Special Sub-Topic: Star Wars
|The headline for the March 15, 2010 edition of "Scientific American" read "Star set to collide with solar system". The orange dwarf star called Gliese 710 is considered 86% likely to collide with what part of our solar system within the next million and a half years?|
Oort Cloud. Reading the actual story somewhat alleviates the sense of panic engendered by the headline; although it could be spectacular, I won't be watching. The Oort Cloud is a hypothetical spherical cloud of comets located almost a light-year away from the sun. Its outer edge is considered to be the boundary of our solar system (hands up if you vaguely thought that it ended around Pluto). Astronomers have not yet made direct observations of the Oort Cloud, but it is confidently believed to exist, and to be the source for a number of long-period comets orbiting our sun. If Gliese 710 does indeed run into this cloud, it could send a shower of comets towards the planetary region of the solar system, with potentially devastating effect. It has been suggested that a similar event may have been responsible for covering our moon with craters around 4 billion years ago.
|In 2010, it was suggested that our sun may be part of a binary system, with an as-yet-unobserved dwarf star lurking nearby that might be responsible for an ostensible 26-million year cycle of mass extinctions on Earth. What nickname was given to this possible star?|
Nemesis. Paleontologists David Raup and Jack Sepkoski have suggested there is evidence that life on earth has been subject to mass extinctions every 26 million years or so. If the Oort Cloud (see explanation in previous question) were to be disturbed by the gravitational pull of a red or brown dwarf star nearby, it would periodically send comet showers towards the inner solar system, the region where planets are found. These could cause immense damage. It is believed that the Tunguska event of 1908 in Russia, a massive unexplained explosion that flattened trees over an area of around 830 square miles, might have been caused by a comet impact. This hypothetical binary partner for our sun was nicknamed Nemesis, and also Death Star. While the theory of cyclic extinctions is not universally accepted, there are a number of other observations, such as the dwarf planet Sedna, that lead some to believe that Nemesis is indeed out there. WISE (Wide-Field Infrared Survey Explorer) has been scanning the heavens since early in 2010, and may yet provide a definite answer on the question of Nemesis.
|According to current astronomical thought, a collision of two white dwarf stars may be responsible for the creation of stars in our galaxy whose spectra show very little of the most common element in the universe. What extremely light element are they missing?|
Hydrogen. In the entire Milky Way galaxy, only a few dozen hydrogen-deficient stars have been found. Unusually, they have virtually no hydrogen, which is the usual fuel source for the nuclear reactions in a star. One explanation is that they are formed when two white dwarf starts collide. A white dwarf is formed when a smallish star (our sun is an example of a star on this pathway) runs out of hydrogen, and starts consuming the helium that has been produced. When that runs out, the next stage for a larger star would be turning to carbon, but stars with insufficient mass to initiate that reaction collapse and cool, forming a white dwarf. Left on its own, it will eventually die out completely; but if two white dwarves happen to come close enough to each other to collide, the process may allow nuclear fusion to start again because their combined mass is large enough to sustain carbon fusion.
|In some parts of the sky, it appears that there are stars which are out of step with main sequence stars in terms of stellar evolution. What are these anomalous stars called?|
Blue stragglers. The main sequence of stars refers to the position of stars on a graph called a Hertzsprung-Russell diagram, which plots a star's brightness (on the vertical axis) against its color (on the horizontal axis). Most stars fall within a wide band called the main sequence, which is considered to represent 'normal' stellar evolution. Blue stragglers are stars in a globular cluster that are brighter, bluer and heavier than the pattern suggests they should be. One explanation for their existence is that they were formed when two old main sequence stars collided. The renewed fusion cycle would produce a younger-seeming star, as is the case for a blue straggler.
|Two stars that orbit each other are called a binary star system. Some binary systems have spectra which show periodic changes. What is the nickname for such a system, based on the currently-accepted explanation that their spectral variability is due to interacting stellar winds?|
Colliding-wind binary. Stars often emit streams of gas from their outer atmosphere, called a stellar wind. If two large stars orbit each other, their stellar winds may interact with each other, which impacts on the stars' spectra. The variable spectra were originally explained in the 1950s by Struve and Sahade as the stellar wind from one star shielding the other star as they rotated. Other explanations have been made since, including the idea that the stellar winds between the two stars produce radio and X-ray emission as they collide. The shock wave produced by the collision also causes the formation of interstellar dust, which can subsequently form a pinwheel nebula such as are seen in the Quintuplet Cluster. One of the best examples of a colliding-wind binary is called WR140, composed of a star roughly 20 times the size of our sun and another approximately 50 times its size, which rotate each other in a highly elliptical orbit with an eight-year cycle. The eccentricity of their orbit increases the magnitude of the variations observed over time.
