Astrosphere

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The "bubble nebula" NGC 7635 is an example of the stellar wind bubble of a star of the spectral class O. The massive star ( BD + 60 2522 ) has displaced the interstellar matter from its surroundings with its strong stellar wind and makes it glow as an emission nebula . The star is not in the center, but in the top left of this picture inside the bubble. The structure is about 7 light years across.
At the astropause of the star LL  Orionis , a bow shock wave indicates the collision of its astrosphere with the comparatively dense interstellar matter of the Orion Nebula .
Image and illustration of the bow shock wave of the star R Hydrae , which is only visible in the infrared

An astrosphere is a bubble-shaped structure in space around a star that is created and filled by its stellar wind .

The size of an astrosphere is very dependent on the type of star and can also be very variable. The sun's astrosphere , called the heliosphere , is currently about 120 astronomical units (AU) in size  . Its limit is thus far outside the known planetary orbits ( Neptune's orbit is about 30 AU from the sun). The astrospheres massive stars of spectral types  O and B whose stellar wind speeds in the range of several thousand km / s, more are light years in size and stellar wind bubbles called ( English for "stellar wind bubbles").

root cause

Even if there are very low particle densities in space , these areas of space still have gas pressure . The pressure of a stellar wind is stronger in the vicinity of the star than the interstellar medium and can displace it without difficulty. With increasing distance from the star, the speed of the stellar wind remains roughly constant, but its density and thus its pressure decrease. At the border of a certain envelope there is a balance of forces between the stellar wind and the interstellar medium. The volume of space up to this limit is the astrosphere or the stellar-wind bubble .

Star winds and interstellar media are plasmas with high proportions of electrically charged components. Therefore, their flow behavior is influenced by magnetic fields and also influence magnetic fields in turn. With the stellar wind, the star's magnetic field is carried far out into space and interacts at the boundary of the astrosphere with external, e.g. B. galactic, magnetic fields.

meaning

Due to its displacement and deflection of the interstellar medium, the astrosphere has a protective effect for a planetary system belonging to the star . Ionized (i.e., charged) particles of the interstellar medium are almost completely deflected around the astrosphere. Only particles that are neutral (non-ionized) and highly energetic can penetrate deep into the astrosphere from outside. This happens for example when hydrogen - ion of the interstellar medium to capture an electron . Even interstellar dust can penetrate an astro sphere.

Astrospheres are by no means rigid and immutable, but they are dependent on both the variable stellar wind and the variable surrounding medium. In the case of the solar system, the comparatively short-term fluctuations in the solar wind , such as those in the approximately 11-year sunspot cycle , only have a relatively minor impact on the heliosphere. On the other hand, as physical models show, changes in the density of the interstellar medium through which the sun moves as it circles the center of the Milky Way can exert a great influence on the shape and size of the heliosphere over geological time periods. For about 100,000 years the Sun has been moving through the interstellar medium of the Local Flake , a denser region within a larger, less dense region. Within geological periods, the sun comes through very different regions. This results in fluctuations in cosmic radiation , which may have had an impact on the Earth's climate history .

Limits

Astrospheres are surrounded by two bounding envelopes:

  • The inner limit is the termination shock ("edge shock wave"). At this shock front , the particles of the stellar wind are abruptly decelerated from supersonic speed (supersonic) to subsonic speed (subsonic) by the interaction with the interstellar gas . The plasma condenses and heats up to temperatures of around 10 6 Kelvin .
  • This compressed, slower plasma mixes outside the termination Shocks in a zone Astro shell (engl. Astro Heath ) is mentioned, with the interstellar matter.
  • The outer boundary layer, at which the mixing of the particles of the stellar wind with the particles of interstellar matter is complete, is called the astropause . Behind it, by definition, begins interstellar space .

The termination shock of a star like the sun is roughly spherical. The Astro casing is on the other hand due to the movement of the star to the surrounding medium by the back pressure significantly deformed. According to models, it generally has an elongated, comet-like shape. It has a blunt end in the direction of movement and a long tail in the opposite direction. Depending on the relative speed of the star to the surrounding interstellar medium, two cases are distinguished:

  • If the relative speed of the star is supersonic the surrounding interstellar medium, a second shock front that occurs in the moving direction, the bow shock ( bow shock ). It is, so to speak, the counterpart to Termination Shock, but seen from the outside: here the interstellar medium is abruptly compressed and slowed down to subsonic speed. If there is a bow shock wave, the astronomical envelope is divided into two areas:
    • the inner astro shell is the zone between the termination shock and the astropause
    • the outer astronomical envelope is the zone between the astropause and the bow shock wave.
  • If the relative speed of the star to the surrounding interstellar medium is subsonic, there is no bow shock wave and no outer astronomical envelope, but only an (inner) astronomical envelope. It can, however, still a bow wave ( bow wave type).

