Galaxy by more_or_less

How many stars can you see at night?

How many stars can you see at night?

There are billions of stars just in our galaxy and it is estimated that in the visible Universe, there are about 200 – 300 billion large galaxies. It gives a HUGE amount of stars – approximately 10^24. But how many of them can we observe with the naked eye? Of course, directly we can only see the stars of our own galaxy, Milky Way. This is because of the large distance to other galaxies which we only observe as faint clouds in the sky. In fact, only few galaxies are visible to the naked eye in Earth’s sky, including famous Andromeda Galaxy M31 and both Magellanic Clouds. But using the powerful optical telescopes we can separate distant galaxies into individual stars.

Galaxy by more_or_less
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How many stars are there in the entire Milky Way Galaxy? We cannot be sure because we are situated inside the Milky Way, and we know the shape of our galaxy only through indirect observations and by comparing it to other observed spiral galaxies. According to scientists Milky Way is an average-sized spiral galaxy measuring up to 120 000 light-years across, containing up to 400 billion stars (some estimations suggest that this number may be slightly smaller.) It is worth noting that these stars occur in many different sizes, masses, colors and luminosities, and the majority of them are low-mass red dwarfs, dimmer even then the quite average star – our Sun.

And how many stars are visible with the naked eye, without any technical aid, just from Earth? This depends on several factors: the weather, the presence of light generated by the city or other human constructions (airports, factories, highways etc.), and more natural celestial phenomena, like full Moon, bright planets on the sky, comets, auroras or the zodiacal light – diffuse glow seen in the night sky along the ecliptic, usually after sunset or before sunrise, caused by sunlight scattered by space dust orbiting in the Solar System. In general the total number of stars visible long after the sunset, in a truly dark sky on a moonless night with clear weather, is about 5000 to 10000. But this is the number of stars we can see from the entire Earth and we should remember that one hemisphere of Earth is always in daylight, so really only about 2500 to 5000 stars are visible at night from any given point on Earth. This number also depends on the topography of the observational site: if it is a place surrounded by mountains, the average horizon level is higher and we cannot see as many stars as from the sailing ship in the middle of the ocean. Another problem is the exact time at night, because as the sky gets brighter near the sunrise, its faintest stars disappear from our view.

One my ask why astronomers cannot agree on the number of visible stars and why the above dispersion of estimates is so high? This is due to the physiology of the human eye. All of us don’t see the sky, or anything else, equally sharp. This may depend on the strength of the vision, our vision defects, or our age. Young kids usually have problems with focalization of sight on a small object, but on the other hand, as we get older, our eyes also become much less sensitive to faint light. Finally, it’s good to remember that our eyes need some time to see the night sky clearly, especially after leaving a bright place. This means that we will not see many stars immediately and one has to wait up to 30 minutes for the eyes to adapt to darkness. But anyway a person with average vision should be able to see more than a thousand stars in the clear night sky.

The Coronet Cluster in X-rays from Chandra (purple) and infrared from Spitzer (orange, green, and cyan)
The Coronet Cluster in X-rays from Chandra (purple) and infrared from Spitzer (orange, green, and cyan) (Source: NASA). Credit: NASA/CXC/JPL-Caltech/CfA

The observational problems due to general light pollution are even more bothering to professional astronomers. The increase in the amount of man-made light is very fast and it strongly affects the observations performed with the small or medium telescopes. Although some efforts have been made to protect the dark sky on Earth, the problem is still widespread. Therefore modern observatories are built in very specific places, such as high mountains situated away from cities or high-lying desert areas, where not only artificial light pollution, but also bad weather and other atmospheric effects are seriously limited. For example, one of the largest and most famous optical telescopes, SALT (Southern African Large Telescope) is located at an altitude of 1,759 meter above sea level just a few miles east of the town of Sutherland, 370 km northeast of Cape Town, on Karoo plateau. In the region, there are about 300 cloudless days per year. Another example is modern observatory situated near the summit of 4,145 meter high Mauna Kea of Hawaii with its twin Keck Telescopes.

The Keck Interferometer, with the telescopes' doors open to equalize temperature inside and outside of the domes (Source: NASA/JPL) Credit: NASA/JPL
The Keck Interferometer, with the telescopes’ doors open to equalize temperature inside and outside of the domes (Source: NASA/JPL) Credit: NASA/JPL

Sometimes this is not enough to perform scientifically significant optical observations of high accuracy. The main reason is atmosphere. Although we can observe thousands of stars through it, its influence on the quality of observations is huge. Watching the stars shining in the night sky may be exciting, but the true power or today’s astronomy is looking far beyond – observation and picturing stars of other galaxies, or just distant galaxies and quasars. And the farther we look into space, the higher sensitivity we need, and the more disturbing are: the seeing (blurring effects of air turbulence), atmospheric fronts, and high, often barely visible clouds. Thus there are so-called space observatories, or orbital telescopes, that are artificial satellites of Earth. The most famous example is Hubble Space Telescope – an optical, near ultraviolet and near infrared instrument carried into low orbit in 1990. How many stars can it see at once? It’s hard to estimate, because its single frame may have angular size of only about 144″ (the size of tennis ball seen from about 100 meters!) and may also contains (for example: see Hubble Deep Field in Ursa Major) several thousand of galaxies.

Author: Elzbieta Kuligowska

References & further reading:

How many stars can you see on a clear, moonless night? (EarthSky)
How Many Stars are There in the Universe? (Universe Today)
How many stars can you see at night? (Cool Cosmos)
Hubble Space Telescope: Pictures, Facts & History (Space.com)
Hubble Deep Field (Wikipedia)