Star collision

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In astronomy, a star collision is understood as a close encounter between two stars at such a short distance that they undergo irreversible structural changes. It is believed that such spectacular events primarily shaped the evolution of globular clusters .

Frequency of collisions with single stars

Star collisions are extremely rare events due to the large distances between individual stars compared to their diameter . As early as the beginning of the 20th century , the astronomer James Jeans estimated that of the 100 billion stars in the Milky Way, hardly a single one would have suffered a collision in the more than 10 billion years of its existence.

In globular clusters , the mean star density of several hundred stars per cubic light year is significantly higher than in the Milky Way - here it is only around 0.01 stars per cubic light year in the vicinity of the sun . This results in a considerably higher probability of star collisions for globular clusters . It is estimated that around 50% of all stars in a globular cluster have collided in the past. In all 150 globular star clusters that are in a spherical environment around the Milky Way, a star collision would occur about every 10,000 years, and in the entire visible cosmos such a star collision takes place about every second.

When estimating the number of collisions, not only mean star distances and diameters must be taken into account, but also the gravitational attraction of the stars, simplified in the mean potential field. Compared to purely ballistic trajectories, the probability of a collision increases by a factor of 100 at the comparatively low speeds of 10 to 20 km / s in globular clusters. This effect is called gravitational focusing . There is a risk of irreversible changes at the latest if at least one of them would fall below the Roche limit at the minimum distance between the stars in the corresponding static situation .

Types of star collisions

The large number of different star classes and the resulting combination options in the event of a collision result in an abundance of different collision types. The course of the collision also depends on the relative speed and the impact parameter , which describes whether the impact is central or just grazing. The latter is far more common. In the following some typical examples of grazing collisions as they appear as the results of computer simulations are described in more detail . However, since the internal structure of the stars involved and their changes must also be taken into account for the collisions, strong simplifications are used due to the associated high computational effort, and the results are therefore still quite uncertain.

Collision of a main sequence star with a white dwarf

A typical white dwarf is about 10,000 km in diameter, but about the same mass as the Sun. In the event of a grazing collision, the white dwarf therefore flies largely unharmed through the main sequence star thanks to its enormous density and then leaves it again. Due to gravity, the relative speed of the collision partners increases to over 500 km / s, so that the actual collision only lasts about an hour.

The fate of the main sequence star depends on the circumstances and has not yet been conclusively researched. If the collision speed is sufficiently high, the shock wave in the main sequence star could also ignite a nuclear fusion outside the core . The energy released in this case would correspond to that which the main sequence star usually converts in 100 million years. The power generated during the collision would be in the range of that which a supernova emits in the days after its eruption. The main sequence star would be completely torn apart and dissolved in a gas cloud. If the sun were to suffer such a fate, the oceans on earth would evaporate and evaporate together with the atmosphere into space .

A red giant collides with a white dwarf

In the case of a red giant instead of a main sequence star, the collision lasts about a month. In a grazing collision, by far the most common case, the core of the red giant remains intact and forms a second white dwarf after a large part of its gas envelope has been lost.

Collision of two main sequence stars

The scenario of a collision between two main sequence stars depends heavily on the initial conditions of the orbits and parameters of the star structure. The two stars can survive as two single stars, but they can also merge into a single star. If the collision speed is sufficiently high, both stars can even completely dissolve. Under certain circumstances, so-called tidal capture is also possible.

Tidal capture

Passages are possible that primarily take place without direct material contact between the partners, but in which there is considerable deformation of one or both stars due to tidal forces . If the associated loss of energy is large enough, the reduced path speed is no longer sufficient to separate the two partners. They orbit each other in a narrow orbit and a collision is then only a matter of time due to continued deformation. The burst of energy released during the merger is called a merger burst .

Collision of two neutron stars

The collision or merging of two neutron stars occurs, among other things, in binary systems in which both partners end up as neutron stars. Due to the radiation of gravitational waves , the mutual distance gradually decreases until both neutron stars merge. The merger itself only takes a fraction of a second. The lighter of the two objects is torn apart, while the heavier object usually collapses into a black hole. Recent research suggests that this is a source of short gamma-ray bursts.

