Safari (web browser)

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Safari

Safari 12 running on macOS Mojave
Developer(s)	Apple
Initial release	January 7, 2003; 16 years ago
Stable release(s) [±]
macOS 12.0.3 / January 22, 2019 iOS 12.0 / September 17, 2018
Preview release(s) [±]
Technology Preview (macOS) Release 77 (12.2) / March 6, 2019
Development status	Active
Written in	C++, Objective-C and Swift
Operating system	macOS iOS Previously supported: Windows, last version 5.1.7 on May 9, 2012.
Engines	WebKit, Nitro
Type	Web browser
License	Freeware; some components GNU LGPL
Website	www.apple.com/safari/

Safari is a graphical web browser developed by Apple, based on the WebKit engine. First released on desktop in 2003 with Mac OS X Panther, a mobile version has been bundled with iOS devices since the iPhone's introduction in 2007. Safari is the default browser on Apple devices. A Windows version was available from 2007 to 2012.

History and development

Until 1997, Apple’s Macintosh computers shipped with the Netscape Navigator and Cyberdog web browsers only. Internet Explorer for Mac was later included as the default web browser for Mac OS 8.1 and later, as part of a five-year agreement between Apple and Microsoft. During that time, Microsoft released three major versions of Internet Explorer for Mac that were bundled with Mac OS 8 and Mac OS 9, though Apple continued to include Netscape Navigator as an alternative. Microsoft ultimately released a Mac OS X edition of Internet Explorer for Mac, which was included as the default browser in all Mac OS X releases from Mac OS X DP4 up to and including Mac OS X v10.2.

Safari 1

On January 7, 2003, at Macworld San Francisco, Steve Jobs announced that Apple had developed its own web browser, called Safari. It was based on Apple's internal fork of the KHTML rendering engine, called WebKit. The company released the first beta version, available only for Mac OS X, later that day. A number of official and unofficial beta versions followed, up until version 1.0 was released on June 23, 2003. Initially only available as a separate download for Mac OS X 10.2, Safari was bundled with Mac OS X v10.3 on October 24, 2003 as the default browser, with Internet Explorer for Mac included only as an alternative browser. Version 1.0.3, released on August 13, 2004 was the last version to support Mac OS X 10.2, while 1.3.2, released on January 12, 2006 was the last version to support Mac OS X 10.3. However, 10.3 received security updates through 2007.

Safari 2

In April 2005, Dave Hyatt, one of the Safari developers at Apple, documented his study by fixing specific bugs in Safari, thereby enabling it to pass the Acid2 test developed by the Web Standards Project. On April 27, 2005, he announced that his development version of Safari now passed the test, making it the first web browser to do so.

Safari 2.0 was released on April 29, 2005, as the only web browser included with Mac OS X 10.4. This version was touted by Apple as possessing a 1.8x speed boost over version 1.2.4, but did not yet include the Acid2 bug fixes. The necessary changes were initially unavailable to end-users unless they downloaded and compiled the WebKit source code themselves or ran one of the nightly automated builds available at OpenDarwin.org. Apple eventually released version 2.0.2 of Safari, which included the modifications required to pass Acid2, on October 31, 2005. 

In June 2005, after some criticism from KHTML developers over lack of access to change logs, Apple moved the development source code and bug tracking of WebCore and JavaScriptCore to OpenDarwin.org. WebKit itself was also released as open source. The source code for non-renderer aspects of the browser, such as its GUI elements, remains proprietary. 

The final stable version of Safari 2, Safari 2.0.4, was released on January 10, 2006 for Mac OS X. It was only available as part of Mac OS X Update 10.4.4. This version addressed layout and CPU usage issues, among other improvements. Safari 2.0.4 was the last version to be released exclusively on Mac OS X until version 6 in 2012.

Safari 3

On January 9, 2007, at Macworld SF, Jobs announced the iPhone. The device’s operating system (later called iPhone OS and subsequently renamed to iOS) used a mobile version of the Safari browser and was able to display full, desktop-class websites.

On June 11, 2007, at the Apple Worldwide Developers Conference, Jobs announced Safari 3 for Mac OS X 10.5, Windows XP, and Windows Vista. During the announcement, he ran a benchmark based on the iBench browser test suite comparing the most popular Windows browsers, hence claiming that Safari was the fastest browser. Later third-party tests of HTTP load times would support Apple's claim that Safari 3 was indeed the fastest browser on the Windows platform in terms of initial data loading over the Internet, though it was found to be only negligibly faster than Internet Explorer 7 and Mozilla Firefox when loading static content from local cache.

The initial Safari 3 beta version for Windows, released on the same day as its announcement at WWDC 2007, had several known bugs and a zero day exploit that allowed remote execution. The addressed bugs were then corrected by Apple three days later on June 14, 2007, in version 3.0.1 for Windows. On June 22, 2007, Apple released Safari 3.0.2 to address some bugs, performance issues and other security issues. Safari 3.0.2 for Windows handles some fonts that are missing in the browser but already installed on Windows computers, such as Tahoma, Trebuchet MS, and others.

The iPhone was formally released on June 29, 2007. It included a version of Safari based on the same WebKit rendering engine as the desktop version, but with a modified feature set better suited for a mobile device. The version number of Safari as reported in its user agent string is 3.0, in line with the contemporary desktop versions of Safari. 

The first stable, non-beta release of Safari for Windows, Safari 3.1, was offered as a free download on March 18, 2008. In June 2008, Apple released version 3.1.2, addressing a security vulnerability in the Windows version where visiting a malicious web site could force a download of executable files and execute them on the user's desktop.

Safari 3.2, released on November 13, 2008, introduced anti-phishing features using Google Safe Browsing and Extended Validation Certificate support. The final version of Safari 3 is 3.2.3, released on May 12, 2009.

