Our Stellar Neighborhood

Our Stellar Neighborhood out to approximately 21 Lightyears. Stars and their relative brightness as well as color, are shown approximately, with the most significant stars labeled. The projection here is a little unusual, in that I have collapsed 3D space into a 2D disk, but in a way that preserves the true spatial distribution of the stars ( @lordneroo feel free to ask questions!)

One of the things I remember growing up within the 1970s, in the midst of the space race, was the prospect that we would one day travel to the stars. Engineers were already thinking of ways of achieving significant fractions of the speed of light necessary to get to the nearest stars. Project Daedalus was one such design.

Although, even half a century later, we are still a long way from achieving interstellar travel it is likely only a matter of time. Naturally, the nearest stars will be our first step when we finally make that step. In this article, I will explore our local stellar neighborhood out to 21 light years which is what we have pretty good data on and is reachable should 20-30% light speed become possible.

In our stellar neighborhood, there are around 150 known stars and a growing list of planets. For Sci-Fi movie buffs the good news is Alpha Centauri (Avatar - 2009) and Altair (Forbidden Planet - 1956) are real stars and are on our list. But thankfully there is no LV-426 (Alien/Prometheus)!

Sirius - The local Powerhouse

Size, brightness and color comparison of a number of the local stars including Sirius and the Sun. Here the brightest star Sirius is shown all the way through to cold Brown Dwarf star WISE 0855-0714

What better place to start our exploration than with the local powerhouse, the star Sirius. Of the 150 local stars, Sirius is the alpha star with 22x the luminosity of the sun. From earth, it is the brightest nighttime star. It is also sometimes called the "Dog Star" because it is the main star of the constellation of Canis Major.

Sirius also has a companion white dwarf star (Sirius B) that can be seen through a good amateur telescope if the atmosphere is sufficiently steady. Below is one of my better photo's of Sirius and it's white dwarf companion.

The white dwarf star is known as a degenerate star as it has exhausted it's nuclear fusion supply so that it can no longer support the mass of the star itself. Sirius B has about the same mass as the sun but has collapsed to a volume less than a millionth of the sun. There are a total of 8 white dwarf stars in the stellar neighborhood, however, to see any of them requires a telescope. Star Omicron2 Eridani B is perhaps the easiest white dwarf star to see.

Sirius and it's white dwarf companion (small star above the main star) is visible in the image made with a 30cm reflector and BW video camera. Note the airy disk diffraction ring around the fainter star.

There are also two other nearby luminous stars called Altair and Procyon, which are much brighter than the sun. These stars also appear as bright stars in our night sky and can be seen easily with the naked eye on any clear night, even from the city.

The Sun, Sunlike stars and Life

For us, the sun is the most important star, but sun-like stars are interesting from the point of view that they might have life-bearing planets. Our sun is a main sequence star of spectral classification G which has some important implications such as it has stable energy output as well a lifetime of approximately 10 billion years. Stars like Sirius have lifetimes under a billion years, which is probably insufficient time for life to develop.

In our vicinity, there are a number of sunlike stars, and below is some information on those stars. The data includes mass and size (relative to the sun), the exterior temperature in Celcius, age in billions of years, number of known planets and the likelihood of finding a planet with the right conditions for life to evolve.

Star	Mass	Size	Temp C	Age	Planets	Life?
Sun	1.0	1.00	5500	4.6	8	Yes
Alpha Centauri A	1.10	1.27	5460	4.4	?	Unlikely
Eta Cassiopeiae A	0.97	1.01	5900	5.4	?	Unlikely
Delta Pavonis	1.00	1.22	5230	7-9	?	Chance
82 Eridani	0.97	0.92	5200	6-12	>3	Chance
70 Ophiuchi A	0.92	1.02	4800	2	?	Unlikely
Tau Ceti	0.92	0.81	5150	6	5?	Chance
Sigma Draconis	0.89	0.80	5000	3	1?	Chance

The second star on this list is the fabled Alpha Centauri, whose primary star is very much like the sun. However, the presence of the second star makes it difficult for stable planet orbits to form.

