View Full Version : Stars

2017-Jan-27, 09:40 PM
Question if stars and planets are different Colors then why do they all look the same to us

2017-Jan-27, 09:54 PM
Question if stars and planets are different Colors then why do they all look the same to us
There's quite a lot to this, surprisingly. Here are some basics:

Stars that are dim do not have enough light flux density (intensity into the eye) to excite our color cones, so our eye's rods do the seeing (scotopic vision), which essentially cannot help our brain discern color. [They can contribute to our ability to see green as the last color visible, however.] Planets are always dim and, surprisingly, there is no way a large telescope will improve the surface brightness that will do much (though a little) to help us see their color. Mars, Saturn & Jupiter are bright enough to reveal some color, of course.

Stars are not just hot, they are very, very hot. The color of a yellowish-white tungsten light bulb is based on the filament's temperature. Not many stars are cooler than this. Even red giant stars will more often look more light-orange than red (e.g. Betelgeuse). [This varies between different sets of eyes, somewhat.]

Hotter stars (except the O and B class stars) produce a more balanced spectrum. A balanced spectrum produces a white result since no one color stands out. [Our Sun is white, if seen from above our atmosphere. Note my avatar which was taken at Kitt Peak while the Sun was relatively high in the sky.] Also, our atmosphere will remove a fair percentage of the violets and blues (Rayleigh Scattering).

The hottest stars (O and B class) produce a great deal of violet and blue, but they also produce much of all the other colors, so the net result will never be a saturated blue but a bluish tint (e.g Rigel, a B8 class).

Jeff Root
2017-Jan-28, 09:00 AM
The tiny apparent size of the stars is also a factor in making their
colors hard to notice. If the light is spread out a bit the color can
be more easily seen, but that also reduces the brightness, which
naturally has the opposite effect when the star isn't very bright to
begin with.

I recall Mars appearing *very* red when it was at one of its closest
oppositions. I think that was in the mid-1970s. It was high in the
sky and I'm quite sure the sky was clear. Unlike a decade or so
later when I was mystified by the bright red light near the horizon
that eventually turned out to be Venus. Mount Pinatubo in the
Philippines had erupted the month before.

-- Jeff, In Minneapolis

2017-Jan-28, 11:58 AM
Bad Astronomy: All Stars Are White (http://www.badastronomy.com/bad/misc/starcolors.html)

This one is easy to disprove by yourself. Go outside on a clear night and look at the stars. The best ones to look at are the brightest. In the Summer, (for the Northern Hemisphere) Vega is a bright star high overhead, and is clearly blue. Antares is another summer star and is also clearly red (or orange). In the Winter, you can see Betelgeuse in the constellation of Orion, which is very red. Aldebaren, a star in Taurus (near Orion) is also very red.

But most of the dimmer stars really do look white. What's going on here?

2017-Jan-28, 02:41 PM
White is not a colour. Humans attribute the local lighting to be white regardless if what a spectrograph sees. If you ever try to match a painted white you will find there are many whites. Then in low light we lose coloured vision but we do not experience literally black and white except perhaps for looking at a dark sky, if we are lucky enough to live away from artificial light. Sometimes a star can be bright enough to stimulate a coloured light receptor cell. The low level rod cells are away from the focal spot of the eyes so we have the tantalising peripheral vision. Cameras allow us overcome this problem by capturing more light over a longer time than our vision system uses.

2017-Jan-28, 05:55 PM
Question if stars and planets are different Colors then why do they all look the same to us
Most of this is about stars, but planets are a different cup of tea.

Planets are not points, but discs. They do not twinkle.

Mars is quite distinctly red. Saturn and Jupiter are more subtlely yellow.

2017-Jan-28, 06:03 PM
To my eyes the sweet spot for seeing colors is in the range between +1 and -1. Much brighter, such as Mars at a close opposition, and the tint looks less saturated. The Moon generally looks neutral white, though its tint is approximately that of a typical K star that looks pale orange. Much fainter than +2 drops below my threshold of color vision. Reddish stars on the edge of this look brownish. I have never noticed a greenish tint for borderline bluish white stars such as Beta Librae, the most commonly mentioned example.

2017-Jan-30, 10:44 PM
Thanks for the answers guys. Makes more sense now