by neufer » Sun Jan 27, 2013 2:18 pm
FLPhotoCatcher wrote:
Does sunlight push the comet's dust, or only the particles from the sun?
Sunlight pushes the comet's dust with a radiation force that drops off with the square of the distance. Since this emulates a negative solar gravity the individual particles end up orbiting in ellipses/parabolas/hyperbolas which differ from that of the comet nucleus.
Solar wind pushes cometary ions away much more vigorously such that the ion tail points almost directly away from the sun.
FLPhotoCatcher wrote:
If sunlight, does that mean that light has mass.
Photons have a mass equal to their energy (
![Image](http://upload.wikimedia.org/math/d/0/7/d07def13d6f88776fe72fd064c75f820.png)
) divided by the speed of light squared.
It is just that photons have no
rest mass (because they have no
rest).
FLPhotoCatcher wrote:
Does light make it's own gravity?
Yes.
FLPhotoCatcher wrote:
Does a magnetic field weigh anything?
Yes:
http://en.wikipedia.org/wiki/Comet#Coma_and_tail wrote:
<<In the outer Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations, but these detections have been questioned, and have not yet been independently confirmed. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.
Both the coma and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System, the dust reflecting sunlight directly and the gases glowing from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma temporarily greatly increases. This happened in 2007 to Comet Holmes.
The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions a short tail pointing in the opposite direction to the ion and dust tails may be seen – the antitail. These were once thought somewhat mysterious, but are merely the end of the dust tail apparently projecting ahead of the comet due to our viewing angle.
While the solid nucleus of comets is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend one astronomical unit (150 million km) or more. The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of the photoelectric effect[dubious – discuss] of solar ultra-violet radiation acting on particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. As the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet, in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.
If the ion tail loading is sufficient, then the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event". This has been observed on a number of occasions, one notable event being recorded on April 20, 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.
Comets were found to emit X-rays in 1996. This greatly surprised astronomers, because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion, leading to the emission of X-rays and far ultraviolet photons.>>
[quote="FLPhotoCatcher"]
Does sunlight push the comet's dust, or only the particles from the sun?[/quote]
[b][color=#FF0000]Sunlight pushes the comet's dust[/color][/b] with a radiation force that drops off with the square of the distance. Since this emulates a negative solar gravity the individual particles end up orbiting in ellipses/parabolas/hyperbolas which differ from that of the comet nucleus.
[b][color=#0000FF]Solar wind pushes cometary ions[/color][/b] away much more vigorously such that the ion tail points almost directly away from the sun.
[quote="FLPhotoCatcher"]
If sunlight, does that mean that light has mass.[/quote]
Photons have a mass equal to their energy ([img]http://upload.wikimedia.org/math/d/0/7/d07def13d6f88776fe72fd064c75f820.png[/img]) divided by the speed of light squared.
It is just that photons have no [b][u]rest[/u] mass[/b] (because they have no [b][u]rest[/u][/b]).
[quote="FLPhotoCatcher"]
Does light make it's own gravity?[/quote]
Yes.
[quote="FLPhotoCatcher"]
Does a magnetic field weigh anything?[/quote]
Yes:
[quote=" http://en.wikipedia.org/wiki/Energy_momentum_tensor"]
The relativistic mass density, i.e. the energy density divided by the speed of light squared
for an electromagnetic field in otherwise empty space is [img]http://upload.wikimedia.org/math/8/e/d/8ede3e1f8a6d138bed238aa0b7d0434b.png[/img][/quote]
[quote=" http://en.wikipedia.org/wiki/Comet#Coma_and_tail"]
[float=right][img3="[b][color=#0000FF]Comet Holmes (17P/Holmes) in 2007 showing blue ion tail on right.[/color]
[color=#FF0000]The sudden explosive eruption of Holmes took place too far from the sun
for there to have been much of an effect upon the [u]dust[/u] "tail"[/color][/b]"]http://upload.wikimedia.org/wikipedia/commons/c/c6/17pHolmes_071104_eder_vga.jpg[/img3][/float]
<<In the outer Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations, but these detections have been questioned, and have not yet been independently confirmed. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.
Both the coma and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System, the dust reflecting sunlight directly and the gases glowing from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma temporarily greatly increases. This happened in 2007 to Comet Holmes.
The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions a short tail pointing in the opposite direction to the ion and dust tails may be seen – the antitail. These were once thought somewhat mysterious, but are merely the end of the dust tail apparently projecting ahead of the comet due to our viewing angle.
While the solid nucleus of comets is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend one astronomical unit (150 million km) or more. The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of the photoelectric effect[dubious – discuss] of solar ultra-violet radiation acting on particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. As the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet, in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.
If the ion tail loading is sufficient, then the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event". This has been observed on a number of occasions, one notable event being recorded on April 20, 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.
Comets were found to emit X-rays in 1996. This greatly surprised astronomers, because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion, leading to the emission of X-rays and far ultraviolet photons.>>[/quote]