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General Information
This page goes over some of the concepts of astronomy. Topics covered include:
- Identification of Stars
- Identification of Other Space Objects
- Classification of Stars
- Classification of Galaxies
- Classification of Nebulae
- The Magnitude System
- Positions
- Interstellar Distances
- Astronomical Terms
Identification
of Stars
Stars are indentified in many ways. One way, devised by Johann Bayer, uses Greek letters and the Latin root of the name of the constellation. The first of the Greek letters are usually given to the brightest stars, but in some constellations, they are not.
Another system was designed using information drawn up by John Flamsteed. He created a catalogue in which he gave certain stars a number. This system used those numbers along with the Latin root of the constellation's name. Another method used is giving stars an SAO (Smithsonian Astrophysical Observatory) number. This way, stars are depicted as "SAO ######" for whatever their respective number is. Some stars have names that most people are familiar with, whether you study the sky or not. The table below shows the different types of classification for certain stars (all stars are brightest for constellation).
| Name | Bayer System | Flamsteed System | SAO Numbers |
| Sirius | a Canis Majoris | 9 Canis Majoris | SAO |
| Rigel | b Orionis | 19 Orionis | SAO |
| Naos | z Puppis | | SAO |
Identification of Other Space Objects
Stars are not the only things in the sky that need identification. Nebulae, clusters, and galaxies also need to
be called something. One method is the Messier system, devised by Messier. This list has an interesting history. Messier was a comet hunter, and during his searches, he found many objects that he thought lookes like comets, but weren't. He created this list as a reference for other comet hunters so that if they found that object, they would know it wasn't a comet, and was already discovered. He gave each object a number, and each is now referred to as that number with a capital "M" in front of it (see table below).
Another method uses the New General Catalog. It is comprised of the letters "NGC" followed by a number (not the Messier number), and is classified that way. But most of these objects have common names that you and I refer to them as, just to make our lives a bit easier. There are many other catalogs that are used, and they are all listed on the first page in the Pictures section of this site.
| Name | Messier Numbers | New General Catalog Numbers |
| Andromeda Galaxy | M31 | NGC 224 |
| Orion Nebula | M42 | NGC 1976 |
| A star cluster (not named) | M50 | NGC 2323 |
Classification of Stars
Stars are generally classified by a spectral class. This classification is set up according to the star's temperature (and therefore color). It looks like this:
| Spectral Class | Color | Temperature (°F) | Example |
| O | Blue-Violet | 50,000-89,500 | Mintaka (d Orionis) |
| B | Blue-White | 17,500-50,000 | Rigel (b Orionis) |
| A | White | 13,000-17,500 | Sirius (a Canis Majoris) |
| F | Yellow-White | 10,300-13,000 | Procyon (a Canis Minoris) |
| G | Yellow | 8,500-10,300 | Sol (our sun) |
| K | Orange | 5,800-8,500 | Arcturus (a Bootis) |
| M | Red-Orange | 4,000-5,800 | Antares (a Scorpii) |
Classification of Galaxies
Classification of Nebulae
The Magnitude System
Each star is given a number that represents its
brightness as seen from the earth, as are all other objects in the night sky. This number is called its magnitude. Actually, there are two different measures of magnitude - apparant magnitude and absolute magnitude.
Apparant magnitude is how bright an object looks to an observer on Earth.
Absolute magnitude is how bright a star would look at a distance of 10 parsecs (32.6 light years). An object's apparant
magnitude is dependent on it's absolue magnitude and it's distance from Earth. Therefore, a moderately bright star that
is relatively close may look brighter than an extremely bright star at a long distance.
The system of magnitudes is a strange one - a brighter star has a lower magnitude than a fainter one.
| Name | Apparant Magnitude | Absolute Magnitude |
| Sol (our sun) | -26.5 | 5.0 |
| Rigel | 0.1 | -7.0 |
| Naos | 2.2 | -7.3 |
Positions
Stars have coordinates, just like a city. Of course, they have different names and different measurements. The equivalent of longitude being called Right Ascension, and the equivalent of lattitude being called Declination. Right ascension is measured in hours, minutes, and seconds (from 0 hours to 23 hours, 59 minutes and 59 seconds, since 24 hours would be the same as 0 hours), from a point called the First Point in Aries. Declination is measured in degrees (from -90 to 90), with 0 being on the Celestial Equator. These two points will be explained
later. Right Ascension is positive to the right of the First Point. Declination is positive above the Celestial Equator, and negative below it, with the extremes being at the Celestial Poles.
