Armed With His Shield And Club, The Mighty Hunter Comes Battle Ready

Legend says that to proclaim His valour, Orion went on to slay a lion and then use the dead carcass as a shield. However, speaking on this head, history does not highlight much on the fact as to whether he held up the dead lion directly up for defence or put its skin on some other shield. Needlessly, modern astronomers fall slightly short of such active imagination and prefer to call the asterism of Orion's shield, sometimes the Hunter's Bow by the generic name of Pi Orionis.  

Star map of Orion constellation. 
Image Credits: Public Domain, via Wikimedia Commons

Numbered 1 - 6 from top to bottom and under the same designation of pi (π), this widely spaced group of six stars position themselves in a curved-vertical line in the north-eastern (this depends strictly on how you draw the constellation) portion of Orion. 

• 𝜋¹ Orionis - The asterism of Orion's shield begins with 𝜋¹ Orionis, an A-type main sequence star having a surface temperature of 8,611 K and a luminosity 16 times that of the Sun. At 100 million years, this relatively young star is swiftly burning away its store of hydrogen as it stays in the main sequence. Located at some 116 light years distant, 𝜋¹ Ori is almost twice as massive as the Sun and 1.67 times larger in radial extent. It is also a rapidly spinning star with a projected rotational velocity of 120 km/s. Our Sun, in comparison, hardly appears to be spinning with a meagre equatorial rotational velocity of 1.997 km/s.  

• 𝜋² Orionis - The second member 𝜋² Orionis sits at an average distance of 224 light years from the Sun. At 2.7 times larger than the Sun and 70 times more luminous, this blue-white star is burning its way through the main sequence with a surface temperature equal to 9,475 K. Just like 𝜋¹ Ori, 𝜋² Orio also spins about its axis at a commendable rate of 261.4 km/s. 

• 𝜋³ Orionis - A break in the typical pool of stars found in Orion comes from 𝜋³ Orionis, for it turns out to be an F-type star. At a distance of 26.3 light years, this star looks similar to the Sun (refer to the stellar classification below to have a rough idea if you are not so familiar with the astronomical vocabulary), having a mass of 1.2 solar masses and a radial extent of 1.3 times the Sun. With a surface temperature of 6,516 K and 3 times the luminosity of the Sun, 𝜋³ Orionis shines bright with a slightly yellowish tinge. At 1.4 billion years, it is quite the oldest member of the Orion constellation. This one also rotates faster than the Sun at an average rotational velocity of 17 km/s. 

• 𝜋⁴ Orionis - Being a B-type main sequence star tucked away at a distance of 1050 light years, 𝜋⁴ Orionis shines with 19,726 solar luminosity and a surface temperature of 21,874 K. At only 15.4 million years old, 𝜋⁴ Ori is not an exception among the populations of young stars in the constellation. As a characteristic B-type star, it is more than 10 times more massive and 9 times larger than the Sun, respectively, and spins appreciably at 38 km/s. 

• 𝜋⁵ Orionis - The fifth candidate turns out to be a spectroscopic binary. The primary component of 𝜋⁵ Orionis is a B-type 12 solar-mass giant star that shines blue with a surface temperature of 11,496 K and a luminosity 11,262 times that of the Sun. In terms of radius, coincidentally, it is also 12 times larger than the Sun. The secondary star is not yet clearly resolved, although, from some calculations, it is likely that the companion is a B-type 5 solar-mass candidate. It is probably 3 times larger than the Sun, 500 times more luminous and has a surface temperature above 15,000 K. The 𝜋⁵ Orionis system is comparatively far at 1,300 light years from the Sun. Lying very close to 5 Orionis, 𝜋⁵ Ori forms a visual double. More than 600 light years away from the Sun, 5 Ori has the spectral classification M1 III, meaning it is an M-type giant star. It looks reddish and shines with a surface temperature greater than 3000 K. 

• 𝜋⁶ Orionis - The last member of the asterism 𝜋⁶ Orionis is a unique one. It is an evolved K-type giant star. At 950 light years from the Sun, 𝜋⁶ Orionis occupies a mass of 4.15 solar masses and extends radially to around 90 solar radii. Although it shines with a luminosity more than 2,185 times the Sun, being a K-type star, its surface temperature stands at only 4,217 K. At 230 million years old, this star has already migrated away from the main sequence into the Horizontal Branch (HB) of the H-R diagram undergoing core-helium burning where it gets brighter every day for a period of 100 million years. 

