"The Closest Known Flyby of a Star to the Solar System"
Mamajek, E.E., Barenfeld, S.A., Ivanov, V.D., Kniazev, A.Y., Vaisanen, P., Beletsky, Y., & Boffin, H.M.J, 2015, Astrophysical Journal Letters, 800, L17

(see FAQ below)

Links to Astrophysical Journal Letters Article, ArXiv, and UR Press Release (Tuesday 17 February 2015 10am)
Previous studies: Scholz (2014), Burgasser et al. (2015), Ivanov et al. (2015)
SIMBAD entry for Scholz's star | Wikipedia entry
Google Sky .kml file for finding Scholz's star and showing a best estimate of its trajectory over the past million years (it's the faint star just above and to the left of the final pin corresponding to its 2010 position)

Artist's conception of Scholz's star and its brown dwarf companion (foreground) during its flyby of the solar system 70,000 years ago. The Sun (left, background) would have appeared as a brilliant star. The pair is now about 20 light years away. Credit: Michael Osadciw/University of Rochester.

FAQ about Scholz's star (WISE J072003.20-084651.2), which had the closest known flyby of a star system to the solar system

What do we know about Scholz's star?

Scholz's star was discovered in 2014 by Ralf-Dieter Scholz (Scholz (2014)). It is a red dwarf + brown dwarf binary system (spectral types M9.5 and T5). Burgasser et al. (2015) reported the discovery of its brown dwarf companion in October 2014, and reported precise radial velocity measurements and an improved distance (6.0 parsecs = 19.6 light years). I estimate the masses of the two components to be roughly 86 and 65 Jupiter masses (0.082 and 0.062 times the mass of the Sun). The SIMBAD database has a new entry for this recently discovered star, which summarizes its basic observables.

Why did you nickname WISE J072003.20-084651.2 "Scholz's star"?

The star was first discovered to be a nearby star by astronomer Ralf-Dieter Scholz of Leibniz-Institut fur Astrophysik Potsdam (AIP) in Germany. The star was first reported by Scholz in a paper that appeared on arXiv in November 2013, and subsequently published in the journal Astronomy & Astrophysics in January 2014. The star is very interesting for multiple reasons: (1) it is a late-M + T dwarf binary, (2) it was very slow proper motion and was previously missed as a nearby star, as it was within a couple degrees of the Galactic equator, and (3) now we know it as the star that came closest to the solar system in the recent past (although I suspect when all is said and done after analysis of the Gaia mission astrometry in the years ahead, probably some other star will win the prize for having come closest (or coming closest) in the future). This star is clearly interesting enough that it warranted some sort of nickname beyond its boring "phone number" designation (based on its celestial coordinates).

Was Scholz's star "seen" before 2014?

It was an anonymous star that appeared in several star catalogs (it is also designated as 2MASS J07200325-0846499, DENIS J072003.2-084650, and USNO-B1.0 0812-00137383 - since it appeared as a source in the 2MASS and DENIS infrared sky surveys and on photographic sky survey plates in the USNO-B1.0 survey). However there was nothing about the star that drew astronomer's attention before 2014. To appreciate just how faint and anonymous it was, check it out in Google Sky. There are many billions of stars on the sky that are similarly anonymous - and barely studied beyond their positions and how bright they are at visual and infrared bandpasses. Red stars near the Galactic plane are a dime a dozen, especially slow moving ones like Scholz's star. What stood out was that it was relatively nearby.

Why was Scholz's star not discovered before 2014? How did it remain hidden for so long?

A few factors probably contributed to it being missed for so long. (1) It is very dim at visual wavelengths - 18th magnitude in the V-band, (2) It is near the Galactic plane (Galactic latitude +2.3 degrees) - so it is in a crowded region of sky, (3) that crowded region of sky has lots of red stars - but most are red giant stars that are much, much further away, (4) the star has a low proper motion (and corresponding tangential velocity) - nearby stars are easier to find if you search the ones that appear to be fast moving. Scholz's star is moving fast with respect to the solar system - but most of its motion is radial (83 km/s), with slow tangential motion (3 km/s). But to measure a radial velocity, you have to measure a spectrum to measure the Doppler shift - and one would need the star to stand out somehow before bothering to take a spectrum. Scholz was the first to notice that this star was interesting. While the star's tangential motion is small - it is not zero. He selected it based on its combination of WISE infrared and 2MASS near-infrared colors - and the fact that it had moderate proper motion (~0.1 arcsecond per year) hinted that it was not simply a more distant red giant, but most likely a nearby red dwarf.

