Um  candidato a <B>buraco negro estelar</B> é a
estrela Cygnus X1, descoberta
pelo
satélite de raios-X Uhuru 
(liberdade em Swahili, a língua do Quênia, onde o satélite foi lançado em 
12.12.1970). 
Tom Bolton descobriu que o sistema
consiste da
<UL>
<LI>estrela supergigante azul O9-B0 HD226868, com 40&#177;5 massas solares, 
<IMG SRC="cygnusX1.jpg" WIDTH="288" HEIGHT="288" ALT="Cygnus X1" ALIGN="RIGHT">
<LI>orbitando uma massa de cerca de 
<A HREF="http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006ChJAS...6a.228Z&db_key=AST&link_type=ARTICLE">13 a 29 massas solares</A>, invisível no ótico,
<IMG SRC="../cigx1.jpg" width="288" height="192" ALT="Simulacao de Cignus X-1" ALIGN="RIGHT">
<LI>com período de 5,5 dias: 
<IMG WIDTH="226" HEIGHT="77" ALIGN="MIDDLE" BORDER="0" SRC="mbn.png"
 ALT="$P^2 = \frac{4\pi^2}{G(M_*+M_\mathrm{BN})}a^3$">
</UL>
Esta companheira compacta é muito mais massiva que o maior limite teórico, 
de 4,3 massas solares, de uma estrela de nêutrons.
Cygnus X-1 &eacute; a mais brilhante fonte de raios X duros (E&gt;20 keV) persistente,
e est&aacute; a cerca de 2500 pc (8000 anos-luz).<BR>
Outros buracos negros estelares são:
<UL>
<LI>GS2000+25, com massa entre 5,5
M<SUB>Sol</SUB> e 8,8 M<SUB>Sol</SUB> (Zacharias Ioannou, 
<A HREF="http://pisces.as.utexas.edu/robinson/">Edward Lewis Robinson</A>, 
<A HREF="http://mintaka.sdsu.edu/faculty/wfw/">William F. Welsh</A>,
e 
<A HREF="http://physics.open.ac.uk/~chaswell/">Carole A. Haswell</A>, 
2004, Astronomical Journal, 127, 481),
<LI>A0620-00 com massa entre 3,3 e 13,6 M<SUB>Sol</SUB> (Cynthia S. Froning e Edward Lewis
Robinson, 2001, Astronomical Journal, 121, 2212)
e
<LI> XTE J1859+226, com massa 7,4&#177;1.1 M<SUB>Sol</SUB>,
</UL>
<b>detectados em sistemas bin&aacute;rios que emitem raio-X, do
g&aacute;s da estrela companheira acelerado pelo buraco negro.</B>
<BR CLEAR="ALL">
<DIV ALIGN="CENTER">
<TABLE BORDER="1">
<TR><TH>Sistema</TH>
<TH>P<SUB>orbital</SUB></TH>
<TH>f(M)</TH>
<TH>Doador</TH>
<TH>Classificação</TH>
<TH>Massa</TH></TR>
<TR><TH></TH>
<TH>[dias]</TH>
<TH></TH>
<TH>Tipo Espectral</TH>
<TH></TH>
<TH>[M<SUB>Sol</SUB>]</TH>
</TR>
<TR><TD>GRS 1915+105
</TD><TD>
33.5</TD><TD>9.5&plusmn;3.0</TD><TD>K/MIII</TD><TD>LMXB/Transiente</TD><TD>14&plusmn;4</TD></TR>