Safari 4

On June 2, 2008, the WebKit development team announced SquirrelFish, a new JavaScript engine that vastly improves Safari's speed at interpreting scripts. The engine is one of the new features in Safari 4, released to developers on June 11, 2008. The new JavaScript engine quickly evolved into SquirrelFish Extreme, featuring even further improved performance over SquirrelFish, and was eventually marketed as Nitro. A public beta of Safari 4 was released on February 24, 2009, with new features such as the Top Sites tool (similar to Opera's Speed Dial feature), which displays the user's most visited sites on a 3D wall. Cover Flow, a feature of Mac OS X and iTunes, was also implemented in Safari. In the public beta versions, tabs were placed in the title bar of the window, similar to Google Chrome. The tab bar was moved back to its original location, below the URL bar, in the final release. The Windows version adopted a native Windows theme, rather than the previously employed Mac OS X-style interface. Also Apple removed the blue progress bar located in the address bar (later reinstated in Safari 5). Safari 4.0.1 was released for Mac on June 17, 2009 and fixed problems with Faces in iPhoto '09. Safari 4 in Mac OS X v10.6 "Snow Leopard" has 64-bit support, which can make JavaScript loading up to 50% faster. It also has built-in crash resistance unique to Snow Leopard; crash resistance will keep the browser intact if a plug-in like Flash player crashes, such that the other tabs or windows will be unaffected. Safari 4.0.4, released on November 11, 2009 for both OS X and Windows, further improves JavaScript performance.

Safari was one of the twelve browsers offered to EU users of Microsoft Windows in 2010. It was one of the five browsers displayed on the first page of browser choices along with Chrome, Firefox, Internet Explorer and Opera.

Safari 4 features

Beginning with Safari 4, the address bar has been completely revamped:

• The blue inline progress bar is replaced with a spinning bezel and a loading indicator attached to it.
• The button to add a bookmark is now attached to the address bar by default.
• The reload/stop button is now superimposed on the right end of the address bar.

Safari on Mac OS X and Windows was made to look more similar to Safari on iPhone than previous versions. 

Safari 4 also includes the following new features:

• Completely passes the Acid3 standards test
• Cover Flow browsing for History and Bookmarks
• Improved developer tools, including Web Inspector, CSS element viewing, JavaScript debugger and profiler, offline table and database management with SQL support, and resource graphs
• Nitro JavaScript engine that executes JavaScript up to eight times faster than Internet Explorer 8 and more than four times faster than Firefox 3
• Native Windows look on Windows (Aero, Luna, Classic, etc., depending on OS and settings) with standard Windows font rendering and optional Apple font rendering
• Support for CSS image retouching effects
• Support for CSS Canvas
• Speculative loading, where Safari loads the documents, scripts, and style information that are required to view a web page ahead of time
• Support for HTML5
• Top Sites, which displays up to 24 thumbnails of a user's most frequently visited pages on startup

Safari 5

Apple released Safari 5 on June 7, 2010, featuring the new Safari Reader for reading articles on the web without distraction (based on Arc90's Readability tool), and a 30 percent JavaScript performance increase over Safari 4. Safari 5 includes improved developer tools and supports more than a dozen new HTML5 technologies, focused on interoperability. With Safari 5, developers can now create secure Safari Extensions to customize and enhance the browsing experience. Apple also re-added the progress bar behind the address bar in this release. Safari 5.0.1 enabled the Extensions PrefPane by default; previously, users had to enable it via the Debug menu.

Apple also released Safari 4.1 concurrently with Safari 5, exclusively for Mac OS X Tiger. The update included the majority of the features and security enhancements found in Safari 5. It did not, however, include Safari Reader or Safari Extensions. Together with Mac OS X 10.7 Lion, Apple released Safari 5.1 for both Windows and Mac on July 20, 2011, with the new function 'Reading List' and a faster browsing experience. Apple simultaneously released Safari 5.0.6 for Mac OS X 10.5 Leopard, excluding Leopard users from the new functions in Safari 5.1.

Safari 5.1.7 has become the last version of Safari developed for Windows.

Safari 5 features

Safari 5 includes the following new features:

• Full-text search through the browser history
• Safari Reader, which removes formatting and ads from webpages.
• Smarter address field, where the address bar autocomplete will match against titles of web page in history or bookmarks.
• Extensions, which are add-ons that customize the web browsing experience.
• Improved support for HTML5, including full screen video, closed caption, geolocation, EventSource, and a now obsolete early variant of the WebSocket protocol.
• Improved Web Inspector.
• Faster Nitro JavaScript Engine.
• DNS prefetching, where Safari finds links and looks up addresses on the web page ahead of time.
• Bing search.
• Improved graphics hardware acceleration on Windows.

Additionally, the blue inline progress bar has returned to the address bar, in addition to the spinning bezel and loading indicator introduced in Safari 4. Top Sites view now has a button to switch to Full History Search. Other features include Extension builder for developers of Safari Extensions, which are built using web standards such as HTML5, CSS3, and JavaScript.

Safari 6

Safari 6.0 was previously known as Safari 5.2 until Apple announced the change at WWDC 2012. The stable release of Safari 6 coincided with the release of OS X Mountain Lion on July 25, 2012, and is integrated into the OS. As Apple integrated it with Mountain Lion, it is no longer available for download from the Apple website or other sources. Apple released Safari 6 via Software Update for users of OS X Lion. It has not been released for OS X versions prior to Lion or for Windows. Regarding the unavailability of Safari 6 on Windows, Apple has stated "Safari 6 is available for Mountain Lion and Lion. Safari 5 continues to be available for Windows." Microsoft removed Safari from its BrowserChoice page. 

On June 11, 2012, Apple released a developer preview of Safari 6.0 with a feature called iCloud Tabs, which allows users to 'sync' their open tabs with any iOS or other OS X device running the latest software. Safari 6 also included new privacy features, including an "Ask websites not to track me" preference, and the ability for websites to send OS X 10.8 Mountain Lion users notifications, although it removed RSS support. Safari 6 has the Share Sheets capability in OS X Mountain Lion. The Share Sheet options are: Add to Reading List, Add Bookmark, Email this Page, Message, Twitter and Facebook. Users can now see tabs with full page previews available.

