THE NEAREST STARS
BACKAmong its many items of interest to amateur and professional astronomers, The Royal Astronomical Society of Canada's annual publication the OBSERVER'S HANDBOOK includes a Table of The Nearest Stars, maintained by Todd J. Henry at Georgia State University. The Table includes all known stars and brown dwarfs within 5 parsecs (pc), or about 16 light years, from the Sun. These videos attempt to show how the contents of the Table would change over a time span beginning 100,000 years ago and ending 100,000 years in the future. Planets included in the Table are neglected.
The videos presented here do not show animations of n-body simulations. Gravity - and the consequential accelerations - are neglected. The players are chosen from a compilation of 478 stars and brown dwarfs within 10 pc which have both accurate parallaxes and proper motions from the Gaia satellite. Radial velocities from ground-based observations are available for 308 stars in The 10 Parsec Sample; since they are needed for the simulation I can use only about 2/3 of the Sample. For more background see Reyle, C., et al., The 10 Parsec Sample in the Gaia Era, 2021 Astronomy & Astrophysics, 650, A201.
Accelerations among The 10 Parsec Sample members are neglected. In the actual universe these stars are subject to accelerations not only from each other but from undiscovered stars within 10 pc and beyond. What you see only approximates reality. Future observations will undoubtedly increase The 10 Parsec Sample and the number of members with radial velocities. New members will be faint - likely spectral types M or brown dwarfs. Furthermore a few stars presently beyond 10 pc might be approaching the Sun fast enough to pass within 5 pc in the next 100,000 years and therefore deserve inclusion; time will tell.
THE REFERENCE FRAME, GENERAL: Reference axes are X(yellow), Y(aqua), Z(orange). In the three "static' videos (i.e. nearest_0,0, nearest_0,90, and nearest_90,0) X and Y are 10 pc long - the diameter of the region included in the Table of The Nearest Stars. The Z axis is 5 pc long. In "The10pc_Sample" video all axes are twice as long. The Sun is stationary, and located at X=Y=Z=0. The origin of reference axes is displaced to minimize interference with the view; In the static videos the origin is at X=Y=-5 and Z=0. In "The10pc_Sample" video the displacements are twice as large. Thus the sphere of interest - 5 pc or 10 pc radius - is always tangent to the X and Y axes. In static videos stars will pop in or out when they pass the surface of the 5 pc radius sphere. If you're familiar with the astronomical reference system - RA,Dec - and want more details, then perhaps the following paragraph will be useful.
THE REFERENCE FRAME, ASTRONOMICAL: At the present time (century zero - see next paragraph) the Z axis points toward The North Celestial Pole, and the Celestial Equator coincides with the XY plane. The direction of the Vernal Equinox - zero point of the Right Ascension (RA) coordinate - is towards positive X as seen from the Sun. If you imagine wrapping your right hand around the Z axis so that your thumb points towards +Z then your fingers are pointing towards increasing RA - an "east-west" angle measured from Earth. Traditionally RA is expressed in hours, minutes and seconds of time from zero to 24 hours. The understanding is that 1 hour = 15 degrees, 1 minute of time = 15 arcminutes, and so on. (ASIDE: The use of seemingly odd "time units" is an historical hangover from the centuries when astronomers were busy compiling ever more accurate tables of star positions. That effort required accurately timing the passage of stars across the observatory's local meridian.) Also as seen from the Sun, the "north-south" Dec coordinate runs from -90 degrees towards -Z to zero in the XY plane to +90 degrees towards +Z.
Alas, nature is often complicated. The Earth's axis of rotation precesses with a period near 258 centuries. So the alignment between X,Y,Z and RA,Dec holds only at times separated by that many centuries, e.g. at centuries -774, -516, -258, 0, 258, 516 and so on - see discussion below. Fortunately, although Precession changes the RA,Dec coordinates of stars it doesn't alter their motion through space. The view is Precession-independent, but the connection between X,Y,Z and RA,Dec is not.
TIME AND POPULATION MARKERS: The lapsed time in centuries from the start 1000 centuries ago is shown in white - negative values indicate past times. The number of objects inside 5 pc at the given century apears in orange. The table below contains information about the plotted symbols. The chosen sizes of the symbols are arbitrary except that more luminous stars appear larger. In the Python language the size of plotted points is proportional to their area on the screen.
| Colour | Size | Spectral Type |
|---|---|---|
| mauve | 500 | A |
| blue | 160 | F |
| yellow | 60 | G |
| orange | 40 | K |
| red | 20 | M |
| white | 15 | white dwarf |
| brown | 15 | brown dwarf |
THE VIDEOS: It's easy to become disoriented during the video and loose track of a particular star if the viewing direction and/or zoom change. The three "static" videos attempt to finesse that difficulty by fixing the viewing direction. The scene is shown from along one of the three axes, as follows. Connection with the RA,Dec system assumes century zero:
"nearest_0,0" is aligned along X, looks toward RA = 12 hours, and is in the plane of the Celestial Equator (CE).
"nearest_0,90" is aligned along Y, also in the plane of the CE, and looking toward RA = 18 hours.
"nearest_90,0" is aligned along Z and looks down on the CE from the North Celestial Pole.
The10pc_Sample video includes the occasional change in viewing direction. Not surprisingly, 1000 centuries ago some stars were farther away than 10 parsecs.
