Pole star project/Red dwarfs

With respect to the color 'red', there are studies of the redness of objects such as the red dwarf AZ Cancri shown in the visual image at right. Cool stars of spectral class M appear red; they are (depending on their size) referred to as "red giants" or "red dwarfs".

A red dwarf is a small and relatively cool star on the main sequence, either late K or M spectral type. Red dwarfs are by far the most common type of star in the Galaxy, at least in the neighborhood of the Sun. Proxima Centauri, the nearest star to the Sun, is a red dwarf. Due to their low luminosity, individual red dwarfs cannot easily be observed. From Earth, none are visible to the naked eye.

Dwarf stars
"Ideally all intrinsic colours should be found from unreddened stars. This is possible for dwarf and giant stars later than about A0 (Johnson, 1964) ... However, it cannot be used for stars of other spectral classes since they are all relatively infrequent in space, and generally reddened."

Red wavelengths
A very important wavelength in this region is the Balmer alpha line, 656.28 nm. It is emitted or absorbed by hydrogen atoms when electrons move between the second and third electron shells. Other Balmer lines, known as beta, gamma and delta, have wavelengths of 486.13, 434.05 and 410.17 nm respectively; these are also in the visual range but are less important than the alpha line.

Exoplanets
"[O]ut [of] a sample of 3,897 red dwarfs ... the Kepler Space Telescope has identified 95 exoplanet candidates circling them. Three of these candidates are roughly Earth-size and orbit within their stars' "Goldilocks zone," where liquid water (and possibly life as we know it) can exist."

Proxima Centauri
Proxima Centauri, the nearest star to the Sun, is a red dwarf, as are fifty of the sixty nearest stars.

Coolest and smallest red dwarfs near the Sun
The coolest red dwarfs near the Sun have a surface temperature of ~2,000 K and the smallest have radii of ~9% that of the Sun, with masses about ~7.5% that of the Sun. These red dwarfs have spectral classes of L0 to L2. There is some overlap with the properties of brown dwarfs, since the most massive brown dwarfs at lower metallicity can be as hot as 3,600 K and have late M spectral types.

Known star systems
The classes of the stars and brown dwarfs are shown in the color of their spectral types (these colors are derived from conventional names for the spectral types and do not represent the star's observed color). Many brown dwarfs are not listed by visual magnitude but are listed by near-infrared J band apparent magnitude due to how dim (and often invisible) they are in visible color bands (U, B or V). Absolute magnitude (with electromagnetic wave, 'light' band denoted in subscript) is a measurement at a 10-parsec distance across imaginary empty space devoid of all its sparse dust and gas. Some of the parallaxes and resultant distances are rough measurements.

Milky Way
According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way.

Gaseous giants
The volume and mass of Jupiter are VJ = 1.4313 x 1015 km3 and MJ = 1.8982 x 1027 kg.

The volume and mass of Saturn are VS = 8.2713 x 1014 km3 and MS = 5.6834 x 1026 kg.

The volume and mass of Uranus are VU = 6.833 x 1013 km3 and MU = 8.6810 x 1025 kg.

The volume and mass of Neptune are VN = 6.254 x 1013 km3 and MN = 1.021 x 1026 kg.

The minimal volume and mass for the small star Helios (H) that may have interacted with the Sun some 13,000 to 100,000 years ago: VH = 2.389 x 1015 km3 (0.00169 V⊙) and MH = 2.656 x 1027 kg (0.00136 M⊙).

Sun
The volume and mass of the Sun are V⊙ = 1.41 x 1018 km3 and M⊙ = 1.9885 x 1030 kg.

Nearby small stars
There are 66 stars within 5.0 parsecs (16.3 light-years) of the Sun.

Past encounters
Over long periods of time, the slow independent motion of stars change in both relative position and in their distance from the observer. This can cause other currently distant stars to fall within a stated range, which may be readily calculated and predicted using accurate astrometric measurements of parallax and total proper motions, along with spectroscopically determined radial velocities. Although predictions can be extrapolated back into the past or forward into the future, they are subject to increasing significant cumulative errors over very long periods. Inaccuracies of these measured parameters make determining the true minimum distances of any encountering stars or brown dwarfs fairly difficult.

One of the first stars known to approach the Sun particularly close is Gliese 710. The star, whose mass is roughly half that of the Sun, is currently 62 light-years from the Solar System. It was first noticed in 1999 using data from the Hipparcos satellite, and was estimated to pass less than 1.3 ly from the Sun in 1.4 million years. With the release of Gaia's observations of the star, it has since been refined to a much closer 0.178 ly, close enough to significantly disturb objects in the Oort cloud, which extends out to 1.2 ly from the Sun.

The second-closest object known to approach the Sun was only discovered in 2018 after Gaia's second data release, known as 2MASS J0610-4246. Its approach has not been fully described due to it being a distant binary star with a red dwarf, but almost certainly passed less than 1 light-year from the Solar System roughly 1.16 million years ago.

Hypotheses

 * 1) The sums of the masses and volumes of Jupiter, Saturn, Uranus, and Neptune are the minima for the small star that may have interacted with the Sun some 13,000 to 100,000 years ago.
 * 2) The small star may have been called Helios or Sol, if this interaction was observed at some point by hominins on Earth.