Re: How many stars we can see in outerspace without telescope?

Hello Chung,

No, it is not true that Earth's atmosphere acts like a magnifying glass and makes the stars brighter. In fact, it dims the light of the stars. As light from a star passes through the atmosphere some of it is absorbed or scattered by the air. How much a star is dimmed depends on through how much air its light passes. If you observe a star directly overhead (the point in the sky named the Zenith) from sea level on Earth, the star will appear about 1.3 times dimmer than it would from outside the Earth's atmosphere (in orbit or on the Moon). If you observe the same star overhead at a dry location at about 16,000 feet above sea level the star would only be about 1.1 times dimmer than from space. There is less air for the starlight to travel through at 16,000 feet. Clearly, from the numbers I have just quoted, the amount of dimming is not excessive at all. So stars would not seem all that much brighter to the naked eye in space as seen from the Earth's surface.

When you observe from Earth's surface and look away from the zenith toward the horizon the stars seen there are dimmed more so than those straight above. Again this is because their light has more air to travel through to reach your eye. From sea level, a star located about 30 ° above the horizon will be dimmed by about 1.7 times. A star near the horizon will be dimmed much more severely: about 6 times. So if you think about it, that means that you can see more stars near the zenith than you can straight overhead from Earth's surface. That is because the dimmest stars visible at the zenith are too faint to be seen at the horizon.

Now imagine you are standing on the night side of the Moon's airless surface and looking out into space. As I said before, the stars will not look significantly brighter. But you will see more stars than you would see from dark location on Earth because there will be no difference from the number of stars visible straight above or near the lunar horizon. The Milky Way Galaxy would be visible with equal brightness all across the sky. I imagine that would be a truly spectacular sight. Astronauts have indeed reported that the stars are spectacular from space. One interesting thing is that in space they would not twinkle. Stars twinkle as seen on Earth because as their light travels through the atmosphere it gets bounced around and sometimes bounces away from the line of sight making the star either dim rapidly or disappear for a brief moment.

You might wonder why, in videos and pictures shown from astronauts in space or on the Moon, does the sky seems so black with almost no stars visible. This is a matter of contrast. Cameras and the human eye can only see a modest range in brightness at one time. But in most of these images the Earth or the Moon's surface is visible reflecting sunlight. They are thousands of times brighter than the stars. In order to set a camera to be able to see the stars in such photos the astronauts would have to badly over-expose the Earth or Moon. Astronauts also report that whenever they look into space with their naked eyes and the Earth, Moon, or Sun is in their field of view they can only make out the brightest stars. For the scenario above where the sky is full of several thousand stars, the sky must be free of those three bright objects.

I hope that satisfies your curiosity. If you have any further questions feel free to ask me.

Bryan Méndez
bmendez@astro.berkeley.edu