Stellar classification is a complicated scheme. To those who are not so familiar with standard notations, you can use this handy catalogue. The Harvard Spectral Classification (above) lists the standard distribution of main sequence stars based on their spectral signatures. O-, B-, and A-type stars are giant and hot entities, whereas F- and K-type stars are larger and smaller than the Sun, respectively, the latter being a G-type object. As you can see, M-type stars are the tiniest individuals. Small and red, M-dwarfs are the most common star types in the Milky Way galaxy. 
Image Credits: Public Domain, via Wikimedia Commons. 

Now that the shield is complete, we can look at His club. Orion's club is rather faint compared to the rest of the constellation. Navigating northwards from Betelgeuse and in the north-western half, we have 𝜇 (Mu) Orionis, allegedly the Hunter's elbow. Then we get to the base of the club, presumably His wrist, looking at 𝜈 (Nu) Orionis and 𝜉 (Xi) Orionis. Further up, when we reach the battering end of the club, we have 𝜒¹ (Chi 1) Orionis and 𝜒² (Chi 2) Orionis. 

• 𝜇 Orionis - Marking the right elbow of the Great Hunter (again, depending on how you draw Him), 𝜇 Orionis is an example of a quadruple star system at 150 light years distant. The primary 𝜇 Ori A and the companion 𝜇 Ori B have their own respective binaries. Altogether 𝜇 Ori system includes four stars, 𝜇 Ori Aa, 𝜇 Ori Ab, 𝜇 Ori Ba, and 𝜇 Ori Bb. The larger of the A system, i.e., 𝜇 Ori Aa, is an A-type dwarf star 2.38 times more massive than the Sun and 2.85 times larger in radius. It is shining through the main sequence as a white-hot star having a surface temperature of 8,300 K and a luminosity equal to 32.2 Suns. Its companion, 𝜇 Ori Ab, is believed to be a G-type dwarf star like the Sun, although a lot smaller at around 0.652 solar masses. 𝜇 Ori Ba and Bb are a pair of F-type dwarf stars approximately 1.4 times heavier than the Sun, with thrice the luminosity and a surface temperature close to 6,600 K. 

• 𝜈  Orionis - Once again, we have another binary candidate located approximately at a distance of 520 light years from the Sun. Marking the handle of the club, 𝜈 Orionis is a spectroscopic binary. From what it seems, the primary is a B-type subgiant 6.7 solar-mass star which could either be on the main sequence or have already evolved. It is almost 2,000 times more luminous than the Sun and has a surface temperature of around 17,880 K. Its binary companion has not yet been properly resolved.  

• 𝜉 Orionis - The second marker of the handle, 𝜉 Orionis, is a spectroscopic binary system with an unknown companion. Similar to 𝜈 Orionis, 𝜉 Orionis is a B-type 6.7 solar-mass main sequence star at a distance of 610 light years from the Sun. Its surface temperature tops 15,476 K, while its luminosity is about 1,390 times that of the Sun. 

• 𝜒¹ Orionis - On the eastern tip of the club lies 𝜒¹ Orionis at a relatively close distance of 28 light years. It is a G-type variable star. 𝜒¹ Ori can be seen as a carbon copy of the Sun, in the sense that it has more or less the same mass and luminosity but differs in terms of surface temperature - 5,955 K compared to 5,778 K and age - 300-400 million years weighed against 4.5 billion years. 𝜒¹ Ori has a faint companion, a tiny 0.15 solar-mass red dwarf star supposedly 70-130 million years old.  

• 𝜒² Orionis - The western tip of the club contains 𝜒² Orionis, one of the most distant and massive stars visible to the naked eye. At a distance of 4,300 light years from the Sun, this B-type blue supergiant weighs more than 42 Suns and shines brighter than 446,000 Suns. Its surface temperature stands at 19,000 K. At 5 million years old, this young star is steadily burning away through the main sequence, and in the long run, it will die out as a brilliant supernova leaving behind a black hole or a neutron star.  

Orion is not a small constellation. As I wrote in the previous issue, not just stars, the entire field of view in and around Orion involves a lot of nebulosity, stellar nurseries and star-forming regions. To give a rough idea, TheSkyLive lists 222 bright stars in Orion. Unfortunately, not all of them are as famous as Betelgeuse or Rigel, and for obvious reasons, lack of sufficient data, a casual search on Wikipedia will yield a stub article. Professional astronomers are working on that, and in future, when I become one of them, I will likely be doing the same. Until then, and to serve our present purpose, I think it is time we move on to some other popular objects in the Orion constellation.