Astroboffin Kurtz ends 40-year quest to find a predicted one-sided vibrating star that was never seen – until now Astronomers have discovered for the first time a bizarre star floating in space some 1,500 light years from Earth that seems to only pulsate on one side. Stars, being giant balls of plasma, are constantly in vibrating motion, with the layers of ionised matter expanding and contracting rhythmically under gravity. At first …
> who says the Red Dwarf has less mass?
Hertzsprung and Russell do. Red dwarfs are not some kind of degenerate matter body like white dwarfs, they are just red-colored (K-L spectral type) "dwarf stars", meaning stars having way less mass than our sun. "Less mass" is part of their definition.
It will have a bulge.
However.... A red dwarf is several shades more dense than the outer layers of the, much hotter, real star that we see the bulge on. So it'll be there,only very much smaller, probably too small to detect from here.
Exactly how much.... Well , that's why they put a math boffin on it. Napkin-fu won't cut it here.
Bigger stars are less dense, so their matter would be pulled more easily by a nearby passing object. Well that's perhaps an oversimplification but I did a quick calculation:
I couldn't find the exact masses and radii in the article but the combined mass was estimated at 2.25 solar masses. If the red dwarf is 0.25 solar masses, its radius would be 0.3 times solar radius. A main series star with 2 solar masses would be in class A, having radius of about 1.7 times the Sun, which is 5.5 times the radius of the red dwarf.
The volume is proportional to the cube of the radius, so the volume of the bigger star is 180 times that of the smaller star while its mass is only eight times the mass of the red dwarf. It follows that the average density of the bigger star is less than 1/20 of the red dwarf's average density. The outer layers would be even thinner. The red dwarf is certainly bulging as well but it's difficult to see in the artist's rendition.
I wonder whether I'm even partially right...
> The volume is proportional to the cube of the radius
You forgot gravity: Stars aren't solids, they are very compressible gas. Density depends on the type (and age) of the star, so the radius alone doesn't tell us much.
Anyway, I don't know where you found that "2.25 solar masses" information, but, red dwarfs clocking at a negligible 0.1 solar masses, that would make the big star a blue B-type star. A cursory Duck search for "HD74423" yielded a more reasonable 1.7 solar masses for the big one, still making it a white/pale blue A-type star, definitely not yellow like on the picture.
So the picture is wrong, which was my point from the beginning... (Red dwarfs don't look like strawberries either...)
/crusade
If I understand this correctly the gravity on the surface of red dwarfs is actually higher than on bigger stars. The simple explanation for this is that they are less hot so more dense. It might help the understanding to think of red giants which the sun will become where the mass remains the same but the radius expands out to Earth.
The little more complex explanation you get from doing the calculation with:
g = Gm/r^2
Punch in the numbers from the table at:
https://en.wikipedia.org/wiki/Red_dwarf#Description_and_characteristics
Where you see that the radius and mass is nearly linked and results in the gravity on the surface of the red dwarf is way higher.
There are some caveats though. Since it is relative to the actual radius to the objects it will decrease much more rapidly when you go out of the gravity well of the smaller star than the bigger one. I.e. you are already so far out from the centre of gravity on the bigger one that any additional distance will have less impact. But I do think this will not counter the overall effect enough so that the gravitational effect from each will be bigger on the larger star than the smaller.
Alternate explanation: The artist got it wrong and mainly wanted to illustrate the bulge in the bigger one.
First, a comment on the red dwarf's bulge. The illustration is an artist's conception, not a telescope image. What's been detected is a pulsation in the larger star, not an actual bulge.
Second, the star is not only pulsing asymmetrically but it is also metal poor but not old which is also unusual. From what I've read the boffins don't think they are related, just a double set of papers to write.
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