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\subsection{Units for the brightness of stars}  The unit of the flux is Watts per square meter. This is very small for astronomical objects. Often used instead: unit 'Jansky' which is defined as $1\,Jy = 10^{-26}$\,W\,m$^{-2}$\,Hz^${-1}$ 10^{-26}$\,W\,m$^{-2}$\,Hz$^{-1}$  Flux is measured on a linear scale, i.e. a source of 10\,Jy is ten times brighter than a source of 1\,Jy. This is inconvenient in astronomy, more useful would be a logarithmic scale $\log{(f)}$. This leads to the concept of magnitudes.  %\subsection{Magnitudes} \subsection{Magnitudes}  %The The  unit magnitudes is derived from a system first used by the Greek astronomer Hipparcox (2nd century BC). In his catalogue of stars, 1st magnitude are the brightest stars, 6th magnitude are the stars just visible for the human eye. This system has now been adopted and extended for modern astronomy. The relation between fluxes and magnitudes is: %\begin{equation}  %m_1 \begin{equation}  m_1  - m_2 = -2.5 \log{(f_1 / f_2)} %\end{equation} \end{equation}  %Or Or  conversely: %\begin{equation}  %f_1 \begin{equation}  f_1  / f_2 = 10^{(m_1 - m_2) / -2.5} %\end{equation} \end{equation}  %This This  is a logarithmic system, but with a scaling factor of 2.5. This factor means that 5\,mag difference correspond to a factor of 100 in flux. The zeropoint for the magnitude scale is Vega at $m = 0.0$. %\subsection{Magnitudes and distances}