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Subjects

Let’s take a closer look at the subjects explored in the Constellation activities!

  1. Sun and the moon
  2. Planets
  3. Stars and constellations
  4. Galaxies and the Universe

Introduction

Astronomy, the study of celestial objects such as the sun and moon, the planets, stars and galaxies, is one of the oldest sciences in the world. All these celestial objects span an enormous range of sizes and timescales, and yet they are all interrelated:

The planets of our solar system turn around the Sun, our closest star; the stars we see in the night sky, are suns of different sizes, in essence just like our own Sun, only much further away. All these stars, a few hundred billions, most with their own planets orbiting around them, make up our home galaxy, the Milky Way. And finally, just like there are billions of stars making up our galaxy, so there are also billions of galaxies in the Universe, all of different shapes and sizes (Figure 0.1).

In the following sections we briefly describe these different celestial objects, grouped in the four subject groups of the Constellation project.

Figure 0.1

Figure 0.1: On the left we see the Hubble Deep Field, an image taken with the Hubble Space Telescope. The image is just one 24-millionth (that’s 0.000…1 with 24 zeros) of the whole sky. Each of those little blobs is an entire galaxy, as can be seen in the middle frame when we zoom in. Each galaxy has billions of stars, most of which have their own planetary system. On the right is an artist’s impression of our Solar system, with the lines tracing the orbits of the planets also depicted. Credit: Hubble Deep Field Team & NASA

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Sun and the moon

Our Sun is a star like the billions of stars we see in the night sky, which make up our home galaxy the Milky Way. The reason we see our Sun as being so much bigger than the other stars, is that we are much closer to it. The Sun together with its planets, one of which is our very own planet Earth, are called the Solar system.

Figure 1.1

Figure 1.1: A schematic illustration of the Earth’s orbit around the Sun, and the Moon’s orbit around the Earth. The moon’s orbit (in red) is slightly tilted compared to the orbit of the Earth around the Sun. (Not to scale) Illustration: Bob King

It appears that the sun moves around the sky, but it is in fact our planet which is spinning around its axis and this is what makes the sun appear to be moving across the sky. Apart from just spinning around itself, the Earth also turns around the Sun, and it takes ~365 days for it to carry out an entire turn. It is due to this turn around the sun that we experience the changing seasons.

But we are not alone in our long journeys around the Sun; we take with us our ever faithful satellite, the moon. The moon not only turns around the Sun with us, it also turns around Earth itself, and it completes a full turn in about 28 days. It doesn’t emit any light of its own, but reflects the light of the Sun. As can be seen in the video below (source: NASA/Goddard Space Flight Center), depending on its position around the Earth (which is shown in the upper left corner of the video) and therefore on the amount of its surface which is illuminated by the Sun, the moon is in a different “lunar phase”. Solar eclipses are another phenomenon caused by the relative positions between the Sun, the Earth and the moon, and in particular, they occur when the moon is in between the Earth and the Sun. If the plane of the Moon’s orbit were perfectly aligned with the plane of the Earth’s rotation around the sun, there would be a solar eclipse every time the Moon completes a turn around the Earth, i.e. once a month! However due to the tilt of the moon’s orbit around the Earth (Figure 1.1), solar eclipses are more rare than that. The moon is thought to have formed in the early days of the life of the Solar system, perhaps from the debris of an epic collision between the Earth and another celestial object about the size of the planet Mars!1

Source: NASA/Goddard Space Flight Center

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Planets

Planets were given their name by the ancient Greeks, derived from the word “Planitis” (Πλανήτης) which means wanderer. The ancients thought that planets were wandering stars, since unlike the other stars in the night sky, they did not stay in one place, but changed their position from night to night.

We now know why that is: planets are in fact very different from the other stars we see in the night sky. They are celestial objects orbiting around our home star, the Sun, just like our own planet Earth does. In contrast, the other stars we see in the night sky are suns which are very far away, so far away, that even though they are moving around the galaxy, they appear to be stationary. In fact stars also change their position in the night sky, but over the span of thousands of years, so slowly that it is practically imperceivable to humans. Planets are relatively nearby, compared to the other stars, and we can thus perceive their progression through the sky.

Figure 2.1

Figure 2.1: Schematic representation of the Solar system (the sizes are to scale but the distances are not.) Credit: “Planets2013” by WP – Planets2008.jpg. Licensed under CC BY-SA 3.0 via Wikimedia Commons

Figure 2.2

Figure 2.2: The planets of our solar system shown scaled between them for comparison. Earth is the little blue-green one in the second row from the front, on the left. Credit: “Size planets comparison” by Lsmpascal – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons

Planets do not emit their own light, unlike stars, and instead they reflect the light of the Sun, as does the Moon. There are 8 planets in our solar system (Figure 2.1) (from closest to furthest to the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. They all have very different sizes, as can be seen in Figure 2.2. Our planet, Earth, is a relatively small planet compared to the gaseous giants Jupiter and Saturn.

As we already mentioned, the stars we see in the night sky are other suns in our galaxy. In the past 20 years we have gone from knowing about the existence of just the planets in our Solar System, to knowing about the existence of over 1800 other worlds around other stars. And that’s only the stars which are close enough to be studied by our telescopes today! We now think that most stars have planets orbiting them, just like our own Sun does.2

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Stars and constellations

Like we mentioned in the previous sections, there are billions of stars like our own Sun in our galaxy. Stars are born in dense clouds of hydrogen and helium gas, which collapse due to the force of gravity. When the gas becomes very dense, it also becomes very hot, and begins to glow, much like a rod of iron glows red when heated.

