The stellar wind (or solar wind for the Sun) is a stream of matter ejecting a star. It is composed of charged ions, electrons and sometimes neutral atoms. The stellar wind is released with a speed from the star and then it is accelerated with the help of photons. Each star has a different type of stellar wind, influencing weather in its planetary system, terraforming possibilities of surrounding planets and space travel.
Overall[]
Stellar winds have a different composition. Around O - type stars and Wolf - Rayet Stars, the winds have speeds exceeding 1000 km/s, while around Red giants, their speed is low, around 10 km/s. Usually, they are made of hydrogen, which is the dominant element in most stars, but in case of Wolf - Rayet stars, they are often made of helium, carbon, oxygen or even silicon.
Through stellar winds, stars lose matter. Red giants lose about one solar mass in a few thousands of years, while O - type and B - type stars lose one solar mass in tens to hundreds of millions of years. The highest mass loss is found to be in Wolf - Rayet stars. Even if on a stellar lifetime the amount of lass lost is significant, usually this is not enough to prevent them from their deaths. For example, a star with 10 times the mass of our Sun, will lose in its lifetime 2 to 3 solar masses. One of the only exceptions are low Wolf - Rayet stars.
Stellar winds are not homogenous. Coronal mass ejections are stellar eruptions in which a stellar wind can increase its matter and energy output even millions of times. This is seen mostly in case of flares that occur in M - type stars and in case of huge mass losses that accompany Red giants. Even without flares and solar storms, the intensity of a stellar wind is not constant.
Around stars like our Sun or smaller, the magnetosphere can keep plasma from solar winds captive for a limited time.
Stellar winds move away from the star that formed them, until they are overwhelmed by the interstellar environment. There, they are decelerated and in the end stopped. Depending on the strength of a solar wind and on the strength of interstellar medium, they form a larger or smaller transit zone, with or without a bow shock area.
The region surrounding a star where stellar winds are dominant and push the interstellar medium away, is known as heliosphere. Recent data from Voyager shows that Sun's heliosphere is really a sphere. Around brighter stars, there is not quite a sphere, but an elongated space, like the coma of a comet.
Violent events, like a nova or a supernova, are also considered stellar winds.
Stellar remnants, like White dwarfs, Neutron stars or Black holes are known not to have stellar winds. However, given their big mass and strong magnetic fields, they can keep huge radiation belts surrounding them, mainly if they have another stellar companion. And even if they are alone, their magnetic fields can keep the interstellar wind away, in the same way the Earth distorts the Solar wind.
Stellar winds by star[]
Each star behaves completely different.
Non main sequence stars[]
Wolf - Rayet Stars have the stronger winds of all. These stars are literally tearing apart, losing one solar mass in hundreds or thousands years. The fact that WR stars are very hot (30000 to 200000 K) also means that their strong radiation pressure accelerates their wind to speeds above 1000 km/s. Such strong winds have the power to erode atmospheres of their planets. It is possible that even Gas Giants are unable to hold their gaseous envelopes in place without a strong magnetic field.
Supergiant Stars and Super-Supergiant Stars lose one solar mass in a few thousand years, but their winds are extremely slow (about 10 km/s). In such conditions, gas giants tend to accumulate matter from the solar winds, while an Earth - like planet would have its atmosphere slowly eroded.
Red Giants are also known to lose matter fast, but at a lower pace.
Young Stars are known to have strong stellar winds and occasional flares. Their winds might be up to 100 times stronger than those of mature, main sequence stars.
Main sequence stars[]
For the Main Sequence Stars, which form the largest group of stars in the Universe, stellar winds are as follows:
O - type stars lose about one solar mass in a few hundred million years, but their winds are very fast, reaching over 1000 km/s. These winds, combined with their powerful UV and X radiation, can erode atmospheres of smaller planets.
B - type stars have smaller winds, losing one solar mass in a few billion years. Their winds are fast (about 1000 km/s) and pose a thread for any Habitable Zone Planet. Still, these winds are far smaller then those surrounding O - type stars.
A - type stars have winds that are about 1000 times more powerful then those of our Sun. This is small enough for a Habitable Zone Planet without a magnetic field to hold its atmosphere long enough for millennia.
F - type stars have winds less powerful then A - type stars, but still more powerful then those of our Sun.
G - type stars are similar to our Sun. They have moderate stellar winds. It is estimated that if our Sun will never change its luminosity, it will lose all its mass in 10E+15 years, which is 10000 times longer then the Sun's lifetime.
K - type stars have some of the slowest stellar winds known to date. As a result, their heliosphere is also far smaller.
M - type stars are very controversial. Some scientists argue that their stellar winds are at least twice as strong as our Sun's, giving a maximum of 10 to 100 times stronger. If this is the case, giving their long lifetime, these stars might lose enough matter, up to the point where they no longer can sustain hydrogen fusion in their cores. On the other hand, it looks like heliospheres around some M - type stars are very small, extending up to only 10 AU, which can be explained only by very small and least powerful winds. What is known for sure is that these stars are flare stars. During flares, their brightness can increase even 100 times for a short period of time (about a few minutes). Flares occur with coronal mass ejections that can feed the stellar winds. So, maybe the intensity of the wind is strongly correlated with the frequency of flares.
