Sky Science

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Part 1 - Objects in Space that Emit and Reflect Light

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1. Recognize that the Sun and stars emit the light by which they are seen and that most other bodies in space, including Earth's Moon, planets and their moons, comets, and asteroids, are seen by reflected light.

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The universe is filled with many different types of objects, such as stars, planets, moons, asteroids, and comets. You can see some of these objects because they produce their own light. Scientists say that these objects emit light. Other objects can only be seen because they reflect the light that comes from objects that emit light.
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​Almost all of the light in space comes from stars. In our region of space, known as the Solar System, the Sun emits most of the light, but that's because the Sun is a star and that's what stars do - they emit light!

Stars, including the Sun, emit light because deep down at the core of a star there are nuclear fusion reactions happening. These reactions produce an enormous amount of heat and light. In the Sun, the nuclear fusion reaction turns hydrogen into helium. In about five billion years, when the Sun gets hotter, another nuclear fusion reaction will begin inside the Sun. This reaction will turn helium into carbon and oxygen. In stars that are even bigger (and hotter) than the Sun, there are nuclear reactions that create neon, magnesium, sulfur, silicon, and iron.
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Sun

Stars have a tendency to gather together into large groups, known as open clusters, globular clusters, and galaxies. You could say that these objects also emit light, but it's really just the stars located within these large groups that are emitting light.
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Open Cluster (M45)

Globular Cluster (M80)

Galaxy (​NGC 6814)

Most objects in space, other than stars, reflect light. For example, planets like Venus, Mars, Jupiter, and Saturn all reflect light. You can look up into the night sky and easily see these four planets for yourself, from your backyard, using just your eyes - no telescope required! All four of these planets look like bright stars, but they're not stars because stars emit light and planets only reflect light. The light that you see when you look at these planets is actually sunlight, which has reflected off the surface of the planet. The same is true when you look at the Moon. The Moon is very bright, but all of that light is just reflected light from the Sun.


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Moon

Asteroids also reflect light from the Sun, but because they're so small they only reflect a small amount of sunlight. That's why astronomers need a telescope to observe asteroids. Comets can sometimes be seen in the sky using just your eyes, once again, no telescope is required. When you observe a comet in the night sky, what you are looking at is sunlight that is being reflected off the gas and dust that surrounds the small icy comet. This gas and dust forms a tail that stretches for millions of kilometres.

If you point a telescope at Jupiter, you should be able to see its four largest moons, which are called Io, Europa, Ganymede, and Callisto. The only reason you can see these moons is because they reflect sunlight. The same is true for Saturn. If you point a telescope at Saturn, you should be able to see it's biggest moon, which is called Titan and the only reason you can see that moon is because it's reflecting sunlight. In fact, this is true for all the moons that revolve around the planets in our Solar System.
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Mars

Asteroid (Vesta)

Jupiter

Comet

Saturn

Jupiter's four largest moons

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This section will contain information regarding emission nebulae, meteoroids, meteors, aurora, and satellites.

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The information below is an extension of the material that has been presented on this webpage. It's well beyond the normal realm of the Grade 6 Sky Science Unit, but has been included here for those students who are looking to go beyond what is required.

The nuclear fusion reaction happening deep inside the Sun (and many other stars too) involves a 3-step process and is called the proton–proton chain reaction. In Step 1 of the process, four protons join together to form two hydrogen-2 nuclei, but this means that two of the protons must be converted into two neutrons. When these two protons undergo that change, they release as by-products a positron (an electron with a positive charge) and a neutrino. In Step 2, each of the hydrogen-2 nuclei fuse with another proton, creating a helium-3 nucleus. This time, the by-product is a gamma ray, which is a form of light with very high-energy. Then, in Step 3, the two helium-3 nuclei smash into each other and they fuse forming helium-4 and two protons are released. These two protons are now able to start the entire process over again.

Proton-Proton Chain Reaction

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The positron released in Step 1 is a particle of anti-matter and when anti-matter comes into contact with a particle of regular matter, both particles are annihilated releasing pure energy in the form of a gamma ray. This gamma ray, as well as the gamma ray created in Step 2, eventually make their way from the core of the Sun to the surface of the Sun, but this process takes about 200,000 years because of the enormous number of collisions the gamma rays have along the way. With each collision, the gamma rays lose a bit of energy and so when they finally reach the surface of the Sun, and are ready to race across the universe at the speed of light, they are no longer high-energy gamma rays, but rather lower-energy x-rays, ultraviolet light, visible light, or infrared light, depending on how much energy they lost on their journey from the core of the Sun to the surface.

The short animated video below describes the long process that the gamma rays must endure as they make their way from the core of the Sun to the surface.