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They say that the system can be seen with the unaided eye. If I were to point my telescope directly at the black hole, what would I see? Nothing? Or a lensing e

• exhilaration 6y ago
  Based on the article it's completely invisible, they only deduced it's there because of the orbit of the two companion stars.
• ebg13 6y ago
  They say that the stars orbiting it can be seen but the hole itself is hidden. I'm curious why there's no lensing, though. Or is it just that the size makes the lensing weak enough to not be noticable?
  • Analemma_ 6y ago
    They say it has a mass of about 4 suns, which translates to an event horizon diameter of about 15 miles, so I think the lensing would be imperceptible unless you were right up close to it or it happened to perfectly occlude a visible star right when you were watching.
  • hannasanarion 6y ago
    It is very hard to observe lensing around a small object. A light source needs to pass directly behind it, and stay behind it for long enough that your camera exposure can capture it (which for distant objects, can take days or months or years). That's why gravitational lensing has only been observed for 1. very bright stars behind the Sun (event that lasts a second detectable with sub-second exposure) 2. Galaxies behind galaxies (event that lasts millions of years detectable with hours-long exposure) 3. an SMBH warping the light generated by its own disk (permanent event detectable with years-long exposure)
    • deleted 6y ago
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    • antognini 6y ago
      Those are not the only ways that gravitational lensing has been observed. Gravitational lensing has also been detected for stars in the Galaxy lensing background stars further away in the Galaxy. While the shift in position can't be observed, the lens increases the apparent brightness of the background star as the two come into alignment over the course of a few hours or so. This phenomenon is known as microlensing [1] and it's one of the ways that exoplanets have been discovered because the exoplanets also lens the background star and produce a characteristic spike on top of the light curve. That said, microlensing is sort of statistical by nature. It's very unlikely that any particular object will be aligned with a background object just right to produce noticeable microlensing. But with a large field of stars and enough observations it becomes possible to observe a microlensing event every now and again. [1]: https://en.wikipedia.org/wiki/Gravitational_microlensing https://en.wikipedia.org/wiki/Gravitational_microlensing
    • nytgop77 6y ago
      If lensing is so hard to observe, why is lack of lensing considered enough to reject "dark matter==black holes"?
      • hannasanarion 6y ago
        Because if the galactic halo was so full of black holes that it accounted for all the dark matter in the galaxy, then lensing would be extremely easy to detect. If the MACHO hypothesis was correct, there would be lensing everywhere you look because the whole galaxy is totally surrounded by enormous black holes. A lensing effect could be constantly seen viewing any object outside of the galaxy, because there's guaranteed to be a black hole in front of it. It would be difficult to get a good picture of any but the closest galaxies because of all the lensing artifacts from the black holes all over the place. Lensing around black holes is hard to detect precisely because black holes are so rare and don't frequently cross in front of very bright objects.
• pjdorrell 6y ago
  You won't see the black hole in your telescope, because it's black, and there isn't any stuff currently falling into it (which is one way that black holes can make themselves very visible). Also, using "your telescope", you won't see the orbits of the black hole's companions with sufficient precision to deduce that the black hole exists and that it's a black hole.