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Known Space

© Copyright 1996, Jim Loy

Known Space is a term invented by science fiction writer Larry Niven, to denote the region of space surrounding the Sun, a region which will be very well known in the early days of interstellar space travel. Known Space (including the Solar System) is in a spiral arm of the Milky Way Galaxy, and is all in orbit around the galaxy's center. This brief survey of Known Space may give some insights into the solar system.

Note: The star classes (G0, for example) are called spectral classes. They merely show the star's color (and therefore temperature, as a star is a nearly perfect "black-body"). The list of colors: O (blue) B (blue) A (blue-white) F (white) G (yellow) K (orange) M (red). Astronomy students use the mnemonic "Oh Be A Fine Girl (or Guy), Kiss Me" to remember this sequence.

See What Is A light year? for the definitions of light year and parsec.

0. The Solar System: The Solar System contains a star (called the Sun) with 4 large planets, 4 small planets (including Earth) which are closer to the Sun than the large planets, and many smaller objects (including Pluto) in orbit around it. Most of the planets have moons orbiting them. The larger objects (planets and minor planets) orbit the Sun roughly in a plane, about the Sun's equator, from West to East, the same direction that the Sun rotates. The Sun is a G2 star, which makes it fairly normal for its mass.

1. Alpha Centauri (Rigil Kentaurus): Alpha Centauri is 1.31 parsecs (4.3 light years) from the Sun. In other words, it takes light 4.3 years to go from Alpha Centauri to the Sun. Alpha Centauri is a G2 star much like the Sun, and is the third brightest star in the sky. It is 60° 44' South of the equator, and is South of the horizon for viewers in the USA. Alpha Centauri is the largest star of a multiple star system. There are theoretical reasons for believing that multiple star systems normally do not have planets. Alpha Centauri A has a somewhat smaller, orange (K0) companion star Alpha Centauri B, orbiting the primary star. Actually, they orbit each other, with the center of mass being between the two stars. Alpha Centauri B would be easily visible, without a telescope, if it were not so close to Alpha Centauri A. A third star, Proxima Centauri (Alpha Centauri C) orbits a great distance away from the other two stars. This star is a small red dwarf (type M5), and is currently closer to the Sun than the other two, which makes it the star which is closest to us, besides the Sun. Proxima is too faint to be seen without a telescope.

Note: The other red dwarfs in this article are very roughly similar in size to Alpha Centauri C.

2. Barnard's Star: Barnard's Star is a red dwarf (M5), 1.83 parsecs (6.0 light years) from us, in the constellation Ophiuchus. It is 4° 37' North of the equator, making it visible in the USA. But, it is too faint to be seen without a telescope. Barnard's Star was discovered because of its large proper motion (as were most of the faint stars on this list); in other words it moves noticeably (from year to year) through the background of distant stars. This motion is 10.34 seconds of arc per year; it takes Barnard's Star about 174 years to move across the angular width of the full moon. The proper motion is the result of its closeness to us, combined with a fairly large velocity through space. In recent years, Barnard's Star has become even more famous, as it has an unseen companion orbiting it (see addendum, below). This object is probably a very large planet, much larger than Jupiter. Although unseen, this companion makes Barnard's Star move from side to side, as they orbit each other. One large planet is likely to be accompanied by other, smaller planets.

3. Wolf 359: Wolf 359 is a very dim red dwarf (M6), 2.32 parsecs (8.1 light years) from us. It is 7° 10' North of the equator, and is too faint to be seen without a telescope. Wolf 359 was discovered because of its moderately large proper motion (as were most of the faint stars in this list).

4. Lalande 21185: This star (also called BD +36° 2147) is a rather large red dwarf (M2), 2.49 parsecs (8.2 light years) from us. It is 36° 8' North of the equator, and is too faint to be seen without a telescope. It too has an unseen companion, which is likely to be a very large planet (see addendum below).

5. Sirius (Alpha Canis Major): Sirius is a White Giant (A1), 2.65 parsecs (8.7 light years) from us. Sirius is the brightest star in the sky, because of its large size, and its closeness to us. It is 16° 41' South of the equator, and can be seen near the Southern horizon, in most of the USA. Sirius is a double star, with a white dwarf companion, Sirius B. These orbit each other. White dwarfs are much tinier than red dwarfs (too tiny to be seen in my diagrams). A white dwarf is normally a star which has died and contracted (maybe after exploding) after burning most of the nuclear fuel in its core.

6. Luyten 726-8: This star (also called UV Ceti) is actually two red dwarfs (M5.5 & M6) in orbit around each other. They are 2.74 parsecs (9.0 light years) from us. Neither star can be seen without a telescope.

