This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

The Extreme Southern Celestial Hemisphere

by Dr Jamie Love Creative Commons Licence 1997 - 2011

You said there was more to ERIDANUS in the south?

Right. I think now is a good time to take the plunge south.

Here's an image of the entire southern celestial hemisphere as seen from the South Pole. The outermost circle is the Celestial Equator so only stars within that circle can be seen from the South Pole. The inner circle is the -45o declination circle. I've also added the four major right ascension lines.

Hey, there's no "South Star"!

That's right. We in the North are lucky to have Polaris as our North Star (at this point in time). Thousands of years from now, as the Earth precesses, a southern star may fit the position but there is no South Star today. (Actually, with a powerful telescope I'm sure you can find "something" as a South Star but it wouldn't be very useful to amateur astronomers.)

There are constellations and stars in this image that you already know but they are positioned in an odd way.

Take a look at the right side of the image. There's a familiar constellation on each side of the 18h line slightly north of the -45o declination line. Can you identify them? (I've colored in one star to help you.) How about the constellation that sits near the 6h line at the edge? That constellation is cut in half but the three stars along the very edge of the Celestial Equator are a clue. And what is that bright star those three stars are pointing too? [By the way, constellations along the "edge", the equator line, will be distorted because it's impossible to display a sphere correctly on a flat screen.]

Here's that image again with some stars identified.

Perhaps you noticed the twisting tail of SCORPIUS and red Antares at its heart. Maybe you found SAGITTARIUS behind the scorpion pouring tea on its tail. The "teapot" of SAGITTARIUS is tipped right over and the very tip of the spout is on the 18h line.

ORION is on the other side of the sky and the Celestial Equator cuts across his belt but you can still use his belt to identify Sirius.

You may recall that ORION can appear very high in our night sky (around midnight in December) but SCORPIUS and SAGITTARIUS are always very close to the southern horizon. If you look at this map (and remember that it is the view as seen from the South Pole) you might understand why some stars are higher up in our northern sky than others. And, of course, you won't see any of our northernmost stars (like Deneb, Vega, etc) in this image.

I've labeled a few of the other bright stars but you would have had difficulty finding them without the Square of Pegasus and other northern markers to guide you. Use your imagination and memory to work out what other constellations are in this image. Now that you've identified Sirius, you'll be able to identify CANIS MAJOR. Do you remember which constellation contains Alphard? (HYDRA!) If you have a very good memory you'll remember that CORVUS makes a nice four-sided box east of Alphard and you can see CORVUS just to the side of the 12h line near Spica. You know that Spica is part of VIRGO. Between VIRGO and SCORPIUS lies LIBRA (very dim). CAPRICORNUS and AQUARIUS follow SAGITTARIUS. They are hard to see but they are north of Fomalhaut. That brings us back to CETUS with dim Diphda and often very dim Mira. [I've included Menkar and Procyon as reference stars, but they cannot be seen from the South Pole because they are north of the celestial equator.]

Is that ERIDANUS leading away from ORION?

Yes, that would be ERIDANUS, the (wandering) River. This is a very dim constellation, and here I am only showing some of the brightest stars, so don't expect to see all the tributaries. You can see that ERIDANUS starts near ORION's left knee (Rigel), drifts over towards CETUS, then back again as it turns south. Notice LEPUS, the Hare. (The hunter and his dogs are chasing the bunny towards the river so they can trap him. ) Again, because I am using only a few of its brightest stars, the constellation looks a bit different.

And, as if that weren't confusing enough, this image is tilted 90o from the position in which you first learned these constellations so these objects might appear a little strange. During this lesson, most of the views of familiar objects will be at odd angles. I hope you understand why.

Eventually ERIDANUS dips below the southern horizon (of northern observers) and continues to meander to its last star Archernar. This is the brightest star in ERIDANUS and, because it is at a declination of -57o, it's impossible to see unless you are very far south. (New Zealanders know this star is circumpolar for their latitude.) Archernar is over 750 times as luminous as the Sun but at a distance of 85 light-years so it has a relative magnitude of 0.5, making it the 9th brightest star in Earth's night sky.

There is little else to see in ERIDANUS, that we haven't discussed before, but it is a great constellation to get you thinking south. It's the only constellation with such a long southerly journey and it ends up inside the -45 declination circle where all the "truly" southern constellations are found.

Let's take a closer look at the objects in this "southern circle".

The bright star just past the 6h line is Canopus and it is part of the constellation of CARINA, the Ship's Keel. Most of the southern constellations were named by sailors and explorers so a ship's keel came easily to their minds. I won't bother drawing CARINA but it includes most of the other stars within that area below Canopus.

Canopus has a magnitude of -0.7 making it the second brightest star in the night sky. You will recall that Sirius is the brightest star in the night sky with a magnitude of -1.5. Sirius is only 26 times as luminous as our Sun and appears so bright because it is only 8.6 light-years away. On the other hand, Canopus is about 650 light-years away so it must be about 200,000 times as luminous as our Sun in order to shine so brightly in our night sky.

