Classifying Stars

Stars differ not only in their brightness, but also in their size, surface temperature, and chemical composition.  The one thing they have in common is that they are all spherical – although some spin so fast they tend to bulge in the middle!


All stars are basically immense balls of intensely hot gas that generate heat and light through a process called nuclear fusion.  The temperature and density in the core of a star are so great that lighter atoms smash into each with enough force to fuse into heavier atoms.  In our own Sun, for example, atoms of hydrogen fuse to produce atoms of helium (this process involves several intermediate steps).  The fusion process releases energy in the form of electromagnetic radiation – light.


By spreading starlight into a spectrum, astronomers can learn the temperature and chemical makeup of stars.  After studying thousands of stars, it became clear that stars fall into various categories, or classes.  Some are massive and bright, and have relatively short, tumultuous lives.  Others are small and dim, and can shine steadily for tens of billions of years.


A letter and number system is used to define stars in terms of their most important physical characteristics, and these designations are displayed when you point to a star in the Sky Chart.  A more complete discussion of spectral classes and the physics of stars can be found in any introductory astronomy text.

Giants and Dwarfs

When you see a bright star in the sky, there are two possibilities: the star is close by and relatively average in size, or it is far away and gigantic.


Rigel is the brightest star in the constellation Orion.  It is nearly 800 light years away, but is the seventh brightest star in the sky.  It is a whopper, with a diameter of about 100 million kilometers.  The Sun, by comparison, is about 1.4 million kilometers across.


Astronomers distinguish between apparent magnitude and absolute magnitude.  Apparent magnitude is how bright a star looks in the sky.  Absolute magnitude refers to how bright a star would appear if it were located exactly 10 parsecs (32.6 light years) away.  The apparent magnitude of Rigel is about 0.2, but its absolute magnitude is nearly –7.0.

More about Constellations

The desire to find order in nature, even where none exists, seems to be built into the human brain.  When you look up at the sky on a dark, clear night, the sheer number of stars can be overwhelming.  Our distant ancestors must have been in awe of those countless lights randomly scattered across the sky like diamonds.


Because of our instinctive need to find order, cultures all across the globe have organized stars into distinctive patterns called constellations.  These patterns are purely a product of the human imagination.  Nature had nothing to do with creating them.


The constellations we recognize today have mostly come down to us from the ancient Greeks.  Many of them represent mythological figures.  Orion, for example, one of the most prominent constellations visible in northern wintertime, represents a heroic hunter who first appeared in one of the great epics of classical Greek literature, The Odyssey.  Orion is accompanied by two hunting dogs that are also immortalized in constellations: Canis Major and Canis Minor, the big and little dogs, respectively.


When you look at Orion, it isn’t hard to imagine the figure of a hunter with a raised arm wielding a club.  You can see one classic representation of this figure by going to the Display menu and selecting Constellations & Asterisms Options.  You can display line drawings, mythical figures, and constellation boundaries by checking the appropriate boxes.  You can also use the slider labeled Transparency to adjust how bright these renderings appear.


For many other constellations, the connection between its array of stars and what it is supposed to represent is difficult to see, to say the least.  They’re a little more like abstract art, intended to represent the idea of a thing rather than the thing itself.


Drawing lines between the stars of a given constellation provides a simple “stick figure” view of that constellation.  When astronomers think about constellations at all, this is how they usually think of them.  The more fanciful mythological drawings of constellations became popular in the early 17th century, especially in the gorgeous star charts engraved by the great German celestial cartographer Johann Bayer (Bayer is also credited with creating the system that designates stars with Greek letters and the genitive name of their constellations, as described previously).


When the constellations we recognize today were originally created, a number of stars were left over – that is, not all stars fit into the established patterns.  To avoid confusion, astronomers designated boundary lines between the constellations.  Not unlike borders between countries, any star that falls within the borders of a given constellation is said to belong to it, whether it was included in the original depiction of that constellation or not.


There are familiar patterns of stars that don’t quite qualify as constellations.  Astronomers call these patterns asterisms.  The Big Dipper and the Pleiades (the Seven Sisters) are probably the two most familiar examples.  In Japan, the Pleiades are called Subaru.  You’ve probably seen them driving around your neighborhood.

Some Tips on Using Star Charts

Learning how to connect what you see on a star chart to what you see in the real sky takes some time.  We’re going to show you a step-by-step process that will make it easier for you to find common stars and constellations.  With a little patience and practice, you’ll soon become an expert.


First of all, when you go outside and look at the sky, you need to know what direction you’re facing.  In particular, you need to know how to find true north.  City streets often lie along north/south and east/west lines, but this isn’t always the case.  If you aren’t sure which way is north at your viewing location, use a magnetic compass to find it.


When hundreds of stars are displayed on your chart, finding individual stars and constellations can be very challenging.  But if you limit the number of stars in the chart to just a few dozen of the brightest stars, you’ll have a much easier time learning the sky.

Printing a Sky Chart

Printing a sky chart to take with you when you go outside is also very helpful.  TheSkyX can print any chart it displays.  You can print an “all sky” chart, or select a particular part of the sky you’re interested in learning. 


Choose the File command from the main menu.  Near the bottom of the menu, you’ll see two items: Print and Print Setup (if you have more than one printer connected to your computer, Print Setup can be used to select the printer you’d like to use).  Select the Print command.


The Print Chart tab of the Export Chart window is displayed on the screen.  In addition to printing charts, TheSkyX allows you export charts as Portable Document  Format (PDF), Scalable Vector Graphic (SVG) and Postscript files. 

To Export a Sky Chart in PDF, SVG or Postscript Format

1.      Select the desired option from the Format list.

2.      Click the Export button.

3.      On the Export Chart dialog, enter the file name to save the chart in this format.

4.      Click Save.



Click the Create Bitmap tab (or, from the main menu, click the Export command from the File menu) to view the options for saving Sky Charts as bitmaps or pixmaps.  Click the Copy Sky Chart button to copy the current chart to the Clipboard.  Click the Save As button to save the chart as a Portable Networks Graphics (PNG) file.


Turn on the Create Custom Size checkbox to specify the Resolution, in dots per inch (DPI), Width and Height, in inches, of the bitmap. 


If you want to create high-resolution star charts for publication,  individual “layers” of the chart can be exported by turning on the desired checkboxes in the Chart Layers tab.



Clicking the Print button on the Print Chart tab sends the chart to the currently selected printer.  TheSkyX uses the current Map Like display settings (page 203) and prints stars in black, leaving the sky white.  The size of the star is proportional to its magnitude.  Non-stellar objects are also printed using the symbols that appear in the Map Like Sky Charts.


You can choose the orientation of the printout and other printing parameters by clicking the Page Setup button.  When you’re ready to print, simply click the Print button.


The best time to start learning the sky is a clear, cloudless night, when there is no Moon or at most a crescent Moon.  Moonlight can interfere as much as city lights when it comes to seeing the stars, and if the Moon is close to Full, you probably won’t be able to find any but the very brightest stars and planets.  You also want to be in an open space, a place where there are no tall buildings, trees, or annoying artificial lights to interfere with your viewing.  Make sure in particular that you have a clear view to the north.


When you get to your observing site, give your eyes at least a few minutes to adapt to the darkness.  You’ll need a flashlight to read the chart of course, but you should use one that has a red filter.  These can be bought at most stores that sell telescopes, or you can simply tape a piece of transparent red film over a standard flashlight.  Using only red light will help preserve your night vision.  If you take your computer outside with you, the Display > Show Night Vision Mode command will help preserve it, too.

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