|A computer simulation at the University of Amsterdam showed that, in dense star clusters, the heaviest stars tend to move towards the centre of the cluster, increasing their chances of colliding with each other, and forming a larger mass. This can lead to what has been termed 'runaway growth', which is speculated to be a possible mechanism for the formation of which of these celestial objects?|
Intermediate black hole. A black hole is a region of space from which nothing can escape once it is within the region called the event horizon. Stellar mass black holes are thought to form when a sufficiently-large star reaches the end of its life cycle. Supermassive black holes, much heavier than stellar mass ones, are thought to be found at the centre of all active galaxies. As yet, there is no good explanation for their formation, but the model of runaway growth that seems to be able to produce intermediate black holes (larger than stellar mass, and not explainable by the collapse of a single star) suggests that an accretion model might work. Just as you can make a very large ball for building a snowman by rolling a small ball around in the snow and adding snow to the outside, these objects could continue colliding with other objects and adding them on to the already-accumulated material.
|The merging of two neutron stars has been proposed as the source of short-duration GRBs. For what does the acronym GRB stand?|
Gamma-ray burst. Gamma-rays are an extremely high-energy form of radiation. Long-duration gamma-ray bursts (lasting around half a minute) are associated with the supernova event in which a young and very heavy star collapses to form a neutron star or a black hole. Short-duration bursts last less than a second, and were first observed in the late 1960s. Until 2005, there was insufficient evidence to provide any explanation of their occurrence. In that year two short-duration GRBs were detected under conditions that allowed a careful study of them, leading to the conclusion that they were definitely not produced by a single-star event, and the proposal that they could be explained by the collision of two neutron stars or of a neutron star with a black hole. If this is so, oservations on them may lead to the detection of the gravitational waves that Einstein predicted should be generated when neutron stars collide.
|About 49 million years ago, in the galaxy called Messier 85, two ordinary stars collided, producing an explosion of a previously-unobserved nature that was detected in 2006. What is the name given by astronomers to this event?|
M85OT2006-1. It has taken 49 million years for the light from M85OT2006-1 to reach Earth and be observed. Astronomers use shorthand to identify stellar observations such as this: M85 shows that it was observed in Messier 85; OT shows that it was an Optical Transient, or short-term visual observation; 2006 is the year of observation, and 1 means that it was the first M85OT observation that year. The galaxy called Messier 85 lies in the constellation Coma Berenices, in the Virgo cluster. Its name refers to its designation as the 85th object in the catalogue compiled by the French astronomer Charles Messier (1730-1817) of 110 permanent deep space objects such as galaxies and nebulae.
The energy released by M85OT2006-1 was classified as a new type of stellar explosion, as it was not consistent with previously-identified novae (too bright), supernovae (not bright enough) and gamma-ray bursts (wrong energy range). The most likely scenario is believed to be the collision of two perfectly ordinary stars, probably similar in size to our own sun.
|A recent computer model suggests that an unusual feature observed near Supernova 1987A, a supernova in the Large Magellanic Cloud, may have been formed when two stars collided prior to the supernova event. What feature is being explained?|
Three-ring nebula. The three-ring nebula is a set of three rings of gas surrounding the star designated as SN1987A. According to the model developed by astrophysicists at the University of Oxford, the story is something like this. Two stars, one about 5 times as large as our sun and one three times as large again, collided to form a red supergiant, which shrank into a blue supergiant referred to as SK-69; this star eventually exploded to produce SN1987A. During the collision, part of the gases from the two stars was blasted out, forming the first two rings. The transition from a red to a blue supergiant caused the emission of the gas that forms the third ring, which is closer to the star and in a different plane. The model has not been fully tested, since it will be many years before the necessary data is available, and is not universally accepted as a definitive explanation of the formation of the three-ring nebula.
|It is not only individual stars that can be seen to collide with each other. The Cartwheel Galaxy is thought to have been a spiral galaxy similar to our Milky Way before it experienced a collision with another galaxy. What galaxy seems to be on a collision course with the Milky Way?|
Andromeda Galaxy. The expansion of the universe means that most galaxies are moving away from each other, but local revolutionary motions can cause some galaxies to approach each other, and even collide. Andromeda is expected to collide with the Milky Way in about three billion years. Don't lose too much sleep over it! All the other galaxies mentioned are or have been involved in inter-galactic collisions. The Cartwheel Galaxy is so named because it has a large ring of stars around its rim, thought to have been formed when an original spiral galaxy, similar to ours, collided with another.
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