Such an event could be observed in the galaxy NGC 4993 on August 17, 2017, 2:41 p.m. CEST through the detection of gravitational waves .

Collisional supernovae

Supernovae are explosions that destroy the precursor star, releasing energies of around 10 51 ergs . An equivalent amount of energy can be released when two sun-like stars collide if they collide centrally at a speed of more than 10,000 km / s. If a binary star system comes near a central, superheavy black hole , the binary star system breaks apart. One star is emitted as a hyper-fast runner, while the other star orbits in a very narrow orbit around the black hole at a speed of up to several tens of thousands of kilometers per second. In the vicinity of a massive black hole like Sagittarius A * in the center of the Milky Way , the high-speed collisions can take place with a low probability and thereafter every 1,000th to 10,000th supernova is caused by the collision of two stars. These collisional supernovae should take place in the center of a galaxy and the light curve should only show a short maximum of a few weeks in length. Subsequently, the released gas should fall into the black hole and lead to an increase in X-rays like an active galactic nucleus .

Evidence for star collisions in globular clusters

Although not a single star collision has so far been directly observed, astronomical observations of globular clusters indicate that the history of these clusters has been strongly shaped by collision events. The following indications are available:

  • Of the stars of a star cluster, all about the same age, that of the pile itself. In globular clusters, however, was discovered as early as the 1950s, many seemingly much younger and unusually hot stars, called blue straggler ( English blue stragglers ). According to more recent studies, they are concentrated in the centers of globular clusters, i.e. where star collisions should be particularly frequent as a result of the higher star concentration. It can be assumed that these are main sequence stars that have combined to form a single star with a correspondingly greater mass during a collision . Since massive stars are particularly bright and short-lived, these stars would be assigned a lower age without knowing their prehistory.
  • The satellite Uhuru , who since 1970 cosmic X-rays examined had more than a hundred X-ray sources discovered in the Milky Way, of which about 10% in the globular clusters. Since the Milky Way is home to 10,000 times more stars than all of its globular clusters combined, this means that the conditions in the globular clusters are significantly more favorable for the formation of X-ray sources. It is assumed that these X-ray sources are binary star systems consisting of a main sequence star and a neutron star or black hole , which orbit each other so closely that material from the main sequence star flows over to its partner and is heated to extreme temperatures in the process . At 1: 1 billion, the probability of two stars formed together in a binary system developing into such an X-ray source is extremely low. However, they could form much more frequently in globular clusters as a result of tidal capture. Another formation mechanism, which is also clearly favored there, are three-body encounters, in which one object is thrown out of the globular cluster and the other two remain in a narrow orbit.
  • Significantly fewer red giants are observed in globular clusters than one would expect based on the known phases of stellar evolution. It is assumed that the cause is that, due to their large size, they are disproportionately often involved in star collisions and mostly transformed into white dwarfs.

Possible collisions with weakly bound binary stars

In large binary star systems with strongly eccentric orbits, a small gravitational disturbance is enough to change the orbital elements. This mechanism can put stars in such a system on a collision course and could be the dominant source of stellar collisions within the Milky Way, producing one collision every 2,500 years. The resulting star is likely to be a rapidly rotating massive single star with a heavily depleted lithium abundance, similar to the FK Comae-Berenices stars .

Web links

literature

Individual evidence

  1. ↑ Robert Gast: Gravitational Waves - The Spacetime Quake of NGC 993 . Ed .: Spectrum of Science. Spektrum der Wissenschaft Verlagsgesellschaft mbH, January 2018, ISSN  0170-2971 , pp. 58–65 .
  2. ↑ Shmuel Balberg, Reem Sari, Abraham Loeb : A new rare type of supernovae: hypervelocity stellar collisions at galactic centers . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.7969v1 .
  3. ↑ DC Heggie, FA Rasio: The effect of encounters on the eccentricity of binaries in clusters . In: Mon. Not. R. Astron. Soc. , 282, 1996, pp. 1064-1084
  4. ^ Nathan A. Kaib, Sean N. Raymond: Very Wide Binary Stars as the Primary Source of Stellar Collisions in the Galaxy . In: submitted to Astrophysics. Solar and Stellar Astrophysics . arxiv : 1309.3272v1 .