The sun emits a constant stream of charged particles (plasma), mostly from high–energy protons and electrons, from the outer atmosphere that is known as the solar wind. Many stars also show a similar phenomenon, which is known as the stellar wind. The solar wind is generally gentle in its motion; however, it can disrupt power on Earth. Stellar winds, even more gentle than the solar wind, are difficult to detect from Earth because these stars are so distant when compared to the sun. However, many stars at certain stages in their evolution have very strong stellar winds. These strong winds produce effects that scientists can observe from Earth. They also can cause the star to lose significant amounts of mass.
The first person to accurately predict the solar wind is acknowledged to be Norwegian scientist Kristian Birkeland (1867–1917). Birkeland made this prediction in 1916. English physicist Frederick Alexander Lindemann (1886–1957), in 1919, suggested that both protons and electrons are involved in the solar wind. Then, in the 1950s, German astronomer Ludwig Biermann (1907–1986), while studying comets, suggested that a comet’s tail always points away from the Sun because of its interaction with the solar wind. The Soviet satellite Luna 1 made the first direct observation of the solar wind in 1959 when it made several measurements of it.
The outermost layer of the sun is the corona, which is a very hot tenuous gas visible only during a solar eclipse. Because the coronal temperatures are typically one or two million degrees Kelvin (K), the individual atoms are ionized and moving very rapidly. The fastest ions are moving faster than the sun’s escape velocity. So they escape, forming the solar wind.
Near the Earth’s orbit, the solar wind particles rush by at speeds of roughly 248 to 310 mi per sec (400 to 500 km per sec). There is considerable variation in the speed because the solar wind is gusty. The solar wind density is also variable, but typically runs to a few particles per cubic centimeter. The solar wind extends well beyond the orbit of dwarf planet Pluto to roughly 100 astronomical units (AUs, 100 times the mean distance from the Earth to the sun). At this point, called the heliopause, the solar wind merges into the interstellar gas and dust.
Analogous to the solar wind, many stars have stellar winds. Because stars are so distant, stellar winds that are as gentle as the solar wind do not produce dramatic effects as seen from the Earth. The stellar winds that scientists observe are, therefore, much stronger than the solar wind. A variety of different types of stars display interesting stellar winds.
The hottest, most massive stars are O spectral class stars, which have at least 15 times the mass of the sun. Wolf–Rayet stars have many characteristics in common with the O stars, but their nature is still not completely understood. Both O stars and Wolf–Rayet stars often have very strong stellar winds.
The surface temperatures of O stars are above 30,000K. Their stellar winds can blow as much as the sun’s mass into space in 100,000 to one million years; the mass of the Earth every year or so. The winds travel outward at speeds as high as 2,175 mi per sec (3,500 km per sec), almost ten times as fast as the solar wind. The Wolf–Rayet stars are hotter, 50,000K (89,541°F; 49,727°C), and their winds are more powerful than the O stars.
These powerful winds from hot stars create huge bubbles around the stars that can be as big as a few hundred light–years across. (One light–year is the distance that light travels in vacuum over a period of one year.) These bubbles form when the stellar wind interacts with the surrounding interstellar medium, the gas and dust between the stars. The stellar wind slows down as it pushes into the interstellar medium. Dragging the interstellar medium along, it creates a region of higher density that is moving more slowly than the stellar wind. However, the stellar wind keeps coming and slams into this region, creating a shock wave. The shock wave heats up the gas in this region and makes it glow, so that astronomers see a bubble around the star. The bubbles are produced from the powerful stellar winds described above. There are also O stars with weaker stellar winds that have less dramatic effects.
The sun and similar stars form from collapsing clouds of gas and dust. After the star forms, the leftover material still surrounds it in a cocoon of gas and dust. How do stars like the sun shed their cocoons? One way is through the stellar winds. Astronomers think that shortly after the sun formed it went through a period when it had a very strong solar wind, which helped blow away the cocoon. It is difficult to know how accurate this scenario is because no humans were around to witness the birth of the solar system.