Figure 3.1

Figure 3.1: Stars of different sizes. Our star, the Sun, is the little dot on the top left. Source: StargazingTonight

Stars come in many different sizes, as depicted in Figure 3.1, and our Sun is actually quite a small star compared to those found in the Galaxy. You can also see a video of the incredibly different sized stars found in the Universe in the link below:

Credit: “How the Universe works” – Discovery Channel

Stars, are born, live out their lives, and one day eventually run out of fuel and die. Depending on their size, some stars die in impressive explosive deaths, called supernova, while others die silently, slowly fading away until they no longer shine. Throughout their lifetime stars consume hydrogen and helium in their cores, and produce heavier elements. These are released in their host galaxies when, the stars which are large enough, undergo violent supernova explosions at the end of their lives. These heavier elements are then mixed in other gas clouds which create new stars, as well as the planets orbiting them. This second hand material is what also made up our own Solar system, our Sun and planet Earth, and eventually us. Everything we are made up of was once cooked up in the core of a large, dense star, so it is fair to say that we are all indeed made of stardust!

Constellations

From the beginning of human history people have recorded the patterns made by stars in the sky. These patterns are called constellations. Each civilisation throughout history has identified its own constellations, and the earliest recordings of constellations are believed to date back some 17 300 years3, in a cave system in the south of France.

Although a group of stars can make up a constellation, that does not necessarily mean that those stars are close together in space. In general they only appear to be close together in the sky due to perspective, i.e. our two-dimensional view of objects positioned in three dimensional space (see an example in Figure 3.2.)

The ancient Greeks described over half of the 88 constellations which are recognised by the International Astronomical Union, and a number of constellations were later added by European astronomers who “discovered” new constellations on their travels to the southern hemisphere3.

Figure 3.2

Figure 3.2: Top row: The Orion constellation on the sky (lines drawn) and on the right an artist’s rendition of Orion’s constellation. Bottom row: The constellation is made up of stars which are not necessarily close to each other in space, they just appear that way when seen in two dimensions. Credit: Top left: “OrionCC” by Till Credner. Top right: “Urania’s Mirror – Orion” by Sidney Hall (via John Dolby). Bottom: Don Dixon

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Galaxies and the Universe

Figure 4.1

Figure 4.1: On the left we see a typical spiral galaxy, and on the right a typical Elliptical galaxy. In the images we also see other distant galaxies. Credit: (Left) European Space Agency & NASA (Right) J. Blakeslee (Washington State University)

As there are billions of stars in our galaxy, so there are also billions of galaxies in the Universe. Galaxies are made up of stars, gas and dust, and the poorly understood dark matter, a form of matter which does not emit electromagnetic waves (i.e. light – hence the name “dark” matter). In the left of Figure 0.1, a picture taken by the Hubble Space Telescope, we see a tiny patch of sky, just a millionth of the entire celestial sphere, in which are contained a few thousands of galaxies of different sizes and shapes. Imagine then how many galaxies there are in the entire Universe!

There are two main families of galaxies: disk-like spirals and ellipticals shown on the left and right of Figure 4.1 respectively. They have different shapes and sizes, because they were formed in different ways, and various events shaped their final appearance. In fact, galaxies interact a lot with one another, and these often violent interactions shape what they look like today (Figure 4.2).

Figure 4.2

Figure 4.2: Interacting galaxies: On the left we see two disc-like spiral galaxies merging together, and on the right we see a giant spiral galaxy in the process of swallowing up a smaller galaxy. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Our home galaxy, the Milky Way, belongs to the family of spiral galaxies, with the shape of a thin disk (right of Figure 4.3). When we look at the night sky and see the Milky Way stretching across the sky, what we are looking at is our galaxy as seen from the inside (left of Figure 4.3).

From the southern hemisphere the sky is that bit more interesting, as from there we can see the centre of our Milky Way galaxy (the bright region in the Milky Way as seen on the left of Figure 4.3).

Figure 4.3

Figure 4.3: Left: The Milky Way as seen from Earth. We can see the dusty, bright disc running along the sky. Right: An artist’s conception of the Milky Way with the position of the Sun marked on it. Credit: Left: Justin Ng – National Geographic. Right: NASA/JPL-Caltech/R. Hurt

So where did it all come from?

It’s thought that the Universe began 13.8 billion years ago, in what is known as the Big Bang. Shortly after the Big Bang the Universe was very hot with the same density everywhere, with just tiny ripples in this density (think of very calm water, with small ripples on its surface). These ripples created a slight accumulation of matter in some places, and from then on it was gravity’s job to do the rest. Wherever there was slightly more matter, the force of gravity accumulated even more matter in those regions, and this soon became a runaway process, with more and more material accumulating together in “clumps”. In these dense regions with lots of matter and gas, stars formed and these were the “seeds” of what are today the galaxies that we see in the night sky with our telescopes. Galaxies also gather together due to their gravitational pull, in what are known as groups and clusters of galaxies. Our own Milky Way is found in what is known as “the Local Group” which contains more than 50 galaxies, and the Local Group itself is found in the Virgo Supercluster. Those are just a few more lines you could add to your address next time you write it down: Earth, the Solar System, Milky Way, Local Group, Virgo Supercluster, The Universe.

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References

1 Staff (7 September 2014). “Revisiting the Moon”. New York Times.
2 The Extrasolar Planets Encyclopaedia
3 “The Constellations” IAU

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