Safari 6 features

Safari 6 introduced the following features, many of which are only available on OS X 10.8 Mountain Lion:

• Unified smart search field, which combines the web address and search fields, similar to Chrome's Omnibox and Firefox's Awesome Bar.
• Tab view (Mountain Lion only), which enables movement between tabs using multi-touch gestures.
• iCloud tabs (Mountain Lion only) synchronizes recent websites across OS X and iOS devices.
• Built-in sharing (Mountain Lion only) to email, Messages, Twitter and Facebook.
• Improved performance
• Support for -webkit-calc()

Additionally various features were removed, including, but not limited to, Activity Window, separate Download Window, direct support for RSS feeds in the URL field and bookmarks. The separate search field is also no longer available as a toolbar configuration option.

Safari 7

Announced at Apple's Worldwide Developer Conference (WWDC) on June 10, 2013, the Safari 7/6.1 developer preview brought improvements in JavaScript performance and memory usage, as well as a new look for Top Sites and the Sidebar, and a new Shared Links feature. Additionally, a new Power Saver feature pauses Plugins which are not in use. Safari 7 for OS X Mavericks and Safari 6.1 (for Lion and Mountain Lion) were released along with OS X Mavericks in an Apple special event on October 22, 2013.

Safari 8

Safari 8 was announced at WWDC 2014 and released with OS X Yosemite. It included WebGL support, stronger privacy features, increased speed and efficiency, enhanced iCloud integration, and updated design.

Safari 8 features

Safari 8 introduced the following features, available on OS X Yosemite:

• WebGL support
• IndexedDB support
• Support for Promises from ECMAScript 6
• CSS Shapes and Compositing
• Support for SPDY
• Encrypted Media Extensions
• APNG support

Safari 9

Safari 9 was announced at WWDC 2015 and released with OS X El Capitan. It included muting tabs and pinned tabs.

• Promise Support

Safari 10

Safari 10 was released alongside macOS Sierra 10.12 for OS X Yosemite and OS X El Capitan. It does not include all of the new features available in macOS Sierra, like Apple Pay on the web and picture-in-picture support for videos, but the update includes the following new functions:

• Safari Extensions such as 1Password, Save to Pocket, and DuckDuckGo
• New Bookmarks sidebar, including double-click to focus in on a folder
• Redesigned Bookmarks and History views
• Site-specific zoom: Safari remembers and re-applies your zoom level to websites
• Improved AutoFill from Contacts card
• Reader improvements, including in-line sub-headlines, bylines, and publish dates
• Legacy plug-ins are turned off by default in favor of HTML5 versions of websites
• Allow reopening of recently closed tabs through the History menu, holding the "+" button in the tab bar, and using Shift-Command-T
• When a link opens in a new tab, it is now possible to hit the back button or swipe to close it and go back to the original tab
• Improved ranking of Frequently Visited Sites
• Web Inspector Timelines Tab
• Debugging using Web Inspector

Safari 10 also includes a number of security updates, including fixes for six WebKit vulnerabilities and issues related to Reader and Tabs.

Safari 11

Safari 11 was released as a part of macOS High Sierra but was also made available for OS X El Capitan and macOS Sierra on September 19, 2017. Safari 11 includes several new features such as an Intelligent Tracking Prevention feature which prevents websites from cross-site tracking.

Safari 12

Safari 12 was released in the lead up to macOS Mojave but was also made available for macOS Sierra and macOS High Sierra on September 17, 2018. Safari 12 includes several new features such as Icons in tabs, Automatic Strong Passwords, Intelligent Tracking Prevention 2.0. An updated Safari version 12.0.1 was released on October 30, 2018 as part of MacOS Mojave 10.14.1 release, and Safari 12.0.2 was released on December 5, 2018, alongside macOS 10.14.2. 

Support for developer-signed classic Safari Extensions has been dropped. This version will also be the last one that supports the official Extensions Gallery, and Apple encourages extension authors to switch to Safari App Extensions. This move triggered negative feedback in the community.

Safari Technology Preview

Safari Technology Preview was first released alongside OS X El Capitan 10.11.4. Safari Technology Preview releases include the latest version of WebKit, incorporating Web technologies to be incorporated in future stable releases of Safari, so that developers and users can install the Technology Preview release on a Mac, test those features, and provide feedback.

Other features

Safari's Web Inspector in macOS Mojave.

 

On macOS, Safari is a Cocoa application. It uses Apple's WebKit for rendering web pages and running JavaScript. WebKit consists of WebCore (based on Konqueror's KHTML engine) and JavaScriptCore (originally based on KDE's JavaScript engine, named KJS). Like KHTML and KJS, WebCore and JavaScriptCore are free software and are released under the terms of the GNU Lesser General Public License. Some Apple improvements to the KHTML code are merged back into the Konqueror project. Apple also releases additional code under an open source 2-clause BSD-like license. 

Until Safari 6.0, it included a built-in web feed aggregator that supported the RSS and Atom standards. Current features include Private Browsing (a mode in which no record of information about the user's web activity is retained by the browser), an "Ask websites not to track me" privacy setting, the ability to archive web content in WebArchive format, the ability to email complete web pages directly from a browser menu, the ability to search bookmarks, and the ability to share tabs between all Mac and iOS devices running appropriate versions of software via an iCloud account.

iOS-specific features

Safari on the iPhone and iPod Touch running iOS 12 in Landscape view

 

Safari on an iPad running iOS 12 in Landscape view

iOS-specific features for Safari enable:

• Bookmarking links to particular pages as "Web Clip" icons on the Home screen.
• MDI-style browsing.
• Opening specially designed pages in full-screen mode.
• Pressing on an image for 3 seconds to save it to the photo album.
• Support for HTML5 new input types.

New in iOS 4

iOS 4.2

• Find feature built into search box.
• Ability to print the current webpage using AirPrint.

iOS 4.3

• Integration of the Nitro JavaScript engine for faster page loads. This feature was expanded to home-screen web applications in iOS 5.0.

New in iOS 5

• True tabbed browsing, similar to the desktop experience, only for iPads.
• Reading List, a bookmarking feature that allows tagging of certain sites for reading later, which syncs across all Safari browsers (mobile and desktop) via Apple's iCloud service.
• Reader, a reading feature that can format text and images from a web page into a more readable format, similar to a PDF document, while stripping out web advertising and superfluous information.
• Private browsing, like in most desktop browsers a feature that does not save the user's cookies and history, or allow anything to be written into local storage or webSql databases.

