Hot star viewed as dimmer?

[Suzy]

I came across this page at Orion's Arm describing how an extrasolar planet's star of a different spectrum might appear to human eyesight. Would a hotter star that had more light in the UV appear dimmer, as stated in the article?

 

[RisingFury]

How bright a star appears is not only a function of where in the spectrum its output peaks, but also how much luminosity it has.

A red giant is colder than our Sun, so its peak will be in the red or IR light, but if it shines a lot more light than our Sun does, it will appear to be brighter.

Same goes for blue stars.

Off the top of my head, I'd say all O type stars appear to be brighter than the Sun because their luminosity is generally much higher.

 

[Linguofreak]

I came across this page at Orion's Arm describing how an extrasolar planet's star of a different spectrum might appear to human eyesight. Would a hotter star that had more light in the UV appear dimmer, as stated in the article?

Yes, if your standing on a planet that is at such a distance from the star that it receives the same amount of sunlight as Earth.

How bright a star appears is not only a function of where in the spectrum its output peaks, but also how much luminosity it has.

But it also depends on the distance. In this case, we are talking about how bright it appears (apparent visual luminosity) when your vantage point is at a distance from the star such that the amount of insolation received (apparent bolometric luminosity) is the same as that which the Earth receives from the Sun.

Given that humanly habitable planets will all tend to receive the same amount of insolation, and assuming that most humans will spend most of their lives on habitable planets, humans will tend to perceive hotter stars as being dimmer. (Of course, stars hotter than the cool end of the F-class won't live long enough to have naturally habitable planets, but Orion's Arm is a setting with a large number of artificially habitable planets).

 

[Suzy]

Thanks! I was just wanting to confirm that. I was thinking of a planet in a star's habitable zone, which for an F-type would be further away, so the luminosity would be a bit less?

 

[Linguofreak]

Thanks! I was just wanting to confirm that. I was thinking of a planet in a star's habitable zone, which for an F-type would be further away, so the luminosity would be a bit less?

There are actually four things you are looking at here:

1) The brightness as seen by a human eye.
2) The brightness as seen by a sensor with equal sensitivity in all parts of the spectrum.
3) The total amount of visible light output by the star.
4) The total amount of electromagnetic radiation ouput by the star.

3) and 4) are generally what is called luminosity.

1) would be less, 2) the same (approximately), and 3) and 4) would be greater.