Production of Light

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When light is passed through a prism or a diffraction grating to produce a spectrum, the type of spectrum you will see depends on what kind of object is producing the light: is it a thick or thin gas, is it hot or cool, is it a gas or a solid? There are two basic types of spectra: continuous spectrum (energy at all wavelengths) and discrete spectrum (energy at only certain wavelengths). Astronomers usually refer to the two types of discrete spectra: emission lines (bright lines) and absorption lines (dark lines in an otherwise continuous spectrum) as different types of spectra.

Continuous Spectrum

A rainbow is an example of a continuous spectrum. Most continuous spectra are from hot, dense objects like stars, planets, or moons. The continuous spectrum from these kinds of objects is also called a thermal spectrum, because hot, dense objects will emit electromagnetic radiation at all wavelengths or colors. Any solid, liquid and dense (thick) gas at a temperature above absolute zero will produce a thermal spectrum. A thermal spectrum is the simplest type of spectrum because its shape depends on only the temperature. A discrete spectrum is more complex because it depends on temperature and other things like the chemical composition of the object, the gas density, surface gravity, speed, etc. Exotic objects like neutron stars and black holes can produce another type of continuous spectrum called ``synchrotron spectrum'' from charged particles swirling around magnetic fields, but I will discuss them in another chapter later on. For now, let's look at a thermal spectrum.

Sometimes astronomers use the term ``blackbody'' spectrum for a thermal spectrum. A ``blackbody'' is an object that absorbs all the light falling on it, reflecting none of it, hence, it appears black. When the ``blackbody'' object is heated, it emits light very efficiently without any gaps or breaks in the brightness. Though no object is a perfect ``blackbody'', most stars, planets, moons and asteroids are near enough to being ``blackbodies'', that they will produce spectra very similar to a perfect thermal spectrum.

picture of a thermal spectrum

Some thermal spectra for objects of different temperatures are illustrated in the figure below.

plots of thermal spectra

Some key features of a thermal (continuous) spectrum are as follows:

  1. There is light from a dense object at all possible l IF the object is above 0 K (absolute zero). Since everything in the universe is above 0 K, all dense objects (solids, liquids, thick gases) will produce a thermal spectrum.
  2. The shape of a continuous spectrum depends on only the temperature of the object NOT its chemical composition. This allows you to determine the temperature of an object from a great distance away.
  3. As the temperature of an object increases, more light is produced at all wavelengths than when it was cooler. You can see this effect with a light bulb wired to a dimmer switch. As you raise the current going to the bulb, the bulb's filament gets hotter and brighter.
  4. As the temperature of an object increases, the peak of thermal spectrum curve shifts to smaller wavelengths (higher frequencies)---cool things appear red or orange, hotter things appear yellow or white, and very hot things blue or purple. This is opposite to what artists use for ``cool'' colors (blues) or ``hot'' colors (reds)! You can also see this effect with the light bulb wired to a dimmer switch. The dim bulb will have an orange color and as you make it brighter, the bulb will turn yellow and even white.

    temperature effect on color and intensity

    Wilhelm Wien (lived 1864--1928) discovered that the peak of the thermal spectrum curve, lpeak in nanometers, is related to the temperature by lpeak = 2.9 × 106 / temperature (in K). This simple relation is now known as Wien's Law. Using this you will find that cool objects like cars, plants, and people radiate most of their energy in the infrared. Very cold objects radiate mostly in the radio band.

  5. A small change in the temperature produces a HUGE change in the amount of energy emitted by every unit area of the object. If you add up all of the energy emitted every second by an area of one square meter on the object's surface, you find it equals sigma×temperature4, where sigma is another universal constant of nature [= 5.67×10-8 J/(m2 K4 s)]. This relation is called the Stefan-Boltzmann law. Because the temperature is raised to the fourth power, a small rise in the temperature of an object will produce a HUGE increase in the amount of energy it emits.

    small temp change makes large luminosity change

    When you add up all of the energy of all of the square meters on the object's surface, you get the luminosity---the total amount of energy emitted every second by the object. The luminosity = (total surface area) × (sigma×temperature4). If our Sun were just twice as hot as it is now, it would produce 24 = 16 times more energy than it does now!

The UNL Astronomy Education program's Blackbody Curves module lets you explore the relationship between temperature and the thermal spectrum by manipulating various parameters with a graphical interface (link will appear in a new window). It also introduces the concept of filters.

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last updated: January 13, 2022

Is this page a copy of Strobel's Astronomy Notes?
Author of original content: Nick Strobel