New in iOS 6

• iCloud Tabs, linking the desktop and iOS versions of Safari.
• Offline Reading Lists allow users to read pages stored previously without remaining connected to the internet.
• Full-screen landscape view for iPhone and iPod touch users hides most of the Safari controls except back and forward buttons and the status bar when in landscape mode.

New in iOS 7

• New icon
• 64-bit build on supported devices using the A7 processor.
• iCloud Keychain: iCloud can remember passwords, account names and credit card numbers. Safari can also autofill them as well. Requires devices that run iOS 7.0.3 and later and OS X Mavericks or later.
• Password Generator: When creating a new account, Safari can suggest the user a long, more secure, hard to guess password and Safari will also automatically remember the password.
• Shared Links
• Do Not Track
• Parental controls
• Tab limit increased from 9 to 36
• New Tab view (iPhone and iPod touch only)
• Unified smart search field
• Sync Bookmarks with Google Chrome and Firefox on Windows.

New in iOS 8

• The Tab view from iPhone is now available on iPads.
• A search function to search through all open tabs has been added in Tab view on iPad and select iPhones.
• Two-finger pinch to reveal Tab view on iPads and select iPhones.
• New Sidebar that slides out to reveal bookmarks, Reading List, and Shared Links on iPads and select iPhones in landscape view.
• Address bar now hides when scrolling down on iPads.
• Spotlight Search is now available from Safari's address bar.
• Option to “Scan Credit Card” when filling out credit card info on a web form.
• WebGL support.
• APNG support.
• Private browsing per tab.
• RSS feeds in Shared Links.
• DuckDuckGo support.
• Option to Request the desktop site while entering in a web address.
• Option to add website to Favorites while entering in a web address.
• Swipe to close iCloud tabs from other devices.
• Hold the "+" (new tab button) in tab view to list recently closed tabs is now available on iPhone.
• Can delete individual items from History.
• Safari now blocks ads from automatically redirecting to the App Store without user interaction.
• Bookmark icon updated.
• Improved, iPad-like interface available on select iPhones in landscape view.

New in iOS 9

Safari on iOS 12, on the Wikipedia mobile landing page

• The option to add content blocking extensions is available to block specific web content.
• Safari view controller can be used to display web content from within an app, sharing cookies and other website data with Safari.
• Improved reader view, allowing the user to choose from different fonts and themes as well as hiding the controls

New in iOS 10

Tab limit increased from 36 to 500

New in iOS 11

• More rounded search bar
• Redesign video player
• Adoption of high level standard for preventing web tracking

New in iOS 12

• Support for stronger password suggestion
• Support for auto-fill from third-party provider
• Third-party can suggest strong password
• Auto-fill of 2FA code sent by email
• More powerful tracking prevention

WebKit2

WebKit2 is a multiprocess API for WebKit, where the web-content is handled by a separate process than the application using WebKit. Apple announced WebKit2 in April 2010. Safari for OS X switched to the new API with version 5.1. Safari for iOS switched to WebKit2 with iOS 8.

Security

Plugins

Apple maintains a plugin blacklist that it can remotely update to prevent potentially dangerous or vulnerable plug-ins from running on Safari. Initially, Apple had blocked versions of Flash and Java, but since Safari 12 support for NPAPI plugins (except for Flash) have been completely dropped.

License

The license has common terms against reverse engineering, copying and sub-licensing, except parts that are open source, and it disclaims warranties and liability.

Apple tracks use of the browser. Windows users may not opt out of tracking, since their license omits the opening If clause. Other users may opt out, and all users can opt out of location tracking by not using location services. "If you choose to allow diagnostic and usage collection, you agree that Apple and its subsidiaries and agents may collect... usage and related information... to provide ... services to you (if any) related to the Apple Software... in a form that does not personally identify you... Apple may also provide any such partner or third party developer with a subset of diagnostic information that is relevant to that partner’s or developer’s software... Apple and its partners, licensees, third party developers and website may transmit, collect, maintain, process and use your location data... and location search queries... in a form that does not personally identify you ... You may withdraw this consent at any time..."

Apple thinks "personal" does not cover "unique device identifiers" such as serial number, cookie number, or IP address, so they use these where allowed by law. "We may collect, use, transfer, and disclose non-personal information for any purpose. The following are some examples of non-personal information that we collect ... unique device identifier... We treat information collected by cookies and other technologies as non‑personal information. However, to the extent that Internet Protocol (IP) addresses or similar identifiers are considered personal information by local law, we also treat these identifiers as personal information."

In September 2017 Apple announced that it will use artificial intelligence (AI) to reduce the ability of advertisers to track Safari users as they browse the web. Cookies used for tracking will be allowed for 24 hours, then disabled, unless AI judges the user wants the cookie. Major advertising groups objected, saying it will reduce the free services supported by advertising, while other experts praised the change.

Browser exploits

In the PWN2OWN contest at the 2008 CanSecWest security conference in Vancouver, British Columbia, an exploit of Safari caused Mac OS X to be the first OS to fall in a hacking competition. Participants competed to find a way to read the contents of a file located on the user's desktop in one of three operating systems: Mac OS X Leopard, Windows Vista SP1, and Ubuntu 7.10. On the second day of the contest, when users were allowed to physically interact with the computers (the prior day permitted only network attacks), Charlie Miller compromised Mac OS X through an unpatched vulnerability of the PCRE library used by Safari. Miller was aware of the flaw before the conference and worked to exploit it unannounced, as is the common approach in these contests. The exploited vulnerability and other flaws were patched in Safari 3.1.1.

In the 2009 PWN2OWN contest, Charlie Miller performed another exploit of Safari to hack into a Mac. Miller again acknowledged that he knew about the security flaw before the competition and had done considerable research and preparation work on the exploit. Apple released a patch for this exploit and others on May 12, 2009 with Safari 3.2.3.

System requirements

Safari 6.0 requires a Mac running Mac OS X v10.7.4 or later. Safari 5.1.7 requires a Mac running Mac OS X v10.6.8 or any PC running Windows XP Service Pack 2 or later, Windows Vista, or Windows 7. Safari 5.0.6 requires a Mac running on Mac OS X 10.5.8.

