The Cosmic Chronicle #6
Hi ,
I hope you’re having a great week! This week’s newsletter is slightly delayed, but it’s here nonetheless.
Recently I was browsing r/Astronomy on Reddit and came across this post. The poster posted a screengrab from Stellarium and asked, “Is Proxima Centauri really this distant from the main system it orbits ?” (I suggest clicking on the link and looking at the video.) This gave me the idea of talking about Proxima Centauri, the nearest star to our solar system.
This star lives in the constellation Centaurus, which is a southern constellation, meaning it lies south of the celestial equator. This makes it difficult to observe it from high latitudes in the northern hemisphere. If you live near the equator, or the southern hemisphere, you should be able to spot it. The brightest star of Centaurus is Alpha Centauri.
Or, is it? (Cue Vsauce music). It turns out Alpha Centauri is actually not a single star but rather a system of three stars. Out of these, two stars - Alpha Centauri A and Alpha Centauri B - form a binary star Alpha Centauri AB, meaning they orbit each other. The third star is Alpha Centauri C aka Proxima Centauri.
Alpha Centauri A and B orbit a common center with a period of 79 years. Their distance varies from 35.6 AU (roughly the distance between the sun and Pluto) to 11.2 AD (roughly the distance between the sun and Saturn). Being so close, these two appear to be a single star (Alpha Centauri) of magnitude -0.27 to the naked eye. In case, you aren’t familiar with how magnitudes are measured, the lower the number, the brighter. A magnitude of -0.27 thus makes Alpha Centauri the third-brightest star in the night sky.
Between the two stars Alpha Centauri A and B, A (aka Rigil Kentaurus) is the bigger and brighter one, with 1.1 times the mass of sun and 1.5 times the luminosity. B (Toliman) is 0.9 times as massive as the sun, and less than half as luminous as the sun.
Proxima Centauri is a weird one. Not only is it very far away from the Alpha Centauri AB system (currently 13000 AD away), it’s also a red dwarf, which makes it one of the most difficult type of star for observation.
A red dwarf is the smallest kind of star on the main sequence. It’s actually the most common type of fusing star in the milky way, but it’s so faint that observing them individually is very challenging. A typical red dwarf has less mass than the sun, with the smallest ones having only 7.5% mass of the sun and radius about 9% of the sun.
Proxima Centauri in particular, has about 12.5% mass of the sun and radius is about one-seventh of that of the sun. It’s also a “flare star”, which means it randomly undergoes dramatic increases in brightness because of magnetic activity. It’s apparent magnitude lies between 10.43 to 11.11, making it extremely faint regardless of the flare.
Proxima Centauri orbits around the Alpha Centauri AB system in a huuuuuge orbit. How huge? The furthest point in its orbit puts it about 13000 AU away from Alpha Centauri AB. That’s about 1.94 trillion kilometres! In it’s nearest point, it’s only about 4300 AU away! Currently it’s about 12947 AU away, nearly at the furthest point.
Because of this huge orbit, Proxima Centauri is the closest star to the solar system, at about 4.2 light years away. However, because of its highly eccentric orbit, it won’t stay the closest star to us forever. Eventually, as it goes to the “other side” of the main system, Alpha Centauri A and B will take the throne of the closest star. Proxima Centauri has been the closest star to the Sun for about 32,000 years and will be so for about another 25,000 years, after which Alpha Centauri A and Alpha Centauri B will alternate approximately every 79.91 years as the closest star to the Sun.
Mind blown? Here’s another fact. Proxima Centauri has a confirmed exoplanet, named Proxima Centauri b. It orbits the star at a distance of roughly 0.05 AU with an orbital period of approximately 11.2 Earth days. It’s also in the habitable zone of the star, making it a likely candidate for containing life!
Finally, here’s a picture to understand how far away and faint Proxima Centauri is. The star on the left is the binary system Alpha Centauri AB, and the star in the circle is Proxima Centauri.
That’s all for this week. Next week I’ll talk about how we could identify Proxima Centauri as a member of the Alpha Centauri star system.