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the life of a star |
Entering the Main Sequence
Once a star-candidate has passed the Brown Dwarf / Protostar phase, and starts fusing Hydrogen into Helium, it is then considered a full-fledged star entering its main sequence. Stars entering their Zero-Age Main Sequence (ZAMS) have passed all the prerequisites to being a star, and now they are burning their primary fuel. This fuel may be different for the more exotic type of star, but it is believed that hydrogen is the principle element that fuels the initial fusion process. The energy that pours out equalizes the forces acting from within and without of the star, which keeps the star from collapsing. (see Classifications).
However, when all the hydrogen fuel in the core is exhausted and there is no longer any heat being generated, the core is crushed by the star's weight, heats up tremendously and ignites unburnt hydrogen in a shell around it. What follows in the events of a stars life depends upon the Masssolar of the star in question. Stars with 0.15 >Msolar<1.4, unless "fed" by ingestion of other protoplanets/protostars, will remain dead and lifeless relics.
Heavier stars live shorter lives because they burn their fusionables at a much higher rate than smaller stars. Again, depending on its mass at this point, this object will become either a red giant, brown dwarf, white dwarf, neutron star, or a black hole after evolving from a nova, type-I, or type II supernova.
The Ingestion of Companion Object
You might think that a planet could have no discernible effect on their stars, however, It is becoming clear that when old stars grow until they are large enough to cannabalize any available planets, their physical makeup can be altered quite significantly. Some scientist think that the "digestion" of a companion planet(s) can have a profound effect on stellar evolution.53d Cannabilism could explain the properties of many giant stars and the beautiful shapes of glowing clouds known as planetary nebulae.
No one dreamed that planets could influence this process, because the only planets likely to be swallowed are close-orbiting ones. Mercury, Venus and the Earth would be little more than a light snack for our Sun, but in the past few years, astronomers have discovered about 20 planets orbiting other stars that are giants at least as massive as Jupiter. At least 5 per cent of nearby stars have planetary companions that orbit uncomfortably close to their stars -- most notably in the case of 51 Pegasi B, which is a hundred times closer to its star than Jupiter is to the Sun. Such planets, if swallowed, could drastically change their stars.
A large planet can continue to orbit inside the star for thousands of years, only slowly being vaporised by the heat. These stars are super-tenuous gas balls with their matter smeared over an absolutely huge volume with their outer regions as rarefied as what we would consider a good vacuum on Earth. Despite the tenuous, aerogel-like nature of the stellar envelope, it still applies a frictional drag that slows anything passing through it. The swallowed planet gradually spirals down towards the core
How close it gets depends on its mass. According to simulations, a planet with the mass of Jupiter would evaporate long before it reached anywhere near the core. Objects more massive than 20 Jupiters stand a statistical chance of survival almost all the way to the core, where the temperature is about 2 million degrees. On approach, they would be dissipated by the combined effects of the heat and the tidal forces exerted by the core, causing the planet to be destroyed.
Gravitational energy dumped by the sinking planet should heat up the star, causing it to puff off its cool outer layers as expanding shells of warm gas and dust glowing with infrared light. Although red giants normally rotate very slowly, the rapidly orbiting planet will raise the spin rate of the star, and seed the star with its heavy elements. Again, these effects depend on the mass of the ingested planet - the more massive the planet, the greater the effect.
Planets are not the only stellar companions that can be cannabalized by a companion star when it becomes a giant. Astronomers using the infrared sky surveys DENIS and 2MASS have discovered a few dozen solitary brown dwarfs which can have a more profound effect on a star. They may even merge with the stellar core. Whether they merge or not depends on the stage in the star's life when they are swallowed. If the star's envelope is thin, containing less than a hundredth of a solar mass of gas, the gravitational energy unleashed by a sinking brown dwarf may be enough to eject the envelope, leaving the dwarf in orbit around the stellar core. If the envelope is thicker, the brown dwarf will either be dissipated or collide and merge with the core.
Fortunately, some of the effects of the ingestion of a planet or brown dwarf, such as a high spin rate and lithium abundance, should persist for hundreds of thousands of years. Lithium is an element that does not normally survive for long in a star, which suggests that it was added by a planet fairly recently. Many red giants have all of these tell-tale traits:
Between 4 and 8 per cent of red giants show evidence of planet swallowing. This agrees with empirical estimates of how common planets are. The planet-swallowing hypothesis seems to be the best explanation for the origin of these lithium-rich giants - at least for now.
