[chris (OP)]

Donald asks:

Jupiter could become a radiating star if it becomes six to ten times larger. How bright would it be?  Could it be seen during the day? At night, could we read by it? What wavelength of light? How many lumens? Can you speculate on nocturnal species, global warming, changes in civilization.

What do you think?

[evan_au]

Compare Proxima Centauri, the nearest star to Earth:
- Proxima is a red dwarf star, 1/8 the mass of the Sun, and puts out 0.005% of the visible light of the Sun (it is much cooler than the Sun, and puts out almost all of its energy at invisible infra-red wavelengths, plus frequent flares that release lots of X-rays).
- If Proxima Centauri were as far away as the Sun-Earth distance, we would see it as a small patch of dim red in the sky, 1/7 the diameter of the Sun, and 1/50 the area.
- Jupiter is 1/1050 the mass of the Sun, so you would need to make it more than 100 times the mass to match Proxima Centauri
- A "sub-brown dwarf" like a 10x Jupiter would not sustain any hydrogen fusion, and would mainly be releasing heat from its original gravitational collapse. If it were as old as our Solar System, it would not put out any visible light from thermal sources (it could still sustain things like lightning and auroras). You would not see it in daylight (when the Sun is out) or at night (when the Moon is out).
- For sizes larger than 13xJupiter, some fusion would occur, but the rate of hydrogen fusion in stars increases as something like the cube of the mass. So it needs to be a lot larger than this to allow you to read by its light.

See: https://en.wikipedia.org/wiki/Brown_dwarf