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My assumption is that stars have significantly larger volume than black holes. And if they spiraled they would would merge before they had the chance to reach high enough velocities to generate gravitational waves detectable by the current LIGO.

Is this the main reason why we know LIGO detected black holes?

Another idea I had was mass. But if black holes can have masses like 30 solar mass then the stars by which they formed would be at least so massive if not more. Right?

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marked as duplicate by Yashas, Kyle Kanos, Jon Custer, SRS, peterh Jul 20 '17 at 15:53

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Have a look at Is it possible to produce gravitational waves artificially? for some info on how the power radiated from merging masses is calculated.

The power is proportional to the quadrupole moment squared and the angular velocity to the sixth power, and the angular velocity is the most important factor because it's raised to such a high power. In the detections so far the period of the orbit was in the tens of milliseconds. Tens of milliseconds! There is simply no way to get anything bigger than black holes to orbit at such a high frequency. Stars, even neutron stars, would merge long before the orbital frequency got that high.

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    $\begingroup$ So in conclusion. Both of the mass and the small volume combined are what separate BH GW from star GW or NS GW. Because to get high enough angular velocity you need to both be very close (small volume) and great gravitational Attraction (high enough mass). $\endgroup$ – Bill Jul 19 '17 at 8:19
  • $\begingroup$ @Bill: yes, that's basically it. $\endgroup$ – John Rennie Jul 19 '17 at 8:30
  • $\begingroup$ As you stress angular velocity (a very nice point) do you know if we expect merging black holes to orbit (maybe spiral is a better word) around each other prior to the merge ? $\endgroup$ – StephenG Jul 19 '17 at 9:05
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    $\begingroup$ @StephenG Yes. If two black holes are in a hyperbolic orbit it is exceedingly unlikely they would collide as the collision cross section is so small. Realistically we only only get mergers if the two black holes are in a bound orbit and the orbital energy can be radiated away as gravitational waves. $\endgroup$ – John Rennie Jul 19 '17 at 9:21
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An other important point is the distance of the two objects before the coalescence. From the form of the signal you can calculate that the final merging distance was really small (only a few Schwarzschild radii of the involved masses). Star systems and even BH-star systems would not be able to come so close without merging, only BHs can do that.

See LIGO explenation.

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  • $\begingroup$ So my initial assumption has some valid points, right? $\endgroup$ – Bill Jul 19 '17 at 7:42
  • $\begingroup$ It has to do with the properties of BHs (Schwarzschild horizont) and not so much with the volume. But even two neutron stars merging or neutron star-BH merging might be possible to observe with LIGO if they are just close enough. $\endgroup$ – Densch Jul 19 '17 at 7:51
  • $\begingroup$ How close the two objects are before merging depends only how big of a Schwarzschild radius they wave (what I meant with volume) and for stars the distance form the core to the surface. Neutron stars have below 3 SM mass so it's easy to identify them. $\endgroup$ – Bill Jul 19 '17 at 7:56
  • $\begingroup$ Yes, it is not volume but the volume/mass ratio to achieve high velocities before merging. And of course the total mass involved is important for a high GW strain (amplitude), as Robert Poenaru pointed out in his answer. $\endgroup$ – Densch Jul 19 '17 at 8:05
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When you say that stars have significantly larger volume that the black holes you must be careful. There are black holes which have volumes bigger than most of the stars (e.g. supermassive black holes, like the one which "lives" in the center of our galaxy).

The key thing of generating gravitational waves is not the volume, but the mass. So, as you already assumed, the answer is indeed the reason of huge masses involved in the merging process.

Also, the stars which created a bh with, say 30 S.M. (solar masses) don't need to have also 30 S.M. The star can have 15, then when it collapses and creates the bh, the bh could start absorbing matter from the cosmic neighborhood, so it will generally become bigger and heavier.

I hope this helps :)

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  • $\begingroup$ So if LIGO tommorow would announce that another set of gravitational waves were detected but this time with 4 and 7 SM BH. How would they know wether they were black holes or stars? $\endgroup$ – Bill Jul 19 '17 at 7:39
  • $\begingroup$ @Bill They know because of the signal waveform. It would be different for stars than for black holes: different frequency, different time evolution / ringdown, amplitude, etc. $\endgroup$ – mpv Jul 19 '17 at 7:42
  • $\begingroup$ Bill, even a binary system of stars can create gravitational waves. Technically, g.waves can be generated if the orbit of the two masses would "suffer" change in the quadrupole moment. But in order for LIGO to detect waves from a binary system of stars, this system should be very close to the earth. $\endgroup$ – Robert Poenaru Jul 19 '17 at 7:59

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