Planet swallowing may also explain another puzzle - At the end of its giant phase, a typical stars' radiation has completely blown away its outer layers, exposing the core, which ends up as a super-dense white dwarf. Intense ultraviolet radiation from the core ionizes the surrounding ejectae, causing it to fluoresce.
This is how a planetary nebula is formed, named for the glowing cloud's resemblance to a planetary disc. There is no evident reason for the gas to be ejected more forcefully in one direction than another, so you might think these nebulae would be spherical. But many are bipolar, with material ejected along a preferred axis.
Twenty years ago it was theorized that this bipolar shape may arise when the nebula has another star as a close companion. The companion becomes enveloped in the main star's outer layers and spirals inwards, dumping enough energy to eject these layers at about 20 kilometres per second. This material stays in the plane of the companion's orbit to form a torus-shaped cloud around the main star. Later, when the star's core is exposed, its radiation drives the ejectae in the stellar wind at velocities up to 1000 kilometres a second.
The wind blows in all directions, but because it is impeded by the slow-moving gas in the doughnut, it emerges perpendicularly as a bipolar outflow. "The doughnut acts like a corset, and the wind blows two bubbles perpendicular to the corset," says Livio.
The problem with Livio's idea was that only a few planetary nebulae appeared to have both a white dwarf and a spiralled-in companion in their hearts. Working with his student Noam Soker, who is now at the University of Haifa in Israel, Livio showed that it's no problem if the companion was a massive planet or brown dwarf. "It could have completely dissipated or merged with the white dwarf," he says.
All this applies to stars near the end of their lives - there are high quantities of heavy metals in the atmospheres of several stars with massive planets -- a sign that they once ate one or more planets ("Death stars", New Scientist, 23 October, p 10). These unfortunate bodies may have been dragged inwards by tides in the disc of gas surrounding each young star.
Lithium-rich Giant Stars
Despite all the successes of the planet-gobbling idea, however, there are still a few puzzles left to solve. One way to do this is to follow the chemical tracer element, lithium, which is normally destroyed inside stars. A newly devoured Jovian planet would provide a fresh supply of lithium to the star, and this shows up as an anomalous excess in the star's spectrum.
Pilachowski points out that the story became more complex with the recent observation of a lithium-rich giant in the globular star cluster M3. Such a star is bigger and brighter because it absorbs gravitational energy from the orbiting companion. This heats the star so that it puffs off expanding shells of dust, which radiate excessive amounts of infrared light. A team led by Robert Kraft of the University of California at Santa Cruz reported finding the star earlier this year. The astronomers argue that it contains far more lithium than a planet could have carried -- in fact almost ten times as much lithium as any planet could have delivered to it. "They argue that the lithium must have actually been made in the star by nuclear reactions," says Pilachowski. "I think the jury is still out. As usual, we need more data."
This apparent discrepancy takes nothing away from Livio and Siess's work. "Their work highlights an important weakness in our theory of stellar evolution," she says. "Namely that planets can have a profound, and until now unappreciated, effect on the evolution of stars." If the planets are the mass of Jupiter, or greater, they will have a profound effect on the red giant's evolution.53e
Giant planets carry the lion's share of angular momentum in a stellar system. For example, Jupiter and Saturn contain 98 percent of the angular momentum in the solar system. The orbiting planet also can transfer angular momentum to the star, causing it to "spin up" to a much faster rate than it would normally have. This is similar to those mechanics that spin-up a Magnetar, millisecond Pulsar, or even a Soft Gamma Repeater / Gamma Ray Burster.
Stars with ~ 1.4 >Msolar<8 experience the surge of new heat as it inflates the outer parts of the star, swelling it into a monstrous red giant often more than a hundred times the diameter of the Sun. The red giant phase lasts about a tenth as long as the main sequence. Any star that becomes a giant can swallow nearby planets, and almost all stars eventually evolve into giants. In our solar system Jupiter is too far from the Sun to be swallowed up when the Sun expands to a red giant in about 5 billion years. However, detections of extrasolar planets do show that Jupiter-sized planets can orbit unexpectedly close to their parent stars. Some are even closer than Earth is to our Sun. These worlds are doomed to be eventually swallowed and incinerated.53e
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