| Name | Coordinates |
| Sirius |
| Right Ascension | Declination |
| 6 hr 45.1 min | -16° 43.1' |
|
| Rigel |
| Right Ascension | Declination |
| 5 hr 14.5 min | -8° 11.9' |
|
| Naos |
| Right Ascension | Declination |
| 8 hr 3.6 min | -40° 0.2' |
|
Interstellar Distances
Space is big. So big that you can't use your ordinary units of measure such as feet or miles (unless you would prefer to say that Alpha Centauri is 25.28 trillion miles or 133.5 quadrillion feet). So, first there's the AU (Astronomical Unit). That is the distance from the Sun to Earth, approximately 93 million miles. Planets' distances from the sun are measured in AUs (Mercury being .39 AUs and Pluto being 39.2). After that comes the light
year. As its name suggests, it is the distance light travels in one year. This measurement is used for interstellar measurements, as is the parsec, which is equal to 3.26 light-years.
The actual calculation of a parsec is the distance at which an object must be in order for its parallax shift to be 1 second of arc.
Astronomical Terms
Here is a glossary of some terms used in astronomy.
Apehelion - A planet's furthest point from the sun.
Apogee - A satellite's furthest point on it's orbit.
Asterism - a recognizable grouping of stars in a constellation (i.e. Big Dipper (in Ursa Major), Northern Cross (in Cygnus).
Astronomical Unit (AU) - the distance from the Sun to the Earth. Approximately 93,000,000 mi.
Celestial Equator - a projection of the Earth's equator out into space. 0° declination.
Celestial Sphere - an imaginary sphere surrounding the Earth on which all heavenly objects (stars, galaxies, nebulae, etc...) seem to lie.
Constellation - an area of the sky, including all stars, galaxies, nebulae, etc. in that area (a constellation is NOT just the star groupings - see Asterism)
Declination - one coordinate that denotes a star's position, similar to latitude on Earth.
Ecliptic - the imaginary line that the sun traces in it's yearly trek across the sky. Also, it is no coincidence that the ecliptic goes through the constellations of the zodiac (and the non-zodiacal constellation Ophiuchus - a result of precession).
First Point in Aries - the point where the sun crosses the Celestial Equator from south to north (vernal equinox).
Actually, the First Point in Aries isn't really in Aries, it has drifted into Pisces (which is next to Aries) due to precession.
Light Year - the distance light travels in one year. 5.88x1012 mi.
Lucida - the brightest star in a constellation.
Meridian - an imaginary line stretching from the north horizon to the south horizon, passing through the zenith.
Nadir - the point on the sky that is directly below your feet where you are standing (if you imagine on the other side of the world).
North Celestial Pole - a projection of the Earth's North Pole onto the celestial sphere. +90° declination.
Parsec - the distance at which an object must be in order for its parallax shift to be 1 second of arc.
Perihelion - A planet's closest point to the sun.
Perigee - A satellite's closest point on it's orbit.
Precession - a slight wobble of the Earth's axis that causes the positions of the stars to change over the course of a very long time (thousands of years). Whereas now the North Star is Polaris, in about 2,000 years, it will be Errai (g Cephei).
Proper Motion - very slowly (at least from our point of view) the stars are moving with respect to each other. If you look at a constellation as it is now and as it will be in 25,000 years, you will notice a significant change in the stars positions.
Right Ascension - one coordinate that denotes a star's position, similar to longitude on Earth.
South Celestial Pole - a projection of the Earth's South Pole onto the celestial sphere. -90° declination.
Transit - to cross the meridian from east to west.
Zenith - the point that is directly over your head where you are standing.
Zodiac - not exactly an astronomical term, but it's existence is explained in astronomy. These constellations are the ones that the sun travels through throughout the year. When the zodiac was created, there were only twelve of these constellations. Due to precession, however, the sun actually travels through 13 constellations, the 13th one being Ophiuchus. The zodiac has not been updated to include this, and will not likely be.
Data collected and tabulated by unknown source.
Report submitted by Gary Conaway of Earth.
(It is based on the Terran point of view.)
