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A First Look At Radiation In Astronomy

If you love Astronomy, then looking at the sky is a regular task for you. Today, I would like to talk about why we see what we do. 


Table of Contents


Radiation is the transmission of energy. It can be between any two points or distance. We often apply the term electromagnetic to radiation, and rightly so. This is because radiation energy is carried by electric and magnetic fields. There are many types of radiation, as denoted by the electromagnetic spectrum.

  • radiation= waves that radiate from a source, like a star.


Light is radiation. We also call this visible light. It is the light we can see with our own eyes. The kinds of radiation or light that we can not see take special instruments to detect and measure. Some examples are radio, infrared, ultraviolet, x-rays, and gamma rays.

  • Radio - these waves are the largest. We use them to carry radio and tv signals.

  • Microwave - these waves have a length between a millimeter and a meter.

  • Infrared - wavelengths between 740 nanometers to 12 centimeters.

  • Visible Light - these wavelengths are between 400 nanometers to 740 nanometers.

  • Ultraviolet - these occupy the region of 10 nanometers to 400 nanometers.

  • X-rays - these range from .01 nanometers to 10 nanometers.

  • Gamma rays - These are the smallest waves at less than a picometer.

We learn a lot by studying light. Analyzing light can tell us what elements are present, how fast or slow its source is moving, and the source’s temperature. Since we can not visit stars, scientists have to rely on methods such as these.


Waves of Light

All forms of light can travel in a wave motion. A wave is a disturbance of the medium in which it travels. That is the simplest definition I can think of. The medium does not move. It is the energy that moves through the medium. That is a wave.

All waves have certain characteristics that can be measured. These are the period, frequency, wavelength, and amplitude.

  • Period= time needed for the wave to repeat itself

  • Wavelength= distance needed for the wave to repeat itself

  • Amplitude= height of the wave

  • Frequency= The number of waves that pass any given point in a certain amount of time

The Parts of Visible Light

All light is a combination of different colors. White light is, in fact, made up of red, orange, yellow, blue, green, and violet. These different hues and their resulting frequencies are how we see light.

  • visible light = electromagnetic radiation that has wavelengths of around 400-700 nanometers.

Each of those colors has a different frequency with red having the lowest and violet the highest frequency. These colors are what we call the visible spectrum. Our eyes can not see any light or radiation outside of these boundaries.

Maxwell and Electromagnetism

James Maxwell was an astounding scientist and, to me, is equal to Einstein in his importance. His theory on electromagnetism is about electric charges and their effects. The electric charges came from particles in an atom.

When charges are opposite they cause motion between the particles. This motion also creates a magnetic field. So, as you can see, electric charges and magnetism go hand in hand. 

Now, when particles do not move, as in particles that attract each other, there is an electric field.

I have mentioned both a magnetic field and an electric field. A field is basically just an effect. An electric field influences something else. The same goes for a magnetic field and also a gravitational field.

Maxwell figured out that  electric and magnetic waves move at the speed of light. This gave him the idea that these forms of radiation were related. He was correct. We call all of this electromagnetic radiation. It all stems from the same interactions.

  • Electromagnetic radiation = radiation comprising waves that move through changing electric and magnetic fields. It travels at the speed of light.

Electric Fields

The forces in an electric field can attract or repel each other. It depends on the charge of each individual component. Whether the particle is positive or negative, it will put out an electrical force around it.

If two particles encounter each other, they can either attract or repel away from each other. If the charges of the two particles are the same, then they will repel each other and if they are different, then they will attract each other.

This happens because the electric field of an object extends out in every direction at once. This means it extends out radially in every direction. The strength of this field also decreases with distance.

Electromagnetic Waves

Maxwell was the first to figure out what these waves were. He developed a set of equations to deal with their effects. He combined the knowledge of electric fields and magnetic fields. You get a magnetic field every time an electric field changes.

They change all the time so you actually have a magnetic field all the time too. So, expanding on our wave model of light, these waves are formed from electric and magnetic fields traveling together.

Interestingly enough, they travel perpendicular to each other. Together these two fields make up what we know as electromagnetism.

These waves move at the speed of light which is roughly 300,000 km/s. Of course, this is in a vacuum and with no friction. That is quick, but it is not instant. Of course you could only tell that at long distances. Just something for you to ponder.

Electromagnetic waves do not require any medium to travel through. They will move through vacuum and medium alike. Since that is the case, we can calculate the different aspects of wave motion with this formula:

\[c = \lambda \times\text f\]

That formula means speed is equal to the product of the wavelength and the frequency. You just manipulate the formula, just like most equations, to solve for the value that you are looking for.

  • frequency = number of waves that travel through a distinct point in a certain amount of time.

  • wavelength = the distance from the tops of separate waves.


Properties of Light

Heinrich Hertz did the first major work in this area. Hertz’s work built on Maxwell’s, but it was innovative. His experiments showed electromagnetic waves could be detected, and they led to huge discoveries later on.

Light takes the characteristics of both photons and waves. These exhibit both particle and wave characteristics. Scientists conducted many experiments that showed this. 

  • photon = this is a packet of electromagnetic energy.

