Circular. You first assume they are that far away to get how big they are, what gravity is expected.. etc etc.The important point is that the model works - it correctly predicts what we would expect to see if those things were very far away and behaved according to the same physical laws that apply here.
Give an example.Our observations verify that these apparently very distant things behave in every way as we would expect if they really were very distant things.
Here is one star for example..
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Kepler observed the star KIC 8462852 for four years starting in 2009. Typically, orbiting planets only dim the light of their host star for a period of a few hours to a few days depending on their orbit. A group of citizen scientists noticed that this star appeared to have two small dips in 2009, followed by a large dip lasting almost a week in 2011, and finally a series of multiple dips significantly dimming the star’s light in 2013.
Tabetha Boyajian, a postdoc at Yale, told The Atlantic: “We’d never seen anything like this star. It was really weird. We thought it might be bad data or movement on the spacecraft, but everything checked out.”
The pattern of dips indicates that the star is orbited by a large, irregular-shaped mass. If it were orbiting a young star, this mass might be a protoplanetary disc, but KIC 8462852 is not a young star. We would also expect to see the presence of dust emitting infrared light, which hasn’t been observed. So what is this orbiting mass? Scientists predict that whatever it is, it had to have formed recently as it would have been pulled in by the star’s gravity and consumed.
Boyajian recently published a paper offering several possible explanations for the bizarre transits. The leading theory is that a family of exocomets passed too close to the star, and were shredded into pieces by the star’s massive gravity. The remaining dust and debris could be left to orbit the star."
Astronomers Have Spotted Something Very, Very Strange Surrounding A Distant Star
Possible explanations all picked from the fishbowl.
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