THE ACTORS: Except for white dwarfs and brown dwarfs the represented objects are normal stars on or near the hydrogen burning Main Sequence. Videos pause for a few seconds at/near the beginning - 1000 centuries ago - to give time for orientation. The Sun is - and remains - in the centre of the field and in the XY plane.
INTERESTING STARS PRESENTLY INSIDE 5 pc: To make use of the table below pause the video "nearest_0,0" at or near century 0. The "Location" of the star is based on imagining an analog clock face centred on the Sun and with 12 O'clock at the top of the image on the screen.
The asterix beside the location of Alpha Aquilae = Altair indicates that this bright star will pass inside 5 pc about 5000 years in the future, i.e. near century 50; check it out. Alpha Centauri is the brightest star in the southern constellation Centaurus. It's also the brightest star in a triple system containing Proxima Centauri, currently the star nearest the Sun. Proxima is represented by the dim red symbol which is always very near Alpha Cen. The third star in this system has a K spectral type (orange), and is also visible near century -500. Barnard's Star is little known outside the astronomical community; this dim M star stands out by currently having the largest proper motion - motion across the sky - of any star in the table of The Nearest Stars. Watch it scoot across the screen towards the right; you'll notice other fast movers as well. It also happens to be the second nearest stellar object - counting the Alpha Centauri triplet as the nearest. Another interesting development is the recent discovery of four small planets orbiting Barnard's star. You can read the discovery paper at the Astrophysical Journal website. The reference is: Basant, R., et. al. 2025, ApJ Letters 982, Number 1. Sirius = Alpha Canis Majoris is currently the brightest star in the winter sky. It's companion, which isn't included in The10pc_Sample, is the nearest known white dwarf. Procyon = Alpha Canis Minoris is prominent and not far from Sirius. Tau Ceti is a nearby star similar to but a tad cooler than the Sun. Finally, 61 Cygni is a double star whose distance was measured by Friedrich Bessel in 1838 - the first demonstration of the parallax method of distance measurement and an astronomical milestone.
MULTIPLE STARS: The 10pc_Sample includes several gravitationally bound systems of 2 or 3 stars, which march across the field together, sometimes maintaining a constant separation, sometimes not. The distinction depends on whether or not kinematic data is available for individual stars in the system - i.e. whether each star has a separately measured proper motion and radial velocity. If not, the stars will appear to move as a unit. Alpha Cen and 61 Cyg are examples of systems where all stars - 3 or 2, respectively - have separate kinematic measurements. In that case the measurements include contributions from the orbital motions of the stars around each other. But these animations do not deal specifically with orbital motions; doing so would require more details on the size, shape, and orientation of the orbits, smaller time steps, and much more computation. Nevertheless, separate measurements will include separate contributions from orbital motions, which can - spuriously - cause the stars to separate or come together over time.
| Name | Location | Colour |
|---|---|---|
| Procyon | 2:45 | blue, dbl |
| Sirius | 3:30 | mauve, dbl |
| Tau Cet | 4:00 | yellow |
| Alpha Cen | 7:00 | yellow+red, triple |
| Alpha Aql | 9:30* | mauve |
| Barnard's Star | 9:15 | red |
| 61 Cyg | 10:30 | orange, dbl |
TWO NOTABLE TRENDS: There are presently 55 stars inside 5 pc. The animation, however, indicates that substantially fewer are present in the remote past, and also in the far future. Why is that so? Part of the reason is that The 10 Parsec Sample includes only stars inside - wait for it - 10 parsecs. Extrapolating over sufficiently long times using the "distance = speed x time" approximation will give some stars time to leave nearby space. But it's more complicated than that - read on.
In the "nearby_0,0" and "nearby_90,0" videos you will notice an odd tendency for more stars to drift towards +Y - i.e. towards the right - than towards the opposite direction. That seems strange! But it results from an arcane fact: The Sun is not at rest relative to the average motion of nearby stars. The average is calculated using stars selected from The 10 Parsec Sample and also more remote stars. But the Sun is excluded . The result constitutes a frame of reference known as The Local Standard of Rest (LSR). It provides our best estimate of the local orbital motion around the centre of our Milky Way galaxy. It's not surprising that the Sun is moving within the LSR. Our star happens to be drifting towards the centre of the Galaxy at several km/second - a rather paltry pace compared to the local orbital speed near 220 km/sec. In the XYZ coordinate system that direction is mostly along the Y axis; the Sun is moving towards -Y. So from our perspective the average nearby star is moving in the opposite direction.
Another consequence of the Sun's drift in the LSR is that, over time, the centre of The 10 Parsec Sample will become appreciably offset from the Sun. In other words, the volume included in the past/future versions of the Table of Nearest Stars becomes closer to the Sample's "edge". One should encounter fewer stars per unit volume near that edge, because of a selection effect: more remote stars are dimmer and more likely to have been missed. Furthermore, those discovered are less likely to have their radial velocities measured - more expensive telescope time per star is required. Recall that I've had to exclude about 1/3 of The 10 Parsec Sample because of the lack of radial velocities. The median distance of the excluded stars is 8.3 parsecs, not far from the 10 parsec edge. Examination of The10pc_Sample video shows consistency with these explanations, namely a deficiency of stars - mostly red K stars - to the right of the Sun in the deep past and to the left of the Sun in the far future. Obtaining radial velocities for the rest of The 10 Parsec Sample should reduce the deficiencies.