Listen to the April 2016 starrytelling podcast here.
This campfire story is dedicated to Stuart @astronomyblog
Welcome to the month of April!
I love the Milky Way!
The Milky Way is the most spectacular feature of the Southern Hemisphere but to say that is such an understatement… The Milky Way is so striking here, that in the absence of a polar star, which I found hard to find in the Northern Hemisphere anyway, people could even orient themselves by it. And why not? We can easily see the Milky Way from Wellington, which according to Lonely Planet is the the coolest little capital in the world. But Wellington is still a city, which means that it does come with light pollution. From most of the cities of the world we are lucky to see just the brightest stars. Yet, I have noticed when walking home at night from the Observatory, from my street, I can still see the Galaxy. I call it My City of Stars. There are times when I look up and gaze straight at the center of it. This time of the year, just after sunset, I can let my eyes wonder from its center to the edge — from Scorpius to Taurus, in one glorious panorama…
So in April, my beautiful City of Stars is stretching through the night sky from northwest to southeast. Allow your gaze to drift along this celestial tapestry, and you will see the brightest stars. Let’s start from West. Lining up onto the celestial river are:
Very low on the horizon, Aldebaran – in Taurus 0.86 magnitude.
Magnitude is the logarithmic measurement of the brightness of the stars. Logarithmic means that each step of one magnitude changes the brightness by a factor of about 2.512. A magnitude 1 star is exactly a hundred times brighter than a magnitude 6 star, as the difference of five magnitude steps corresponds to (2.512)5 or 100.
Castor and Pollux – in Gemeni @ 1.93 mag/ 1.14
Betelgeuse – in Orion @0.42 mag
Procyon – in the Small Dog 0.34
And Sirius – in the Big Dog. @-1.46 Sirius is the brightest stars in the sky.
By convention, the brighter the star, the smaller the number and so some stars and objects have negative magnitudes, like Sirius, or like the International Space Station which can reach up to -6 magnitude, or to have an idea, the full Moon, which has -13. The big dog constellation finally looks the right way up heading also to the western horizon too. From it, turn your gaze left.
Nearby comes Canopus -0.72, the second brightest star in the sky. Canopus is not in the white band of the Milky Way. Standing tall, Canopus is high in the sky as it likes to be at this time of the year after sunset … Canopus is a circumpolar star from Wellington. Which means that it goes around in circles in 23 hours and 56 minutes, riding the celestial Ferris Wheel of the Southern Skies, a giant wheel that never stops, day after day, in a sidereal time cycle, as long as the Earth is turning. Clockwise goes the Ferris Wheel in the southern hemisphere. There is a name for that region of the sky that the astronomers use, we call it the southern circumpolar region. All stars within that region of the sky will never set below the horizon nor will they ever rise, from beneath it but will seem to silently turn clockwise throughout the night.
Besides Canopus, there are other stars lighting the gondolas of the big wheel but not each and every gondola has a bright star inside. If Canopus is on the top of the big wheel then just imagine the diameter of the wheel is from Canopus to the horizon. Looking clockwise from Canopus in the 4 o’clock position on the wheel is the Lone Star, Achernar. Achernar marks the end of the grand river Eridanus, the river-asterism that flows all the way from Orion to the southern world. At 0.4 magnitude it shines bright in a region that seems devoid of other stars. Lower down, a peacock (Pavo) takes a ride on the wheel. It’s main star, which carries the mundane name of Alpha Pavonis (which literally means the brightest star in Pavo), is in the 7 o’clock position on the giant turning wheel almost as if is just hanging on a side.
Following the imaginary curve of the wheel, two very bright stars show up closer to the nine o’clock position. Firstly, the third brightest star in the sky and our closest neighbour, Alpha Centauri, and then Beta Centauri. They point up at the Southern Cross which is even higher than them in the sky at this time of the year. And one of my favourites, the hypergiant eta Carinae is somewhere in between Canopus and the Southern Cross. All these stars make the imaginary big wheel.
If you imagine connecting the Southern Cross and Achernar, half way through you will find the hub of the wheel. That is marked by the South Celestial Pole. The north and south celestial poles are the two imaginary points in the sky where the Earth’s axis of rotation, indefinitely extended, intersects the celestial sphere. Most modern earth bound telescopes are firstly installed so that they oriented on the N-S axis. We call it polar alignment and such mounts are called equatorial. From Wellington, the south celestial pole is located at an altitude of 41 degrees above the horizon. This coincides with the latitude of Wellington.
