To better determine H$_0$, rate at which the universe is expanding

  \ \ \ - redshift $\rightarrow$ $v$ OK

  \ \ \ - distance uncertain

\bigskip

Use objects that we know absolute magnitudes (these objects are called
'standard candles') to compare with apparent magnitudes

  \begin{itemize}
	 Cepheid variables (M$_v \sim -6$) $\rightarrow$ d $\lap$ 20~Mly
	 brightest supergiants (M$_v \sim -8$) $\rightarrow$ d $\lap$ 50~Mly
	 brightest globular clusters (M$_v \sim -10$) $\rightarrow$ d $\lap$ 130~Mly
	 brightest H~II regions (M$_v \sim -12$) $\rightarrow$ d $\lap$ 300~Mly
	 supernovae (M$_v \sim -19$) $\rightarrow$ d $\lap$ 8~Gly
  \end{itemize}

\clearpage %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\underline{Quasar（似星體）}

\begin{itemize}

   1936: first radio telescope by Grote Reber

   1950s, 1906s: radio sources across sky,

   1959: e.g. listing of the Third Cambridge Catalogue of 471 sources

   1960: 3C48, starlike with unidentified spectral lines

   1962: 3C273, same with jet

   1963: 3C273 spectral lines perhaps hydeogen lines with large redshift
	      v~=~0.15~c i.e. distance=2~Gly
\end{itemize}

$\longrightarrow$ starlike appearance plus strong radio emission, hence named
\underline{quasi-stellar radio sources} $\rightarrow$ quasars.	Sometimes also
called \underline{quasi-stellar objects}, or QSOs

3C48, v~=~c/3, $d$=4 Gly if H$_0$=80~km/s/Mpc

  \ \ \ Note: if H$_0$ is larger, $d$ is greater

\hugeskip

Quasars fluctuate in brightness, time scale $\rightarrow$ size limit \\
e.g. an object of diameter of 1~ly cannot change brightness with a period of
less than 1~yr.

$\longrightarrow$ Huge amount of anergy (equivalent to 100 galaxies) emitted
within 1~light-day in diameter

\clearpage %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

normal galaxies $\rightarrow$ active galaxies $\rightarrow$ quasars

Active galaxies: luminous with bright, starlike nuclei, often associated with jets

\begin{itemize}
   Seyfert galaxies: spiral galaxies with starlike nuclei and strong emission
	lines
   BL Lac objects ("blazars"): elliptical galaxies with starlike nuclei and
	featureless spectra; rapid emission variation (as much as a hundred times
	in days)

\end{itemize}

\hugeskip

	\cl{ A supermassive blackhole may be the central engine.}

\hugeskip

	\cl{ Quasars, BL Lac objects, double radio sources ....}

	\cl{\bf ALL UP TO HOW WE SEE IT }

\end{document}