<TR><TD>V404 Cyg</TD><TD>6.471</TD><TD>6.09&plusmn;0.04</TD><TD>K0IV<TD></TD><TD>12&plusmn;2</TD>
</TR>
<TR><TD>Cyg X-1</TD><TD>5.600</TD><TD>0.244&plusmn;0.005</TD><TD>09.7Iab</TD><TD>HMXB/Persistente</TD><TD>10&plusmn;3</TD>
</TR>
<TR><TD>LMC X-1</TD><TD>4.229</TD><TD>0.14&plusmn;0.05</TD><TD>07III<TD></TD><TD>&gt;4</TD>
</TR>
<TR><TD>XTE J1819-254</TD><TD>2.816</TD><TD>3.13&plusmn;0.13</TD><TD>B9III</TD><TD>IMXB/Transiente</TD><TD>7.1&plusmn;0.3</TD>
</TR>
<TR><TD>GRO J1655-40</TD><TD>2.620</TD><TD>2.73&plusmn;0.09</TD><TD>F3/5IV<TD></TD><TD>6.3&plusmn;0.3</TD>
</TR>
<TR><TD>BW Cir
</TD><TD>
2.545</TD><TD>5.74&plusmn;0.29</TD><TD>G5IV</TD><TD>LMXB/Transiente</TD><TD>&gt;7.8</TD>
</TR>
<TR><TD>GX 339-4</TD><TD>1.754</TD><TD>5.8&plusmn;0.5<TD></TD><TD></TD>
</TR>
<TR><TD>LMCX-3</TD><TD>1.704</TD><TD>2.3&plusmn;0.3</TD><TD>B3V</TD><TD>HMXB/Persistente</TD><TD>7.6&plusmn;1.3</TD>
</TR>
<TR><TD>XTE J1550-564</TD><TD>1.542</TD><TD>6.86&plusmn;0.71</TD><TD>G8/K8IV</TD><TD>LMXB/Transiente</TD><TD>9.6&plusmn;1.2</TD>
</TR>
<TR><TD>4U 1543-475</TD><TD>1.125</TD><TD>0.25&plusmn;0.01</TD><TD>A2V</TD><TD>IMXB/Transiente</TD><TD>9.4&plusmn;1.0</TD>
</TR>
<TR><TD>H 1705-250</TD><TD>0.520</TD><TD>4.86&plusmn;0.13</TD><TD>K3/7V</TD><TD>LMXB/Transiente</TD><TD>6&plusmn;2</TD>
</TR>
<TR><TD>GS 1124-684</TD><TD>0.433</TD><TD>3.01&plusmn;0.15</TD><TD>K3/5V<TD></TD><TD>7.0&plusmn;0.6</TD>
</TR>
<TR><TD>XTE J1859+226
</TD><TD>
0.382</TD><TD>7.4&plusmn;1.1<TD></TD><TD></TD>
</TR>
<TR><TD>GS 2000+250</TD><TD>0.345</TD><TD>5.01&plusmn;0.12</TD><TD>K3/7V<TD></TD><TD>7.5&plusmn;0.3</TD>
</TR>
<TR><TD>A 0620-003</TD><TD>0.325</TD><TD>2.72&plusmn;0.06</TD><TD>K4V<TD></TD><TD>11&plusmn;2</TD>
</TR>
<TR><TD>XTE J1650-500</TD><TD>0.321</TD><TD>2.73&plusmn;0.56</TD><TD>K4V<TD></TD><TD></TD>
</TR>
<TR><TD>GRS 1009-45</TD><TD>0.283</TD><TD>3.17&plusmn;0.12</TD><TD>K7/M0V<TD></TD><TD>5.2&plusmn;0.6</TD>
</TR>
<TR><TD>GRO J0422+32</TD><TD>0.212</TD><TD>1.19&plusmn;0.02</TD><TD>M2V<TD></TD><TD>4&plusmn;1</TD>
</TR>
<TR><TD>XTE J1118+480</TD><TD>0.171</TD><TD>6.3&plusmn;0.2</TD><TD>K5/M0V<TD></TD><TD>6.8&plusmn;0.4</TD>
</TR>
</TABLE>
<CAPTION>
Buracos Negros Estelares de acôrdo com Jorge Casares, 2007,
Proceedings IAU Symposium No 238, p.3</CAPTION>
</DIV>
<HREF="EtacarinaeSGS.jpg">
<IMG SRC="../etacar.jpg" width="479" height="480" ALT="Eta Carinae" ALIGN="LEFT"></A>