Stellar collision

Under a star collision is understood in astronomy a close encounter of two stars in such a small distance that they may suffer irreversible structural changes. It is believed that such spectacular events have shaped the evolution of globular clusters in particular.

Frequency of collisions with single stars

Star collisions are due to the large distances between individual stars in comparison to its diameter extremely rare events. So the astronomer James Jeans estimated already from the beginning of the 20th century that is likely to have suffered in the more than 10 billion years of its existence, scarcely a single collision of the 100 billion stars of the Milky Way.

In globular clusters, the mean stellar density with a few hundred stars per cubic light-year is significantly higher than in the Milky Way - here it is in the solar neighborhood is only about 0.01 stars per cubic light year. Thus results for globular clusters significantly greater probability for star collisions. It is estimated that about 50 % of all stars of a globular cluster have suffered a collision in the past. In all about 150 globular clusters, which are located in a spherical environment around the Milky Way, would thus occur about every 10,000 years a star collision, and in the entire visible universe is about every second such star collision instead.

In estimating the number of collisions not only middle star distances and diameters must be taken into account, but also the gravitational pull of the stars, simplified averaged potential field. Compared to purely ballistic paths, the probability of a collision between the in globular clusters quite low speeds 10-20 km / s increases by a factor of 100 This effect is called gravitational focusing. Irreversible changes threaten latest when at least one of them would be in the corresponding static situation exceed the Roche limit at the minimum distance of the stars.

Types of stellar collisions

From the large number of different classes of stars and the consequent possible combinations in a collision to a plethora of different types of collisions result. The course of the collision also depends on the relative velocity and the impact parameter that describes whether the impact is centrally or just roaming. The latter is far more common. The following are some typical examples of grazing collisions are described in detail, as they appear as results of computer simulations. As for collisions but also the internal structure of the stars involved and the changes need to be considered, strong simplifications are used because of the associated high computational complexity, the results are therefore still quite uncertain.

Collision of a main sequence star with a white dwarf

A typical white dwarf has a diameter of about only 10,000 km, but about the same mass as the sun. In a grazing collision therefore flies the white dwarf due to its enormous density largely unscathed by the main sequence star through and then leaves again. Due to the gravitational case, the relative velocity of the collision partners rises to more than 500 km / s, so that the actual collision takes only about an hour.

The fate of the main sequence star depends on the circumstances and is not yet fully explored. At sufficiently high collision rate could be achieved by the shock wave in the main sequence star outside the core ignite a nuclear fusion. The energy released in this case would correspond to the one that usually implements the main sequence star in 100 million years. The power dissipated during the collision would be in the range of those who radiates a supernova in the days after its onset. The main sequence star would be completely torn up and disappear in a cloud of gas. The sun would befall such a fate, they would vaporize on earth, the oceans and evaporate along with the atmosphere into space.

Collision of a red giant with a white dwarf

In the case of a red giant rather than a main sequence star, the collision takes about a month. In glancing collision, by far the most common case, however, the core of the red giant is maintained and after the loss of a large part of its gaseous envelope a second white dwarf.

Collision of two main-sequence stars

The scenario of a collision between two main-sequence stars depends strongly on the initial conditions of the paths and parameters of the stellar structure. The two stars can survive as two single stars, but they can also merge into a single star. At sufficiently high collision rate even both stars may dissolve completely. Under certain conditions, a so-called Gezeiteneinfang is possible.

Gezeiteneinfang

There are passages possible that occur primarily without direct physical contact of the partners, in which there is but by tidal forces considerable deformation of one or both stars. If the energy loss associated large enough so the reduced train speed ranges then no longer sufficient for a separation of the two partners. They circle each other on a close orbit and a collision is then due to ongoing deformations only a matter of time.

Collision of two brown dwarfs

Brown dwarfs may merge into a single object. If the initial mass limit for hydrogen fusion ( about 75 Jupiter masses ) is exceeded, generated a main sequence star. This is then due to the small mass of a red dwarf.

Collision of two neutron stars

The collision or merger of two neutron stars is for example found in binary systems in which both partners end up as a neutron star. Due to the strong attraction of the mutual distance decreases gradually until the two neutron stars merge. The merger itself takes only a fraction of a second. The lighter of the two objects is torn here, while the heavier object collapses usually a black hole. Recent research results indicate that this is a source of short gamma-ray bursts.

Collisional supernovae

Supernovae are explosions that destroy the progenitor star, releasing energies of about 1051 erg. An equivalent amount of energy can be released in the collision of two solar-like stars when they collide centrally at a speed of more than 10,000 km / s. Device is a binary star system in the vicinity of a central supermassive black hole, then breaks apart the double star system. The one star is emitted as hyper -speed engines while the other star in a very close orbit around the black hole at a speed of up to several tens of thousands revolves kilometers per second. In the vicinity of a massive black hole as Sagittarius A * at the center of the Milky Way high-speed collisions can occur with a low probability and thereafter every 1,000 until 10,000 th supernovae caused by the collision of two stars. This collisional supernovae should take place in the center of a galaxy and show the light curve only a short maxima from several weeks in length. Subsequently, the released gas should fall into the black hole, leading to an increase in the X-rays as an active galactic nucleus.

Evidence for star collisions in globular clusters

Although there is yet no single star collision could be observed directly interpret astronomical observations of globular clusters suggest that the history of this pile of collision events is strongly influenced. The following circumstantial evidence are:

• The stars of a star cluster all have about the same age, namely the cluster itself in globular clusters, however, it was discovered already in the 1950s, many seemingly much younger and unusually hot stars, so-called blue stragglers (English: blue stragglers ). According to recent studies, they focus just in the centers of globular clusters, that is, where star collisions should be particularly common as a result of the higher concentration of stars. It can be assumed that this is to main-sequence stars, which have united in a collision at a single star with correspondingly larger mass. Since massive stars are particularly bright, short-lived, one would assign these stars without knowledge of their history a lower age.

• The satellite Uhuru, who studied cosmic X-rays since 1970, had discovered over one hundred X-ray sources in the Milky Way, of which about 10 % in the globular clusters. Since the Milky Way but 10,000 times more stars houses, as all of their globular clusters together, it means that the conditions are much more favorable in the globular clusters for the development of X-ray sources. It is believed that it is in these X-ray sources is a binary star system consists of a main sequence star and a neutron star or black hole, orbiting so close that material from a main sequence star she flows over to his partner and thereby heats up to extreme temperatures. The likelihood of development of two jointly incurred in a binary star system star to such an X-ray source is extremely low with 1:1 billion. In globular clusters, however, they could make much more frequently by Gezeiteneinfang. Another there also significantly favored formation mechanism are three-body encounters in which an object is ejected from the globular clusters and the other two remain in a close orbit.

• In globular clusters significantly less red giants are observed than would be expected based on the known phases of stellar evolution. The cause is thought that they are there disproportionately likely to be involved because of their large expansion in star collisions, while mostly turns into white dwarfs.

Possible collisions in weakly bound binary stars

In many binary systems with highly eccentric orbits already enough a small gravitational perturbation to change the orbital elements. This mechanism can bring star of such a system on a collision course and could be the dominant source of stellar collisions within the Milky Way and produce a collision every 2,500 years. The resulting star is expected to be similar to the FK Comae Berenices - stars - a rapidly rotating massive single star with a strongly depleted lithium abundance.