As we move across the Earth, at different latitudes the height in the sky of the north and south celestial poles changes to such extremes that they would appear permanently directly overhead to an observer at the Earth’s North Pole and South Pole respectively and would seem to lay on the horizon for someone at the equator. I love imagining them both every time I have the chance to have a stopover in Singapore, a place located one degree from Earth’s equator. As the Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other points appear to rotate around them, completing one circuit per day, that is per sidereal day. Sidereal is time measured by the stars, which is four minutes shorter than time measured by the position of the Sun.
I have my own special and personal relation with both celestial poles because I always felt like a navigator across the hemispheres. My own version of axis mundi goes through the North and the South celestial pole. Also known as the cosmic axis, world axis, world pillar, center of the world, world tree, in certain beliefs axis mundi is the connection between Heaven and Earth. The famous philosopher Mircea Eliade who also wrote the history of religions, thought that “Every Microcosm, every inhabited region, has a Centre; that is to say, a place that is sacred above all.” From the center, one may still venture in any of the four cardinal directions, make discoveries, and establish new centers as new realms become known and settled.
It is more common that a mountain or an elevated place is chosen as the axis mundi, the portal between the heavens and the Earth, so you might find more often references to that. For instance, Mount Kunlun in China, Mount Zion for the ancient Hebrews, the Black Hills for the Sioux, Uluru for the PitJantJatJara people in central Australia they are all a different version of axis mundi. In the ancient Mesopotamia, the cultures of Sumer and Babylon made their own artificial mountains, or ziggurats, on the flat river plain. These supported staircases leading to temples at the top and many times these temples held astronomical observatories as well.
Because the axis mundi is an idea that unites a number of concrete images, there are multiple spots as “the center of the world” and you can have your own axis mundi too and put it anywhere you like.
The Large and Small Magellanic Clouds
Back to my celestial Ferris Wheel, the sky looks almost devoid of stars anywhere inside it, with two exceptions. Let’s split it in two with a diametral line that links the Alpha and Gamma Crucis, stars of the Southern Cross to lonely Achernar. On the same side as the pointers of the Southern Cross, you will find the Small Magellanic Cloud a beautiful bright galaxy, that looks to the untrained eye (like I was) like a cirrus cloud hanging at 200,000 light years in space. On the other side of the semicircle, another galaxy, the Large Magellanic Cloud compensates its loneliness by its size, from 150,000 light years away. Here is why: I always remember which one is the small and which which one is the large cloud by thinking that the smaller cloud of Magellan plus the two pointers on one side balances (at least visually it does) the large Magellanic cloud on the other. This simple mnemonics has been very useful for me to memorise these galaxies. These so called clouds that neighbour our galactic presence are visually two thirds away from the Southern cross and one third from Achernar.
There is nothing else too bright within the big wheel, maybe because the wheel it’s inhabited by this giant spider, the Tarantula Nebula that has its nest inside the Large Magellanic Cloud. You can see its beautiful wisps through a telescope although it is very faint. Tarantula nebula is a star-forming region, also known as 30 Doradus, and according to NASA is one of the largest star forming regions, located close to the Milky Way. About 2,400 massive stars in the center of 30 Doradus, produce intense radiation and powerful winds as they blow off material into space.
The Large Magellanic Cloud enormous on a human scale is in fact less than one tenth the mass of our home galaxy. It spans just 14,000 light-years compared to about 100,000 light-years for the Milky Way and it is classified as an irregular dwarf galaxy. The ESO astronomers believe that its irregularity, combined with its prominent central bar of stars suggests to astronomers that tidal interactions with the Milky Way and fellow Local Group galaxy, the Small Magellanic Cloud, could have distorted its shape from a classic barred spiral into its modern, more chaotic form.”
