ATOMS AND STARLIGHT: I and IIStars Have Absorption Line Spectra
The interior of the star is hotter and denser and the outer atmosphere of the star is less dense and cooler. The interior generates a continuous spectrum (Law 1) and the cooler outer atmosphere absorbs certain photons (Law 3), depending upon which atoms are in the star.
Mostly, it is the hydrogen gas/atoms in the stellar atmosphere that make the prominent absorption lines in stellar spectra.
Temperature and Absorption Lines
As we shall now see, the strength of the absorption lines is temperature dependent.
Temperature measure the internal energy, the motions of the atoms in a solid. The scale of absolute temperature is called Kelvins. Zero Kelvin ( T = 0 ) means zero energy in an object or gas, i.e., no motions of the atoms. This is a theoretical idea. Everything in Nature has energy- nothing has absolute zero temperature
First, there is a sequence of star types, called spectral types, and this sequence is a temperature sequence...
"O", "B", "A", "F", "G", "K", "M", from hottest to coolest.
"A" stars have the strongest hydrogen lines. "O" stars are the hottest stars. "K" and "M" stars are the coolest stars. The strength of the absorption lines are temperature dependent. The strength depends upon what fraction of the atoms are ionized. When an atom is ionized, it means that one or more electrons are stripped off the atom. They get "knocked out" of the atom by very high energy photons.
The Balmer Thermometer
"A" stars have a temperature of 10,000 Kelvin (K). Hydrogen absorption is maximized at 10,000 K. Consider hotter and cooler stars and why they have weaker hydrogen absorption lines in their spectra.
Most of the hydrogen is ionized in hotter stars. The photons are too energetic and they strip the electrons off the atoms. Since the electrons are gone... obviously the photons that would otherwise be absorbed in the hydrogen atoms cannot interact with the atoms. So, the absorption lines become very weak because very few of the hydrogen atoms are participating.
Hydrogen is not excited in cooler "K" and "M" stars, (the photons do not have enough energy to raise the electron to higher energy orbits). So, most of the electrons in hydrogen are stuck on the ground orbit because they cannot absorb the lower energy light. This results in weaker absorption lines, because very few hydrogen atoms are participating.
You cannot measure the temperature of the star based only upon its hydrogen line strengths. This is because the strength can be the same for two different temperatures (cooler than 10,000 K and hotter than 10,000 K, as described above). You need more information. For example, absorption lines from calcium, which are strong for temperature below 10,000 K. Or, absorption lines from helium, which are strong for temperatures greater than 10,000 K. In practice, we have many elements to look at simultaneously and they all have their own unique temperature dependence.. We measure the strengths of all absorption lines from the different elements and constrain the temperature. We call this technique the Balmer thermometer.
The Doppler Effect
When an object is moving toward you, its spectrum is blueshifted. This means that wavelengths of the photon as measured by you are shorter than the wavelengths of the photons when they were emitted by the object! When an object is away from you, its spectrum is redshifted. This means that wavelengths of the photon as measured by you are longer than the wavelengths of the photons when they were emitted by the object! This is called the Doppler effect and the shifting of the spectrum is called the Doppler shift. The amount the spectrum is shifted is proportional to the velocity of the object. That is, the faster the object moves toward you, the more its spectrum is shifted toward the blue. Or, the faster the object moves away from you, the more its spectrum is shifted toward the red.