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Archive:Hoag's Object

2007-07-31T17:13:10Z

168.215.251.1: added distance units

{{Galaxy
| name = Hoag's Object
| image = [[Image:Hoag'sobject.jpg|300px]]<br />
<small>Hoag's Object taken by the [[Hubble Space Telescope]].<br>
Courtesy of [[NASA]]/[[ESA]]</small>
| epoch = [[J2000]]
| ra = {{RA|15|17|14.4}}<ref name="ned">{{cite web
| title=NASA/IPAC Extragalactic Database
| work=Results for Hoag's Object
| url=http://nedwww.ipac.caltech.edu/
| accessdate=2006-12-21}}</ref>
| dec = {{DEC|+21|35|08}}<ref name="ned" />
| constellation name = [[Serpens Caput]]
| z = 12,740 ± 50 [[kilometer|km]]/[[second|s]]<ref name="OConnelletal1974">{{cite journal
| author=O'Connell, Robert W.; Scargle, Jeffrey D.; Sargent, W. L. W.
| title=The Nature of Hoag's Object . .
| journal=Astrophysical Journal
| year=1974
| volume=191
| pages=61-62
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1974ApJ...191...61O }}</ref><ref name="ned" />
| dist_ly = 600 ± 30 [[light-year|Mly]] (183 ± 8 [[parsec|Mpc]])<ref name="OConnelletal1974" />{{Ref_label|A|a|none}}
| type = SA0(r){{Fact|date=February 2007}}
| appmag_v = 16.0<ref name="ned" />
| size_v = 0′.28 × 0′.28<ref name="ned" />
| notes = Ring galaxy
| names = [[Principal Galaxies Catalogue|PGC]] 54559,<ref name="ned" /> PRC D-51<ref name="ned" />
}}

'''Hoag's Object''' is a non-typical [[galaxy]] of the type known as a [[ring galaxy]]. The appearance of this object has interested amateur astronomers as much as its uncommon structure has fascinated professionals. The galaxy was discovered in [[1950]] by astronomer [[Art Hoag]] who identified it as either a [[planetary nebula]] or a [[peculiar galaxy]].<ref name="Hoag1950">{{cite journal
| author=Hoag, Arthur A.
| title=A peculiar object in Serpens
| journal=Astronomical Journal
| year=1950
| volume=55
| pages=170
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1950AJ.....55Q.170H
}}</ref>

==Characteristics==
A nearly perfect ring of young hot blue [[star]]s circle the older yellow nucleus of this ring galaxy ~600 million [[light-year]]s away in the constellation [[Serpens]]. The diameter of the 6″ inner core of the galaxy is about 17 ± 0.7 kly (5.3 ± 0.2 kpc) while the surrounding ring has an inner 28″ diameter of 75 ± 3 kly (24.8 ± 1.1 kpc) and an outer 45″ diameter of 121 ± 4 kly (39.9 ± 1.7 kpc), which is slightly larger than the [[Milky Way Galaxy]].<ref name="OConnelletal1974" />{{Ref_label|A|a|none}} The gap separating the two stellar populations may contain some [[star cluster]]s that are almost too faint to see. As rare as this type of galaxy is, oddly another more distant ring galaxy can be seen, between the nucleus and the outer ring at the 0100 (one o'clock) position.

==History and formation==
Interestingly, even though Hoag's Object was clearly shown on the [[Palomar Star Survey|Palomar ''Star Survey'']], it was not included in either the ''[[Morphological Catalogue of Galaxies]]'', the ''[[Catalogue of Galaxies and Clusters of Galaxies]]'', nor the ''[[Catalogue of Galactic Planetary Nebulae]]''.<ref name="OConnelletal1974" />

In the initial announcement of his discovery, Art Hoag proposed the hypothesis that the visible ring was a product of [[gravitational lensing]]. This idea was later discarded due to the fact that the nucleus and the ring have the same [[redshift]], and because more advanced telescopes revealed the knotty structure of the ring, something that would not be visible if the ring were the product of gravitational lensing.<ref name="Schweizeretal1987">{{cite journal
| author=Schweizer, Francois; Ford, W. Kent, Jr.; Jederzejewski, Robert; Giovanelli, Riccardo
| title=The structure and evolution of Hoag's object
| journal=Astrophysical Journal, Part 1 (ISSN 0004-637X)
| year=[[September 15]] [[1987]]
| volume=320
| pages=454-463
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1987ApJ...320..454S
}}</ref>

Many of the details of the galaxy remain a mystery, foremost of which is how it formed. So-called "classic" ring galaxies are generally formed by the collision of a small galaxy with a larger disk-shaped galaxy. This collision produces a density wave in the disk which leads to a characteristic ring-like appearance. However, there is no sign of any second galaxy that would have acted as the "bullet", and the core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely.