64-bit builds

The version of Safari included in Mac OS X v10.6 (and later versions) is compiled for 64-bit architecture. Apple claims that running Safari in 64-bit mode will increase rendering speeds by up to 50%. 

On 64-bit devices, iOS and its stock apps are 64-bit builds including Safari.

Criticism

Distribution through Apple Software Update

An earlier version of Apple Software Update (bundled with Safari, QuickTime, and iTunes for Microsoft Windows) selected Safari for installation from a list of Apple programs to download by default, even when an existing installation of Safari was not detected on a user's machine. John Lilly, former CEO of Mozilla, stated that Apple's use of its updating software to promote its other products was "a bad practice and should stop." He argued that the practice "borders on malware distribution practices" and "undermines the trust that we're all trying to build with users." Apple spokesman Bill Evans sidestepped Lilly's statement, saying that Apple was only "using Software Update to make it easy and convenient for both Mac and Windows users to get the latest Safari update from Apple." Apple also released a new version of Apple Software Update that puts new software in its own section, though still selected for installation by default. By late 2008, Apple Software Update no longer selected new installation items in the new software section by default.

Security updates for Snow Leopard and Windows platforms

Software security firm Sophos detailed how Snow Leopard and Windows users were not supported by the Safari 6 release at the time, while there were over 121 vulnerabilities left unpatched on those platforms. Since then, Snow Leopard has had only three minor version releases (the most recent in September 2013), and Windows has had none. While no official word has been released by Apple, the indication is that these are the final versions available for these operating systems, and both retain significant security issues.

Failure to adopt modern standards

While Safari pioneered several now standard HTML5 features (such as the Canvas API) in its early years, it has come under attack for failing to keep pace with some modern web technologies. In the past, Apple did not allow third party web browsers under iOS, but now there are plenty of web browsers available for iOS, including Chrome, Firefox, Opera and Edge. However, due to Apple developer's policies, browsers like Firefox for iOS needed to change its internal browser engine from Gecko to WebKit. There are ongoing lawsuits in France related with Apple policies for developers.

Safari Developer Program

The Safari Developer Program was a free program for writers of extensions and HTML5 websites. It allowed members to develop extensions for Apple's Safari web browser. Since WWDC 2015 it is part of the unified Apple Developer Program, which costs $99 a year.

Olbers' paradox

From Wikipedia, the free encyclopedia

As more distant stars are revealed in this animation depicting an infinite, homogeneous and static universe, they fill the gaps between closer stars. Olbers's paradox argues that as the night sky is dark, at least one of these three assumptions about the nature of the universe must be false.

 

In astrophysics and physical cosmology, Olbers' paradox, named after the German astronomer Heinrich Wilhelm Olbers (1758–1840), also known as the "dark night sky paradox", is the argument that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe. The darkness of the night sky is one of the pieces of evidence for a dynamic universe, such as the Big Bang model. In the hypothetical case that the universe is static, homogeneous at a large scale, and populated by an infinite number of stars, then any line of sight from Earth must end at the (very bright) surface of a star and hence the night sky should be completely illuminated and very bright. This contradicts the observed darkness and non-uniformity of the night.

History

The first one to address the problem of infinite number of stars and the resulting heat in the Cosmos was Cosmas Indicopleustes, a Greek monk from Alexandria, who states in his Topographia Christiana: "The crystal-made sky sustains the heat of the Sun, the moon, and the infinite number of stars, otherwise, it would have been full of fire, and it could melt or set on fire".

Edward Robert Harrison's Darkness at Night: A Riddle of the Universe (1987) gives an account of the dark night sky paradox, seen as a problem in the history of science. According to Harrison, the first to conceive of anything like the paradox was Thomas Digges, who was also the first to expound the Copernican system in English and also postulated an infinite universe with infinitely many stars. Kepler also posed the problem in 1610, and the paradox took its mature form in the 18th century work of Halley and Cheseaux. The paradox is commonly attributed to the German amateur astronomer Heinrich Wilhelm Olbers, who described it in 1823, but Harrison shows convincingly that Olbers was far from the first to pose the problem, nor was his thinking about it particularly valuable. Harrison argues that the first to set out a satisfactory resolution of the paradox was Lord Kelvin, in a little known 1901 paper, and that Edgar Allan Poe's essay Eureka (1848) curiously anticipated some qualitative aspects of Kelvin's argument:

Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy – since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all.

The paradox

The paradox is that a static, infinitely old universe with an infinite number of stars distributed in an infinitely large space would be bright rather than dark.

A view of a square section of four concentric shells

 

To show this, we divide the universe into a series of concentric shells, 1 light year thick. A certain number of stars will be in the shell 1,000,000,000 to 1,000,000,001 light years away. If the universe is homogeneous at a large scale, then there would be four times as many stars in a second shell, which is between 2,000,000,000 and 2,000,000,001 light years away. However, the second shell is twice as far away, so each star in it would appear one quarter as bright as the stars in the first shell. Thus the total light received from the second shell is the same as the total light received from the first shell. 

Thus each shell of a given thickness will produce the same net amount of light regardless of how far away it is. That is, the light of each shell adds to the total amount. Thus the more shells, the more light; and with infinitely many shells, there would be a bright night sky. 

While dark clouds could obstruct the light, these clouds would heat up, until they were as hot as the stars, and then radiate the same amount of light. 

Kepler saw this as an argument for a finite observable universe, or at least for a finite number of stars. In general relativity theory, it is still possible for the paradox to hold in a finite universe: though the sky would not be infinitely bright, every point in the sky would still be like the surface of a star.

The mainstream explanation

The poet Edgar Allan Poe suggested that the finite size of the observable universe resolves the apparent paradox. More specifically, because the universe is finitely old and the speed of light is finite, only finitely many stars can be observed from Earth (although the whole universe can be infinite in space). The density of stars within this finite volume is sufficiently low that any line of sight from Earth is unlikely to reach a star. 

However, the Big Bang theory introduces a new paradox: it states that the sky was much brighter in the past, especially at the end of the recombination era, when it first became transparent. All points of the local sky at that era were comparable in brightness to the surface of the Sun, due to the high temperature of the universe in that era; and most light rays will terminate not in a star but in the relic of the Big Bang. 

