Now, it’s time to go out of the Solar System. Although the system is a few light years across, space is much, much vaster.
The closest star system — Alpha Centauri
Alpha Centauri is the closest star system, only 4.3 light years away. Although it isn’t very bright, they are visible in the night sky.
The visible primary components, Alpha Centauri A and B, consists of a yellow dwarf (G-type main-sequence star) and an orange dwarf (K-type main-sequence star). The invisible red dwarf, which is the closest star to Sun, is Proxima Centauri. You may wonder why Proxima Centauri is invisible, that is because Proxima Centauri is a dim red dwarf (M-type main-sequence star), too dim for us to see even at close proximity.
Proxima Centauri has one known planet its habitable zone. It is rocky so it is considered potentially habitable. But a lot of it is not known because it is too far away. Also, planets orbiting red dwarf might not be habitable because the star might tidally lock it.
Brown Dwarfs
Within 8 light years from Sun, there are no stars more luminous than Alpha Centauri A. There are already brown dwarfs that close. Brown Dwarfs are celestial bodies that can kickstart deuterium fusion, but hydrogen cannot fuse at that mass. Their masses range from 13 to 75 Jupiter masses. Sub-brown dwarfs formed like stars and brown dwarfs, but they are just planets floating in space. We use infrared telescopes to detect brown dwarfs because it is not so hot to emit a lot of visible light.
Luhman 16, a brown dwarf system consisting of 2 brown dwarfs, are just a mere 6.5 light years from Sun. They are the closest brown dwarfs to Earth.
Sirius System and White Dwarfs
Sirius is the fifth-closest star system to Earth, with a distance of 8.6 light years away. It is also the brightest star within 10 light years from Sun.
Sirius’s Bayer designation is Alpha Canis Majoris, and it is the brightest star in the night sky. But it is composed of 2 stars, with an A-type main-sequence star and a white dwarf. Sirius A has 1.711 times the diameter of Sun, and it radiates at 25.2 times the luminosity of Sun. Sirius B is a white dwarf, and a white dwarf is an interesting star type. There are no known planets in this system.
Although white dwarfs are hot, they are dim because they are very small. They are at the size of Earth or even smaller. White dwarfs form when low-mass stars, which are stars having a mass lower than 8 Solar masses die and its core collapses. Electron degeneracy pressure stops the collapse in that density so that they have these properties.
Sirius B is the closest white dwarf to Sun, and is the only one within 10 light years.
Procyon System and subgiants
Procyon is only about 11 light years from Earth, and it is one of the brightest stars we see in the night sky. It is composed of an F-type subgiant and a white dwarf. Procyon A has a diameter of 2.048 times that of Sun, and it is a subgiant. A subgiant is a transition state of the main-sequence and the giant phase. It also means a star is more luminous than a main-sequence star, but not as bright as giants.
General Highlights of the interstellar neighbourhood
8 out of 15 brightest stars in the night sky are within 100 light years.
There are about 50 stars within 15 light years, and about 38 of them are red dwarfs and none are B type or O type stars. That means our interstellar neighbourhood is relatively quiet.
The nearest giant star is just 34 light years away. It is called Pollux and it is one of the brightest stars visible.
Habitable Planets
There are 8 rocky planets in the habitable zone within 40 light years. They are Earth, Mars (Uninhabitable because of a thin atmosphere), Proxima Centauri b, GJ 667 Cc, Teegarden’s star c, TRAPPIST-1e-g.
55 Cancri e
A planet 40 light years away, called 55 Cancri e, orbits a K-type star so close that its temperature can reach 2,000 degrees celsius on the side facing the star. It only takes 0.7 days to orbit the star once, and its diameter is 1.91 times that of Earth.
TRAPPIST-1 system
A planetary system, also 40 light years away, is called the TRAPPIST-1 system. It is the only planetary system known to contain 7 planets. They are Earth-sized and rocky, and 3 of them are potentially habitable.
TRAPPIST-1 is an ultra-cool dwarf, with a temperature of 2,500 K, it is just barely a star. It has 84 times the mass of Jupiter but its diameter isn’t much larger than it. That makes it extremely dense, but not as dense as white dwarfs.
Star Highlights
Lacaille 8760 is a red dwarf, also called AX Microscopii. It is exceptionally luminous for red dwarfs, and it would be an orange dwarf when it gets a bit hotter. Its apparent magnitude is 6.69, just invisible to the naked eye.
Meanwhile, 2MASS J0523-1403 is far less luminous than Lacaille 8760, even TRAPPIST-1. Its mass is in the boundary between stars and brown dwarfs, about 80 Jupiter masses, and it is just capable of hydrogen fusion.