I could keep watching the south celestial circle stars turning around all night long (for real) but we have more stars to visit tonight and so I jump off the big wheel at zenith where Canopus is watching and I go back onto the Milky Way. I’m preparing to slide down toward the eastern horizon… I bump into the False Cross, then almost fall into the enormous gravitational field of the famous hypergiant star Eta Carinae that attracts me with its silent beauty, and the memory of my most magical starry sight, one late night at the Carter Observatory as I looked at it through our 41 cm Boller and Chivens and saw the tiny hourglass or homunculus, of its outer shells of blasted solar matter… But there is another pull, much stronger that drags me away from eta Carinae. I’m falling towards our own Milky Way center, that rises magnificently on the Eastern Horizon. I’m not there yet tho. I have to pass through the stars of the Southern Cross, completely engulfed into the Milky Way: Alpha Crucis, Beta Crucis, Gamma Crucis and Delta Crucis and it’s famous fifth star, Eta Crucis. What’s with these names? The brightest stars in a constellation are named using the letters of the Greek alphabet followed by the Latin genitive of that constellation.
Crux, the Southern Cross, is no stranger to the northern hemisphere and it was entirely visible as far north as Britain in the fourth millennium BC. The Greeks could see it too but since then, the precession of the equinoxes, the wobble of Earth, its gyroscopic dance on the orbit has changed the skies a lot so that now Crux is only visible in the Northern Hemisphere from as far south as 25 degrees latitude north. Florida Keys, Puerto Rico, the islands of the Caribbean, as well as Hawaii are its northern limit of visibility. Near the Southern Cross, there is a dark patch of dust that masks the light that comes from the stars behind it and that is known as the coalsack. Inside the coalsack, the Jewel Box is one of my favourite sights that I visit over and over with the telescope.
The Rope of stars
Lower down on the path of the Milky Way the two pointers look now as if they are hanging from the Southern Cross. First comes Beta Centauri (the genitive for Centaurus, the name of the constellation) then the famous Alpha Centauri. For Maori they are also known in a different time of the year as the rope of an anchor and I can’t stop but thinking that this is the end of my rope of stars. If I let it go now, I will fall into the center of the galaxy which is slowly and majestically climbing on the Eastern Horizon. Here in Aotearoa, the Maori have three names for the same asterisms (groupings of stars) at different times of the year. What we know as Scorpius is now called Manaia Ki Te Rangi, the guardian of the skies. The messenger between the earthly world of mortals and the domain of the spirits, Mania also resembles to a seahorse and its symbol is used as a guardian against evil. Often you will see Maori people wearing a greenstone in Maori named pounamu Manaia as a taonga, a necklace.
Lower on the horizon, at a magnitude of +0.95 red giant Antares shines as the brightest star in Scorpius. Right next to it, you can sometimes see its rival, Ares by its Greek name, or Mars as we all know it better, is challenging the giant’s red hue with its own red glimmer. This is how Antares got its name, as being the rival of Ares, Ant-Ares, the rival of Mars.
Welcome back to Earth!
What you can’t see:
- Pisces (Sun in Pisces from 12th of March to 18th of April) and
- Aries (Sun in Aries from 19th of April until 14th of May)
This is the Zodiacal band, an awesome drawing by Eugene Georgiades. Since one thousand years ago, when people stopped taking precession into account, the zodiacal constellations have shifted. Yes, we are once again not what we think we are. Here is an excellent site with more details about your real star sign.
Featured sky: the ecliptic
As the Milky Way splits the sky into two sectors, through the northeastern horizon runs the ecliptic, a lower arch, the plane of our solar system bearing the zodiacal constellations. They intersect the Milky Way right on the horizon. First to set on the western horizon, is Taurus and of it, just Aldebaran is left gleaming faintly as it passes beyond the edge of the world. The arch of the ecliptic climbs through Gemini, holder of the two bright stars Castor and Pollux, then higher up, Cancer is almost invisible to the untrained eye, a good peripheral vision training object. Leo, is next with the Royal Star Regulus, then comes Virgo with its bright star Spica, then Libra with Zubenelgenubi and Zubeneschamali the severed claws of Scorpius repurposed into a balance for Justice by the Roman Emperor Julius Caesar. Finally the arch curves down onto the western horizon where Scorpius holds red Antares.
Special Thanks go to Peter Detterline, Chief Astronomer of the Mars Society, Alan Gilmore from University of Canterbury and to Toa Nutone Wii Te Arei Waaka from the Society for Maori Astronomy and Traditions.