How many visible stars

My home town of San Francisco, California has a lot of hills, and many hilltops happen to be parks. Getting to the top of a hill above most of the streetlights significantly improves the darkness of the sky, at least toward the zenith.

The horizon in all directions is still overwhelmed by light pollution. So why do we get the impression that there are many more? Millions even? I postulate it is because our mind notes that there are more faint stars than bright ones. There has been an odd explosion in the accounting on naked eye stars in the past ten or twenty years. It used to be standard to say that there are about naked eye stars all across the celestial sphere. This was based on a "normal" limiting magnitude of 6.

More recently, some sources have gotten carried away with the possibility that the "real" limiting magnitude for human vision is around magnitude 8.

Since the total number ramps up so sharply with a small change in limiting magnitude, there's no real hard answer on this one. The actual number of naked eye stars at a given site also depends on the atmospheric extinction and for dark sites the brightness of the natural airglow, both of which are significant factors below about 30 degrees above the horizon.

While most people wouldn't think of looking for faint stars in this region, one half of the area of the hemisphere of the sky is below the 30 degree line. Even in a sky with no artificial sky glow, the number can go down drastically in hazy conditions or on bright airglow nights. The absolute best that is theoretically possible on a foot mountain top where the air is free of dust and under very low airglow skies is about 5, stars, if you can see 7.

I wonder how many of these are doubles where you would only see them as one. Bob, don't your calculations assume that the 9, stars are evenly distributed through both hemispheres? Are they, or does the Southern Hemisphere maintain its usual advantage here too?

Further to Frank Reed's point--I'm 63, with astigmatism and yellowing corneas, and I think that as a practical matter I can't get past mag. I live in Boston proper and most nights I would peg the limiting magnitude in my backyard at 2.

Allow me picky here, if I may. This article mentions "north and south polestars" Is there a south polestar? If so it's news to me. There is a southern pole star--sigma octantis, but it's magnitude 5. You divided the total number equally into 2 hemispheres. David Fried already pointed out that such equal distribution is unlikely.

But you also say that if you're at mid latitudes you can see some of the other hemisphere. True, but you lose part of your own. I know that careful reading yields that this is over the course of the year, but your " From the Earth's surface, you can never see more than a half of the celestial sphere a hemisphere at any one time.

So I'm afraid the number remains at to assuming reasonably even distribution. Note that I'm ignoring that if you're up a mountain the horizon is slightly below the horizontal plane, as well as atmospheric refraction which has a similar end-result. Great article. But where did 6. Myself I've seen much fainter than mag 6. Does that mean there could be closer 20, stars over the whole sky?

Or closer to 10, stars at any one time. Cheers - Frank. Astronomy and Stargazing Projects. By: Roger W. If you can estimate the rate at which stars have formed, you will be able to estimate how many stars there are in the Universe today.

In , an image from the Hubble Space Telescope HST suggested that star formation had reached a peak at roughly seven thousand million years ago. Recently, however, astronomers have thought again. The Hubble Deep Field image was taken at optical wavelengths and there is now some evidence that a lot of early star formation was hidden by thick dust clouds. Dust clouds block the stars from view and convert their light into infrared radiation, making them invisible to the HST.

But Herschel could peer into this previously hidden Universe at infrared wavelengths, revealing many more stars then ever seen before. Soon Gaia will launch, which will study one thousand million stars in our Milky Way. It will build on the legacy of the Hipparchus mission, which pinpointed the positions of more than one hundred thousand stars to high precision, and more than one million stars to lesser precision.

Gaia will monitor each of its one billion target stars 70 times during a five-year period, precisely charting their positions, distances, movements, and changes in brightness. Combined, these measurements will build an unprecedented picture of the structure and evolution of our Galaxy. Thanks to missions like these, we are one step closer to providing a more reliable estimate to that question asked so often: "How many stars are there in the Universe?

From the core of a large city, you may only be able to see stars of magnitude 1 or brighter, for a total of 11 or 12 stars. If 1 were the magnitude limit around the world, there would be just 22 stars visible overhead. Even on nights when there is no visible cloud cover, poor transparency total transparency of the atmosphere and poor seeing steadiness of the atmosphere may reduce the number of visible stars.

To learn more about good seeing, check out our detailed article on the subject. A hypothetical middle-aged human with perfect vision may be able to detect stars brighter than magnitude 6 or 6.

However, near-sighted astronomers will see fewer stars than average, and far-sighted astronomers will see more than average. Reduced pupil size, decreased color vision and loss of peripheral vision, are all normal, age-related changes to our vision.

So older astronomers will see fewer stars and observe less contrast between the colors of those stars. Under such conditions, estimates around the Web vary between about 7, magnitude 6 or brighter stars worldwide on Star in a Star to c.

In this scenario, the total number of visible stars worldwide is We can only see half the sky, or celestial sphere, at a time, so the maximum number of stars you would see at any one time with the naked eye ranges between 3, and 4, The more light you collect, the fainter the limiting magnitude you can see.

Keep in mind that the faintest magnitude easily observed will be considerably less than the faintest theoretical magnitude. You can calculate the limiting magnitude of your own telescope using the Telescope Limiting magnitude Calculator.