Have you ever wondered the following question: are the brightest stars in the sky so noticeable because they are so big themselves, or because they are close to us? The answer to this question is a bit complicated and confusing. But let’s try to figure it out.
Does brighter mean closer?
Usually people, especially those, who are still familiar with astronomy, but at an elementary level, are very surprised when asked to name the 10 nearest star systems to us, receive a list in which most of the names do not tell them anything. The fact is that 8 places in it are occupied by red and brown dwarfs, which are sometimes not only with the naked eye, even with binoculars often difficult to see.
And even if we cross off the list of the 10 nearest star systems, brown dwarfs that many people don’t consider stars, it still leaves only three luminaries visible to the naked eye: Alpha Centauri, Sirius and Epsilon Eridani.
One might think that the Sun is in some special region of the Galaxy where there are few bright stars. But studies show that this ratio of red dwarfs to other stars is quite typical.
At the same time, both Sirius and Alpha Centauri are among the ten brightest stars in our sky. And that makes things even more confusing. So how far away are the other bright stars? What influences their good visibility more – high luminosity or distance from Earth? After all, you can go to the other extreme and decide that the brightest stars in our sky are scattered throughout the Galaxy.
How far away are the 10 brightest stars from us?
Before looking at how far away the 10 brightest stars are, it’s worth defining the terms that relate to their visibility. There are three of them: apparent magnitude, absolute magnitude and luminosity. The first of these is a characterisation of how visible stars are in our sky when light has already travelled the distance between us and all possible obstacles. It is a dimensionless value that can even be negative. The smaller it is, the brighter the star.
Absolute stellar magnitude is the magnitude that these stars would have if we observed them from a distance of 10 parsecs without any obstacles. It is also measured in dimensionless units and can be negative.
Finally, there’s luminosity. This is the amount of energy that a star emits per unit of time. It is usually measured in the Sun’s luminosity and, unlike the stellar magnitude, it is not zero or negative.
So here goes, the brightest luminary in our sky is Sirius, or Alpha Canis Majoris, a white star with a luminosity 25.4 times that of the Sun. It also has a companion – a white dwarf, but it does not make a special contribution to the visibility of the star in our sky. This pair is located very close to each other – 8.6 light years away from Earth.
Second place goes to Canopus (Alpha Carinae). It’s also a white star, but there’s a twist. It is much more massive than Sirius, and its luminosity exceeds that of the Sun by a factor of 15,100. Its distance is 310 light years.
The third place is Alpha Centauri. This is a system of three stars: yellow, orange and red, with the mass of only the first of them by 52 per cent more than the solar, and the other two are significantly inferior in all parameters to our luminary.
The fourth brightest star is Arcturus (Alpha Boötis). It is an orange giant with a luminosity 2015 times that of the Sun. It is located at a distance of 36.7 light years away from us – further away than Sirius or Alpha Centauri, but much closer than Canopus.
Fifth place is Vega (Alpha Lyrae). Another white star with 40 times the luminosity of the Sun. It is located at a distance of 25 light years away from us.
Sixth place is Capella (Alpha Aurigae). This is a double system of two giant yellow stars orbiting around a common centre of mass. The luminosity of each is 77 to 78 times that of the Sun. This system is located at a distance of 42 light years away from us.
Seventh place – Rigel (Beta Orionis) This is a giant blue star that is extremely hot. Its luminosity is 120,000 times that of the sun. It’s located about 870 light years away from us.
Eighth place – Procyon (Alpha Canis Minoris). A yellowish-white star (spectral class F) with a luminosity 7 times higher than that of the Sun. Like Sirius, it also has a companion, a white dwarf. The distance to it is 11.4 light years.
Ninth place is Achernar (Alpha Eridani). This is a double system consisting of a large blue star with a luminosity of 3150 solar and its companion, which has a spectral class A and a luminosity 17 times solar. The distance between them is 139 light years.
Finally, the tenth place is Betelgeuse (Alpha Orionis). It is a huge red star, very bloated and cold. Its luminosity is variable and varies within a few tens of thousands of solar. Its distance is very roughly estimated at 450-600 light years.
What conclusions about the dependence of the brightness of a star in the Earth’s sky on distance, temperature or luminosity can be drawn? Practically none, except that all these are really stars, which by the last parameter far exceed the luminosity of the Sun.
But in all other respects they are completely different luminaries, and it is even impossible to say which of them are more: those that are moderately brighter than the Sun, but are located at a distance of only tens of light years, where our radio signals have already managed to reach, or real giants, which are much more distant from us, but we still see them.
And most importantly, why in the list there is not a single star that would be more than 1000 light years away from us. After all, the Milky Way is 10 thousand light years thick, and its diameter is 100 thousand light years. So where are the more distant stars?
The 100 brightest stars
To make the situation a little clearer, we should abandon the consideration of each individual star, take a large sample and just look at how the luminaries are distributed in it by different parameters.
For example, let’s take all the stars in our sky whose apparent magnitude is greater than 2.5. There are only 93 such stars.
If we look at the distances to them, we find that 36 of them are no more than 100 light years away from us and 50 are between 100 and 1000 light years away.
Only 6 bright stars – Naos (Zeta Puppis), Alnilam, Alnitak (Epsilon Orionis and Zeta Orionis), Sadr (Gamma Cygni), Wezen and Aludra (Delta Canis Majoris and Eta Canis Majoris) – are between 1000 and 2000 light years away. And only one of the 93 brightest stars, Deneb (Alpha Cygni), is more than 2000 light years away.
The distribution by spectral classes (at least for the main component of the system) looks no less interesting. Only six luminaries belong to red (the coldest), 21 – to orange, two – to yellow, 7 – to white-yellow (class F), 18 – to white, 30 – to blue (class B), 3 – to hot blue (class O, the hottest). And in one more system, Regor (Gamma Velorum), the main component is the extremely rare and hot Wolf–Rayet star.
That is, hot stars dominate among the brightest luminaries of our sky. Bloated red giants make up a small fraction of the brightest luminaries, and we do not see anything even slightly comparable in mass and luminosity to the Sun among the main decorations of the night sky.
The situation is rather ambiguous. Indeed, the main role in the visibility in the sky of the Earth plays just luminosity. Stars, which by this parameter exceed the Sun no more than a few tens of times, are a minority among those to which the eye involuntarily clings at night.
However, the vast majority of them, although not close to us, are no more than 1% of the diameter of the galactic disc. Perhaps something would change if we introduced more faint stars into the sample?
More stars!
Let’s see how the statistics looks like if we take the 300 brightest stars in this sky. To do this, we will have to include stars with a stellar magnitude less than 3.0, including those that, although not badly visible to the naked eye, even experienced amateur astronomers do not immediately find them in the sky.
Among these three hundred, only 80 are closer than 100 light years away from the Sun. 202 luminaries are between 100 and 1000 light years away, only 15 are between 1000 and 2000 light years away, and 3 are further than 2000 light years away.
If we look at the spectral classes, we get the following picture: class M (red) – 18, class K (orange) – 66, class G (yellow) – 37, class F – 23, A (white) – 54, class B (blue) – 92, class O – 6 and one Wolf–Rayet star.
That is, cooler stars become much more numerous among the less bright stars, and the fraction of very hot stars decreases. But this does not mean that there are very many Sun-like stars among them. There are only 15 that are no more than 10 times brighter than the Sun. All of them are located within a radius of 50 to 60 centimeters from the Earth.
The general conclusion can be stated as follows. The vast majority of the bright stars in the sky have much greater luminosity than the Sun. However, even they would remain hardly noticeable if they were at a distance of at least a few thousand light years.
