Showing posts with label Puppis. Show all posts
Showing posts with label Puppis. Show all posts

Wednesday, April 4, 2012

Southern Winter Milky Way Skies.

Today I want to do something a little different and share a picture taken by my housemate and good friend Jonathan. He's recently gotten into astrophotography using his DSLR camera, but has been too busy to do much (it being his last semester at UHHilo and all).

Two weeks ago he was able to make it up to the Vis and took the following picture, which he posted on his blog (which you can find at Life is a Zwitterion). I saw it and offered to label the constellations and a few other objects visible in it for him, and he suggested I post it on my blog, so here it it. Thanks Jon!



This is a 30-second exposure taken from the Visitor Information Station on Mauna Kea looking south at 8:25 on March 19th, 2012. Visible near the bottom is the flank of Mauna Loa, with part of the rim of Puʻu Kalepeamoa visible in the foreground (ka lepe a moa means "the comb of the rooster" in Hawaiian, which is  an apt description for the shape of the range of hills it is part of). In the background lies a stretch of the winter Milky Way, with several prominent constellations visible.

Also visible in this picture are the two brightest stars in the night sky, Sirius and Canopus. Sirius is one of the twenty closest stars us, a mere 8.6 light-years away. It is actually a binary system composed of a white main sequence star (Sirius A) of spectral type A1 about twice as massive as the Sun and about 25 times as luminous, and a white dwarf (Sirius B) first discovered in 1862. Sirius B has a diameter of about 12,000 kilometers (7,500 miles), nearly the same as Earth, but contains 98% of the mass of the Sun.

The other bright star, Canopus, is much further away: 310\(\pm\)10 light-years. Canopus is a supergiant star of spectral type F, and it is 13,600 times more luminous than the Sun. In fact, it's the most intrinsically luminous star within ~700 light-years. Unfortunately, it's far enough south as not to be visible from much of the continental United States, although south of 37\(^\circ\)18' it may be visible under very good conditions.  Its radius is fully one-third of the distance from the Earth to the Sun, about 50 million kilometers or 30 million miles.

Along with stars, there are quite a few constellations (or parts of them) visible in this image. Prominently placed is Canis Major, the Big Dog (Canis Minor is out of the top of the picture, however). The three constellations that made up the old constellation Argo Navis are also visible here; Puppis, the Poop Deck, Carina, the Keel, and Vela, the Sail. Also visible is Pyxis, the Ship's Compass, though it is a modern addition by Nicolas Louis de Lacaille in the 18th century. Below Canis Major is Columba, the Dove, and Pictor, short for Equuleus Pictoris the Painter's Easel (another modern addition courtesy of de Lacaille). Finally, just at the bottom of the picture is Dorado, the Swordfish. Dorado is interesting because it contains both the majority of the Large Magellanic Cloud (one of the Milky Way's satellite galaxies) and the south celestial pole. In fact, if it weren't for Mauna Loa getting in the way, the Large Magellanic Cloud would be visible at the bottom of the picture.

Finally, there are at least three Messier objects visible in this picture (there are probably even more that I missed): M46, M47, and M41. Messier 46 you may recall as the open cluster with superimposed planetary nebula I showed a picture of a few weeks ago.

Tuesday, March 20, 2012

Planetary Nebulae, and the White-Hot Dwarfs that Light Them

Monday night I was able to get some images with the imager, and conditions were great nearly the entire evening. One of the objects I imaged was Messier 46 (or, rather, I walked a volunteer through imaging it, as I was doing some training). M46 is an open cluster in the constellation Puppis the Poop Deck not too far away from Sirius, the brightest star in the night sky.

Messier 46 in Puppis, with the planetary nebula NGC 2438 visible near the top.

M46 by itself is a fairly unremarkable (if fairly rich) open cluster of perhaps up to 500 stars about 30 light-years across, located about 4,500 light-years from us and receding at a speed of 41.4 km/s. What's interesting is that at first glance it appears to have a planetary nebula (cataloged as NGC 2438, visible above the middle of the cluster) embedded in it. However, the nebula does not appear to share the motion of the cluster, and is just a chance alignment along our line of sight.

The nebula itself is turns out to be pretty interesting. It is located closer to us than M46 at about 2,900 light-years, and has the amazing property that its central white dwarf star is one of the hottest stars known, measuring in at an astounding 75,000 K. (That's about 74,700 °C, or 134,672 °F.)