The constellation of CARINA was once part of a larger constellation called ARGO NAVIS, the Argo Ship. However, astronomers have "dismantled" the ship producing a complicated group of stars I like to call the "Shipwreck"! (Like the "Teapot", the "Shipwreck" is NOT a true constellation.) Scattered around CARINA (the Keel) are the "modern" constellations of VELA (the Sails), PUPPIS (the Deck but more specifically the "Poop Deck"), MALUS (the Mast) and PYXIS (the Compass). Maybe that compass was broken because its ship certainly is. None of these bits of boat look like anything recognizable and they overlap with other constellations anyway!

As if that were not enough, Lacaille stepped in and stirred up more trouble by naming several more small (and totally unnecessary) constellations in this part of the sky including ANTLIA (the Pump) - now THAT is something useful! Too bad it's far from the deck or keel - it's above the sails!

Lacaille also thought that this ship had an artist on board because he pulled PICTOR (the Painter) out of the wreckage. PICTOR is stuck on the end of CARINA (the Keel) near Canopus. It's brightest star, Alpha () Pictoris, has a magnitude of only 3.3 and the constellation is nothing more than two lines! The only reason I point it out is because Beta () Pictoris is a well-known star among astronomers. By the way, Beta and Gamma Pictoris create a line that runs very close to the 6 hour RA line.

Beta () Pictoris is a type A star with a visual magnitude of 3.8. It's 59 light-years away and it has a large dust ring revolving around it - but it's invisible. Two astronomers (Smith and Terril) used coronographic images to estimated that the ring extends more than 1100 Astronomical units (AUs) from the star. They examined over 100 nearby stars and concluded that none have the "visible optical properties" of a disk as good as Beta Pictoris. Importantly, astronomers have detected a "void" in the disk - a ring within the disk between 10 and 30 AUs from the star that is depleted of dust. They suggest that a planet (maybe two) is making a path through the ring as it collects up dust by gravitational force. Two other astronomers (Roques and Lazzaro) have used computer models to understand the ring and they conclude that there might be a planet, about five times as massive as the Earth, in a nearly circular orbit (eccentricity of 0.01) around Beta Pictoris at a distance of 20 AUs.
Also, astronomers have recorded short-lived absorption lines similar in composition and behavior to that of comets! They conclude that there are comets orbiting Beta Pictoris in eccentric orbits which occasionally cause them to approach the star and become warm enough to produce a coma that is detected by its absorption spectrum.

That's nice (to know) but is there anything else down south?

Oh, goodness, yes!

Slightly south (towards the South Pole) of a line between Archernar and Canopus you will find the Magellanic Clouds. You recall that these are irregular galaxies 160,000 light-years away but still within our Local Group.
Ferdinand Magellan was the first European to record these objects so they are named after him.
The Large Magellanic Cloud (LMC) is so big (8 degrees in diameter) and bright that it can be seen when the moon is bright.
The Small Magellanic Cloud (SMC) is fainter and smaller.

Wait a minute! You told me the Milky Way cannot be seen (well) during a bright Moon. Are you telling me that our own galaxy isn't as bright as the Magellanic Clouds?!

Well, yes and no. The Milky Way is larger and more luminous than the Magellanic Clouds, but the Milky Way has a lower relative magnitude. It's difficult to appreciate the absolute magnitudes of galaxies but our Milky Way Galaxy has an estimated absolute magnitude of -20.5! (And remember, these magnitudes are do not "scale" linearly.) The Large Cloud of Magellan has an absolute magnitude of -18.5 and the Small Cloud has an absolute magnitude of -16.8. So we live in the brighter galaxy. Remember, we only see a small portion of the Milky Way's disk and it is spread out over a HUGE area of night sky. That gives the Milky Way a dim appearance compared to our more compact view of the distant Clouds of Magellan.

Let's move over to the other side of this southern circle and look at the two most famous southern constellations.

CRUX AUSTRALIS is the Southern Cross and it's the easiest constellation to identify in the Southern Celestial Hemisphere. The dimmest of its four stars has a magnitude of 2.8 so it really stands out.
Of course, there are many stars in this constellation but we use only four of them to make the cross.
Unfortunately, there is no central star in the cross so it isn't quite like a miniature CYGNUS.

CENTAURUS is the Centaur, a mythical half-man half-horse.
Here he is upside down with his human arms extending away from his body and his horse rump leaning through the 12h line. The CRUX is between his horse legs.
The brightest star in this constellation has had several names through the centuries (and cultures) but most folks know it as Alpha () Centauri. That's a good name because this is the brightest star in CENTAURUS with a magnitude of -0.3.