What about HIP 85605 - the "Rogue Star"/"Deadly Dwarf Star"/"Death Star" that is supposed to wipe out Earth in 240,000 to 470,000 years?

We discuss HIP 85605 in Section 3 of our paper. Bailer-Jones (2014) conducted an excellent survey of about 50,000 stars in the Hipparcos catalog to try to identify cases of stars that came very close to the solar system (or will in the future). His closest flyby was for a star HIP 85605 - a previously uninterested orange (K-type) dwarf. If the revised Hipparcos trigonometric parallax for HIP 85605 was taken at face value (placing the star about 7 parsecs of 22 light years away), then the star's trajectory would bring it to within 0.10 parsec (0.33 light year = 21,000 astronomical units) - near the inner Oort cloud - about 300,000 years in the future. Bailer-Jones discusses the distance to the star at some length - since it had large uncertainties, and its astrometric solution may have been affected by another unrelated neighboring star. Unfortunately, as we show in Mamajek et al. (2015), the Hipparcos distance to HIP 85605 is almost certainly wrong - and it is probably about 10x further away than previously thought. The reason is that the Hipparcos distance forces the star to have a very dim absolute magnitude (Mv = 11.9+-0.5), much dimmer than plausible for a typical orange dwarf of the same color (main sequence K dwarf would have Mv of around 7!). So the Hipparcos parallax places HIP 85605 about 5 magnitudes below the main sequence - and in a region of color-magnitude space where no known stars lie. It would be too faint to be even a very metal poor orange dwarf, and it is too bright to be a cool white dwarf. I illustrate this point in a plot posted to figshare. Furthermore, David Latham (CfA) has confirmed that spectra of HIP 85605 taken over a decade ago do show it to be a typical orange dwarf star. So the Hipparcos distance is unphysical. At the more likely distance of ~60 parsecs (nearly 200 light years), the trajectory of the star doesn't bring it much closer than 10 parsecs (~30 light years) of the solar system. So we conclude that HIP 85605's close flyby is probably an artefact of a poor Hipparcos astrometric solution for HIP 85605. Scholz's star is then currently the record holder for the nearest known flyby of a star to the solar system.

So despite the unfortunate headlines from December 2014 about HIP 85605 - e.g. "Is a Death Star Coming at US? Study Says It's Possible But Don't Panic" - I'm very confident that HIP 85605 will not be making a close flyby of the solar system in the distant future (triggering a comet shower, etc. etc.).

Is Scholz's star bound to the Sun? i.e. part of our solar system?

No, we're entirely certain Scholz's star is NOT on a bound orbit with our Sun - it is NOT part of our solar system. It is not in orbit around the Sun - indeed it appears to have traversed *20 light years* in about 70,000 years since its "flyby" of the solar system.

How do we know it was not bound to the Sun? Some observations and basic Newtonian physics. When it passed ~52,000 AU from the Sun ~70,000 years ago, its velocity was about 83 kilometers per second. We can easily calculate the escape velocity for a body at that distance from the Sun using basic Newtonian physics: The escape velocity of an object is:

V_esc = root(2 G Msun / r)

where "G" is the Newtonian gravitational constant, "M" is the mass of Sun (really the mass of solar system, but the Sun completely dominates the first 3 digits anyway), and "r" is the separation between the Sun and Scholz's star at its minimum flyby separation. G = 6.67-11 m^3/kg/s^2, Msun = 1.99e30 kg, and r = 52,000 AU = 7.8 trillion km = 7.8e15 meters. This translates to an escape velocity of only 185 meters per second (0.185 km/s) at that separation! If Scholz's star had a velocity lower than this, then it could be on a bound orbit. But, the velocity of Scholz's star was actually ~450 times too fast to remain bound to the Sun. So like every other star we know, it is unbound to the Sun, and merrily going about its business orbiting the Galaxy. Scholz's star and our solar system were simply ships passing in the night.

Is Scholz's star actually any of the following hypothetical objects: "Tyche", "Nemesis", "Nbiru", or [other made-up object supposed to invoke fear and foreboding, and sell silly new age books]?

