BL Lac object

Called BL Lacertae objects, even shorter BL Lac objects, or BL- lacertids are active galaxies and are included in the most luminous quasars known cosmic objects.

Discovery

The first time was discovered in 1929 by Cuno Hoffmeister such an object and as a variable star in the constellation lizard ( Lacerta ) with the standard variable name BL classified, whose variations in brightness, however, were irregular. Only in 1968 it was found in measurements with a radio telescope that it is in this BL Lac object is a strong radio source, which is also variable. At the same time they found several other objects of this type, although there were the characteristics of these objects.

Properties

The BL Lacertae objects are particularly distinguished by three characteristics:

1 Continuous spectrum without lines

In the observation of stars is usually stated in astrophysics based on the absorption and emission lines found in the spectrum of a star about its physical and chemical nature. The spectrum of BL Lac and other objects of this type, however, has no such lines and thus leaves no conclusions about the composition of the object. Only a small nebula around BL Lac later settled ray spectra and turned out to be a giant galaxy that was assigned from 800 million light years due to their redshift of 0.0688 a distance. So are these objects - similar to the quasars, but which have a line spectrum - so highly luminous, that it can outshine an entire galaxy.

2 brightness variation across the entire spectrum

All previously known BL Lac objects have a non- periodically varying brightness over its entire spectrum, this spectrum so far only by the possibilities of measurement ( radio to hard X-rays from ) can be limited. These changes can be in the order of hours, and thus limits the size of these objects, a strong, since they can not be greater than the distance traveled by light during this period. Therefore, one must assume that it is very compact objects that are no larger than our solar system.

Third polarization of the emitted light

The emitted radiation of the BL Lac objects is polarized across the entire spectrum greatly (up to 20 %), both the polarization and the brightness is variable.

Explanations

Your incredibly high luminosity receive the BL Lac objects (like quasars ) do not like stars through nuclear fusion of hydrogen, helium, etc. to form heavier elements, but BL Lac objects contain black holes with masses reaching the billion times the sun's mass can. The radiated energy is released when matter to be attracted to these black holes plunges into it. The crash of the black hole, this matter is partially converted directly into energy, which approximated released reaches the value mc2. This is a more than ten-fold higher amount of energy released than when the same amount of matter would be fused inside the star. In addition, the energy is emitted in a much shorter time than the lifetime of stars, which drastically increases the brightness in addition.

Some BL Lac objects emit so strong that they outshine the entire galaxy emitted by the surrounding light. Thus, it is not possible for these objects to provide a distance due to the red-shift of the galaxy.

Based on the characteristics described above was a consistent model to describe the BL- Lac- objects can be found. It is like the quasars around massive black holes at the centers of active galaxies. Due to the non-thermal spectrum and the sharp polarization may not have to be ordinary stars themselves. It is now believed that the source of radiation are relativistic plasma flows which are collimated ejected from the center of the object, the so-called jet. The emitted radiation has the properties of a synchrotron radiation. As with the quasars also subject to thermal radiation is also emitted by the accretion disk, but which has its maximum at very short wavelengths. The main difference between BL Lac objects and radio-loud quasars is in the perspective from which we can observe the objects or in the galaxy in which they reside. In a BL Lac object you look directly into the jet: the angle between the jet axis and the direction of observation is very low. The jet dominates the spectrum of the object. In quasars, you do not look directly into the plane and there are additionally observed broad emission lines. These emission lines are emitted by gas clouds, which are excited by the radiation of the accretion disk. But instead must be sufficiently high gas concentrations are present in the vicinity of the center. From the radio- weak quasars, the BL Lac objects also differ by the above- mentioned plasma jet, which is orthogonal to the accretion disk.

• Galaxy