How many stars are there on the night sky?

September 19, 2019 - Starter Pack
How many stars are there on the night sky?
Photo by Nicole Avagliano from Pexels

At first sight, watching the night sky on a clear summer night would leave you under the impression that you’re looking at countless stars with different brightness levels. But let’s think this trough more thoroughly, it’s a thought that deserves more attention.

After a closer look, anyone could be easily convinced that the total number of visible stars with the naked eye is around 3000 on the visible hemisphere – for an observer with a good eyesight, plus another 3000 on the other hemisphere. By contrast, with a regular binocular you could see more than 60000 stars an with a modest telescope around 200000.

Apparent magnitude

Getting back to the star brightness issue, I must say that humans made astronomical observations since ancient times and introduced the notion of stellar apparent magnitude in the following way: the brightest visible star was given a magnitude of 1 and the least bright star (visible with the naked eye) was given a magnitude of 6. Back to the present day we know that we can see fainter stars with the help of telescopes. Depending on the sky quality and the type of telescope – stars with magnitudes between 7 and 15 are visible even in amateur instruments (6-20 inches) . The visible light limit of Hubble Space Telescope is an apparent magnitude of 32. Keep in mind that brighter objects than stars with 1 magnitude exist. This means that we’ll use 0 or even negative magnitudes for really bright objects.

Star luminosity

This represents the quantity of energy emitted by the star per unit of time. As said before, stars are different from each other based on their brightness. But this brightness depends both on the star’s physical properties and also by how far away it actually is from the observer. That’s why the distance is critical when we want to determine some physical parameters like emitted energy, spectral characteristics, physical structures.

The distance to a nearby star can be calculated using a method called stellar parallax – a nearby star’s apparent movement against the background of more distant stars as the Earth revolves around the Sun.

After our Sun, the nearest star is Alpha Centauri at a distance of 4.37 light years. Cosmic objects located at billions of light years are known. Considering the fact that stars are really far away, their movement is detected really difficult. That’s why in our human lifetimes we can’t really notice any change in the positions of the stars (compared to each other). But they actually change a lot with time (thousands of years) – the constellations will look very different compared to the way they are now.

Stars changing the form of Ursa Major constellation during a 100 thousand years interval

Absolute magnitudes

Now that we know that stars can have different luminosity and they’re at wide physical distance from us, we understand why some appear brighter than others. Going further, in order to ease the process of studying them, the absolute magnitude of a star M was introduced – as the magnitude the star would have if it was placed at a distance of 10 parsecs form Earth. By considering stars at a fixed distance, we can compare the real brightness of different stars.

Here are the apparent, the absolute magnitudes and distance of some stars:

Star m M Distance (parsecs)
Sun -26.8 4.83 0
Alpha Centauri -0.3 4.1 1.3
Canopus -0.72 -3.1 30.1
Rigel 0.14 -7.1 276.1
Deneb 1.26 -7.1 490.8

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