• Question: What is the difference between orbiting a star and orbiting a black hole with the same mass? Will they lead to time delays?

    Asked by Harry to Daniel, Hannah, Maggie, Ry, Scott on 13 Nov 2017.
    • Photo: Ry Cutter

      Ry Cutter answered on 13 Nov 2017:

      Fantastic question!
      There is no difference! The only thing that determines orbits is the mass of the thing you’re orbiting.
      But there would be no light with a black hole so the system would be much much colder.

      Ryan

    • Photo: Maggie Lieu

      Maggie Lieu answered on 13 Nov 2017:

      A star that has the same mass as a black hole would also be a black hole. This is because what makes a black hole a black hole is that it’s mass is so high that its gravity is strong enough to prevent any light from escaping it. So orbiting them would be exactly the same and yes the gravity is strong enough to cause time delays

    • Photo: Scott Melville

      Scott Melville answered on 13 Nov 2017:

      The precise answer to this depends on how that same mass is distributed (I think!).
      If you pile it all in one place, then Maggie is right, it’ll always be a black hole. But if you spread it out very thin, it might look something like a star.
      So how can we tell the difference (besides, you know, just looking at it :P)? Ryan’s right that if you’re just sitting at one point nearby, you actually would feel the same gravity (it’s called Gauss’s law).
      But if you had some width (if you were sitting at a few different points), then you could probably tell the difference because of tidal forces – some of your points would be pulled a little bit differently to the other points.
      Regardless, the strong gravity will indeed make time behave a bit funny – a few seconds for you near the mass will feel much longer for someone far away from any masses.

    • Photo: Hannah Middleton

      Hannah Middleton answered on 13 Nov 2017:

      If the Sun turned into a black hole, then the Earth would keep orbiting on the same path as normal.
      .
      The Sun is about 1.4 million km across. To turn it into a black hole, you would have to compress it’s mass down into a diameter of just 6km across – about the size of a town.
      .
      Although the Earth is far enough away that it will keep orbiting (albeit in the cold and dark), some strange things happen if you have an object orbiting very close to the black hole. Around a black hole there is a distance called the “Innermost Stable Circular Orbit” or ISCO. This is the point where an object orbiting will start to plummet towards the black hole heading for the event horizon (the point of no return). For a black hole with the same mass as our Sun, this would be at about 6km above the event horizon of the black hole. The ISCO is something that only really applies to black holes, as for our Sun as it normally this position lies well inside the surface of the Sun – an object would burn up well before reaching there!
      .
      As for time delays, at the Earth the effect will be the same as with the normal Sun. But as you can now get much closer to the black hole than you can get to the Sun, then time will pass much slower there – this phenomena is known as time dilation.

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