This star is our nearest known star (not counting the Sun) - only 4.35 light-years away. In fact it's a multiple star system and lots of folks are confused about it so let's explore Alpha Centauri in some depth. Like all multiple star systems, the primary star is given the letter "A". Alpha Centauri A is a type G star slightly more luminous than our Sun but otherwise a close match. It's secondary, Alpha Centauri B, is a type K star (so it's cooler than A) and is the larger of the two in mass but dimmer with only half our Sun's brightness. These two stars are separated by only 23 Astronomical Units. (Recall that 1 AU is the distance from the Earth to the Sun.) That's not very far apart. For comparison, Uranus is 20 AU from the Sun. These stars take about 80 years to orbit each other and their apparent separation can be as close as 2 second of arc or as distant as 22 second of arc. To the naked eye these two stars are combined as a single point of light but southern amateur astronomers can easily see this pair though a small telescope.

They definitely need a telescope to see the third member of this multiple group. Alpha Centauri C is a dim red dwarf (M-type) with a magnitude of only 11. It's so dim it was not discovered until 1915. There is some debate as to its association with the other two stars. Alpha Centauri C lies 13,000 AU away from "Alpha Centauri" (the two stars - A and B). We don't know if that is so far away as to be beyond the gravitational grip of the other two stars. If Alpha Centauri C is in orbit around A and B it will take many years before we will detect that because they are so far apart that it will take thousands of years to complete an orbit! Through a good telescope Alpha Centauri C is found to be separated from the other two by 2o. (That's two whole degrees! ) Not only that, but parallax measurements show it's closer to us than the pair so Alpha Centauri C is our nearest neighbor at a distance of only 4.22 light-years.

If Alpha Centauri C orbits the other two then we have a three star system in "Alpha Centauri" and we would be correct to call this star Alpha Centauri C. But, if this dim red dwarf is not connected to that system it "earns" a name of its own. Due to its position relative to us, the name Proxima Centauri is often used to describe this star.
So, if you want to have an argument with someone about which star is nearest to our own, you can talk about "Alpha Centauri" as a single star system, Alpha Centauri C as a member of that system which currently happens to be closest to us in its orbit around the other two stars, or call it Proxima Centauri in order to highlight that it may not be associated with the other two at all!

Sadly, if you live further than 25 degrees north of the equator, you won't be able to see Alpha Centauri.
But, the next time you watch a science fiction program that shows Alpha Centauri try to see if the creators got the system right. Does it have a yellow star like our own and an orange star separated by about the distance from here to Uranus? (I think "Babylon-5" got it right.) And what about Proxima?

Is Beta Centauri the same as Alpha-Centauri B?

No!

Beta () Centauri is a B-type star 460 light-years away and over 10,000 times as luminous as our Sun. Don't get it confused with the Alpha Centauri system. They are very different.

Beta Centauri happens to lie close to Alpha Centauri in our view of the night sky but that's just coincidence (and confusing).

Here's a good view of the southern sky showing the stars within the -45 declination circle.

There are many other constellations here (and near by) that we have not discussed. Most of them are very small and inconspicuous.

CENTAURUS is the only large constellation in the Southern sky (not counting ERIDANUS) and one of the few that is named after an ancient, mythical beast. By the time the southern skies were identified and named by Europeans that kind of naming had fallen out of favor. Here you will find small constellations named after scientific instruments (telescope and microscope), ship equipment and parts (net, sextant, keel) simple figures (triangles and crosses) and animals (several birds, a swordfish, a lizard and even a fly!).

Use this image to identify Archernar, Canopus, the two Clouds of Magellan, CRUX and CENTAURUS.

Here's that image again with the stars identified and the constellations drawn in.
I've also added in the names of some of the many other constellations but I haven't drawn in the lines. (It would get really crowded and there is no real need for you to learn them anyway.)

DORADO is the Swordfish.
HYDRUS is the Little Snake.
TUCANA is the Toucan.
PHOENIX is the mythical bird that rises from its ashes.
GRUS is the Crane you learned earlier.
PAVO is the PEACOCK.
TELESCOPIUM is the Telescope.
ARA has no common name that I know!
TRIANGULUM AUSTRALE is the Southern Triangle and easy to identify.
LUPUS is a Wolf (attacking CENTAURUS).
MUSCA is the Fly (bothering the hoof of CENTAURUS).
VELA is the Sails of the ship.
PUPPIS is the Deck (Poop Deck) of the ship.
CARINA is the Keel of the Ship.

I could go on and on but these names and this image are simply an introduction to the sights in the southern sky.

By the way, the Milky Way also extends into the southern celestial sphere (obviously). I haven't drawn it in because, well, this image is complicated enough! You may recall that it dips south from the "teapot" and through the tail of SCORPIUS. It then travels through LUPUS, CENTAURUS, CRUX, VELA and PUPPIS before winding its way back north along the side of CANIS MAJOR and into CANIS MINOR. Like its northern half, the southern portion of the Milky Way is very dim and can only be seen on a clear moonless night.

So I guess I now know all the stars.

Well, not all of them but certainly the brightest ones and the ones most talked about in astronomy and among amateur astronomers.
This lesson concludes your course in observational astronomy (but don't forget to study the last two lessons in "academic" astronomy). I hope you will make an effort to get outside and see these beautiful sights. And maybe find an excuse to travel south to see the "wonder down under"!




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.