To put those number in perspective, our Sun's surface is about 5,778 K (or about 9,940 °F), while the very hottest O-type main-sequence stars (ones that are in their hydrogen-fusing phase, about 90% of the lifetime of all stars) might get up to 30,000 K (53,540 °F). Note that all these temperatures refer to the star's surface temperature – even in our fairly low-on-the-temperature-scale Sun the core temperature is over 15 million K (about 27 million °F). That still means that the Earth-sized ultra-dense sphere of electron-degenerate carbon and oxygen at the center of NGC 2438 is over two-and-a-half times hotter than the hottest stars operating under fusion power alone, and nearly thirteen times hotter than our Sun.

Planetary nebulae are so-named not because they have anything to do with planets, but because in the early telescopes through which they were discovered they looked somewhat like them (sort of a faint, fuzzy disc-shape). They come about when a star less than ten times the mass of the Sun runs out of hydrogen in its core to fuse, at which point it begins to work its way up the periodic table fusing heavier and heavier elements in its core while its outer layers puff up and the star becomes a red giant. The outer layers slowly puff off into space to form the nebula, while the core eventually stalls at carbon and oxygen (or maybe neon, if the star has between eight and ten times the Sun's mass) and forms a white dwarf, which is basically just the exposed core of a former star.

At this point no new energy is being released, so the white dwarf enters a long period of cool down. Due to their composition of electron-degenerate matter, temperatures inside the white dwarf are a fairly steady 1,000,000 K or so (about 18 million °F), while the outside cools off as it radiates energy into space, lighting up the outer layers it puffed off earlier and making the planetary nebula visible. Eventually the nebula disperses into the interstellar medium, and the white dwarf is left to gradually get cooler, and cooler, and cooler...

To end this post, I thought I'd mention that the hottest white dwarf found to date had a surface temperature of an incredible 200,000 K (360,000 °F) – almost three times hotter than the white dwarf in NGC 2438, nearly seven times hotter than the hottest O-type stars, and a whopping thirty-five times hotter than the surface of our own star. Pretty amazing, huh? On the flip side, the coolest white dwarfs found have surface temperatures of just about 4,000 K (6,740 °F) – they've already cooled off to less than the Sun's surface temperature.

Check back tomorrow, when I'll show you a picture of what happens when a star with more than ten times the mass of the Sun runs out hydrogen in its core. Hint: it's a lot more explosive!

Edit (3/24/12): As you can probably tell, it's way past tomorrow and I still haven't gotten that post up. That's because it turned out to be much longer than I anticipated when I started it, and then I went to work for two days. I will have it up in the next few days, though.

Wednesday, January 11, 2012

Conventional Constellations

I haven't been up to posting anything for the last few days due to a raging cold and some exhaustion after my friend's wedding, but recently I came across a list of constellations no longer officially recognized by the International Astronomical Union (IAU), and thought I'd share some of the more amusing ones with you.

To a modern astronomer, the word "constellation" has a very specific meaning: it's a region of the celestial sphere as formalized by the IAU in 1922. Nowadays, there are 88 official constellations, formed so that between them they cover the entire night sky so that any celestial object may unambiguously be found to be located in one or another of them.

Prior to 1922, this was not the case. There was no standard set of constellations, so different authors could (and did) publish star charts with their own favorite constellations on them. Since many of the authors came from different European states at a time when patriotism and national identity were on the rise, you can imagine that there was a bit of one-upping and brinkmanship going on between various star chart publishers. An example of such was Robur Carolinum, Charles' Oak, named in honor of the tree where Charles II of England is said to have hidden from Oliver Cromwell's troops after the Battle of Worcester, which was put on the map by Sir Edmund Halley, of eponymous comet fame. Other examples were Frederici Honores, Frederick's Honors, created to honor Frederick II of Prussia, and Psalterium Georgii, George's Harp, in honor of  George III of Britain (yes, the one who ended up losing the American colonies). See a bit of a trend? Actually, though, these kind of overt advances were fairly rare, and most of the proposed constellations were much more neutral in tone.

Even though they may not all have been national-glory-seeking advances, many of the constellations proposed in the 18th and 19th centuries were mutually incompatible because they incorporated stars used by constellations in other star charts. But to fully understand why people were making up constellations in the first place, it is necessary for me to back up and show how the original constellations came about.