This paradox is explained by the fact that the Big Bang theory also involves the expansion of space, which can cause the energy of emitted light to be reduced via redshift. More specifically, the extreme levels of radiation from the Big Bang have been redshifted to microwave wavelengths (1100 times the length of its original wavelength) as a result of the cosmic expansion, and thus forms the cosmic microwave background radiation. This explains the relatively low light densities present in most of our sky despite the assumed bright nature of the Big Bang. The redshift also affects light from distant stars and quasars, but the diminution is minor, since the most distant galaxies and quasars have redshifts of only around 5 to 8.6.

Alternative explanations

Steady state

The redshift hypothesised in the Big Bang model would by itself explain the darkness of the night sky even if the universe were infinitely old. In the Steady state theory the universe is infinitely old and uniform in time as well as space. There is no Big Bang in this model, but there are stars and quasars at arbitrarily great distances. The expansion of the universe causes the light from these distant stars and quasars to redshift, so that the total light flux from the sky remains finite. Thus the observed radiation density (the sky brightness of extragalactic background light) can be independent of finiteness of the universe. Mathematically, the total electromagnetic energy density (radiation energy density) in thermodynamic equilibrium from Planck's law is

${\displaystyle {U \over V}={\frac {8\pi ^{5}(kT)^{4}}{15(hc)^{3}}},}$

e.g. for temperature 2.7 K it is 40 fJ/m³ ... 4.5×10⁻³¹ kg/m³ and for visible temperature 6000 K we get 1 J/m³ ... 1.1×10⁻¹⁷ kg/m³. But the total radiation emitted by a star (or other cosmic object) is at most equal to the total nuclear binding energy of isotopes in the star. For the density of the observable universe of about 4.6×10⁻²⁸ kg/m³ and given the known abundance of the chemical elements, the corresponding maximal radiation energy density of 9.2×10⁻³¹ kg/m³, i.e. temperature 3.2 K (matching the value observed for the optical radiation temperature by Arthur Eddington). This is close to the summed energy density of the cosmic microwave background (CMB) and the cosmic neutrino background. The Big Bang hypothesis predicts that the CBR should have the same energy density as the binding energy density of the primordial helium, which is much greater than the binding energy density of the non-primordial elements; so it gives almost the same result. However, the steady-state model does not predict the angular distribution of the microwave background temperature accurately (as the standard ΛCDM paradigm does). Nevertheless, the modified gravitation theories (without metric expansion of the universe) cannot be ruled out as of 2017 by CMB and BAO observations.

Finite age of stars

Stars have a finite age and a finite power, thereby implying that each star has a finite impact on a sky's light field density. Edgar Allan Poe suggested that this idea could provide a resolution to Olbers' paradox; a related theory was also proposed by Jean-Philippe de Chéseaux. However, stars are continually being born as well as dying. As long as the density of stars throughout the universe remains constant, regardless of whether the universe itself has a finite or infinite age, there would be infinitely many other stars in the same angular direction, with an infinite total impact. So the finite age of the stars does not explain the paradox.

Brightness

Suppose that the universe were not expanding, and always had the same stellar density; then the temperature of the universe would continually increase as the stars put out more radiation. Eventually, it would reach 3000 K (corresponding to a typical photon energy of 0.3 eV and so a frequency of 7.5×10¹³ Hz), and the photons would begin to be absorbed by the hydrogen plasma filling most of the universe, rendering outer space opaque. This maximal radiation density corresponds to about 1.2×10¹⁷ eV/m³ = 2.1×10⁻¹⁹ kg/m³, which is much greater than the observed value of 4.7×10⁻³¹ kg/m³. So the sky is about five hundred billion times darker than it would be if the universe was neither expanding nor too young to have reached equilibrium yet. However, recent observations increasing the lower bound on the number of galaxies suggest UV absorption by hydrogen and reemission in near-IR (not visible) wavelengths also plays a role.

Fractal star distribution

A different resolution, which does not rely on the Big Bang theory, was first proposed by Carl Charlier in 1908 and later rediscovered by Benoît Mandelbrot in 1974. They both postulated that if the stars in the universe were distributed in a hierarchical fractal cosmology (e.g., similar to Cantor dust)—the average density of any region diminishes as the region considered increases—it would not be necessary to rely on the Big Bang theory to explain Olbers' paradox. This model would not rule out a Big Bang, but would allow for a dark sky even if the Big Bang had not occurred. 

Mathematically, the light received from stars as a function of star distance in a hypothetical fractal cosmos is:

${\displaystyle {\text{light}}=\int _{r_{0}}^{\infty }L(r)N(r)\,dr}$

where: 

r₀ = the distance of the nearest star. r₀ > 0;

r = the variable measuring distance from the Earth;

L(r) = average luminosity per star at distance r;

N(r) = number of stars at distance r. 

The function of luminosity from a given distance L(r)N(r) determines whether the light received is finite or infinite. For any luminosity from a given distance L(r)N(r) proportional to r^a, ${\displaystyle {\text{light}}}$ is infinite for a ≥ −1 but finite for a < −1. So if L(r) is proportional to r⁻², then for ${\displaystyle {\text{light}}}$ to be finite, N(r) must be proportional to r^b, where b < 1. For b = 1, the numbers of stars at a given radius is proportional to that radius. When integrated over the radius, this implies that for b = 1, the total number of stars is proportional to r². This would correspond to a fractal dimension of 2. Thus the fractal dimension of the universe would need to be less than 2 for this explanation to work. 

This explanation is not widely accepted among cosmologists since the evidence suggests that the fractal dimension of the universe is at least 2. Moreover, the majority of cosmologists accept the cosmological principle, which assumes that matter at the scale of billions of light years is distributed isotropically. Contrarily, fractal cosmology requires anisotropic matter distribution at the largest scales. Cosmic microwave background radiation has cosine anisotropy.

Type Ia supernova

From Wikipedia, the free encyclopedia

This artist’s impression video shows the central part of the planetary nebula Henize 2-428. The core of this unique object consists of two white dwarf stars, each with a mass a little less than that of the Sun. They are expected to slowly draw closer to each other and merge in around 700 million years. This event will likely create a Type Ia supernova and destroy both stars.

 

A type Ia supernova (read "type one-a") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white dwarf.

