Starship Asterisk*

APOD Robot wrote, on January 26, 2013:
when stars get hairy

http://adsg.syix.com/linda/cecilia/comets.htm
Since the beginning of history people have seen comets appear in the night sky, rising and setting with the stars. The ancient Greeks called them "hairy stars", and the name "comet" comes from the Greek word for hair. This is because the tail of a comet was thought to look like long hair streaming from a woman's head.

http://en.wikipedia.org/wiki/Coma_Berenices wrote:
Coma Berenices is a traditional asterism that has since been defined as one of the 88 modern constellations. It is located near Leo, to which it formerly belonged, and accommodates the North Galactic Pole. Its name means "Berenice's Hair" (in Greek, via Latin), and refers to the legend of Queen Berenice II of Egypt, who sacrificed her long hair.
...
Coma Berenices is one of the few constellations to owe its name to a historical figure, in this case Queen Berenice II of Egypt, wife of Ptolemy III Euergetes (fl. 246 BC–221 BC), the king under whom Alexandria became an important cultural center.

Berenike II. Source: Wikipedia

In 243 BC, during the Third Syrian War, Ptolemy undertook a dangerous expedition against the Seleucids, who had murdered his sister. His newlywed bride, Berenice, swore to the goddess Aphrodite to sacrifice her long, blonde hair, of which she was extremely proud, if her husband returned safely. He did, so she cut her hair and placed it in the goddess's temple. By the next morning the hair had disappeared.

Coma Berenices.
Source:hs-2008-24-e-small_web.jpg
hubblesite.org

To appease the furious king the court astronomer, Conon, announced that the offering had so pleased the goddess that she had placed it in the sky. He indicated a cluster of stars that have since been called Berenice's Hair.[1]

This incident inspired the court poet Callimachus of Cyrene to write a poem entitled Βερενίκης πλόκαμος (Greek "Berenice's braid"). About 2/3 of the Greek original is now lost, but the full version was translated to Latin by the Roman poet Catullus, and his version exists to this day.

FLPhotoCatcher wrote:
Does sunlight push the comet's dust, or only the particles from the sun?

FLPhotoCatcher wrote:
If sunlight, does that mean that light has mass.

FLPhotoCatcher wrote:
Does light make it's own gravity?

FLPhotoCatcher wrote:
Does a magnetic field weigh anything?

http://en.wikipedia.org/wiki/Energy_momentum_tensor wrote:
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

http://en.wikipedia.org/wiki/Comet#Coma_and_tail wrote:

[img3="Comet Holmes (17P/Holmes) in 2007 showing blue ion tail on right.
The sudden explosive eruption of Holmes took place too far from the sun
for there to have been much of an effect upon the dust "tail""]http://upload.wikimedia.org/wikipedia/c ... er_vga.jpg[/img3]

<<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.>>

FLPhotoCatcher wrote:If sunlight, does that mean that light has mass? Does light make it's own gravity?

C/2011 L4 (PANSTARRS) most likely originated in the Oort cloud, a cloud of cometlike objects located in the distant outer solar system. It was probably gravitationally disturbed by a distant passing star , sending it on a long journey toward the sun¹.

Joe Stieber wrote:
The bright star above-right of the picture’s center (above-left of the moon) is 1st magnitude Fomalhaut, Alpha Piscis Austrini (in Piscis Austrinus, the Southern Fish). This star is easily visible from mid-northern latitudes. Above the comet’s arching tail, the pair of brighter stars in a slightly tilted vertical line are 2nd magnitude Alpha and Beta Gruis (in Grus, the Crane). Beta is the upper star, Alpha the lower. The brighter star to the left of the comet’s tail is 3nd magnitude Alpha Indi (in Indus, the Indian).

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Ann wrote:
In Fomalhaut's case, it is remarkable in that it has a planet that has been directly imaged by Hubble! See this and this.

http://en.wikipedia.org/wiki/Fomalhaut wrote:

<<Fomalhaut (Alpha Piscis Austrini, α PsA) is the brightest star in the constellation Piscis Austrinus and one of the brightest stars in the sky. Fomalhaut can be seen low in the southern sky in the northern hemisphere in fall and early winter evenings or all year round in the southern hemisphere. Another name in Western use is "the Lonely Star of Autumn", because it is the only first-magnitude star in the autumn sky of mid-northern latitudes. Fomalhaut/Earthwork B in Mounds State Park near Anderson, Indiana lines up with the rising of the star Fomalhaut in the fall months, according to the Indiana Department of Natural Resources.

