The Nebula-class Star Destroyer, which also goes by the designation Defender-class Star Destroyer, represented the New Republic's most formidable and expansive warship design within the scope of the New Class Modernization Program.
Despite its length of merely 1,040 meters—amounting to approximately 65% of the Imperial-class Star Destroyer's size—the Nebula-class was engineered to effectively rival an Imperial II. The Nebula often got confused as an Imperial design, due to its angled design, despite its structure deviating significantly from the Imperial-class, most notably in its exclusion of a command tower. Due to its comparatively compact dimensions, it frequently earned the moniker "pocket Star Destroyer", a designation later applied to the Turbulent-class as well.
Its design allowed it to triumph over a single enemy Star Destroyer, a pair of heavy cruisers, or a complete formation of smaller Imperial support vessels. The defenses it possessed were powerful enough to withstand even some of the smaller Super Star Destroyers or similar warships.
The Nebula-class Star Destroyer's arsenal included 40 heavy turbolaser batteries, 40 turbolaser cannons, 20 ion cannons, 8 tractor beam projectors, 8 assault concussion missile tubes, each loaded with 30 missiles, along with pulse cannons that were turbolaser-sized, providing substantial firepower. Even though it was touted as having far better firepower than the Republic-class, the two had almost identical armaments; the Nebula had eight concussion missile launchers, while the Republic had two extra tractor beams. The Nebula was a much better design than the Republic because of the power generation and gun models. Moreover, the Nebula's hull armor and shield systems surpassed those of the majority of other ships in the galaxy, including the Republic.
These ships had the capacity to accommodate up to sixty starfighters, commonly including an assault wing composed of K-wing bombers for precise strikes and Defender starfighter for close-quarters support. Nebulas could also support a space superiority wing, which consisted of hyperdrive-equipped E-wings, A-wings, or upgraded X-wings. Furthermore, they carried 1,600 troops and enough consumables to last for five to six months, contingent on the configuration.
For faster-than-light travel, Nebula-class Star Destroyers used a Class 1 hyperdrive. The design also featured lateral thrust vents to improve the ship's maneuverability, giving it much more agility than the majority of large capital ships.
In keeping with the New Class's strategy of using the same hull for a variety of configurations, the Nebula-class Star Destroyers and the Endurance-class fleet carriers shared a basic spaceframe design.
The Nebula-class Star Destroyer's development started soon after Coruscant fell. Upon becoming a component of the Defender design program, it was formally renamed Defender. The Nebula-class improved the fundamental aspects of Walex Blissex's Republic-class design by incorporating it into a more compact, angular hull. Political distractions, Grand Admiral Thrawn's campaigns, and the resurrected Darth Sidious all caused the Defender program to be delayed. When work eventually restarted, the design was included in the New Class program, and its hull design was used for the Endurance-class carrier. The Nebula-class entered service in 15 ABY, making it the last of the New Class's eight new starship designs. The ship's debut was reportedly poorly timed because it took place during what was thought to be the most tranquil year since the New Republic was founded.
The Obi-Wan, named in honor of Obi-Wan Kenobi, one of the Rebellion's heroes, was the first of its kind to enter service. Captain Whyrrryk, a female Wookiee, was in charge of the ship. Due to the procedure of creating small yet potent warships of this nature, only a small number of Nebula-class ships were ever made. Around 17 ABY, there were only ten in service, with plans to launch one more each year until the New Class construction program was finished. When the Yuuzhan Vong invaded, the New Republic's few, but valuable, Nebula-class ships were held in reserve fleets close to the Core Worlds due to political pressure. Despite this, _Nebula_s performed admirably in combat with the extra-galactic warships once they were finally deployed. The Nebula-class was still the pinnacle of contemporary, efficient Star Destroyer design during the Yuuzhan Vong War, even in light of its age.
By the time of Darth Krayt's Galactic Empire, at least one Defender-class Star Destroyer was still in service. Pol Temm kept the ship as a component of his The Wheel defense fleet. He engaged in a space battle with a Mandalorian group known as the Mandalorian Supremacists in an attempt to uphold his no-weapons policy, and he was allegedly killed when his Defender exploded.
The official Expanded Universe has a convoluted and lengthy history regarding this ship class. The Star Destroyer twin for the Endurance-class carrier was dubbed the "Nebula-class" by Michael P. Kube-McDowell, the author of the Black Fleet Crisis novels. However, the RPG sourcebook Cracken's Threat Dossier designated this ship the Defender-class Star Destroyer and gave it a complex internal history as part of a "Defender" project, which also included the Defender-class assault carrier (another "New Class" design created by Kube-McDowell) and the Defender starfighter (an older WEG ship based on a toy from the animated series Star Wars Droids).
The sourcebook also featured the only official images of the Nebula-class and Endurance-class that were available at the time. Unlike the images of the other New Class ships in the book, these two ship classes appeared to have been constructed on the same spaceframe. Consequently, they could not be disregarded as easily as the other images of New Class ships, even though it was difficult to reconcile the Endurance-class with the description of Intrepid as "fat-hulled" in Before the Storm.
The Defender-class made another appearance in a later RPG sourcebook, Starships of the Galaxy, but with slightly different statistics: the ship's consumables were increased from 5 to 6 months, and it now carried 60 "starfighters and shuttles" as opposed to the single wing from the earlier resource. The Saga Edition of the same sourcebook would eventually increase the starfighter compliment even further to 60 starfighters and 12 "space transports". In addition, Starships of the Galaxy featured a brand-new image of the Defender, which was streamlined, dagger-shaped, and far more evocative of other Star Destroyer designs.
The New Essential Chronology added to the confusion by stating that the official name is the Nebula-class Star Destroyer, which is a retcon of its name to the scheme that Michael P. Kube-McDowell originally established.
Invasion: Revelations 3, which was released in 2011, featured a pair of ships that were explicitly referred to as Defender-class and had the appearance from Cracken's Threat Dossier. This may have turned the Nebula and Defender into entirely distinct classes of Star Destroyer.
Jason Fry, the author of 2012's The Essential Guide to Warfare, jokingly referred to a section as "continuity hell" because it attempted to resolve a number of the loose ends pertaining to the New Class program. It was determined that the Nebula-class was formally renamed Defender (without the use of "class" or italics) when it was incorporated into the Defender program. That program came to a standstill, and when work on the ship was restarted as part of the New Class program several years later, it appeared to be back under the original Nebula name.
It is noteworthy that the Nebula-class Star Destroyer and Endurance-class fleet carrier closely resemble the design of the Victory-class Star Destroyer and Venator-class Star Destroyer of the Galactic Republic. In both instances, the two destroyers possess comparable power levels, but one is more suited to ship-to-ship combat, while the other functions as an escort carrier.