Physically, carbon–oxygen white dwarfs with a low rate of rotation are limited to below 1.44 solar masses (M_☉). Beyond this, they reignite and in some cases trigger a supernova explosion. Somewhat confusingly, this limit is often referred to as the Chandrasekhar mass, despite being marginally different from the absolute Chandrasekhar limit where electron degeneracy pressure is unable to prevent catastrophic collapse. If a white dwarf gradually accretes mass from a binary companion, the general hypothesis is that its core will reach the ignition temperature for carbon fusion as it approaches the limit. 

However, if the white dwarf merges with another white dwarf (a very rare event), it will momentarily exceed the limit and begin to collapse, again raising its temperature past the nuclear fusion ignition point. Within a few seconds of initiation of nuclear fusion, a substantial fraction of the matter in the white dwarf undergoes a runaway reaction, releasing enough energy (1–2×10⁴⁴ J) to unbind the star in a supernova explosion.

This type Ia category of supernovae produces consistent peak luminosity because of the uniform mass of white dwarfs that explode via the accretion mechanism. The stability of this value allows these explosions to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance. 

In May 2015, NASA reported that the Kepler space observatory observed KSN 2011b, a type Ia supernova in the process of exploding. Details of the pre-nova moments may help scientists better judge the quality of Type Ia supernovae as standard candles, which is an important link in the argument for dark energy.

Consensus model

Spectrum of SN 1998aq, a type Ia supernova, one day after maximum light in the B band

 

The Type Ia supernova is a subcategory in the Minkowski–Zwicky supernova classification scheme, which was devised by German-American astronomer Rudolph Minkowski and Swiss astronomer Fritz Zwicky. There are several means by which a supernova of this type can form, but they share a common underlying mechanism. Theoretical astronomers long believed the progenitor star for this type of supernova is a white dwarf, and empirical evidence for this was found in 2014 when a Type Ia supernova was observed in the galaxy Messier 82. When a slowly-rotating carbon–oxygen white dwarf accretes matter from a companion, it can exceed the Chandrasekhar limit of about 1.44 M_☉, beyond which it can no longer support its weight with electron degeneracy pressure. In the absence of a countervailing process, the white dwarf would collapse to form a neutron star, in an accretion-induced non-ejective process, as normally occurs in the case of a white dwarf that is primarily composed of magnesium, neon, and oxygen.

The current view among astronomers who model Type Ia supernova explosions, however, is that this limit is never actually attained and collapse is never initiated. Instead, the increase in pressure and density due to the increasing weight raises the temperature of the core, and as the white dwarf approaches about 99% of the limit, a period of convection ensues, lasting approximately 1,000 years. At some point in this simmering phase, a deflagration flame front is born, powered by carbon fusion. The details of the ignition are still unknown, including the location and number of points where the flame begins. Oxygen fusion is initiated shortly thereafter, but this fuel is not consumed as completely as carbon.

G299 Type Ia supernova remnant.

 

Once fusion begins, the temperature of the white dwarf increases. A main sequence star supported by thermal pressure can expand and cool which automatically regulates the increase in thermal energy. However, degeneracy pressure is independent of temperature; white dwarfs are unable to regulate temperature in the manner of normal stars, so they are vulnerable to runaway fusion reactions. The flare accelerates dramatically, in part due to the Rayleigh–Taylor instability and interactions with turbulence. It is still a matter of considerable debate whether this flare transforms into a supersonic detonation from a subsonic deflagration.

Regardless of the exact details of how the supernova ignites, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf fuses into heavier elements within a period of only a few seconds, with the accompanying release of energy increasing the internal temperature to billions of degrees. The energy released (1–2×10⁴⁴ J) is more than sufficient to unbind the star; that is, the individual particles making up the white dwarf gain enough kinetic energy to fly apart from each other. The star explodes violently and releases a shock wave in which matter is typically ejected at speeds on the order of 5,000–20,000 km/s, roughly 6% of the speed of light. The energy released in the explosion also causes an extreme increase in luminosity. The typical visual absolute magnitude of Type Ia supernovae is M_v = −19.3 (about 5 billion times brighter than the Sun), with little variation.

The theory of this type of supernova is similar to that of novae, in which a white dwarf accretes matter more slowly and does not approach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a hydrogen fusion surface explosion that does not disrupt the star.

Type Ia supernova differ from Type II supernova, which are caused by the cataclysmic explosion of the outer layers of a massive star as its core collapses, powered by release of gravitational potential energy via neutrino emission.

Formation

Formation process

 

Gas is being stripped from a giant star to form an accretion disc around a compact companion (such as a white dwarf star). NASA image

 

Simulation of the explosion phase of the deflagration-to-detonation model of supernovae formation, run on scientific supercomputer.

Single degenerate progenitors

One model for the formation of this category of supernova is a close binary star system. The progenitor binary system consists of main sequence stars, with the primary possessing more mass than the secondary. Being greater in mass, the primary is the first of the pair to evolve onto the asymptotic giant branch, where the star's envelope expands considerably. If the two stars share a common envelope then the system can lose significant amounts of mass, reducing the angular momentum, orbital radius and period. After the primary has degenerated into a white dwarf, the secondary star later evolves into a red giant and the stage is set for mass accretion onto the primary. During this final shared-envelope phase, the two stars spiral in closer together as angular momentum is lost. The resulting orbit can have a period as brief as a few hours. If the accretion continues long enough, the white dwarf may eventually approach the Chandrasekhar limit. 

The white dwarf companion could also accrete matter from other types of companions, including a subgiant or (if the orbit is sufficiently close) even a main sequence star. The actual evolutionary process during this accretion stage remains uncertain, as it can depend both on the rate of accretion and the transfer of angular momentum to the white dwarf companion.

It has been estimated that single degenerate progenitors account for no more than 20% of all Type Ia supernovae.

Double degenerate progenitors

A second possible mechanism for triggering a Type Ia supernova is the merger of two white dwarfs whose combined mass exceeds the Chandrasekhar limit. The resulting merger is called a super-Chandrasekhar mass white dwarf. In such a case, the total mass would not be constrained by the Chandrasekhar limit. 