Photons carry energy. The energy of a photon depends on its frequency. Energy levels of a gamma-ray photon are much more energetic than a photon of visible light. 

While photons carry energy, that energy lessens with the distance it travels. Your car headlights are much brighter right in front of your car than a hundred feet away. The light disperses in many directions at once. So, that is why it gets weaker the further away it is from your headlight.

There is an equation that governs this type of phenomenon. It is called the inverse square law.

  • inverse square law = the amount of light energy moving through an area lessens in proportion to the square of the distance away from the source.

If we go stand in front of the headlight of the car, we will see a certain amount of light. Then if we move three times further away, we will see nine times less light. That is how light propagates and the inverse square law works.

Electromagnetic Spectrum

As you now know, there are several kinds of electromagnetic radiation. Some of it is visible but most of it is not. They have their own characteristics too, meaning we can tell them apart with certain tests.

These differences are wavelength and frequency. So we have to measure these things with instruments and tests. Radio frequencies are on the low end of the spectrum and gamma rays are on the top end.

Almost everything in space emits some kind of radiation. This includes planets, stars, nebulae, and galaxies.

Gamma rays have the shortest wavelengths and so are the most energetic. They are at one end of the electromagnetic spectrum. They are only created in the hottest of stars and phenomena. It takes severe conditions in space to generate them. However, Earth’s atmosphere absorbs them, which is lucky for us because they are very harmful.

The next longest wavelength is an x-ray. This type of radiation still carries a lot of energy. When you get an x-ray at the hospital, you usually have to wear a shield. This is because it can damage some parts of your body more easily than others. You only want the x-rays to hit what it has to so doctors can get the information they need.

Next up are ultraviolet rays. This is also known as black light. It is close to visible light, so that is why it is used in portable devices. Humans can’t see ultraviolet rays. People can see reflections if the objects contain phosphorous. That is what makes objects glow under ultraviolet radiation.

Visible light is the light that we see. It is also the most common type of radiation to get through our atmosphere.

Infrared radiation has slightly longer wavelengths than visible light. They get close to overlapping, just like some others.

Microwaves are the next longest type of radiation. We use them in our ovens. We can also use them in communications. Water absorbs microwaves easily but other substances do not. Therefore, your coffee cup does not burn you when you take it out of your microwave oven, even though your coffee is now hot.

Radio waves are at the other end. They have the longest wavelengths and are the least energetic. There are several categories of radio waves, with different wavelengths. These radio waves can travel great distances. There is even a branch of astronomy dedicated to radio waves.

Opacity of Objects

The opacity of an object is how little light or radiation it lets through. This concept directly affects what we can perceive on this planet. How this affects us is that even in the visible spectrum there will be more opacity and lower amounts of radiation that is let through. Then in other parts the opposite will be true and it will be less opaque.

Thermal Radiation

Thermal radiation is energy given off by heat. Every object gives off a little heat. This happens because electromagnetic radiation is given off at the particle level. This is also directly related to the object’s temperature.

The hotter the temperature, the more electromagnetic radiation is given off. Every object has a heat signature then. This is how you can detect what kind of object you are seeing when you look at its heat analysis.

The type of radiation that is given off, whether ultraviolet or infrared, is determined by the object’s temperature. The temperature can vary wildly from objects in space.

What is temperature then?

Temperature is the particle energy in an object, as they move around. Atoms in every object move constantly, even solids. We just can’t see them. This is because they are too small. It is this motion and movement, at the atomic level, that creates energy and heat.

This temperature and color is how we can identify objects in space. For example, if a star is glowing at a certain color, we know it is roughly a certain temperature. Objects that glow red are less hot than those that glow white or blue. That is just how nature works. I don’t know how or why artists came up with the exact opposite; it makes little sense.


Blackbody Analysis

No physical object emits all of its radiation at one frequency, it is actually scattered around a lot. This is the heat signature I referred to above. Every object will look a little different.

While signatures have a lot in common usually, there are always differences. Just don’t expect them to be radically different because they are not. When graphed this is called a blackbody curve.

Characteristics of Radiation

When graphing blackbody curves of objects you will notice that hotter items shift the curve upwards. Stars are a splendid example of this. Hotter temperatures are higher frequencies too. Therefore, we see objects when they are very hot.

Objects with less temperature or cooler emit radiation in the non-visible spectrum. It is all still radiation though. There is also a simple relationship between the temperature of the object and the wavelength the radiation is emitted at.

 \[ wavelength=\frac {1}{temperature} \]

As you might infer from this, the hotter the temperature the greater the amount of energy is involved. This is summarized by the equation:

\[ energy = temperature^4 \]


Electromagnetic radiation is the basis for a lot of interesting science. Understanding a little about it will tell us a lot about objects in space where we can not go. Thanks to James Maxwell, we know that charged particles, like electrons, give off waves of energy. This discovery was instrumental in our further progress in science.

The spectrum of radiation comprises many categories. These range from the large radio waves to the tiny gamma rays. Each type has its own characteristics and uses. We also learned that the temperature of the object dictates the radiation it gives off.


I appreciate you reading this and I hope I was able to teach you something. Electromagnetic radiation is a wonderful subject and I just barely scratched the surface.

If you would like to read more about astronomy, here are some options for you.