Astronomers can, however, watch the birth of other stars similar to the sun. T Tauri stars are stars in the process of forming that astronomers think will eventually have properties similar to the sun’s properties. Among other properties, T Tauri stars show evidence of strong stellar winds. They also show a range of thick and thin circumstellar cocoons. Studying the stellar winds from T Tauri stars will help scientists understand how the Sun and similar stars shed their initial cocoons.
Many young stars show bipolar outflows, which are two streams of material blowing away from the star in opposite directions. They usually occur in the birth of stars more massive than the sun. The bipolar outflow stage only lasts about 10,000 years or so, but during that time astronomers see a strong stellar wind from the newly forming star. Why is the outflow bipolar? One theory suggests that the outflow is bipolar because an equatorial disk of material surrounding the star constrains the wind to flow out from the two polar regions. This disk may be the material that will eventually form planets around the star. Another possibility is that the star’s magnetic field forces the outflow into a direction perpendicular to the equatorial disk. The study of stellar winds from newly forming stars will eventually provide scientists with clues to help understand how the sun and solar system formed.
Old stars in the process of dying can also have very strong stellar winds. When stars like the sun exhaust the hydrogen fueling their nuclear fires, they expand into red giants. A typical red giant is about the size of the Earth’s orbit around the sun. Because red giants are so large and the gravitational force decreases with distance from the center, the gravitational force at the surface of a red giant is much less than at the surface of the sun. Hence only a gentle push is needed to allow matter to escape and form a stellar wind. This push might come from the light leaving the star. Light or other radiation striking an object will produce a very small but non–zero force that is called radiation pressure. The radiation pressure on dust grains might provide the needed push. Similarly, this radiation pressure might also play a role in causing the stellar winds from the hot O stars mentioned in a previous paragraph. In addition, many red giants pulsate. They expand and contract in periods of a few years. These pulsations can also provide the needed push to cause significant stellar winds and to cause the loss of quite a bit of mass.
In some cases red giants form planetary nebulae, glowing shells of gas around the star. According to one model, the pulsations create a gentle wind moving at about 6 mi per sec (10 km per sec). This wind is gentle but can carry away the mass of the Sun in as little as 10,000 years. Removing this much mass from the shell of the star exposes the more violent core and unleashes a wind blowing out thousands of miles per second. The fast wind slams into the slow wind and creates a shock wave that heats up the shell of gas until it glows as a planetary nebula. The remaining core of the star collapses into a white dwarf star about the size of the Earth.
The single most important property affecting the evolution of a star is its mass. Therefore, when the stellar wind causes a star to lose mass, its evolution is
Interstellar medium —The matter between the stars.
O stars —The hottest most massive stars when classified on the basis of the star’s spectrum.
Planetary nebula —A shell of hot gas surrounding a star in the late stages of its evolution.
Red giant —An extremely large star that is red because of its relatively cool surface.
Solar wind —A stream of charged and neutral particles that emanates from the Sun and moves into the solar system.
Stellar wind —A stream of particles blowing out from a star.
T Tauri star —An early stage in the evolution of stars like the Sun.
White dwarf —A star that has used up all of its thermonuclear energy sources and has collapsed gravitationally to the equilibrium against further collapse that is maintained by a degenerate electron gas.
Wolf–Rayet star —A very hot energetic star that ejects a shell of gas.
affected. In some cases, these effects are still poorly understood. One reasonably well understood effect occurs for red giants collapsing into white dwarfs as described above. A star having more than 1.4 times the mass of the sun cannot collapse into a stable white dwarf. Stars that exceed this mass limit collapse into neutron stars or black holes. Stellar winds can cause red giants with masses up to about eight times the mass of the sun to lose enough mass to collapse into a white dwarf having less than 1.4 times the mass of the Sun.
Roughly half of all stars occur in binary systems. When a star in a binary system has a stellar wind, it loses mass. Some of this mass is transferred to the other star in the system, so it gains mass. Hence, the mass of both stars in the system changes when one of the stars has a stellar wind. The evolution of both stars in the system is affected.
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