eso8703 — Komunikat naukowy
Bubbles from a Dying Star
26 lutego 1987
Stars do not live forever - they are born and they die. All stars are born by contraction of matter in large interstellar clouds of gas and dust, but they do not all die in the same way. Heavy stars appear to end their active lives in gigantic supernova explosions (eso8608) while less massive stars (like our Sun) follow another path.
For instance, once the Sun - perhaps 5 billion years from now - has exhausted its nuclear fuel, it will increase more than a hundredfold in size and become a red giant star. Thereafter it is expected to very rapidly shred its outer layers, leaving only a shrunken core behind. This core is a white dwarf star, that is a very small and dense object which slowly cools and ultimately fades away from sight as it loses the little energy left.
Although this scenario appears plausible to most astronomers, nobody had ever directly witnessed how a solar-like star, towards the end of its life, undergoes the dramatic transformation from a red giant star to a white dwarf.
Now, however, observations at the European Southern Observatory for the first time show a dying star at this crucial evolutionary stage.
Using four different telescopes at the La Silla observatory, ESO staff astronomer Bo Reipurth has just concluded a detailed study of the peculiar star OH231.8+4.2 which is visible as a faint object in the southern constellation of Puppis, inside the Milky Way band. The distance is about 4000 light years. This object is known to have rather strong radio emission from OH molecules (hydroxyle) and it is also a source of infrared radiation, as observed with the satellite IRAS. With the Danish 1.5 m telescope, Reipurth obtained deep CCD (Charged-Coupled Device) pictures of OH231.8+4.2 in the red light of hydrogen, nitrogen and sulphur and discovered a most unusual, complicated structure (see attached photograph). The interpretation of this picture, supported by spectroscopic and infrared observations with the ESO 3.6 m, 2.2 m and 1 m telescopes, now shows that we are here watching the rapid transformation of a red giant star at the end of its active life.
The star itself cannot be seen on the picture; it is embedded in and obscured by dense matter. The bubble nebula (shaped like a figure 8 or an hourglass) consists of matter which has been ejected from the star. The dark band around the `waist' is the shadow of an extremely dense disk of dust particles, rotating around the central star. The matter in the nebula has been ejected in directions perpendicular to the disk. The light received from the innermost part of the nebula is reflected light from the star, but the light from the outer part (the `bubbles') is emitted from `shocked' regions where the rapidly moving stellar matter rams into the surrounding interstellar gas.
It is obvious that the star is losing mass very fast. Since the size of the southern part of the nebula is about 50000 Astronomical Units (0.8 light years) and the outward velocity is about 140 km/sec, the ejection must have started very recently (in astronomical terms). Together with similar data for the northern part of the nebula, this leads to an estimated age of only 1400 years. At the end of this short-lived process, the star which was originally rather similar to our Sun, will have lost much of its material and will have been transformed into a very small and dense object.
Since normal stars live billions of years and the mass-loss phase only lasts a few thousand years, our chances of observing a dying star in this phase are very slim indeed; this readily explains why such observations have never been done before. The present observation is therefore of the greatest importance for our understanding of the late stages of stellar evolution; it constitutes a long-sought missing link.
When OH231.8+4.2 sometime in the near future has shred a large part of its mass, the surrounding nebula becomes less and less dense as it slowly expands. Soon the remaining, inner parts of the star (the core) become visible as a white dwarf star. The entire object will then appear as a planetary nebula, consisting of a diffuse mass of gas that glows because of the irradiation by ultraviolet light from the white dwarf star at the centre. Much later, when the surrounding gas has dispersed into space, only the white dwarf will be left as a ghostly memorial to the once so brilliant star.
An article, which describes the details of this discovery, is appearing in the February 26 issue of the scientific journal Nature.
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