Ideas? Need a star that is visibly unique, or rather rare, for story line

Does this all take place in interstellar space?

Maybe a stellar dust ring?

Red giant being siphoned by an orbiting neutron star.

https://astroquizzical.com/astroquizzical/when-you-have-a-binary-star-system-with-the-stars

If you are talking about interstellar space, just a visual cue will be quite difficult if it's only a star. I mean milky way has hundreds of billions of stars, so even if it were in a location that contained, say 0.01% of those stars, you would still have to siphon from millions of stars. There aren't that many configurations: dwarves, neutrons, reds and so on, and their binary combinations only come to a few dozen different possibilities if you take size in to account, and such. Check out Yerkes classification for types of stars here.

A possible candidate is the rarest, O-type or blue hypergiant. If it's on a route, this could be a viable option - they account for 0.00003% of all stars, which is only in the tens to hundreds of thousands in our galaxy. However, it might be visually a bit difficult to recognize from a white-blue supergiant. Put an o-type into a binary with a red dwarf, that should limit it a lot and is believable.

One other thing you could use is a nebula on the background.

I know you said no variable stars, but what about a star that orbits something in close enough binary to vary its light?
Say a normal star orbits a black hole, a stellar mass sized one, that its detected when the black hole warps light when it passes behind it, or the hole gravitationally tugs on the star to cause it to wiggle, akin to a method of how exoplanets are detected.
The star could be a fairly common one that orbited another that has been entirely consumed by a wandering black hole or something before, effectively replacing its partner.

There's always the "cold" white dwarf who's surface has crystallized (still visible, tho). Punchline is that this thing would be a diamond. Literally.

AR Cassiopeiae is a star system consisting of seven stars. That's certainly extremely rare. There is also Sagittarius A*. It's the super-massive black hole at the center of the Milky Way. There is normally only one of those per galaxy. A nebula formed from a system of three Wolf-Rayat stars would also be quite unique. Wolf-Rayat stars are rare as it is. If three of them were found together it would be unique.

CindyRae said:

The protagonist sees a ship in front of a dim reddish star.

Click to expand...

Presumably at interplanetary distances instead of interstellar distances.

I think that your best choice is a close binary star, a "Tatooine star". If the two stars have habitable planets, then one of them can be Sunlike and the other one Sunlike to red dwarf. Some exoplanets are known to orbit such stars, so orbits around them can be stable. If they do not, then the system can be an X-ray binary. The X-ray emitter will glow blue-white in the visible part of its spectrum, with that not being much of its total luminosity.

I'll use Cygnus X-1 as an example. From C: - apj_742_2_84.pdf, NASA - Cygnus X-1: A Stellar Mass Black Hole, and Cygnus X-1 - Wikipedia, the X-ray source has a total luminosity between 3,000 and 10,000 times the Sun's luminosity, and its companion star 300,000 - 400,000 times the Sun's. The companion, like the source, will look blue-white. The companion's surface temperature is about 31,000 K, meaning that much of its light is ultraviolet light. The companion's size is 20 solar radii, and the X-ray source orbits at 40 solar radii away from the companion's center. The companion's mass is about 19 times the Sun's and the black hole's mass 15 times, making its black-hole radius about 45 km.

Let's see how safe it would be to approach it. To get the Sun's brightness at the Earth, one would have to be at about 600 AU. At that distance, the companion's radius will be about 0.5 minutes of arc, with the source being 0.5 min above it. That gives a diameter of 1 min, and that is a little smaller than our visual acuity. But the BH's diameter would be 0.2 milliarcseconds, VERY hard to see. So it would look like a bright point source.

There is also the question of what one might be doing near Cyg X-1, other than researching black holes and accretion disks, as they are called.

ETA: Cygnus X-1's orbit period is about 5.6 days, so it won't change very fast when one looks at it.

Neutron star with planets.

I've tried to estimate how much of Cyg X-1's light comes off as infrared, visible light, and ultraviolet light, and I've found 1% infrared, 4% visible, and 95% ultraviolet. For the Sun, I calculate 51% infrared, 37% visible, and 12% ultraviolet, not far off from the actual numbers, which are something like 52% infrared, 40% visible, and 8% ultraviolet.


I'll now consider the case of a Sunlike star with a red dwarf orbiting it close by and an Earthlike planet orbiting it at the Earth's distance from the Sun. I'll use Proxima Centauri as a reference. Its radius is about 0.15 times the Sun's, so it would look about 5' across as opposed to the Sun's 32' at the Earth's distance. So it would be hard to resolve. It may also be hard to see against the glare of the sunlike star, except when the two stars are close to the horizon, and also if this planet has a moon like ours that produces total eclipses.

The red dwarf's surface temperature is a little more than half the sunlike star's surface temperature, and less than the temperature of the sunlike star's starspots. So it would look dark as it transits the sunlike star, something that it is likely to do from that planet.

Its mass is about 0.12 the Sun's, so that earthlike planet will have to orbit a bit faster to stay in orbit: 344.8 solar days instead of 365.25. Its luminosity is much less: 0.0017 total, 0.00005 in visible light, all relative to the Sun.