Also among the brightest stars are Rigel A, a young supergiant in the constellation Orion , and Canopus , a bright beacon in the Southern Hemisphere often used for spacecraft navigation. It has been found that stars of many types are active and have stellar winds analogous to the solar wind. The importance and ubiquity of strong stellar winds became apparent only through advances in spaceborne ultraviolet and X-ray astronomy as well as in radio and infrared surface-based astronomy. X-ray observations that were made during the early s yielded some rather unexpected findings. They revealed that nearly all types of stars are surrounded by coronas having temperatures of one million kelvins K or more.
Furthermore, all stars seemingly display active regions, including spots, flares, and prominences much like those of the Sun see sunspot ; solar flare ; solar prominence. Some stars exhibit starspots so large that an entire face of the star is relatively dark, while others display flare activity thousands of times more intense than that on the Sun. The highly luminous hot, blue stars have by far the strongest stellar winds. Observations of their ultraviolet spectra with telescopes on sounding rockets and spacecraft have shown that their wind speeds often reach 3, km roughly 2, miles per second, while losing mass at rates up to a billion times that of the solar wind.
The corresponding mass-loss rates approach and sometimes exceed one hundred-thousandth of a solar mass per year, which means that one entire solar mass perhaps a tenth of the total mass of the star is carried away into space in a relatively short span of , years. Accordingly, the most luminous stars are thought to lose substantial fractions of their mass during their lifetimes, which are calculated to be only a few million years. Ultraviolet observations have proved that to produce such great winds the pressure of hot gases in a corona , which drives the solar wind, is not enough.
Instead, the winds of the hot stars must be driven directly by the pressure of the energetic ultraviolet radiation emitted by these stars. Aside from the simple realization that copious quantities of ultraviolet radiation flow from such hot stars, the details of the process are not well understood.
History of observations
Whatever is going on, it is surely complex, for the ultraviolet spectra of the stars tend to vary with time, implying that the wind is not steady. In an effort to understand better the variations in the rate of flow, theorists are investigating possible kinds of instabilities that might be peculiar to luminous hot stars.
Observations made with radio and infrared telescopes as well as with optical instruments prove that luminous cool stars also have winds whose total mass-flow rates are comparable to those of the luminous hot stars, though their velocities are much lower—about 30 km 20 miles per second. Because luminous red stars are inherently cool objects having a surface temperature of about 3, K , or half that of the Sun , they emit very little detectable ultraviolet or X-ray radiation; thus, the mechanism driving the winds must differ from that in luminous hot stars.
Winds from luminous cool stars, unlike those from hot stars, are rich in dust grains and molecules.
Astronomy for Kids: Stars
Since nearly all stars more massive than the Sun eventually evolve into such cool stars, their winds, pouring into space from vast numbers of stars, provide a major source of new gas and dust in interstellar space, thereby furnishing a vital link in the cycle of star formation and galactic evolution. As in the case of the hot stars, the specific mechanism that drives the winds of the cool stars is not understood; at this time, investigators can only surmise that gas turbulence, magnetic fields, or both in the atmospheres of these stars are somehow responsible.
Strong winds also are found to be associated with objects called protostars , which are huge gas balls that have not yet become full-fledged stars in which energy is provided by nuclear reactions see below Star formation and evolution. Radio and infrared observations of deuterium heavy hydrogen and carbon monoxide CO molecules in the Orion Nebula have revealed clouds of gas expanding outward at velocities approaching km 60 miles per second.
Furthermore, high-resolution, very-long-baseline interferometry observations have disclosed expanding knots of natural maser coherent microwave emission of water vapour near the star-forming regions in Orion, thus linking the strong winds to the protostars themselves. The specific causes of these winds remain unknown, but if they generally accompany star formation, astronomers will have to consider the implications for the early solar system. After all, the Sun was presumably once a protostar too.
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As a protostar gets smaller, it spins faster because of the conservation of angular momentum—the same principle that causes a spinning ice skater to accelerate when she pulls in her arms. Increasing pressure creates rising temperatures, and during this time, a star enters what is known as the relatively brief T Tauri phase. Most of the stars in our galaxy, including the sun, are categorized as main sequence stars. They exist in a stable state of nuclear fusion, converting hydrogen to helium and radiating x-rays.
This process emits an enormous amount of energy, keeping the star hot and shining brightly. Some stars shine more brightly than others. Their brightness is a factor of how much energy they put out—known as luminosity —and how far away from Earth they are. Color can also vary from star to star because their temperatures are not all the same. Hot stars appear white or blue, whereas cooler stars appear to have orange or red hues. By plotting these and other variables on a graph called the Hertzsprung-Russell diagram, astronomers can classify stars into groups.
Along with main sequence and white dwarf stars, other groups include dwarfs, giants, and supergiants. Supergiants may have radii a thousand times larger than that of our own sun.
Stars spend 90 percent of their lives in their main sequence phase. Now around 4.
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As stars move toward the ends of their lives, much of their hydrogen has been converted to helium. Helium sinks to the star's core and raises the star's temperature—causing its outer shell of hot gases to expand. These large, swelling stars are known as red giants. The red giant phase is actually a prelude to a star shedding its outer layers and becoming a small, dense body called a white dwarf.
Star Facts: The Basics of Star Names and Stellar Evolution
White dwarfs cool for billions of years. Some, if they exist as part of a binary star system , may gather excess matter from their companion stars until their surfaces explode, triggering a bright nova. Eventually all white dwarfs go dark and cease producing energy. At this point, which scientists have yet to observe, they become known as black dwarfs. Massive stars eschew this evolutionary path and instead go out with a bang—detonating as supernovae.
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While they may appear to be swelling red giants on the outside, their cores are actually contracting, eventually becoming so dense that they collapse, causing the star to explode. These catastrophic bursts leave behind a small core that may become a neutron star or even, if the remnant is massive enough, a black hole. In cities and other densely populated areas, light pollution makes it nearly impossible to stargaze.