This is the archive of all Topics of the Week whose end date was more than 30 days ago.
7 Answers
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Index:
- October 20-26, 2013 - meteor and orionids
- October 27-Nov 2, 2013 - cmb
- November 10-16, 2013 - dwarf-planets and stellar-dynamics
- November 24-30, 2013 - planetary-formation
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meteor (October 20 - October 26, 2013)
A meteor or "shooting star" is the visible streak of light from a meteoroid or micrometeoroid, heated and glowing from entering the Earth's atmosphere, as it sheds glowing material in its wake. Meteors typically occur in the mesosphere at altitudes between 76 km to 100 km (46–62 miles). The root word meteor comes from the Greek meteōros, meaning "suspended in the air".
from Wikipedia
Meteor may also be used commonly as a shortened form of meteoroid.
A meteoroid is a small rocky or metallic body travelling through space. Meteoroids are significantly smaller than asteroids, and range in size from small grains to 1 meter-wide objects. Most are fragments from comets or asteroids, while others are collision impact debris ejected from bodies such as the Moon or Mars.
from Wikipedia
Example questions:
Have we ever observed a large meteor hit the Moon?
Why did the Chelyabinsk meteor explode?
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Upvotes: 2
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dwarf-planets (used November 10 - November 16, 2013)
A dwarf planet is a planetary-mass object that is neither a planet nor a satellite. More explicitly, the International Astronomical Union (IAU) defines a dwarf planet as a celestial body in direct orbit of the Sun that is massive enough for its shape to be controlled by gravitation, but that unlike a planet has not cleared its orbital region of other objects. The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of trans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies. Dwarf planets come in between.
from Wikipedia
Example questions:
What physical features determine if a planet is a major, minor or dwarf planet?
How many planets are there in this solar system?
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Upvotes: 1
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orionids (used October 20 - October 26, 2013)
The Orionid meteor shower, usually shortened to the Orionids, is the most prolific meteor shower associated with Halley's Comet. The Orionids are so-called because the point they appear to come from, called the radiant, lies in the constellation Orion, but they can be seen over a large area of the sky. Orionids are an annual meteor shower which last approximately one week in late-October. In some years, meteors may occur at rates of 50-70 per hour.
from Wikipedia
I suggest that this tag run next week during the Orionid meteor shower.
Posted by:
Upvotes: 2
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cmb (cosmic microwave background radiation) (used October 27 - November 2, 2013)
The cosmic microwave background (CMB) is the thermal radiation left over from the "Big Bang" of cosmology. In older literature, the CMB is also variously known as cosmic microwave background radiation (CMBR) or "relic radiation." The CMB is a cosmic background radiation that is fundamental to observational cosmology because it is the oldest light in the universe, dating to the epoch of recombination. With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark. However, a sufficiently sensitive radio telescope shows a faint background glow, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum. The CMB's serendipitous discovery in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned the discoverers the 1978 Nobel Prize.
The CMB is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380,000 years old. It shows tiny temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all future structure: the stars and galaxies of today.
from Wikipedia
Example questions:
What is the mass of hotspots in the Cosmic Microwave Background radiation?
Future of CMB observations: How will our knowledge of the early universe change?
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Upvotes: 3
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stellar-dynamics (used November 10 - November 16, 2013)
Stellar dynamics is the branch of astrophysics which describes in a statistical way the collective motions of stars subject to their mutual gravity. The essential difference from celestial mechanics is that each star contributes more or less equally to the total gravitational field, whereas in celestial mechanics the pull of a massive body dominates any satellite orbits. Stellar dynamics is usually concerned with the more global, statistical properties of several orbits rather than with the specific data on the positions and velocities of individual orbits. The motion of stars in a galaxy or in a globular cluster are principally determined by the average distribution of the other, distant stars, and little influenced by the nearest stars.
from Wikipedia
Example question:
Relativistic effects in stellar dynamical systems
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Upvotes: 1
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planetary-formation (used November 24-30, 2013)
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation is now thought to be at work throughout the universe. The widely accepted modern variant of the nebular hypothesis is Solar Nebular Disk Model (SNDM) or simply Solar Nebular Model.
According to the nebular hypothesis, stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds (GMC). They are gravitationally unstable, and matter coalesces to smaller denser clumps within, which then proceed to collapse and form stars. Star formation is a complex process, which always produces a gaseous protoplanetary disk around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form.
The protoplanetary disk is an accretion disk which proceeds to feed the central star. Initially very hot, the disk later cools in what is known as the T tauri star stage; here, formation of small dust grains made of rocks and ices is possible. The grains may eventually coagulate into kilometer-sized planetesimals. If the disk is massive enough the runaway accretions begin, resulting in the rapid—100,000 to 300,000 years—formation of Moon- to Mars-sized planetary embryos. Near the star, the planetary embryos go through a stage of violent mergers, producing a few terrestrial planets. The last stage takes around 100 million to a billion years.
from Wikipedia
Example questions:
- Why haven't asteroid belts turned into new large bodies?
- Can protoplanetary disks form main-sequence stars?
- Generalised planets?
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Upvotes: 1