Stars and Magnitudes
Basics on the types of stars and how to observe them.
1. Stellar Basics
Stars come in different colors, sizes, and brightness. The average person in the darkest of skies is able to see arounf 10,000 stars while an experienced oberserver with sharp eyes may be able to see 50,000. Using a telescope the number of stars visible quickly rises into the millions.
Stars range in colors from blue violet to deep reds. The largest stars are hundreds of times laregr than the mass of the sun and the smallest are around 8% the mass of the sun. Stars vary in mass by a factor of 10,000 but vary in brightness by a factor of over 1 trillion. The brightest stars can be seen from the opposite side of the galaxy while the dimmest ones are invisible to the eye even when right next door.
The color of a star depends on the temperature of the star’s surface. Stars with higher surface temperatures emit light at high energies and appear bluer while cooler stars emit light at lower energies and appear redder.
Since stars have a large range of luminosities a bright star may be close or extremely luminous and much farther away. The brightest stars in the night sky are blue and red giants, with many being very bright and in the astronomical perspective just next door.
Credit and Copyright : T. Jittasaiyapan
2. Magnitudes
The origin of the magnitude system used to measure the brightness of stars is credited to the Greek Astronomer Hipparchus who made one of the first catalogues of the stars in the night sky. He ranked the stars in terms of brightness on a scale from 1 to 6. The brightest stars in the sky had a magnitude of 1 and the dimmest that could be seen have a magnitude of 6. Since this method of measuring stellar magnitudes relies on the human eye, it is subjective and not absolute.
With the development of photography to measure the brightness of stars new stellar catalogues were developed with more rigorous measurements of stellar brightness. The magnitude system developed by Hipparchus was adopted into the system we use today.
The current magnitude system is exponential with 1st magnitude stars defined as 100 times brighter than 5th magnitude stars. This choice sets the brightest stars at -1.5 ( 10 times brighter than magnitude 1), and the dimmest stars for the average person around 6.5. The brightest stars are over 1500 times brighter than the dimmest visible to the naked eye. These magnitudes are referred to as apparent magnitudes and describe how stars appear to be, not how luminous they are intrinsically.
The dimmest stars able to by experienced observers with sharp eyes is around 8.0. Seeing stars around 6.5 - 8th magnitude requires a very dark sky with no light pollution.
Stars of various magnitudes in Orion