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Stellar Black Holes

Blackhole Model Most black holes are said to be stellar: formed from stars. It is estimated that the Milky Way contains 10 million of these black holes. Their mass can be 10 times that of the sun and the radius of event horizon can be a few kilometers. Because not even light can escape from inside the event horizon, it is hard to detect black holes. Astronomers get around this problem by indirect observations on some signatures, which are peculiar to a black hole. It usually involves the interaction of the black hole with its environment, e.g., a companion star. Figure 08-21 is a model of a stellar black hole drawing material (the accretion stream) from the companion star. The accretion stream forms an accretion disc before finally spiraling into the black hole and generates bursts of X-rays.

Figure 08-21 Blackhole Model
[view large image]

Table 08-06 below shows the fate of stars at the end of their life as they become brown dwarfs, white dwarfs, neutron stars or black holes depending on the initial mass.

Final Event Initial Mass (Msun) / Type Final Mass (Msun) Life Time (109 yrs.) Heaviest Element Synthesized Residual Core
Gradual Cooling < 0.1 / M7 same > 1000 Helium Brown Dwarf
Stellar Wind < 0.4 / M5 ~ same > 200 Helium White Dwarf
Stellar Wind or
Planetary Nebula
< 1.0 / G2 < 0.7 > 10 Helium or Carbon White Dwarf
Planetary Nebula < 3.0 / A0 < 0.8 > 0.35 Oxygen White Dwarf
Supernova, Type I / II < 10 / B5 < 1.5 > 0.02 Oxygen or Silicon White Dwarf or Neutron Star
Supernova, Type II < 15 / B1 < 10 > 0.01 Silicon or Iron Neutron Star or Black Hole
Supernova, Type II < 30 / O8 < 20 > 0.004 Iron Black Hole

Table 08-06 End of Stars

For high mass stars, there are three ways to end their life. Depending on the initial mass and the metallic content Z (elements
High Mass Supernova Types of  Supernova other than hydrogen and helium), their fates are depicted in Figure 08-24b. The Iron-core-collapse supernova turns the iron core into neutrons and neutrinos leaving a neutron star behind. The pair-instability supernova (as discussed earlier) blows the whole thing apart leaving nothing behind (as recently observed in SN2007bi). The dotted line marks the point above which pair-unstable stars are thought to form black hole instead of exploding. The blue shaded area denotes a transition region in which stars first become 'pair unstable', but eventually undergo iron-core collapse. Triangles and

Figure 08-24b High Mass Supernovae

Figure 08-24c Types of Supernovae

squares denote values obtained by different theoretical studies; green triangles for zero metallicity. Figure 08-24c displays a pictorial explanation for these types of supernovae.

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