No. "Tyche" and "Nemesis" are names attached to particular predictions about hypothetical companion to the Sun (neither of which has ever been seen and probably do not exist). Sensitive surveys of the sky in the infrared have ruled out distant companions to the Sun down to the mass of Jupiter or Saturn (depending on distance; see WISE survey paper by Luhman (2014)). However, given the eccentric orbits of some of the distant objects many dozens of AU away from the Sun (e.g. Eris, Sedna, etc.), it wouldn't shock me if a body the size of Mars or Earth were discovered beyond the Kuiper Belt (but seems unlikely if a giant planet, brown dwarf, or star will be found in the Oort Cloud).

"Nibiru" is apparently related to some bizarre pseudoscience doomsday hoax. Needless to say - it's crap, and without the internet to perpetuate silly ideas, it probably would have died years ago. From what I've seen on various youtube videos (what passes for scholarship in the Nibiru-phile community), Nibiru appears to be a meme propogated by people noticing what appear to be lens flares and reflections (e.g. "ghost" images, but not ghosts in the sense of Slimmer or Casper). It seems ridiculous to consider the idea of a large body orbiting anywhere close to Earth and the other major planets without the notice of (1) >10,000 professional astronomers worldwide, (2) >millions of amateur astronomers regularly observing the night sky, (3) dynamicists that fit the positional data for the major bodies in the solar system, integrating their orbits (taking into account the gravitational effects of all the bodies), and (4) thousands of all of these folks that regularly tweet, facebook, blog, gossip, post their images and observations, etc. All of these parties would take great pleasure being the first to discover a new large planetary body in the solar system, if they had observations that supported the discovery of such a body! The orbits of the major planets and satellites are observationally constrained well enough that NASA can land spacecraft on Mars within a couple hundred meters of the original targe. The orbits were known well enough 25 years ago, that the Voyager 2 probe passed Neptune within 100 kilometers of its intended target position. You simply can't hide something as big as an Earth-like planet orbiting anywhere near the major planets in the solar system.

Some things to keep in mind: (1) stars pass through the Sun's Oort cloud "all the time" - about 10 stars every million years(!), however extremely few are massive or slow-moving enough or come close enough to produce any significant impact on the comets in the Oort Cloud. Close flybys of <20,000 AU (<0.1 parsec, <0.3 light year) that pass through the denser parts of the Oort Cloud (the inner Oort Cloud) are very rare - with ones whose combination of mass and velocity are strong enough to greatly perturb the cloud may occur at intervals of something like once every ~100,000,000 years or ~billion years, or so. Fortunately, space is a *very* *big* place - there is a lot of space between the stars - even during these "flybys".

How bright was Scholz's star at its closest? Was it visible to the naked eye? How intrinsically bright/luminous/massive would it have had to have been to be visible to the naked eye?

Scholz's star is currently V = 18.3 magnitude at distance 6.0 parsecs, so it has absolute V magnitude of Mv = 19.4. At its closest distance of 0.25 parsecs (52,000 AU) it would have been at magnitude V = 11.4 (there is a typo in Sec. 4 of the paper - the predicted V magnitude should be 11.4, not 10.3). This is roughly 5 magnitudes (factor of 100x) fainter than the faintest naked eye stars. As we mention in the paper, Scholz's star is a magnetically active M9.5 star - similar stars have been seen to flare by more than 9 magnitudes (Schmidt et al. 2014), so it possible that Scholz's star may have occasionally been a naked eye object for minutes or hours during rare bright flare events.

At distance 0.25 pc (52,000 AU), for a star to be naked eye with V magnitude brighter than 6, a star would have to have absolute V magnitude brighter than Mv ~ 14, roughly corresponding to a main sequence star of M5 type or hotter (~15% the mass of the Sun).

Does a low tangential velocity of a nearby star suggest it's likely either coming towards or has already been quite near or in the solar system? (Couldn't it have a low tangential velocity AND a low radial velocity, or if that's an unlikely combo, why so?)