It has been suggested that Hoag's Object might be a product of an extreme "bar instability" which occurred a few billion years ago in a [[barred spiral galaxy]]. Schweizer, et al. (1987)<ref name="Schweizeretal1987" /> claim that this is an unlikely hypothesis due to the fact that the nucleus of the object is spheroidal, whereas the nucleus of a barred spiral galaxy is disc-shaped, among other reasons. However, they admit evidence is somewhat thin for this particular dispute to be settled satisfactorily. Interestingly, a few galaxies share the primary characteristics of Hoag's Object, including a bright detached ring of stars, but their centers are elongated or barred, and they may exhibit some spiral structure. While none match Hoag's Object in symmetry, this handful of galaxies are known to some as Hoag-type galaxies.

While there is much still to be explained about the formation of Hoag's Object, it is suggested that a major "accretion event", in other words a collision or a capturing of one galaxy by another, caused the formation of the galaxy we see today. The event likely took place at least 2-3 billion years in the past, and may have resembled the processes that form [[polar-ring galaxy|polar-ring galaxies]].

==External links==

* [http://heritage.stsci.edu/2002/21/big.html Hubble Space Telescope image of Hoag's Object.]
* http://nedwww.ipac.caltech.edu/level5/Rings/Rings17_2.html
* http://oposite.stsci.edu/pubinfo/PR/2002/21/index.html
*[http://www.spacetelescope.org/images/html/opo0221a.html ESA/Hubble image of Hoag's Object]

== Notes ==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}}1974 O'Connell paper<ref name="OConnelletal1974" /> assumes a [[Hubble constant]] of 75 this figure adjusts for 2006's 70{{±|2.4|3.2}} (km/s)/Mpc.</li>
</ol>
</div>

==References==

<div class="references-small"><references/></div>

[[Category:Ring galaxies]]
[[Category:Serpens constellation]]
[[Category:PGC objects|54559]]

[[de:Hoags Objekt]]
[[lb:Hoag's Objet]]
[[sk:Hoagov objekt]]

Archive:Project fork

2007-07-06T12:03:46Z

168.215.92.130: /* Proprietary software */

{{otheruses|fork (disambiguation)}}

In [[software engineering]], a '''project fork''' happens when developers take a copy of [[source code]] from one [[software package]] and start independent development on it, creating a distinct piece of software. [[free software|Free]] or [[open source]] software is inherently prone to forking.

A kind of fork that is standard practice in many projects is a ''stable'' or ''release'' version, modified only for bug fixes, while a ''development'' tree develops new features. This is common practice in the [[Linux kernel]], for instance, but has been misrepresented occasionally in the trade press as the more problematic sort of fork described above.[http://www.groklaw.net/article.php?story=20041121124609671] Such forks are often referred to instead as "branches" both to avoid the negative connotations of a fork and because it is closer in intent and function to the common software engineering meaning of [[branching (software)|branching]].

==Free software==

In free software, forks often result from a [[schism (religion)|schism]] over different goals or personality clashes. In a fork, both parties assume nearly identical code bases but typically only the larger group, or that containing the original [[software architecture|architect]], will retain the full original name and its associated user community. Thus there is a reputation penalty associated with forking. The relationship between the different teams can be cordial (''e.g.'' [[Ubuntu Linux|Ubuntu]] and [[Debian]]), very bitter ([[XOrg Server]] and [[XFree86]]) or none to speak of (most branching [[Linux distribution]]s).

Forks are considered an expression of the freedom made available by free software, but a weakness since they duplicate development efforts and can confuse users over which forked package to use. Developers have the option to collaborate and pool resources with free software, but it is not ensured by [[free software licenses]], only by a commitment to cooperation. That said, many developers will make the effort to put changes into all relevant forks, ''e.g.'' amongst the [[BSD]]s.

''[[The Cathedral and the Bazaar]]'' stated in 1997 [http://www.catb.org/~esr/writings/cathedral-bazaar/homesteading/ar01s03.html] that ''"The most important characteristic of a fork is that it spawns competing projects that cannot later exchange code, splitting the potential developer community."'' However, this is not common present usage.

In some cases, a fork can merge back into the original project or replace it. The [[EGCS|Experimental/Enhanced GNU Compiler System]] was a fork from [[GNU Compiler Collection|GCC]] which proved more vital than the original project and was eventually "blessed" as the official GCC project. Some have attempted to invoke this effect deliberately, ''e.g.'' [[Mozilla Firefox]] was an unofficial project within [[Mozilla]] that soon replaced the [[Mozilla Suite]] as the focus of development.

On the matter of forking, the [[Jargon File]] says:
:"Forking is considered a [[Bad Thing]] — not merely because it implies a lot of wasted effort in the future, but because forks tend to be accompanied by a great deal of strife and acrimony between the successor groups over issues of legitimacy, succession, and design direction. There is serious social pressure against forking. As a result, major forks (such as the [[GNU Emacs|Gnu-Emacs]]/[[XEmacs]] split, the fissioning of the [[386BSD]] group into three daughter projects, and the short-lived GCC/EGCS split) are rare enough that they are remembered individually in hacker folklore."

