Starship Asterisk*

gvann wrote: ↑Wed Jan 25, 2023 3:39 am

johnnydeep wrote: ↑Tue Jan 24, 2023 10:33 pm The Wikipedia article about red dwarfs has this to say:

https://en.wikipedia.org/wiki/Red_dwarf wrote:Because low-mass red dwarfs are fully convective, helium does not accumulate at the core, and compared to larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence.

Why does helium NOT building up in the core prolong the period of hydrogen fusion?

The Wikipedia page for stellar evolution gives more details about what happens: In a low-mass star, since convection continually brings hydrogen down into the core, where pressure and temperature are highest, hydrogen can continue to fuse and generate energy.

In contrast, a larger star will accumulate only helium (which is heavier than hydrogen) in its core, and hydrogen fusion will continue in a shell above the surface of the helium core. At some point, pressure and temperature in the accumulating helium core will become large enough to start helium fusion. That's when the star evolves into the red-giant stage and hydrogen fusion stops.

Ann wrote: ↑Wed Jan 25, 2023 4:48 am

Avalon wrote: ↑Wed Jan 25, 2023 3:41 am What is the explanation for the apparent red disc belting the star in the AI generated rendering?

The illustration was made by AI. AI learns by copying. While searching for space images to be inspired by, AI undoubtedly came across pictures of Saturn, among them certainly Chesley Bonestell's View of Saturn from Titan.

After seeing Chesley Bonestell's Saturn picture and other Saturn images, AI probably "thought" that it was a good idea to portray red dwarf star LHS 475 as a reddish orb with a disk crossing its face, in the same way that the rings of Saturn can be seen crossing the face of Saturn.

Ann

Avalon wrote: ↑Wed Jan 25, 2023 3:41 am What is the explanation for the apparent red disc belting the star in the AI generated rendering?

johnnydeep wrote: ↑Tue Jan 24, 2023 10:33 pm The Wikipedia article about red dwarfs has this to say:

https://en.wikipedia.org/wiki/Red_dwarf wrote:Because low-mass red dwarfs are fully convective, helium does not accumulate at the core, and compared to larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence.

Why does helium NOT building up in the core prolong the period of hydrogen fusion?

https://en.wikipedia.org/wiki/Red_dwarf wrote:Because low-mass red dwarfs are fully convective, helium does not accumulate at the core, and compared to larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence.

Cousin Ricky wrote: ↑Tue Jan 24, 2023 4:27 pm Do AIs know that “red” is relative, and that red dwarfs are more or less the color of incandescent light bulbs? Well, I guess not, if they’re learning from human artists. I have seen very, very few red or yellow stars depicted realistically by human space illustrators.

I understand that some artistic license is required to cope with the dynamic range (stars are very bright!), but space illustrators don’t even seem to be working from a realistic baseline.

Chris Peterson wrote: ↑Tue Jan 24, 2023 3:36 pm
How large a star or moon appears in an image is entirely a product of the focal length of the imaging system and the relative distances between foreground objects. Without knowing that there is no way of knowing how this scene would look to our eyes.

rstevenson wrote: ↑Tue Jan 24, 2023 12:09 pm When I saw this image a few minutes ago, my initial reaction was that the star was much too small in the sky.

Our Sun appears to be about 1/2° in diameter, if memory serves. We are, by definition, 1 AU away from our Sun which is 1 M☉ in radius. The equivalent figures for this exoplanet are 0.0206 AU and 0.2789±0.0014 R☉. A more knowledgeable person than I can probably use those figures to calculate how many degrees wide the star would be in the sky of the exoplanet. I gave it a try, taking the inverse of each of those figures (since I am comparing them to 1 AU and 1 M☉) and multiplying by 1/2°, …
(1/0.0206) (1/0.2789) (1/2°) = 1.52°
If that’s the right way to do it, then the star should appear to be about 3 times the size of our Sun, given an equivalent viewing angle on each planet. So now I think the AI illustrated the star much too large, as opposed to my initial reaction. But I’ve no idea if that calculation I just did is correct. Can someone check that for me, please?
—————- a few minutes pass —————
ACK! Found a mistake already. The answer 1.52 is in radians, not degrees. It should be 87°, according to Wolfram Alpha (which knows a great deal more than I about how to calculate.) Of course, garbage in, garbage out, so it remains to see if what I calculated makes sense. If it does, then the star should be much larger in the sky.

Rob

RJN wrote: ↑Tue Jan 24, 2023 2:20 pm

rstevenson wrote: ↑Tue Jan 24, 2023 12:09 pm It should be 87°

Thanks, Rob! Interesting point. However, the angular diameter across the entire frame was not given. It is common in astrophotography, as you know, to zoom in or zoom out of objects to make them appear angularly big or small compared to the angular size of the frame. This APOD (https://apod.nasa.gov/apod/ap200802.html) shows an example involving the Sun being made to look small, while this APOD. (https://apod.nasa.gov/apod/ap200322.html) depicts Earth's Moon as looking unusually big. It seems to me that if we allow astrophotographers this freedom, we should allow astro-artists the same freedom, even if they are AI. - RJN

rstevenson wrote: ↑Tue Jan 24, 2023 12:09 pm It should be 87°

De58te wrote: ↑Tue Jan 24, 2023 12:14 pm Something puzzles me. If the Earth-like planet LHS 475 b is orbiting its red star at a distance of 40 light years from the red star, than why does its sun look bigger in the sky than our Sun does to us which I believe is much closer to the Earth than even 1 light year away??