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\section*{Stars \& Galaxies, Galaxies example class 1}
\noindent{\bf Question 1.}

\begin{enumerate}
\item What are the two main galaxy types? Describe three
  characteristics for each.

\item Describe the three main stellar components of a spiral galaxy.

\item What is an HII region? How do you recognize them in a picture of
  a galaxy? Why do they tend to follow spiral arms?

\item Why can you not see the MWs bulge in the night sky?

\item Derive the equation $m-M=5\log(r)-5$ that relates the apparent
magnitude, $m$, the absolute magnitude, $M$, and the distance to a
star, $r$, when obscuration is neglected.


\end{enumerate}

\noindent{\bf Question 2.}

The absolute magnitude of the sun in the B-band is $M_{\rm B}=+5.58$,
and its luminosity $L_{\rm B}=3.86\times 10^{26}$~W.


\begin{enumerate}
\item Compute the flux from a solar type star in the Andromeda galaxy
  at distance $r=710$ kpc. Compute its apparent magnitude in the
  B-band.

\item Assume that the density of stars in Andromeda falls exponentially
  with height above the disk, $n(z)=n_0\,\exp(-|z|/z_0)$, with $n_0=1$
  pc$^{-3}$ and the scale-height $z_0=0.1$\, kpc. Compute the surface
  density ($\sigma$) of stars (the number of stars seen per unit area),
  when the disk is seen face on.

\item Consider the flux, $F$, received from Andromeda from a square with
  side-length $\theta=1$ arc seconds. Converts this to magnitudes.
  Show that the surface brightness is independent of distance. Assume
  Andromeda's disk is seen face-on.

\end{enumerate}

\center{[1\,pc=3.08$\times 10^{16}$ m.]}

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