February 2014 – Orion and the Birth of Stars

Heavens Above!

Heavens Above! is the astronomy section of the Sci@StAnd website, updated each month to bring you a different constellation of focus. Last month, we focused on the constellation of Gemini and the planet Jupiter. This time, we focus on the constellation of Orion and its association with the birth of stars.

Constellation IX – Orion

The constellation of Orion is arguably the most famous of all the constellations. Chances are, if you are in any way interested in space, you will have heard of Orion or, maybe more specifically, Orion’s belt (figure 1). This is, in part, thanks to the original Men in Black film starring Will Smith and Tommy Lee Jones where the main story evolves around the quote “to avoid war, the Galaxy is on Orion’s belt”.


Figure 1: The constellation of Orion, the hunter, highlighting the location of Orion’s Belt, made famous by the Men In Black movie.

In Greek mythology, Orion was a giant, given the ability to walk on water by his father, Poseidon, god of the seas. This ability allowed him to walk across the sea to the different Greek islands and even across oceans to distant kingdoms. When on these faraway lands, Orion would spend his time hunting the multitude of different animals that he encountered. He became boastful of his skills as a hunter and proudly declared that he would be able to kill all of the animals that inhabited the Earth.

Meanwhile, Gaia, the Greek equivalent of Mother Earth, was worried. The animals that she bore and cared for were being slaughtered by this great hunter. Upon hearing Orion’s boastful claims, she sent up a giant scorpion to the Earth in order to defeat him. The gods took Orion’s body and placed him in the sky where we see him now. They then took the scorpion and placed him securely on the opposite side of the sky so that both he and Orion would be safe from one another. This is said to be the reason why Scorpio and Orion can never be seen in the sky at the same time.

Depictions of Orion, such as that shown in figure 2, usually show him with a shield held outstretched before him while he raises a club or draws an arrow to defend himself against a charging bull, known as Taurus, located in the sky nearby.


Figure 2: Depiction of Orion holding his shield out in front of him and raising his club above his head.

Orion is important to astronomers as he is host not only to sites of star formation but also late-stage stellar evolution. Next month, we will look in more detail at Orion’s links to the latter stages of a star’s life but for now, we turn our focus to Orion’s links to their formative years.

The Birth of Stars

Stars, such as our Sun, are not eternal entities. They have not been around forever. Instead, stars are born and they die, just like the rest of us. We only refer to a celestial object as a star when it is actively burning hydrogen gas, the lightest (and most abundant) of all the chemical elements, in its core. For the star to be able to do this, the temperature and pressure inside it need to be large enough to squeeze the hydrogen particles close enough together that they fuse.

As two hydrogen particles fuse together, they form a helium particle and also release a large amount of energy. It is for this reason that stars shine: just a simple form of stellar alchemy. This process, called fusion, continues until the hydrogen is used up, just like an oil lamp will only burn while there is fuel inside it. Our Sun has used up approximately half of its hydrogen fuel but this is nothing to be concerned about. Stellar lifetimes are typically of the order of millions of years and our Sun is only halfway through its 9 billion year lifetime.

So if stars are not around forever and the fusion of hydrogen into helium (with which we base our definition of a star) requires high temperatures and pressures then there must be a time before a star forms when it is cooler and under less pressure. Since we know that stars are made of gas, requiring the interior of the star to be under less pressure means that the particles of that gas must be more spread out. This is the same as saying that we would expect the gas to be less dense. If we continue this extrapolation backwards, getting cooler and cooler and more and more sparse, we end up with a large expanse of gas at the same ambient temperature as the rest of space. This inter-stellar gas is where stars begin their lives.

By thinking about stars in this way we have imagined what happens as they form but with one fundamental difference: we’ve imagined it in reverse. The precursors to stars start off their lives in massive expanses of inter-stellar gas which, if the conditions are right, collapse and contract to form the stars that we see in constellations such as Orion. But what causes their collapse in the first place?

Over large distances, the most dominant force is that of gravity. The same force that acts to pull us down onto the surface of the Earth and stop us floating away into space also keeps the Moon in orbit around the Earth and the Earth in orbit around the Sun. In fact, anything in the Universe that has a mass will experience the force of gravity. These regions of inter-stellar gas are no different. However, the gravity in these regions is weak as gravity depends not only on mass but also on the distance between all the different objects (which in this case are all the individual particles of gas). This means that, for most of the time, the particles of gas just happily trundle about, minding their own business. Their motions are rather random and this causes the number of particles of gas in a particular volume to vary across the cloud. The interesting thing happens when a large number of particles of gas end up randomly entering the same volume. If the density in this volume of gas exceeds a critical value, the gas will begin to collapse under gravity and forms stars.


Orion as a Site of Star Formation


Figure 3: The horsehead nebula, located just below the belt of the constellation of Orion. Image credit: Hubble Space Telescope, NASA, ESA

For the ease of explanation, we have only thought about the formation of a single, solitary star so far but, in reality, stars don’t form in isolation. Instead, they form in extensive nurseries known as nebulae. The constellation of Orion is host to two prominent nebula which you can see with the aid of a small telescope or a pair of large binoculars. In Orion’s belt, located below the left-hand star, there is the horsehead nebula (figure 3) and, located in Orion’s sword is the Orion Nebula (figure 4).


Figure 4: The Orion nebula, located in the sword of the constellation of Orion. Image credit: Hubble Space Telescope, NASA, ESA

Both of these images were taken with the magnificent Hubble Space Telescope and highlight the vastness of the inter-stellar gas compared to the individual stars that are forming. For instance, hidden in the wisps of gas in the image of the Orion nebula are over 3000 proto-stars!


Locating Orion and its Nebulae

The chances are that if you have looked up into the sky during the winter in the northern hemisphere, you will have gazed upon the constellation of Orion whether you knew it or not. The constellation is one of the easiest to find as it dominates the southern sky at this time of year. Even on a not-so-clear night, Orion’s belt should still be prominent. Keep your eye out for three, bright stars in a line.

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Figure 5: Finding the Orion nebula in the sword of Orion, and the horsehead nebula below the left star in the belt of Orion. Image credit: Stellarium with own editing.

If you are in doubt about where to look, remember that you can always use the Plough for guidance. Just as we did last month to find the constellation of Gemini, use the stars in the handle of the Plough to guide your eye across the sky (figure 6). The first two bright stars you will come to will be the Gemini twins but if you continue along the same line you will see a bright orangey star. This is Betelgeuse, which sits at Orion’s left shoulder (figure 5). Continuing the same line further in the same direction should guide your eyes through the three bright stars in Orion’s belt and down to a large bluish star. This is Rigel which sits at the right knee of Orion (figure 5).


Figure 6: Using the plough to find the constellations of Gemini and Orion. Image credit: Stellarium plus own editing.

To find the horsehead and Orion nebulae, you need to look at the half of Orion that is closest to the horizon. Just down from the left of the three stars in Orion’s belt, you should be able to find the horsehead nebula and, in the sword of Orion, you will be able to find the Orion nebula but you will need a telescope or large binoculars to see these regions. However, if you are located in a region with really dark skies, you may be able to see a fuzzy patch in Orion’s sword even with your naked eye.

Coming next month: Orion, part II: the death of stars


Happy Stargazing!








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