A low tangential and low radial velocity would mean the star's velocity vector is very similar to that of the Sun's. The velocities of local stars in the solar neighborhood are smeared out over several tens of kilometers per second in each dimension (3 dimensions: one towards the Galactic Center "U", towards the direction of Galactic rotation "V", and towards the north Galactic pole "W"). The smaller one draws one box in velocity space (e.g. the velocity of such-and-such star must be within x kilometers/second of such-and-such velocity) the fewer field stars will satisfy that criterion (unless the velocity you originally select is close to that of a nearby stellar cluster or association). So very few stars have velocities within a few km/s of that of the Sun (i.e. ones that would show both tiny tangential motions and radial motions). Stars with really tiny radial velocities and tiny tangential motions may be scientifically interesting, however, as such velocities are what you'd expect for "solar siblings" (i.e. stars that would have shared the Sun's same birth cluster) - but this is getting off topic. Scholz's star is certainly not such a star, as it is moving ~83 km/s with respect to the Sun.

How likely is it that Scholz's star could have flared enough to be occasionally visible 70,000 years ago?

Good question: while cases of late M-type stars with whopping flares have been recorded (discovered in all-sky surveys), I'm not sure if there have been long-term surveys of enough late-M-type stars to get a good statistical answer (I think stars of M9 and M9.5 type were only discovered for the first time in the 80s or 90s, and then discovered in more numbers in the 2MASS and Sloan surveys in the 2000s ). What is seen among the hotter red (M) dwarf stars is that the most energetic flares are very rare (months, years), and the least energetic flares occur more frequently. Schmidt et al. 2014 reported a whopping flare of a M8 star. Late M-type stars are faint, so they really haven't been monitored like hotter/brighter stars have been. In fact the Schmidt paper is an example of a previously unknown M8 star that was discovered *because it flared*. The closing statement in Schmidt et al.'s paper tells the story: "Overall, however, there are not yet sufficient observations to characterize the flare frequency distribution of M7-M9 dwarfs and investigate the similarity of their emission mechanisms to those on more massive M dwarfs." So a good answer to "how often might have Scholz's star flared brightly enough to be visible?" is "I don't know". If you held my feet to the fire to make a bet, I'd probably estimate a frequency of less than once a year. So I would say that ultracool red dwarf stars like Scholz's star have been to seen to brighten by factors of ~4000x, and we know spectroscopically that Scholz's star is magnetically active, so it seems reasonable that it probably undergoes similar flares.

Also, are there any remaining alternatives to this possibility that have not been ruled out - could the current motion of Scholz's star be the result of more recent interactions with other objects for example?

Extremely unlikely. Space is a big place. Its high velocity is not unusual for old stars. And if it had been involved in a recent interaction with another star that was enough to be responsible for its high speed - then it probably would have had its companion brown dwarf stripped away. Nothing is physically odd about the velocity of Scholz's star itself - the only thing odd is that its motion brought it so close to the solar system in the "recent" past (~70,000 years ago).

How close does a star have to come into the solar system to perturb enough to trigger comets coming into the inner solar system? How close in would this star have had to come and how about for a star as massive as Proxima Centauri?

The "danger zone" so to speak is apparently within 20,000 AU of the Sun - the so-called "inner Oort cloud" or "Hills cloud". In 99.999% of the simulations of the trajectory of Scholz's star passed well beyond 20,000 AU (only 1 out of 10,000 simulations brought it within 20,000 AU). The likelihood that Scholz's star passed through the "outer", low-density Oort Cloud (based on the current velocity and distance data) is about 98%.
The answer to "How close does a star have to come into the solar system to perturb enough to trigger comets coming into the inner solar system?" A star within a parsec or so could perturb "some" comets towards coming into the inner solar system, but there are fewer comets in the Oort Cloud that far out to perturb. One way of quantifying this was proposed by Feng & Bailer-Jones (2014) -- they scale their results by defining a proxy indicator of the encounter-induced flux of Oort Cloud comets as \gamma = (mass of star)/(velocity of star X flyby distance). Their simulations are suggestive that gamma < 10^-5.3 (Msun * s / km / AU) are unlikely to generate an enhancement in the flux of long-period comets. For all of the simulations, none of them resulted in an encounter-induced flux of Oort Cloud comets that came close to generating a significant enhancement in the flux of long-period comets.