Brown Dwarfs are sometimes theorized to have small stellar winds. Bright, M - type brown dwarfs, are known to have star-like coronas and have winds, while cooler T and Y type brown dwarfs are unable to produce stellar winds.
Stellar remnants[]
White dwarfs and Neutron stars don't have their own stellar winds, but giving their fast rotation period, they have magnetic fields. Given their strong gravity, they can accrete matter from the wind of a nearby star and they also can have radiation belts surrounding them.
Formulas[]
First of all, one must acknowledge that the intensity of a stellar wind will never be that of a theoretical model, because of many parameters: star rotation speed, convection currents, a stellar companion and metalicity (amount of heavier elements then hydrogen and helium). Still, there are two general formulas that can give a terraformer a general view of what to expect near a star:
For red giants and WR stars:
Log(-M) = -8.16+1.77Log(L/Ls)-1.68Log(Teff/K)
where M is the amount of mass loss (in solar masses), L is star's luminosity, Ls is the luminosity of our Sun and Teff is the surface temperature of the star, in degrees K.
For main sequence stars, the formula is:
LogM = -12.76+1.3*Log(L/Ls)
where M is the amount of mass loss, L is star's luminosity and Ls is our Sun's luminosity.
If we consider Sun's Solar Constant (total energy output) as 1, the formula becomes:
M = (10^(-12.76+1.3*LOG10(Ks)))*5754400000000
where Ks is the target star's solar constant (again, considering our Sun's as equal to 1) and M is the mass loss through stellar wind. The formula is adapted to show a value close to 1 for our sun, Sol.
If we want to calculate the energy of the stellar wind, it is very complicated, but there is a much simple formula which gives nearly accurate results:
E(wind) = (10^(-12.76+1.3*LOG10(Ks)))*5754400000000*((t/5778)^4)
where E(wind) is the energy of the wind, Ks is the Solar Constant (rounded for Sun at the value of 1) and t is temperature of the star in degrees K.
This formula can be used to calculate the energy of the stellar wind around a planet:
E(local) = E(wind)/(d^2)
where E(local) is the local wind energy, E(wind) is the wind energy calculated for a star and d is the distance of the planet, given in AU.
Theoretical models[]
Using these formulas, we can calculate the stellar winds expected to exist around stars:
For the Sun (used as an example here):
- Wind mass loss: 1
- Wind energy: 1
- Wind force on a habitable zone planet: 1
O1 star:
- Wind mass loss: 6470000
- Wind energy: 35100000000
- Wind force on a habitable zone planet: 202000
O5 star:
- Wind mass loss: 1384000
- Wind energy: 3650000000
- Wind force on a habitable zone planet: 68900
B1 star:
- Wind mass loss: 31200
- Wind energy: 12810000
- Wind force on a habitable zone planet: 4470
B5 star:
- Wind mass loss: 1152
- Wind energy: 62800
- Wind force on a habitable zone planet: 277
A1 star:
- Wind mass loss: 46.0
- Wind energy: 309
- Wind force on a habitable zone planet: 16.24
A5 star:
- Wind mass loss: 16.84
- Wind energy: 65.1
- Wind force on a habitable zone planet: 7.41
F1 star:
- Wind mass loss: 8.22
- Wind energy: 17.91
- Wind force on a habitable zone planet: 3.54
F5 star:
- Wind mass loss: 4.46
- Wind energy: 7.37
- Wind force on a habitable zone planet: 2.33
G1 star:
- Wind mass loss: 1.229
- Wind energy: 1.301
- Wind force on a habitable zone planet: 1.110
G5 star:
- Wind mass loss: 0.868
- Wind energy: 0.800
- Wind force on a habitable zone planet: 0.891
K1 star:
- Wind mass loss: 0.359
- Wind energy: 0.230
- Wind force on a habitable zone planet: 0.506
K5 star:
- Wind mass loss: 0.1421
- Wind energy: 0.0496
- Wind force on a habitable zone planet: 0.222
M1 star:
- Wind mass loss: 0.0279
- Wind energy: 0.00450
- Wind force on a habitable zone planet: 0.0705
M5 star:
- Wind mass loss: 0.001211
- Wind energy: 0.0000953
- Wind force on a habitable zone planet: 0.01669
Please note that these results are close to reality if we exclude flares. Many of the M-Type Stars have extensive flare activity and are known as Flare Stars. Because of these flares, their stellar winds are usually about 20% the intensity of the solar wind. During massive flares, which release plenty of UV and X energy, the wind can achieve high velocities and erode atmospheres of nearby planets.
For Young Stars like Epsilon Eridani, the stellar wind is 100 to 200 times stronger (150 times for Epsilon Eridani).