7. Ross 154: This is a red dwarf (M4.5), 2.90 parsecs (9.3 light years) from us, and cannot be seen without a telescope.

8. Ross 248: A red dwarf (M5.5), 3.16 parsecs (10.3 light years) from us, and cannot be seen without a telescope.

9. Epsilon Eridani: An orange star (K2), 3.28 parsecs (10.8 light years) from us, and is easily seen (as a fairly dim star) without a telescope. It is 9° 33' South of the equator.

10. Luyten 789-6: A red dwarf (M5.5), 3.31 parsecs (11.1 light years) from us, and cannot be seen without a telescope.

11. Ross 128: A red dwarf (M5), 3.32 parsecs (11.1 light years) from us, and cannot be seen without a telescope.

12. 61 Cygni: A double orange star (K5 & K7), 3.43 parsecs (11.2 light years) from us. It can just barely be seen without a telescope. 61 Cygni A has an unseen companion, probably a very large planet (see addendum below).

13. Epsilon Indi: An orange star (K5), 3.44 parsecs (11.4 light years) from us, and can be seen without a telescope as a fairly faint star.

14. Procyon (Alpha Canis Minor): A white giant star (F5), 3.48 parsecs (11.3 light years) from us. It is the 8th brightest star in the sky. It has a companion white dwarf.

15. BD +59° 1915: A double red dwarf (M4 & M5), 3.52 parsecs (11.6 light years) from us, and cannot be seen without a telescope.

16. BD +43° 44: A double red dwarf (M2.5 & M4), 3.55 parsecs (11.6 light years) from us, and cannot be seen without a telescope.

17. CD -36° 15693: A red dwarf (M2), 3.59 parsecs (11.7 light years) from us, and cannot be seen without a telescope.

18. Tau Ceti: A star somewhat more yellow the Sun (G8), 3.67 parsecs (12.0 light years) from us, and is easily visible without a telescope (as a fairly dim star). It has an unseen companion, which may be a large planet (see addendum below).

19. BD +5° 1668: A red dwarf (M4), 3.76 parsecs (12.3 light years) from us, and is not visible without a telescope.

20. CD -39° 14192: A large red dwarf (M0), 3.85 parsecs (12.6 light years) from us, and is not visible without a telescope.

21. CD -45° 1841: A large red dwarf (M0), 3.91 parsecs (12.7 light years) from us, and is not visible without a telescope.

22. Kruger 60: A double red dwarf (M3 & M4.5), 3.94 parsecs (12.8 light years) from us, and is not visible without a telescope.

23. Ross 614: A double red dwarf (M4.5 & M?), 4.02 parsecs (13.1 light years) from us, and is not visible without a telescope.

24. BD -12° 4523: A red dwarf (M4.5), 4.02 parsecs (13.1 light years) from us, and is not visible without a telescope.

25. von Maanen's Star: A white dwarf (dZ7), 4.28 parsecs (14.0 light years) from us, and is not visible without a telescope.

26. Wolf 424: A double red dwarf (M5.5 & M6), 4.37 parsecs (14.2 light years) from us, and is not visible without a telescope.

What about all of those bright stars you see every night which I didn't list? Most of those are large Blue Giants, and others are huge Red Giants. These stars are very bright, and are more distant than the above stars. An example is Betelgeuse (See the diagram on the left), which is a red giant about 150 parsecs (500 light years) away (close, really), the brightest star in the constellation Orion, and the 12th brightest star in the sky. In the diagram, we see Betelgeuse with the Sun next to it for comparison. Betelgeuse varies in size, from the red disk in the diagram, down to the black circle (labeled "smallest") at the right side of the diagram. At its largest, Betelgeuse (one of the largest stars known) is more than 900 times larger than the Sun. Betelgeuse is about 20 times more massive than the Sun.

Years ago, the Sun was (and still is, sometimes) called a yellow dwarf, because it is smaller and dimmer than most stars that you can see without a telescope. The Sun is certainly not anywhere as large as Betelgeuse or as bright as Sirius. But, as you can see from the above list, the Sun is larger and brighter than most stars in Known Space. Of the 26 nearest stars and star systems in my table, only seven can be seen without a telescope. There are a lot of red dwarfs out there. And Known Space can be assumed to be fairly representative of most of galactic space.

Sky & Telescope did a similar article in their August 1996 issue. But, I seem to have beaten them to the punch, as I say at parties. Theirs uses more recent data (see below).

Addendum:

Since I made the above list, all of the possible planets that I mentioned have been found to be either non-existent or uncertain. In particular, the planet around Barnard's star was found to be due to an error in the star's position. More accurate data made Barnard's Star's oscillations go away. Many planets have been found out there. Of the stars above, only Epsilon Eridani (not suspected of having a planet, in the above article) is definitely known to have a planet. Lalande 21185 may have a couple planets.

Here is a more up-to-date list (2001) of the nearest stars:

Each of these stars has several names. See other sources for more accurate positions (RA and decl.). In particular, see The 100 Nearest Star Systems.

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