Stars

Stellar Black Holes

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]

The observational techniques for identifying stellar black holes:


• Eclipsing Binary - Eclipsing binary stars are just one type of variable stars known as extrinsic variables. They are a special group of spectroscopic binaries. These stars appear as a single point of light to an observer, but based on its brightness variation and spectroscopic observations we can say for certain that the single point of light is actually two stars in close orbit around one another. The variations in light intensity from eclipsing binary stars are caused by one star passing in front of the other relative to an observer. A brightness versus time (or phase) plot for a variable star is know as light curve. In addition to the measurement of stellar masses, the light curves of eclipsing binaries give much information about the physical characteristics of the stars that compose the system. It points to the presence of a black hole if one of the star is small, invisible and has a mass greater than 3 M_sun.
• X-ray Binary - X-ray observations, coupled with optical identifications established that the bright X-ray sources in the Milky Way are binary systems containing a neutron star or black hole orbiting around a companion star. If the companion star is very young and massive, the system is called a High Mass X-ray Binary. These high mass O or B stars usually have an intense wind that is easily captured by the neutron star or black hole to release X-rays. Such is the case for the most famous X-ray binary, the black hole system Cygnus X-1, where a black hole with a mass of 10 M_sun orbits an O star called HD226868 (see Figure 08-22).
• Soft X-ray Transients - Soft X-ray transients (SXTs, also know as X-ray Novae) belong to a special class of X-ray binaries; they typically brighten by many orders of magnitude quickly, and decay exponentially over the next several months. SXTs probably experience such outbursts once every few decades. In quiescence, the mass-donor can be studied in detail, without the complication due to accretion. Typical orbital period is 5-20 hrs and the secondary are often low mass main sequence stars, which can shred material to the black holes only when they are close to each other. Recent theoretical work indicates that a black hole system with "advective accretion flows" (ADAFs - energy transferred as heat instead of radiation) would appear much darker than a similar neutron star system, because the energy would disappear into the black hole instead of transforming into radiation by the neutron star. Observations of V404 Cyg (see the animation in Figure 08-23) have yielded a spectrum much like an ADAF onto a black hole. This star seems to be swallowing nearly hundred times as much energy as it radiates, and the only way this can happen is if the star is a true black hole.
• Micro-quasar - Figure 08-24a is a series of radio images for GRS1915, which is a "Hard X-ray Transient" source probably associated with a 33 M_sun black hole and a high mass companion star. As the gas plunges into the black hole, the bubbles are formed from excessive matter moving along the rotational axis. It ejects one plasma bubble almost directly toward the line of sight at 90% the speed of light, and the other moving away. This kind of stellar black hole is also known as micro-quasar because it mimics, on a smaller scale, many of the phenomena seen in quasar.

			GRS1915 is probably the largest stellar black hole discovered; while the X-ray source GRO J0422+32 near V518 Persei seems to be the smallest with a mass of 3 to 5 Msun.
Figure 08-22 Cygnus X-1 [view large image]	Figure 08-23 V404 Cyg	Figure 08-24a GRS1915

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

		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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