While astrospheres are invisible per se, bow shock waves can be observed astronomically if they have a sufficient particle density. The compression of the interstellar matter heats it up, which can be detected by infrared telescopes. When streams of matter hit a star, the bow shock wave can also be observed in visible light.

Heliosphere

The astrosphere around the sun is called the heliosphere . Analogous uses the terms heliopause and Helio shell ( heliosheath ). The NASA - probe Voyager 1 reached the termination shock at about 94 AU distance from the Sun and the heliopause at 121.7  AE . Voyager 2 reached the termination shock at 84 AU and the heliopause at 119.0 AU. Contrary to earlier assumptions, there does not seem to be a bow shock wave in the solar system.

Super bubbles

Even larger bubbles that are hundreds of light years may have in diameter, called Super Bubbles ( super bubbles ). They are not only created by stellar winds, but also by supernovae :

In OB associations , stars of the spectral classes O and B are so close together that their stellar wind bubbles combine to form a super bubble. Larger super-bubbles are created by the explosion pressure of supernovae: since stars in OB associations have short life spans, most of their supernovae occur within the common bubble. They do not form supernova remnants , but their energy is converted into sound waves that inflate the bubble many times over.

The sun is currently passing through such a super-bubble, the local bubble .

literature

  • Henry J. Lamersm Joseph P. Cassinelli: Introduction to stellar winds. Cambridge University Press, 1999, ISBN 0-521-59398-0 . (English)
  • K. Scherer among others: Cosmic rays in astrospheres. In: Astron. Astrophys. Volume 576, April 2015, p. A97. (English)

Web links

  • Astrospheres (website in English on the topic of astrospheres)

Individual evidence

  1. ↑ Ashley Morrow: Hubble Sees a Star 'Inflating' a Giant Bubble . In: NASA . April 21, 2016 ( nasa.gov ).
  2. ↑ There are, however, trans-Neptunian objects whose orbits extend beyond the heliopause, and the Oort cloud (which has not yet been detected) is much further away .
  3. ↑ Bruce T. Draine: Physics of the Interstellar and Intergalactic Medium . Princeton University Press, 2010, ISBN 1-4008-3908-4 , pp. 422 ff . ( books.google.de ).
  4. ↑ NASA Administrator: NASA - Did You Know ... In: NASA . June 6, 2013 ( nasa-usa.de ).
  5. ↑ a b Klaus Scherer, Horst Fichtner, Hans-Jörg Fahr, Eckart Marsch: Astrophysics: The heliosphere - protective shield for the earth: The particle density in space influences the expansion of the solar atmosphere . In: Physics Journal . tape 57 , no. 4 , April 1, 2001, p. 55-58 , doi : 10.1002 / phbl.20010570415 .
  6. ↑ Stefan Frech, Susana Frech: Technical dictionary astronomy English-German. 1st edition. Books on Demand, Norderstedt 2011, ISBN 978-3-00-050182-1 .
  7. ^ J. Castor, R. McCray, R. Weaver: Interstellar Bubbles . In: Astrophysical Journal Letters . tape 200 , 1975, pp. L107 – L110 , doi : 10.1086 / 181908 , bibcode : 1975ApJ ... 200L.107C .
  8. ↑ RS Steinolfson: Termination shock response to large-scale solar wind fluctuations . January 1, 1994.
  9. ↑ Klaus Scherer: Astrospheres. In: rub.de. astrospheres.tp4.rub.de, accessed on November 8, 2016 .
  10. ↑ NASA Administrator: NASA - Did You Know ... In: NASA . June 6, 2013 ( nasa-usa.de ).
  11. ↑ Benjamin Knispel: Heliosphere. The discovery of slowness. In: ASTROnews. May 11, 2012, Retrieved May 14, 2012.
  12. ↑ New Interstellar Boundary Explorer data show heliosphere's long-theorized bow shock does not exist. Phys org., May 12, 2012, accessed October 1, 2017 .
  13. ↑ Kohji Tomisaka, Satoru Ikeuchi: Evolution of superbubble driven by sequential supernova explosions in a plane-stratified gas distribution . In: Publications of the Astronomical Society of Japan . tape 38 , January 1, 1986, pp. 697-715 , bibcode : 1986PASJ ... 38..697T .
  14. ^ Mordecai-Mark Mac Low, Richard McCray: Superbubbles in disk galaxies . In: The Astrophysical Journal . tape 324 , January 1, 1988, doi : 10.1086 / 165936 , bibcode : 1988ApJ ... 324..776M .