page_title: AB7
section_title: Properties
hierarchical_section_title: AB7 / The stars / Properties
caption_attribution_description: English: This picture of the star cluster and surrounding nebula NGC 371 was taken using the FORS1 instrument on ESO’s Very Large Telescope, at the Paranal Observatory in Chile. NGC 371 lies in the Small Magellanic Cloud, one of the closest galaxies to the Milky Way. The vivid red cloud in this new image from ESO’s Very Large Telescope is a region of glowing hydrogen surrounding the star cluster NGC 371. This stellar nursery lies in our neighbouring galaxy, the Small Magellanic Cloud. The object dominating this image may resemble a pool of spilled blood, but rather than being associated with death, such regions of ionised hydrogen — known as HII regions — are sites of creation with high rates of recent star birth. NGC 371 is an example of this; it is an open cluster surrounded by a nebula. The stars in open clusters all originate from the same diffuse HII region, and over time the majority of the hydrogen is used up by star formation, leaving behind a shell of hydrogen such as the one in this image, along with a cluster of hot young stars. The host galaxy to NGC 371, the Small Magellanic Cloud, is a dwarf galaxy a mere 200 000 light-years away, which makes it one of the closest galaxies to the Milky Way. In addition, the Small Magellanic Cloud contains stars at all stages of their evolution; from the highly luminous young stars found in NGC 371 to supernova remnants of dead stars. These energetic youngsters emit copious amounts of ultraviolet radiation causing surrounding gas, such as leftover hydrogen from their parent nebula, to light up with a colourful glow that extends for hundreds of light-years in every direction. The phenomenon is depicted beautifully in this image, taken using the FORS1 instrument on ESO’s Very Large Telescope (VLT). Open clusters are by no means rare; there are numerous fine examples in our own Milky Way. However, NGC 371 is of particular interest due to the unexpectedly large number of variable stars it contains. These are stars that change in brightness over time. A particularly interesting type of variable star, known as slowly pulsating B stars, can also be used to study the interior of stars through asteroseismology [1], and several of these have been confirmed in this cluster. Variable stars play a pivotal role in astronomy: some types are invaluable for determining distances to far-off galaxies and the age of the Universe. The data for this image were selected from the ESO archive by Manu Mejias as part of the Hidden Treasures competition [2]. Three of Manu’s images made the top twenty; his picture of NGC 371 was ranked sixth in the competition. Notes [1] Asteroseismology is the study of the internal structure of pulsating stars by looking at the different frequencies at which they oscillate. This is a similar approach to the study of the structure of the Earth by looking at earthquakes and how their oscillations travel through the interior of the planet. [2] ESO’s Hidden Treasures 2010 competition gave amateur astronomers the opportunity to search through ESO’s vast archives of astronomical data, hoping to find a well-hidden gem that needed polishing by the entrants. Participants submitted nearly 100 entries and ten skilled people were awarded some extremely attractive prizes, including an all expenses paid trip for the overall winner to ESO’s Very Large Telescope (VLT) on Cerro Paranal, in Chile, the world’s most advanced optical telescope. The ten winners submitted a total of 20 images that were ranked as the highest entries in the competition out of the near 100 images. Colours &amp; filters Band Telescope Optical HeII Very Large Telescope FORS1 Optical HeI Very Large Telescope FORS1 Optical H-alpha Very Large Telescope FORS1 .
context_page_description: AB7, also known as SMC WR7, is a binary star in the Small Magellanic Cloud. A Wolf–Rayet star and a supergiant companion of spectral type O orbit in a period of 19.56 days. The system is surrounded by a ring-shaped nebula known as a bubble nebula.
context_section_description: The total visual brightness of AB7 can be determined fairly accurately at absolute magnitude (MV) −6.1, 23,500 times brighter than the sun. The components cannot be observed separately and the contribution from each component can only be estimated. The O star dominates the visual spectrum and produces around 70% of the brightness, leading to MV −5.7, and −4.4 for the primary.
  The temperature of a star can be determined in several different ways: from the spectral type; directly from atmospheric models; and from the ionising effects of its radiation. Accurate calibrations are available for the temperatures of class-O stars, although these are slightly different for SMC metallicity and for stars of different luminosity classes.  The temperatures for WR spectral classes are less precisely defined, especially for the SMC and especially for the hottest classes. AB7 completely ionises the surrounding interstellar material to a distance of 20 parsecs and this can be used to derive the temperature and luminosity of the ionising star. This level of ionisation cannot be achieved by an O6 star, so will be almost entirely due to the WR component. Unfortunately, the ionisation is beyond what would be caused by the hottest model, a 120,000K star.  An earlier attempt at the same calculation gave a blackbody temperature of 80,000K.  The temperatures can be calculated directly by modelling the atmospheres of both stars to reproduce the observed spectrum in detail. This method results in a temperature of 106,000 K for the WR component and 36,000 K for the O companion.  The effective temperature is useful for modelling the atmosphere and comparison between stars, but a typical "observed" temperature at optical depth 2/3 can be significantly different for stars with a dense stellar wind.  In the case of the WR primary star, the optical depth temperature is 96,000 K.
  The simplest way to measure the luminosity of a star is to observe its radiated output at all wavelengths (the spectral energy distribution or SED) and sum them together. Unfortunately this is impractical for AB7 because the majority of the radiation occurs in the far ultraviolet. A more common method is to measure the visual luminosity and apply a bolometric correction to give the total luminosity at all wavelengths, although the size of the bolometric correction is extremely sensitive to the effective temperature. Following this method gives a luminosity of 1,270,000 L☉ for the primary. The luminosity can also be derived from the observed levels of ionisation. Assuming the older temperature of 80,000K gives 1,000,000 L☉. Modelling the atmospheres gives luminosities for the WR and O component of over 1,000,000 L☉ and 316,000 L☉ respectively.
  The radius of a star with strong stellar wind is poorly-defined since any strong density discontinuity that might be defined as a surface is entirely hidden from view. Commonly used definitions of the radius in such cases include: a temperature radius; an optical depth radius; and a transformed radius. The differences are only significant in the case of the WR component. The temperature radius is the radius of a uniform disc that would produce the known luminosity at the calculated effective temperature, and is 3.4 R☉. The radius at optical depth 2/3 is 4.0 R☉. The transformed radius is a value used in the modelling of the atmosphere and is 5.6 M☉. The O-component radius is 14-15 R☉.
  The masses of each component in the AB7 system can be determined from the binary orbit. The minimum masses are found to be 18 M☉ and 34 M☉ respectively for the primary and secondary. With the assumption of an inclination of 60°, the actual masses are 28 M☉ and 54 M☉.  The secondary is more massive and visually brighter, but not more luminous.
  Both components of AB7 have powerful stellar winds and are losing mass rapidly. Wind speeds of 1,700 km/s for the primary and 1,500 km/s for the secondary are calculated, with mass loss from the primary a billion times higher than the sun, and 100 million times for the secondary star.  The WR wind is sufficiently dense tha
caption: N76 in Hα (red), HeI (green), and HeII (blue)
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