Human beings are wired to look for patterns everywhere, and there have been recorded patterns in the stars for thousands of years. The earliest recorded star catalogs that we know of belonged to the Babylonians; the Three Stars Each catalog appear to have been recorded around the 12th century BC (near the boundary between the Late Bronze Age and the Iron Age, sometime during the period of the Judges in the Biblical chronology). After this, perhaps sometime around 1000 BC, the MUL.APIN catalog was compiled using more accurate observations.

(For the curious, MULAPIN refers to the first constellation of the Babylonian year, "The Plough" [not associated with the asterism known by the same name in Britain, which is usually known as the Big Dipper in North America]. The difference between an asterism and a constellation is fairly subtle. An asterism is just any pattern in the stars. A constellation, as mentioned before, is one of the 88 officially-recognized patches of sky that contains a particular asterism. So the Big Dipper is technically not a constellation but an asterism, located in the constellation of Ursa Major which contains additional stars and whatnot.)

The Babylonians weren't the only ones coming up with patterns in the sky, however. The ancient Chinese came up with their own system of constellations, as did the ancient Indians, the Mayans, and the Polynesians. In the southern hemisphere where the Milky Way is more visually striking, both the Inca and the Aborigines came up with constellations composed not of stars, but of dark nebulae in the Milky Way.

After the conquest of the former Persian empire by Alexander the Great in the 4th century BC, Babylonian astronomical knowledge became available to the Greek-speaking world which adopted it readily. In the 2nd century BC an otherwise little-known individual by the name of Claudius Ptolomy, a Roman-era scholor in Egypt, wrote what ranks right up there among the most influential scientific texts of all time: the Almagest. (The origin of the name is a fascinating aside: the original name in Greek meant "Mathematical Treatise", which became "The Great Treatise" [not undeservedly] and eventually just something like "The Greatest", which sounds like magisti in the Greek of the time, which the Arabs transliterated as al-majisti when they preserved the ancient Greek writings in the Middle Ages, and which was taken into English [finally] as Almagest.  Phew.)

The Almagest is a fascinating piece of work for many reasons (not least of which being that it helped set up the Aristotelian geocentric cosmology that was to reign in astronomy for the next 1800 years), but for our purposes the most interesting part of it is that it formalized a system of 48 constellations. Twenty of these were taken directly from the Babylonian system, while another ten had the same stars but different names. What's interesting is that all but one of these constellations have survived to the present day, as official IAU-sanctioned constellations (and the single one that wasn't, Argo Navis the Argonaut's Ship, was simply broken up into three separate constellations because it was considered "too big". It now exists as Vela, the Sails, Carina, the Keel, and Puppis, the poop deck).

This long, roundabout treatise on the history of constellations is mostly to give you a feel for how they have changed (and how they haven't) through most of history. In the 17th to 19th centuries there was a huge explosion in the number of new constellations being added to star charts. You see, due to his location, Ptolemy couldn't see below a certain southern declination, so he had no constellations for the area around the south celestial pole. Likewise, he didn't bother filling in every area of the sky with a constellation, leading to bare patches even in the northern hemisphere. After the invention of the telescope and the explosion of new objects found in the centuries that followed, astronomers naturally wanted an easy way to roughly locate where something was on the sky, akin to being able to give the state a particular city is located in, rather than having to come up with the exact latitude and longitude all the time. This state of affairs led to people trying their hand at coming up with new constellations to both fill in the gaps left by Ptolomy and to create new constellations where he had never set eye. To make a long story short: some were successful, others not so.

And that's what I originally wanted this much-longer-than-anticipated post to be about: constellations that just didn't make it. The "also rans". They are objects of historical interest, and some of them are just plain funny. Looking over the list, one name in particular stands out as the source of amusing constellations, an English botanist (of all things) by the name of John Hill, who introduced constellations representing the Seahorse, Toad, Leech, Slug, Earthworm, Limpet, Mussel, Rhinoceros Beetle, Eel, Long-legged Spider, Tortoise, Star-Gazer Fish, and two species of extinct mollusk. He even introduced one representing the Pangolin, which just might be my favorite animal, and which I kinda feel bad now isn't an official constellation. Oh, well, not much to do about it. Now that the sky has been officially divided among the 88 official constellations, there's no more room for someone to create their own versions and market them, except for fun. Which isn't bad, really, science needs order and stability to flourish, and an internationally-agreed-upon map of the entire sky has been quite beneficial to astronomy overall.

And with that I think I need to end this rambling post, seeing as it's gone way beyond what I meant when I started, and get some sleep. A hui hou!