Alpha Centauri is actually 3 stars, the largest components A and B are visible in this image. Component A is very similar in brightness, mass, and temperature to our own sun. This image is from an article I posted earlier this year titled The Alpha Centauri Star System

In the table above some of the stars are marked as unlikely as harboring life-bearing planets. The reason for this is that there additional factors at play, including the following:

• High Metallicity - A star with high amounts of heavier elements such as Silicon and Iron are more likely to have formed with planets.
• Old - An older star system will have had more opportunity for life to form.
• Single Stars - Stars orbited by other stars can destabilize planetary orbits.

One of the above stars, Delta Pavonis, is considerably older than the sun and is now starting to reach the Helium fusing stage which will soon cause it to evolve into a Red Giant star. Therefore any intelligent life there might want to start prepping!

Red Dwarf Stars

The red dwarf star Proxima Centauri is the C component of Alpha Centauri. This image is from an article I posted earlier this year titled The Alpha Centauri Star System

Red dwarf stars are by far the most numerous of all true stars. These stars, generally have between 8-50% of the suns mass but are still large enough to fuse Hydrogen into Helium. However, they are typically cooler than the sun at around 3000 Celcius at exterior and several orders of magnitude fainter.

In our stellar neighborhood out to 21 light years, nearly 100 of the 150 or so stars are classed as red dwarfs. Our nearest star Proxima Centauri is a red dwarf star. However, in spite being so numerous not one of them is bright enough to be seen with the naked eye.

Many red dwarf stars are known to possess planets. But there are some potential problems that could reduce the likelihood of life, such as:

• A number of red dwarfs are known to be flare stars, the star's output can increase several times for several minutes.
• Planets close enough to the star for sufficient warmth will likely be tidally locked to the star, meaning the atmosphere could be frozen out on the perpetually night facing side.

Brown Dwarf's

Artist impression of nearby brown dwarf WISE J085510.83-071442.5. It is believed to possess water vapor/ice clouds and has an exterior temperature below freezing point. Image Credit: NASA

Brown dwarfs are stars that have insufficient mass to support significant Hydrogen fusion. They emit almost all their light in infrared and can be very difficult to detect even with large sensitive telescopes.

Within 21 light-years some 30 brown dwarf's have been identified. However, it is likely many more will be found. One newly discovered brown dwarf, WISE J085510.83-071442.5, is 100 times less massive than the sun. Its output is so low, the exterior is below freezing and is believed to have water clouds.

Other identified brown dwarfs are warmer and brighter, for example, Luhman 16 a nearby star has an exterior temperature of 1000 degrees Celcius. If the sun was replaced with Luhman 16, it would look like a tiny bright red iridescent disk against a dark starry sky.

3D animation

Finally, this article gave me an opportunity to model the nearest stars in Blender 2.79. So here is an animation of flying around the sun from approximately 50 light years. Star colors and brightnesses are somewhat exaggerated for effect.

Can you identify some of the stars in this animation from the article above? As a hint the sun is the stationary yellow star, Alpha Centauri is the slightly brighter yellow star orbiting close in and Sirius is the Brightest star.

Orbiting from a distance of 50 lightyears. The sun is the stationary yellow star at the center, SIrius is the brightest white star. Can you identify any other stars?

Conclusions and Further Reading

I hope the above article gave you some insight into what interesting objects can be found in our local stellar neighborhood. For more information please consult the following references.

1. Nearby Stars: To 21 Light years

2. Astrobiologically Interesting Stars within 10 parsecs of the Sun. G. Porto de Mello, E. F. del Peloso1, L Ghezzi

Finally, note all images and diagrams in this article are the authors unless otherwise noted. Please feel free to use them so long as you credit me (i.e. Image Credit:@terrylovejoy).