It is easy to declare a fork, but can require considerable effort to continue independent development and support. As such, forks without adequate resources can soon become inactive, ''e.g.'' [[GoneME]], a fork of [[GNOME]] by a former developer, which was soon discontinued despite attracting some publicity. Some well-known forks have enjoyed great success, however, such as the [[X.Org Server|X.Org]] [[X Window System|X11]] server, a fork from [[XFree86]] which gained widespread support and notably sped up X development.

==Proprietary software==

In [[proprietary software]], the copyright is usually held by the employing entity, not by the individual software developers. Proprietary code is thus more commonly forked when the owner needs to develop two or more versions, such as a [[Window (computing)|windowed]] version and a [[command line interface|command line]] version, or versions for differing operating systems, such as a [[wordprocessor]] for [[IBM PC]] compatible machines and [[Apple Macintosh|Macintosh]] computers. Generally, such internal forks will concentrate on having the same look, feel, data format, and behavior between platforms so that a user familiar with one can also be productive or share documents generated on the other. This is almost always an economic decision to generate a greater [[market share]] and thus pay back the associated extra development costs created by the fork.

A notable proprietary fork not of this kind is the many varieties of proprietary [[Unix]] — all derived from AT&T Unix and all called "Unix", but increasingly mutually incompatible. ''See'' [[UNIX wars]].

The BSD license permits forks of Free /Open Source software to become proprietary software, and some say that commercial incentives thus make proprietarisation almost inevitable. Examples include
[http://www.enterprisedb.com EnterpriseDB] (a version of postgresql with Oracle compatibility features), [http://www.fastware.com.au/docs/FujitsuSupportedPostreSQLWhitePaperV1_02.pdf Fujitsu Supported PostgresSQL]
with their proprietary ESM storage system, and [http://www.netezza.com/media/2004/Weblog.htm Netezza]'s proprietary highly scalable derivative of postgresql. Some of these vendors contribute back changes to the community project; while some keep their changes as their own competitive advantages.

==Other notable forks==

*Most [[Linux distribution]]s are descended from other distributions, most being traceable back to [[Red Hat Linux]], [[Debian]] or [[Slackware]]. Since most of the content of a distribution is free software, ideas and software interchange freely as is useful to the individual distribution. Merges (''e.g.'', [[United Linux]] or [[Mandriva]]) are rare.
*[[Pretty Good Privacy]] was forked outside of the United States{{Fact|date=June 2007}} to free it from the restrictive laws on the [[export of cryptography|exportation of cryptographic software]].
*The game [[NetHack]] has spawned a number of variants using the original code, notably [[Slash'EM]], and was itself a fork of [[Rogue (computer game)|Rogue]].
*[[OpenBSD]] was a fork of [[NetBSD]] 1.0 by Theo de Raadt
*[[OpenSSH]] was a fork from [[SSH]], which happened because the license for SSH 2.x was [[non-free]] (even though the source was available), so an older version of SSH 1.x, the last to have been licensed as [[free software]], was forked. Within months, virtually all Linux distributions, BSD versions and even some proprietary Unixes had replaced SSH with OpenSSH.
*[[DragonFly BSD]] was forked from [[FreeBSD]] 4.8 by long-time FreeBSD developer [[Matt Dillon (computer scientist)|Matt Dillon]], due to disagreement over FreeBSD 5's technical direction.
*[[Adempiere]] is a community maintained fork of [[Compiere]] 2.5.3b, due to disagreement with commercial and technical direction of Compiere Inc.
*[[Enciclopedia Libre]], a fork of the [[Spanish Wikipedia|Spanish-language Wikipedia]], was created to evade the possibility of advertising.
*[[X.org]] is a fork of XFree86, an implementation of the [[X Window System ]] after disagreement with the new license for the final release version of XFree86
*Numerous forks of the [[PostgreSQL]] database have gone on to become successful commercial products such as Netezza's data warehouse appliance and Fujitsu's FSP database which uses PostgreSQL's SQL front end over their proprietary back-end storage system.
*[[NeoOffice]] is a fork of [[OpenOffice.org]], with an incompatible license, due to disagreements about licensing and about the best method to [[porting|port]] OpenOffice.org to [[Mac OS X]].

==References==
* [http://linuxmafia.com/faq/Licensing_and_Law/forking.html Fear of forking] - An essay about forking in [[free software]] projects, by Rick Moen
* [http://www.dwheeler.com/oss_fs_why.html#forking Forking] ([[David A. Wheeler]])
* [http://www.usemod.com/cgi-bin/mb.pl?RightToFork Right to Fork] at [[Meatball Wiki]].

[[Category:Software engineering]]
[[Category:Software forks| ]]

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