A “Gravity Star” is a huge, relatively low temperature star, whose radiance is fueled not by nuclear fusion, but by the compressive heating of the star’s matter by gravity. In the days before nuclear fusion was understood, Lord Kelvin theorized that the heat caused by such compression could account for the Sun’s energy. Later, the rather oddball physicist Osborne Reynold theorized that such stars could arise by accretion of mater in a dense cloud of gas and debris-as exists in the center of certain galaxies. A Gravity star would be just below the mass limit that would cause it to collapse into a “black hole”.
Has anyone ever detected them?
If it’s just short of being a black hole, what prevents fusion from taking place?
I guess if it was made of heavier elements, fusion might not happen, but isn’t that then the point at which things go boom?
I think you need to establish that they can exist in the first place.
Brown dwarfs have been observed and fit your definition (gravitational heating without fusion) - there is one 6.5 light years from earth. However, any bigger than 75 to 80 Jupiter masses and fusion starts to occur, and it becomes a main sequence star. Nothing bigger than that can avoid fusion (unless it is made up of high-mass elements, when other effect occur).
We like the stars, the stars that go boom.
They are hot because we fuse them.
Do tell! I mean, what about a solar-mass object comprised almost entirely of iron or lead?
Reynold also did his theorizing before nuclear fusion was understood. The initial heat in any star is caused by gravity, but once the core reaches a sufficient temperature for fusion that’s what powers the star.
So proto-stars could be considered to be gravity stars, as well as the aforementioned brown dwarfs: Pre-main-sequence star - Wikipedia
what about jupiter? is that not a gravity star, heating up its insides from the intense pressure
How many zeppelins is that?
Well, since a solar mass is below the Chandrasekhar limit (about 1.44 solar masses), it isn’t going to collapse into a neutron star (i.e the mass isn’t sufficient to overcome the electron degeneracy pressure). It will get hot due to compression, then slowly cool down again.
The point is that as the dominant atomic mass increases in a star, less energy is released by fusion, until Fe-Fe, when fusion becomes endothermic (requiring energy input). At that point, the mass (and possibly the spin) of the star determines it’s future.
If you started with a cool nebula primarily consisting of heavy elements - Fe and heavier (maybe all the light elements have been blown off by a nearby supernova) that collapsed, it would not form a protostar with H-H fusion. It would just collapse to form a central mass heated by compression. Then the mass (and possibly the spin) of the star determines it’s future, as to whether it exceeds the Chandrasekhar limit to collapse into a neutron star, black hole or core-collapse supernova.
Thanks for the replies…could a “Brown Dwarf” have habitable planets orbiting it?
Maybe, but they won’t be Earth-like habitable planets because you won’t have that kind of solar radiation. You’re more likely to have planets that have chances for life like Europa and Titan do, maybe relying on heat generated through tidal forces on the planet’s crust rather than on the sun.