It’s a well-known and widely-publicized fact that the Milky Way galaxy is made up of billions of stars, and that the Milky Way itself is one among innumerable galaxies. Looking up at the night sky, these figures are not surprising; it is filled with stars of all colours and brightnesses. However, this typical image of the multitudes of our galaxy betrays its true size. An astronomer at Yale University, Dorrit Hoffleit, looked at the intensities of all the stars in her star catalogue and determined that only around 9100 stars can be seen with the naked eye; the vast majority of the stars in the galaxy are too dim. Using binoculars, several hundred thousand stars are revealed, and powerful armature-level telescopes can probably see a few hundred million.

What’s more, even with the thousands of stars humans can see from Earth, it’s hard to get a scale of the Milky Way galaxy. While the galaxy is around one hundred thousand light years across, the vast majority of the stars we can see are quite close to us, within 1000 light years. There are a few reasons for this. One is that the light from stars decays with the square of the distance, so distant stars are difficult to see. Another is that light from a distant star can be scattered or absorbed by gas and dust along its path. A final reason why many stars cannot be seen from Earth is that most stars are very dim.

Proxima Centauri and Alpha Centauri

The closest star to Earth (with the obvious exception of the Sun) is Proxima Centauri, 4.25 light years away. Despite its incredible closeness, Proxima Centauri is far too dim to be seen with the naked eye, or even with the help of some binoculars. As a red dwarf star, it has very little mass and is much cooler than the Sun. Therefore, it gives off much less radiation than any of the stars that are visible to unaided humans.

The Earth’s closest visible stellar neighbor is Alpha Centauri, a smidgen further than Proxima Centauri: 4.35 light years. Alpha Centauri is actually a double star system, consisting of Alpha Centari A and Alpha Centauri B. As the names would suggest, Prixima Centauri is very close to Alpha Centauri in the sky; all three stars are named “Centauri” because they appear on top of the constellation Cantaurus in the southern hemisphere. The three stars are so close, in fact, that Proxima Centauri may be gravitationally-associated with the other two stars, orbiting them every five hundred thousand years. As a result, following astronomical naming convention, Proxima Centauri is also referred to as Alpha Centauri C.

Polaris

Polaris, also known as The North Star, is so named (regardless of which name is used) because it is nearly coincident with the axis of rotation of the Earth; that axis, extending out of Earth’s north pole, passes almost straight through Polaris. Earth is, somewhat famously, on a 23.5° tilt relative to its plane of orbit, which is why there are seasons; as the Earth orbits the Sun, its axis of rotation remains fixed pointing in one direction: towards Polaris. Depending on the time of year, this arrangement causes either the northern or southern hemisphere to receive more direct sunlight, and therefore more heat.

The unique position of Polaris in the night sky gives it some interesting geometric properties. For instance, long-exposure photographs of the night sky produce a pattern known as “star trails”. As the Earth rotates about its axis once each day, the stars appear to spin, leaving circle-shaped trails in the photograph. Every star, that is, except Polaris. If Polaris is in the camera’s field of view, it appears as a single point, with every other star trail a concentric circle centered on it.

Polaris was a god-send for explorers for a long time, essentially until the advent of advanced navigation systems like gyroscopes and GPS. Today, airplane pilots are still taught to navigate using the stars in case of an emergency. Due to its unique geometry, Polaris is always due north of one’s position on the globe. Furthermore, since its position in the night sky does not change between nights or over the course of a night, it is easy to determine one’s latitude using Polaris and a sextant (provided one is in the northern hemisphere).

Cepheid Variable Stars

As epitomized in the classic nursery rhyme “Twinkle Twinkle, Little Star,” stars are frequently observed twinkling because of the way that Earth’s atmosphere interacts with the incoming starlight. However, there are some stars that actually do change over time, albeit slower than the twinkling observed by human eyes. There are many different classifications of variable stars, the Cepheid variables among them. These stars, which include Polaris, vary in intensity in regular ways that are proportional to their luminosity.

The link between a Cepheid’s luminosity and its period of variability was first discovered by Henrietta Swan Leavitt in 1908 and published by her in 1912. This is an important finding, because it allows astronomers to determine the distance to Cepheid variables; by observing the period of a Cepheid, its luminosity can be found, and the difference between its luminosity and apparent luminosity as observed from Earth can be used to estimate the distance. While there are many complications to the math, this is essentially an application of the inverse square law: an object’s apparent luminosity decays  with the square of the distance from object to observer.

The Cosmic Distance Ladder

Cepheid variables make up one of the lowest rungs on what is known as the “cosmic distant ladder.” The universe is so big, it can be very difficult to determine distances and impossible to determine all distances using only one method. Astronomers use a variety of techniques that can measure overlapping ranges of distances to build up a sense of scale.

The first rung on the ladder is parallax method, where the distance to nearby stars can be found based on their apparent movement relative to distant stars as the Earth goes around the Sun. A common method for understanding this technique is to hold a pencil in at arms length and close either eye. Depending on which eye is closed, the pencil will have a different location relative to any objects in the background. This is the same principle used by astronomers, who observe the star twice over Earth’s orbit.

The parallax method can be used to estimate the distance to nearby Cepheid variable stars, validating the technique of using period and apparent magnitude to estimate the distance. Agreement of the two methods in gauging the distance to different stars gives astronomers confidence in both methods. Cepheid variables can gauge the distance to stars that are further away than parallax, so the method can be used to validate and be validated by other more distant range-finding techniques.

This article started with the surprising fact that there are only 9100 stars visible to the naked eye, and that the view from Earth betrays the full majesty of the cosmos. The rest of this article, however, has been an argument against that thesis. Only four stars have been discussed , along with two interesting behaviours those stars can exhibit: parallax and Cepheid variability. The depth of this discussion has, furthermore, been shallow and simplified. There are nearly 10 000 other stars visible on particularly dark nights. This article does not begin to delve into so many topics, like the magical constellations that fill the sky, inspiring and being inspired by lore for thousands of years. Nor does it touch on the multitude of exo-planets that have been discovered around those stars. There is something to be said about seeing the billions of stars in the Milky Way galaxy, or the billions of galaxies beyond that. However, the telescope-less amateur stargazer need not fret; the stars that are visible without magnification are quite enough to get started.