General comments on the Oort cloud: Some reviews on the Oort Cloud can be found in Weissman 1996 and Rickman 2014. Based on the number of long-period comets that enter the inner solar system, I've seen estimates that there are probably something like ~trillions of comets in the Oort Cloud. The origins of the Oort cloud have been debated - with older papers suggesting that these icy bodies were ejected from the solar system during its formation, however recent papers (Levison et al. 2010, Brasser et al. 2012) have suggested that interactions with other stars in the Sun's original birth cluster were part of the story, and indeed many or most of the Oort cloud comets may have even been "stolen" from the vicinity of other stars in the Sun's birth cluster. Our paper summarizes my understanding of the Oort Cloud so far as I was able to glean from the literature. My understanding is that there are more comets within the inner Oort Cloud ("Hills cloud") within about 20,000 AU of the Sun (however it probably does not have a particularly hard boundary).

Nathan Kaib, author of a recent Science paper on the Oort Cloud and the effects of stellar flybys, said in a recent email: "As you state in your paper, I think the effect [of the Scholz's star flyby] on the Oort Cloud and LPC [long period comet] flux will be minimal." Seeing as Nathan does dynamical simulations of the Oort cloud and comet populations for a living, I put some weight in his statement.

How long a duration was the star a solar system "resident"?

This was a journalist's question. I guess by "resident", one could call it a resident as it passed through as a gravitationally unbound object passing within the Sun's tidal radius. The Sun's tidal radius (where its gravity dominates that of the Galactic gravitational potential) is roughly 1.35 parsecs (calculating discussed in Mamajek et al. 2013). Scholz's star passed through the Oort Cloud ~70,000 years ago, as close as ~52,000 AU (0.82 light years). These numbers are approximate due to uncertainties in the distance, proper motion, and radial velocity of Scholz's star (discussed in paper). If one defines the "solar system" using the rather generous definition of going out to the edge of the Sun's tidal radius - then Scholz's star spent about ~30,000 years within this radius (between ~56,000 amd ~86,000 years ago). The star was within 100,000 AU (0.48 parsec) of the Sun for ~10,000 years (between roughly 66,000 and 75,000 years ago).

How fast was Scholz's star moving across the sky during its flyby? Where would it have appeared in the sky (even though it would have normally been too faint to see)?

At its closest, Scholz's star would have been moving across the sky at an astounding 70 arcseconds per year (it could traverse a full moon in about 26 years). Compare this to Barnard's star - the star with the current highest proper motion (~10.7"/year). It would have passed closest roughly in the direction of Ursa Major (the Big Dipper) - however the position on the sky where it was the closest has large uncertainty (roughly +-30 degrees in RA, +-14 degrees in Declination).

This is a best estimate of the past trajectory for Scholz's star based on the available velocity and distance measurements. The crosses plot the star at 100 year intervals - one can see that the angular motion was much greater near its nearest pass. Its motion is now mostly radial as it is now moving away (far right, bottom). The red part of the trajectory shows when the star was within the Sun's tidal radius (roughly 1.35 parsecs), roughly the maximum possible extent of the Oort Cloud. Note that 98% of the simulated trajectories brought it within the outer Oort Cloud.

You can also see the trajectory plotted on the sky using this Google Sky .kml file. Pins are shown at regular intervals going back 1,000,000 years. Please excuse the extra significant digits in the calculated distances - those should be rounded down a bit (indeed the distances are not constrained to better than ~17% at best, given the current astrometric accuracy). If you load the .kml file in Google Earth, you can see where the star's trajectory brought it closest to the solar system. It sailed through Ursa Major ~70,000 years ago, passing close to the modern position of the star Dubhe during its closest point. Note that all the stars are moving. Several of the Big Dipper stars are part of the same cluster (Ursa Major open cluster) - 70,000 years ago they would have moved a few degrees but still been in the same general region of the sky. A Google Sky screen shot of the star's trajectory through Ursa Major is shown below:

Here is an image from Google Sky showing the past 67,000 years or so of the path of Scholz's star across the sky (showing it passing through Cancer, Gemini, Canis Minor, and into Monoceros, where it has spent the past 55,000 years).

What was the effects of Scholz's star's gravitational force on the Earth? Its tidal force?

