Star 1st magnitude
Since ancient Greece, the 20 brightest fixed stars have been referred to as 1st magnitude stars . In the brightness scale strictly defined by Pogson in 1850, all stars that are brighter than +1.5 mag are counted as the first size today.
There are 22 1st magnitude stars in the entire starry sky , 10 of which are north of the celestial equator and 12 are south of it. Particularly noticeable among them are four groups of stars :
- in the northern sky the winter hexagon and the great summer triangle
- in the southern sky the Southern Cross and the neighboring Centaurus .
history
The star brightness scale was introduced into astronomy by Hipparcos (190-120 BC) . He defined the weakest stars visible to the naked eye as 6th magnitude stars .
Stars that are slightly fainter than the brightest were called 2nd magnitude stars by the Greek astronomers Hipparchus (190–125 BC) and Ptolemy (100–175 AD) (e.g. Big Dipper , Cassiopeia and Orion Belt ).
Apparent brightness scale
This classification have the astronomers of modern times, a defined physically accurate logarithmic scale adapted to them as apparent brightness designate (or "magnitude" or Magnitudo, abbreviated like ):
Size class | Magnitude | number | annotation |
---|---|---|---|
1. size | <1.5 mag | 22nd | the brightest two actually have "−1st size" (−1.5 to −0.5 mag), the eight following "0. Size "(−0.5 to +0.5 mag) |
2. size | 1.5 to 2.5 mag | 70 | |
3rd size | 2.5 to 3.5 mag | 170 | |
4. size | 3.5 to 4.5 mag | 430 | to Argelander / Kapteyn |
5. size | 4.5 to 5.5 mag | 1200 | |
6. size | 5.5 to 6.5 mag | 4000 |
This scale is designed so that each level corresponds to an intensity ratio of the light of , and therefore 5 levels exactly 1: 100. A first magnitude star is 100 times as bright as a 6th magnitude point of light, but this appears to the eye as fewer steps. The reason for this is the Weber-Fechner law of our visual sense , which on the other hand enables us to perceive differences in brightness between day and night of 1:10 billion.
The number of stars increases by a factor of 2½ to 3 per size class (cf. population index for meteor streams ), which continues for several brightness levels and is related to the visibility of distant stars in the Milky Way . Only between magnitude 14 and 18 does the factor drop to 2.3 per level, from which the total number of stars has been extrapolated to at least 30 billion .