIBLE LIGHT AND THE ELECTROMAGNETIC SPECTRUM
*ALL OBJECT SHINE BY THEIR OWN LIGHT...we can't usually see this, we can only see very hot object shining.
Wein's Law states that as the temperature of an object increases, the wavelength of light that is the maximum brightness of an object increases. This type of light, emitte by all objects, is called the black body radiation.
Where it peaks depends on the temperature. There is a color associated with the temperature. The simple version of this law is: Brightest color of the emitted radiation changes with temperature.
INFRARED (cool)> RED (warm)> YELLOW (hot)> BLUE (very hot)> ULTRAVIOLET (extremely hot)
We also see Stefan's Law- where As an object heats up it emits more light at all wavelengths. Hot object are cooler than bright objects. This brighttness is also reffered to as the FLUX. This diagram demonstrates both laws.
Next we study Kirchoff's Laws, because in addition to an object emitting light due to their temperatures, atoms can absorb and emit light. We considr three different cases:
1. A black body source emitts a continuous spectrum, or a rainbow of color.
2. A hot rarified gas produces an emission line spectrum. This is a series of bright spectral (color bands) against a black back ground.
3. A cool gas infront of a continuous source of light produces an absorption line spectrum. A series of dark spectral lines among the colors of the continous spectrum.
The dark lines from Kirchoff's third law tell us what atoms there are in that cloud of gas. In our case, the gas would be from the atmosphere of a star. This is thought of kind of like the star's fingerprint.
Have a dense central nucleus, almost all of the mass of the atom is in this nucleus. There are the positive protons, nuetral nuetrons, and orbiting this nucleus are the negatively charge electrons.
or, for a simplified diagram:
Atoms have very specific energy levels corresponding to the allowed orbits of electrons. Every type of element is different. Electrons can either be in their ground state, where they have not absorbed any energy, or when they absorb energy, they will be in their excited state, where they jump either one or several levels of orbit. They don't tend to hold onto the energy for too long though, and when they do let it go, they can eith fall one level, or several, and even go back to their ground state. So we know that electrons can capture a proton, go up a level, and then randomly go back down. This shows that incoming light is abosrbed if it is at the correct wavelength.
Given all of this information, and remeber the Doppler effect from our last chapter, we can conclude that the lines in the spectrum of a star (or planet, galaxy, etc.) are blue shifted if it is moving toward the Earth. It is red shifted if it is moving away from the Earth.