Galaxy Names

The Tadpole Galaxy
The Tadpole Galaxy

Galaxy names come in a bewildering range of forms; from descriptive (e.g. Whirlpool Galaxy, Black Eye Galaxy, The Eyes), to ones that seem to relate to a constellation (e.g. Andromeda Galaxy, Hydra A, Leo I), to ones named after a person (e.g. Stephan’s Quintet, Malin I, Mayall’s Object), to letter+number combinations (e.g. the Messier catalog galaxies such as M33 and M87), to letters+number combinations (e.g. NGC 3115, DDO 185), to impossible-to-remember stings-with-dashes-dots-and-pluses like MCG-06-07-001, 4C37.11, and SDSS J002240.91+143110.4!

And sometimes a galaxy has LOTS of different names, such as M87, Virgo A, NGC 4486, Arp 152, 3C274, IRAS 12282+1240, WMAP J123051+1223 (there’s, like, about another 20!).

However, of the estimated 100 billion galaxies we could observe, with current astronomical facilities, only a few million have names, and most of those are unique (i.e. only one name per galaxy). Of course, almost all the single-name galaxies are little more than faint smudges in an optical or infrared image … and that gives a clue to where the names come from!

Most galaxy names come from the catalog, or catalogs, in which they appear. The catalogs have many sources, but most recent ones have been put together as a key output of a dedicated survey or mission, and the galaxy name reflects that. So, for example, SDSS stands for Sloan Digital Sky Survey (one of the most amazing optical/NIR galaxy surveys of all time), IRAS for InfraRed Astronomy Satellite, DDO for David Dunlap Observatory (where a catalogue of dwarf galaxies was put together), and 4C for 4th Cambridge survey (a radio survey). Some of the older catalogs, or lists, were put together from previously known galaxies, or objects (the Messier list is perhaps the most famous example).

More to explore, on galaxy names. The online dedicated, searchable database NED (NASA/IPAC Extragalactic Database) is astronomers’ essential resource; SEDS’ (Students for the Exploration and Development of Space, hosted by the University of Arizona LPL) Messier galaxy section is amateurs’ favorite; and Galaxy names are identified by a group of letters and numbers. What do they stand for? (Hubblesite).

Universe Today articles on galaxy names? Sure! Here is a small sample: This Where in the Universe Challenge, Astrophoto: NGC 4631 by Bernd Wallner, and Have a Cigar! New Observations of Messier 82.

Astronomy Cast’s Milky Way episode has more on galaxy names; well worth a listen!

Sources: Hubblesite, SDSS, IRAS, DDO, NASA/IPAC

Universe Today Guide to the Messier Objects

Well, Tammy’s done it again. Remember the Universe Today Guide to the Constellations? Well now Tammy has completed another monster volume. The Universe Today Guide to the Messier Objects. This is a guide to all 110 Messier Objects, from M1 (the Crab Nebula) to M110 (a satellite galaxy to Andromeda), and everything in between.

In addition to descriptions of the individual Messier Objects, there’s also a nice introduction to the Messier Objects, a guide to doing a Messier marathon, and suggestions for stretching your Messier marathon out to a week.

If you’ve got any questions, comments or feedback, please let us know. I’m sure there are going to be some bugs in there.

Thanks. And thanks again to the wonderful Tammy Plotner for grinding through this monster project.


P.S. If you want to use any part of this information for any reason whatsoever, you’ve got my permission. Be my guest. Print them off for your astronomy club, turn it into a PDF and give it away from your site. Republish the guides on your own site. Whatever you like. All I ask is that you link back to Universe Today and the specific page, so people can find out where it came from.

Planetary Nebulae

No, planetary nebulae are not nebulae found around planets; nor are they nebulae produced by planets … rather, they got stuck with this name because the first ones to be observed (and written about) look like planets (well, they did through the eyepieces of the telescopes of the time … somewhat).

Charles Messier – yep, the comet hunting guy – listed M27 in his famous catalog; that’s the Dumbbell Nebula, and the first planetary nebula recorded (1764). It was Herschel – the guy who discovered Uranus – who dreamed up the name ‘planetary nebula’; and why? Because, to him, they looked a bit like the gas giants Jupiter, Saturn, and Uranus (Neptune wasn’t discovered then). There are four planetary nebulae in Messier’s list; in addition to M27, there’s M57 (the Ring Nebula), M76 (Little Dumbbell Nebula), and M97 (Owl Nebula). So why did Herschel say planetary nebulae looked like giant planets, including Saturn? Because, in 1781, he discovered one – NGC 7009 – that looked like Saturn! Guess what it’s called? The Saturn Nebula.

When spectroscopes were used to observe planetary nebulae, they caused excitement; unlike stars and (what we today call) galaxies – which have dark absorption lines in their spectra – planetary nebula have bright emission lines (and essentially nothing else, i.e. no continuum emission). Further, the brightest of the lines (actually two, close together), in most planetary nebulae, corresponded to nothing ever seen in any laboratory spectrum … so they were thought to be caused by an as yet undiscovered element, called nebulium.

Today we understand planetary nebulae to be a short-lived phase of (most) stars … after the red giant phase, when the star’s fuel has been exhausted, it shrinks to become a white dwarf. The gas expelled during the red giant phase become heated and ionized by the intense UV radiation of the new white dwarf (these central objects, in most planetary nebulae, are among the hottest stars). The plasma has an extremely low density, which means that certain excited, meta-stable states of ions such as O2+ can jump to a lower energy state by emission of ‘forbidden’ radiation (rather than by collision).

Such spectacular objects … no surprise that Universe Today has many stories and articles on planetary nebulae! Here are just a few Found: Planetary Nebula Around Heavy Stars, Planets May Actually Shape Planetary Nebulae, Will We Look Like This in 5 Billion Years?, and Penetrating New View Into The Helix Nebula.

Astronomy Cast’s Nebulae has more on planetary nebulae; the following episodes put planetary nebulae into a broader astronomical context: The End of the Universe Part 1: The End of the Solar System, The Life of the Sun, and The Life of Other Stars.

Source: SEDS