Collisions of solitary stars within the Milky Way occur only once every 10⁷ to 10¹³ years; far less frequently than the appearance of novae. Collisions occur with greater frequency in the dense core regions of globular clusters. A likely scenario is a collision with a binary star system, or between two binary systems containing white dwarfs. This collision can leave behind a close binary system of two white dwarfs. Their orbit decays and they merge through their shared envelope. However, a study based on SDSS spectra found 15 double systems of the 4,000 white dwarfs tested, implying a double white dwarf merger every 100 years in the Milky Way. Conveniently, this rate matches the number of Type Ia supernovae detected in our neighborhood.

A double degenerate scenario is one of several explanations proposed for the anomalously massive (2 M_☉) progenitor of SN 2003fg. It is the only possible explanation for SNR 0509-67.5, as all possible models with only one white dwarf have been ruled out. It has also been strongly suggested for SN 1006, given that no companion star remnant has been found there. Observations made with NASA's Swift space telescope ruled out existing supergiant or giant companion stars of every Type Ia supernova studied. The supergiant companion's blown out outer shell should emit X-rays, but this glow was not detected by Swift's XRT (X-ray telescope) in the 53 closest supernova remnants. For 12 Type Ia supernovae observed within 10 days of the explosion, the satellite's UVOT (ultraviolet/optical telescope) showed no ultraviolet radiation originating from the heated companion star's surface hit by the supernova shock wave, meaning there were no red giants or larger stars orbiting those supernova progenitors. In the case of SN 2011fe, the companion star must have been smaller than the Sun, if it existed. The Chandra X-ray Observatory revealed that the X-ray radiation of five elliptical galaxies and the bulge of the Andromeda galaxy is 30–50 times fainter than expected. X-ray radiation should be emitted by the accretion discs of Type Ia supernova progenitors. The missing radiation indicates that few white dwarfs possess accretion discs, ruling out the common, accretion-based model of Ia supernovae. Inward spiraling white dwarf pairs are strongly-inferred candidate sources of gravitational waves, although they have not been directly observed.

Double degenerate scenarios raise questions about the applicability of Type Ia supernovae as standard candles, since total mass of the two merging white dwarfs varies significantly, meaning luminosity also varies.

Type Iax

It has been proposed that a group of sub-luminous supernovae that occur when helium accretes onto a white dwarf should be classified as Type Iax. This type of supernova may not always completely destroy the white dwarf progenitor, but instead leave behind a zombie star.

Observation

Supernova remnant N103B taken by the Hubble Space Telescope.

 

Unlike the other types of supernovae, Type Ia supernovae generally occur in all types of galaxies, including ellipticals. They show no preference for regions of current stellar formation. As white dwarf stars form at the end of a star's main sequence evolutionary period, such a long-lived star system may have wandered far from the region where it originally formed. Thereafter a close binary system may spend another million years in the mass transfer stage (possibly forming persistent nova outbursts) before the conditions are ripe for a Type Ia supernova to occur.

A long-standing problem in astronomy has been the identification of supernova progenitors. Direct observation of a progenitor would provide useful constraints on supernova models. As of 2006, the search for such a progenitor had been ongoing for longer than a century. Observation of the supernova SN 2011fe has provided useful constraints. Previous observations with the Hubble Space Telescope did not show a star at the position of the event, thereby excluding a red giant as the source. The expanding plasma from the explosion was found to contain carbon and oxygen, making it likely the progenitor was a white dwarf primarily composed of these elements. Similarly, observations of the nearby SN PTF 11kx, discovered January 16, 2011 (UT) by the Palomar Transient Factory (PTF), lead to the conclusion that this explosion arises from single-degenerate progenitor, with a red giant companion, thus suggesting there is no single progenitor path to SN Ia. Direct observations of the progenitor of PTF 11kx were reported in the August 24 edition of Science and support this conclusion, and also show that the progenitor star experienced periodic nova eruptions before the supernova – another surprising discovery. However, later analysis revealed that the circumstellar material is too massive for the single-degenerate scenario, and fits better the core-degenerate scenario.

Light curve

This plot of luminosity (relative to the Sun, L₀) versus time shows the characteristic light curve for a Type Ia supernova. The peak is primarily due to the decay of nickel (Ni), while the later stage is powered by cobalt (Co).

 

Type Ia supernovae have a characteristic light curve, their graph of luminosity as a function of time after the explosion. Near the time of maximal luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star, heavy elements synthesized during the explosion; most prominently isotopes close to the mass of iron (iron-peak elements). The radioactive decay of nickel-56 through cobalt-56 to iron-56 produces high-energy photons, which dominate the energy output of the ejecta at intermediate to late times.

The use of Type Ia supernovae to measure precise distances was pioneered by a collaboration of Chilean and US astronomers, the Calán/Tololo Supernova Survey. In a series of papers in the 1990s the survey showed that while Type Ia supernovae do not all reach the same peak luminosity, a single parameter measured from the light curve can be used to correct unreddened Type Ia supernovae to standard candle values. The original correction to standard candle value is known as the Phillips relationship and was shown by this group to be able to measure relative distances to 7% accuracy. The cause of this uniformity in peak brightness is related to the amount of nickel-56 produced in white dwarfs presumably exploding near the Chandrasekhar limit.

The similarity in the absolute luminosity profiles of nearly all known Type Ia supernovae has led to their use as a secondary standard candle in extragalactic astronomy. Improved calibrations of the Cepheid variable distance scale and direct geometric distance measurements to NGC 4258 from the dynamics of maser emission when combined with the Hubble diagram of the Type Ia supernova distances have led to an improved value of the Hubble constant. 

In 1998, observations of distant Type Ia supernovae indicated the unexpected result that the universe seems to undergo an accelerating expansion. Three members from two teams were subsequently awarded Nobel Prizes for this discovery.

Types

Supernova remnant SNR 0454-67.2 is likely the result of a Type Ia supernova explosion.

 

It has been discovered that Type Ia supernovae that were considered the same are in fact different; moreover, a form of the Type Ia supernova that is relatively infrequent today was far more common earlier in the history of the universe. This could have far reaching cosmological significance and could lead to revision of estimation of the rate of expansion of the universe and the prevalence of dark energy.