This is a straightforward calculation. Force goes as mass times acceleration. The acceleration due to gravity goes as:
a = (Gravitational Constant)*(mass)/distance^2
It will be constructive if we calculate this due to Scholz's star at its closest point, and that for the MOON. Units are in brackets. The mass of the Scholz's star binary is about 15% the mass of the Sun (1.99e30 kg) or 3.0e29 kg. Its distance at closest separation was 52,000 AU, where 1 AU is 150 million km (so 7.8e15 meters).

a(Scholz's star) = (6.67e-11)[m^3/kg/s^2]*3.0e29[kg]/(7.8e15[m])^2
a(Scholz's star) = 3.3e-13 m/s^2

And the acceleration due to the Moon? The moon has mass 7.35e22 kg and it is roughly 384,000 km away.

a(Moon) = (6.67e-11)[m^3/kg/s^2]*7.35e22[kg]/(3.84e8[m])^2
a(Moon) = 3.3e-5 m/s^2

So the ratio of the gravitational accelerations is:
a(Moon)/a(Scholz's*) = 3.3e-5/3.3e-13 = 100,000,000 times!
So the MOON is 100,000,000 x more important gravitationally than Scholz's star was even at its closest point! The situation is even more extreme if one compares *tidal* forces, which go as the distance to the 3rd power!
Let's take the ratio of the tidal accelerations by the moon and by Scholz's star:
a_tidal(Moon)/a_tidal(Scholz's*) = [(Mass_Moon)/(Mass_Scholz)]*[dist_Scholz^3/dist_Moon^3]
= (7.35e22 kg/3.0e29 kg)*(7.8e15[m])^3/(3.84e8[m])^3
= (2.45e-7)*(8.4e21)
= 2e15 = 2,000,000,000,000,000!

So the tidal acceleration of the MOON on the Earth is 2 QUADRILLION times stronger than that of the Scholz's star system at its closest point.

The gravitational force and/or acceleration and tidal force and/or acceleration by Scholz's star on the Earth at its closest point was completely and utterly negligible compared to nearer smaller objects like the Moon!

Did our solar system pass through the "Oort Cloud" of the Scholz's binary? Does Scholz's star have a planetary system?

We only know a little about Scholz's star and its brown dwarf at this point. Scholz's star was only discovered/reported in late 2013 (paper by Ralf-Dieter Scholz was published early 2014) and its brown dwarf companion was only discovered by Adam Burgasser et al. in late 2014. In our paper (Mamajek et al. 2015) we estimate the masses of Scholz's star and its companion - roughly 8.2% and 6.2% the mass of the Sun (total mass of system ~14.4% of the Sun). The two stars orbit each other at a separation of about 0.8 AU (Burgasser et al. 2015). The shape of the brown dwarf's orbit ("B") around Scholz's star ("A") is not known yet. If we assume a circular orbit, then we can estimate the size of the regions around these objects where there may be stable orbits where planets could exist. Following the orbital stability simulations of Holman & Weigert (1999), these numbers translate to a region about 0.37 AU in radius around "A" where stable planetary orbits may exist. The stable zone around "B" is slightly smaller. There could also be objects that orbit *both* A and B, if they are situated >1.3 AU away from the center-of-mass for the system. But these numbers depend on the eccentricity of B's orbit around A. Given the low-mass of both objects - and based on what we know about circumstellar disks and the types of planets being discovered around red dwarfs, if there are planets in the Scholz system, they are likely small (i.e. unlikely to be gas giants, but there could be ice giants or smaller rocky or icy planets). Who knows, maybe future Doppler spectroscopy surveys will be able to detect planetary companions around Scholz's star?

Why is Scholz's star moving so fast?

Probably because it is a somewhat older star. Its velocity with respect to the Sun (about 80 km/s) is not that unusual so an "old thin disk star". Stars are generally classified by their chemical composition, age, and kinematics/velocity into three rough categories: "thin disk", "thick disk", and "halo" (in approximate order of age, with "halo" being the oldest population). The Sun is a medium-aged (~5 billion years) "thin disk" star. Scholz's star appears to be a "thin disk", but probably somewhat older than the Sun given its higher velocity. There are stars that move way faster than Scholz's star that are still bound to our Milky Way. A good famous example is Barnard's star, a nearby ~10 billion-year-old metal-poor star, which is moving at ~140 km/sec with respect to the Sun.

As mentioned previously, the interaction between Scholz's star and the Sun was very light. Scholz's star probably "sped up" by about 0.2 km/sec as it got within a light year of the Sun, but then decreased its speed by the same amount as it sped away from the Sun's gravitational field.

Long story short: there is nothing magical about the velocity of Scholz's star - stars in the solar neighborhood tend to have velocities that differ from that of the Sun by tens of km/sec, and sometimes over 100 km/s. But the velocity of Scholz's star is not that unusual compared to other stars.