In Walter Tevis' (1983) novel The Steps of the Sun , Fomalhaut is visited by the protagonist and two potentially inhabitable planets are found (and described). Parts of Philip K Dick's 1964 novel _The Unteleported Man_ are set on the fictional planet Fomalhaut IX.

Fomalhaut is a member of the 16 stars belonging to the Castor Moving Group. This is an association of stars that share a common motion through space and have been claimed to be physically associated. Other members of this group include Castor and Vega. This moving group has an estimated age of 200±100 million years and originated from the same location.

The star has been recognized by many cultures of the northern hemisphere, including the Arabs, Persians and Chinese. It marked the solstice in 2500 BC. It was also a marker for the worship of Demeter in Eleusis. Ptolemy put it in Aquarius as well as Piscis Austrinus. Following Ptolemy, Flamsteed in 1725 denoted it 79 Aquarii as well as 24 Piscis Austrini.

The name Fom al-Haut comes from scientific Arabic فم الحوت fam al-ħūt (al-janūbī) "the mouth of the [Southern] Fish". The Latin names are ōs piscis merīdiāni, ōs piscis merīdionālis, ōs piscis notii "the mouth of the Southern Fish". The name Difda al Auwel comes from the colloquial Arabic الضفدع الأول aḍ-ḍifdiˤ al-’awwal "the first frog" (the second frog is Beta Ceti).>>

Sinan İpek wrote:Why are there streaks all pointing at the Sun in the comet's tail.

My hypothesis: While the comet is orbiting around the sun, the sun's wind varies in its intensity, hence the comet's tail gets brighter and dimmer according to it. Is that true?

Chris Peterson wrote: As Art explained, light does have mass, since mass and energy are equivalent. In the case of photons, however, the analysis doesn't typically involve mass, but momentum. The physical pressure that photons exert is the result of a transfer of momentum. In quantum mechanics, the momentum of a photon is inversely proportional to its wavelength. Of course, in classical mechanics, momentum is just the mass times the velocity, and even though photons aren't strictly described by classical mechanics, we can see here a connection between photons and what we generally describe as mass.

stephen63 wrote:So it seems that light exhibits both particle and wave properties, Wave–particle duality(Wiki). What I always fail to grasp is the ENTIRE electromagnetic spectrum. Please bear with me! At the long wavelength end are Radio waves and the short wavelength end are Gamma rays. Somewhere in between lies the visible spectrum. So, at what point or frequency do EM waves suddenly become photons? It seems to me like I'm comparing apples to oranges. But, not completely, because light also behaves like an EM wave in the double-slit experiment.

Chris Peterson wrote:
Duality doesn't mean that electromagnetic energy is wave-like at some parts of the spectrum and particle-like at others. Photons are the particles that carry electromagnetic energy. This is as true for very short wave radiation like gamma rays, as for very long wavelength radiation like radio. Whether a particular wavelength of radiation seems to us more like a wave or a particle is really a consequence of the specific technology we use to generate or detect it. It's all photons.

http://en.wikipedia.org/wiki/Hydrogen_line wrote:
<<The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum wavelength of 21.10611405413 cm in free space. This wavelength or frequency falls within the microwave radio region of the electromagnetic spectrum, and it is observed frequently in radio astronomy, since those radio waves can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light. The microwaves of the hydrogen line come from the atomic transition between the two hyperfine levels of the hydrogen 1s ground state with an energy difference of 5.87433 µeV. The frequency of the quanta that are emitted by this transition between two different energy levels is given by Planck's equation.>>

APOD Robot wrote: Comet McNaught Over Chile

After making quite a show in the northern hemisphere in early 2007 January

cljohnston108 wrote:

APOD Robot wrote: Comet McNaught Over Chile

After making quite a show in the northern hemisphere in early 2007 January

Ummm, in what universe is Australia in the Northern Hemisphere?