Stars of the Orion Constellation
By Andrew Wood
Standing (rather acrobatically on his head from our southern hemisphere perspective) high in the north at this time of year, the easily recognisable constellation of Orion, The Hunter, possesses a wealth of information, even to the naked eye.
I looked up and there it was, unmistakable. From my backyard in suburbia, a teenager in the 1970s with little knowledge but a fledgling interest in the night sky, the constellation Orion looked down at me exactly as the astronomy book showed it; and I wasn’t particularly looking for it, having just gone outside for a casual glance at the stars. This obviousness of Orion is a rarity among the constellations.
A constellation is a group of stars that historically formed a shape that could be related to by a human observer; in the case of Orion the ancient Greeks saw a hunter with a belt and sword, a shield and a club [Figure 1], lustily pursuing the nearby Seven Sisters of the Pleiades star cluster.
The Yolngu people of the Northern Territory have a similar story, in which the three stars making up the belt of Orion (Djulpan to the Yolngu) chase the group of sisters.
We now tend to, less artistically though more practically, simplify the representation of constellations into stick-figures [Figure 2].
Figure 2
In modern times the constellations have been standardised; some historic names have remained, and more modern configurations added.
Orion is one of eighty-eight of the officially recognised modern constellations. The constellations in modern star atlases are shown as bordered regions similar to countries in an Earth atlas.
In Figure 3 (below) the stars of the constellation lie inside the set boundary that makes up Orion.
Figure 4 (below) is an image of Orion taken with a digital camera. The major stars of Orion do shine brightly, giving the constellation its easy recognition which no doubt inspired the stories of ancient cultures. Wonderful as their star literature is, however, these cultures had no knowledge of what stars actually were. In fact, it was only recently, with the use of specroscopy in the early 20th century, that anyone knew what stars actually were.
During the 20th century, scientific investigation and developments in technology enabled us to interpret the true nature of stars. We can now look at an area of the night sky like Orion and understand the differences we see in the stars’ appearances.
The obvious difference, as the Figure 4 shows, is the degree of variability in star brightness. Hipparchus in ancient Greece classified star brightness, or magnitude. His method of labelling brighter stars with lower numbers has, like some of the ancient original constellations, persisted, and today the magnitudes of all visible stars have been measured accurately. The stars Bellatrix and Alnitak (Figure 3) appear of similar brightness in Figure 4. Hipparchus most likely classified them as magnitude 2 stars. Today, astrometry measurements give their magnitudes as 1.62 and 1.71, respectively. Using methods such as parallax and spectroscopy, we now know that the stars don’t inhabit a ‘celestial sphere’ but are spread throughout the galaxy. Bellatrix and Alnitak are respectively 243 and 825 light years from Earth’s position in the galaxy. That they appear to be almost equal in brightness logically means that Alnitak must in fact be brighter.
Absolute magnitude is the measure of a star’s actual brightness when astronomers compare them as if they were at the same distance. The absolute magnitudes of Bellatrix and Alnitak are -2.74 and -5.31 (brighter magnitudes do go negative). The values of 1.62 and 1.71 given earlier for these stars are their apparent magnitudes. Our perception of a star’s brightness, therefore, depends not only on how bright it actually is, but also how far away it is.
Orion’s two brightest stars are Rigel and Betelgeuse (see Figure 3 and compare with Figure 4). Respectively, they are: approximately 850 and 500 light years from Earth, apparent magnitudes 0.15 and 0.45, and absolute magnitudes of -6.73 and -5.17. This makes them sound very similar. But they are not.
The surface temperature of Rigel is 10,000k, much hotter than Betelgeuse, which has a surface temperature of 3,500K.
The reason for this difference can be seen by looking at Figure 4 – and it is much more apparent if you can see the stars outside yourself. Betelgeuse is noticeably red.
A star’s colour is related to its surface temperature, in the same way that heated metal changes colour as it gets hotter. Metal glows red up to a temperature of about 3,500K; which is the surface temperature of Betelgeuse. If metal can be heated beyond this, it will turn orange, then yellow, white and finally blue, at over 20,000K.
So how do two stars of similar brightness vary so much in surface temperature. Figure 5 explains why.
Betelgeuse, although not the largest star known, is certainly a big star in comparison to Rigel. It is therefore a very luminous star, despite it being at the cool, red end of the temperature spectrum. Betelgeuse has a radius of 667 solar radii (a solar radius being the radius of the Sun), which allows it to be nearly as luminous as the much hotter Rigel at 92 solar radii.
A clue to how these differing properties of stars arise can also be seen in the Orion constellation. Shown diagrammatically in Figure 3, and easily seen if you go out and look at the area of Figure 4 yourself, especially through binoculars, is a visually ‘fuzzy’ area. This is the Sword of Orion, hanging below the three belt stars. The ‘fuzziness’ is the Great Nebula of Orion, a diffuse cloud of thin but widespread gas and dust. Stars form from the gravitational collapse of nebulae such as this, which consist mainly of hydrogen.
The large nebula collapses into many smaller denser regions called protostars, in which the temperature eventually becomes hot enough for hydrogen atoms to smash together to become helium atoms, at which point stars are born. Star forming regions such as the Orion nebula are of different masses; therefore, there is a large range of properties of new stars, which will set them on evolutionary paths that hundreds of millions to billions of years in the future will see them become quasars, neutron stars, white dwarfs, black holes or supernovae. The gas leftover from supernovae explosions will start the whole process again.
As a snapshot of stars as they are now, however, the easily recognisable constellation of Orion has a wealth of information about the different natures of stars and how they come about. Something to contemplate while taking in the wonderful visual spectacle. And don’t forget to check out the nearby Pleiades cluster, and another notably red star, Aldebaran in the direction of another star cluster, the triangular-shaped Hyades.
References
Figures 1-3 were created using the Stellarium planetarium program: see stellarium.org
Figure 4 is a digital camera image taken by the author
Figure 5 was taken from: https://images.saymedia-content.com/.image/t_share/MTc0NjQxMzA4OTY1NTQ1OTc0/the-size-of-the-sun-as-compare-to-the-other-stars.jpg
Information regarding the story form the Yolngu people was found at: http://www.fireballsinthesky.com.au/wp-content/uploads/sites/7/2014/02/Aboriginal-interpretations-of-the-night-sky.pdf
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