H3
h3h3Productions, commonly known as H3, is a YouTube channel and podcast series created by American-Israeli couple Ethan Klein and Hila Klein, featuring satirical reaction videos, sketch comedy, and commentary on internet trends and cultural phenomena. Launched in 2011, the channel gained early traction through critiques of viral content and absurd online behaviors, such as the 2016 video "VAPE NATION," which highlighted the excesses of vaping culture.[1][2] The Kleins, who met during Ethan's birthright trip to Israel where Hila served in the Israel Defense Forces, built a following by defending fair use in transformative commentary, notably prevailing in a 2018 copyright infringement lawsuit brought by video creator Matt Hosseinzadeh, which set a precedent for reaction content creators. This legal victory, coupled with consistent uploads blending humor and skepticism toward mainstream narratives, propelled the channel to millions of subscribers and established H3 as a counterpoint to polished influencer content. The associated H3 Podcast, debuting in 2017, expanded into long-form interviews with comedians, activists, and public figures, often delving into politics, social media drama, and personal stories, amassing over 2.6 million subscribers by 2025.[3][4][5] Despite its influence in online comedy, H3 has been marked by controversies, including a high-profile fallout with collaborator Trisha Paytas that ended their joint "Frenemies" podcast in 2021 amid accusations of toxicity and financial disputes, as well as a 2020 lawsuit from Triller CEO Ryan Kavanaugh alleging copyright violations. More recently, in September 2025, resurfaced posts from Klein's old Reddit and X accounts revealed crude and inappropriate comments, sparking backlash over hypocrisy given his public persona's emphasis on accountability for others. These incidents have fueled debates about consistency in H3's critiques of internet hypocrisy, though the platform retains a dedicated audience for its unfiltered takes.[1][6][7]Scientific and chemical uses
Trihydrogen cation (H₃⁺)
The trihydrogen cation, denoted H₃⁺, is the simplest stable polyatomic molecular ion, consisting of three hydrogen nuclei (protons) sharing two electrons in a symmetric, equilateral triangular configuration with bond lengths of approximately 0.87 Å and a bond angle of 60°.[8] This structure arises from the delocalized bonding where the electrons occupy a molecular orbital spanning the three protons, conferring high stability despite the ion's overall positive charge.[9] Its ground state electronic configuration is 1a₁'², with a dissociation energy into H₂⁺ + H of about 1.67 eV, making it metastable under low-density conditions but highly reactive in denser environments.[10] First observed in 1911 by J. J. Thomson through mass spectrometry of a hydrogen plasma discharge, where peaks corresponding to mass-to-charge ratios indicated the presence of H₃⁺ alongside H⁺ and H₂⁺, the ion's existence was initially debated but confirmed by subsequent early-20th-century experiments.[11] Theoretical predictions of its infrared spectrum emerged in the 1970s, enabling laboratory measurements in 1980 using high-resolution spectroscopy, which identified rovibrational transitions in the 2.5–4 μm range critical for astronomical detection.[12] Precision saturated absorption spectroscopy later refined these levels, achieving uncertainties below 10⁻⁶ cm⁻¹ for key lines like the ν₂ band at around 2520 cm⁻¹.[13] In the interstellar medium, H₃⁺ forms primarily via the reaction H₂⁺ + H₂ → H₃⁺ + H, initiated by cosmic ray ionization of H₂, and serves as a cornerstone of ion-molecule chemistry, reacting exothermically with over 80 neutral species (except noble gases like He and Ne) to produce H₂ + MH⁺, where M is the neutral atom or molecule.[8] This reactivity propagates chain reactions forming complex molecules, with H₃⁺ abundances reaching 10⁻⁵ relative to H₂ in diffuse clouds and up to 10⁻⁴ in denser regions, as inferred from absorption spectroscopy toward infrared sources.[10] First spectroscopically confirmed in space in 1988 via infrared observations of its ν₂ fundamental band in ionospheres of planetary atmospheres and later in diffuse interstellar clouds in 1996, H₃⁺ observations have mapped cosmic ray fluxes and probed conditions in regions like the Central Molecular Zone of the Milky Way.[12] [14] Laboratory studies emphasize its role in plasma physics and astrochemistry simulations, where H₃⁺ densities exceed 10¹⁰ cm⁻³ in low-pressure discharges, and its radiative cooling via infrared emission influences non-local thermodynamic equilibrium conditions.[15] Quantum chemical calculations, such as those using configuration interaction methods, accurately predict its potential energy surface, validating formation pathways and dissociation dynamics observed in crossed-beam experiments.[16] Despite its ubiquity in space, H₃⁺ evades detection on Earth due to rapid neutralization by electrons and neutrals, underscoring its preference for low-density, ionized hydrogen-rich plasmas.[11]Tritium (³H)
Tritium, denoted as ³H or T, is a rare radioactive isotope of hydrogen with one proton and two neutrons in its nucleus, giving it an atomic mass of approximately 3.0160492 u.[17] It decays via beta emission to stable helium-3, releasing an electron with a maximum energy of 18.6 keV and no accompanying gamma radiation.[17] The isotope's half-life is 12.323 ± 0.004 years, making it suitable for studies spanning decades but requiring careful handling due to its weak beta emissions that pose minimal external radiation hazard yet can incorporate into biological molecules.[17] [18] Tritium was first produced in December 1934 by physicists Ernest Rutherford, Mark Oliphant, and Paul Harteck at the Cavendish Laboratory, through deuteron bombardment of deuterium gas, which yielded small quantities of ³H via the reaction D + D → ³H + p (where D is deuterium and p is a proton).[19] This artificial synthesis preceded its detection in nature, where trace amounts arise from cosmic ray interactions with atmospheric nitrogen and oxygen, producing about 4 kg annually in the Earth's hydrosphere at concentrations around 10⁻¹⁸ relative to protium.[20] Commercial production occurs primarily in heavy-water moderated nuclear reactors, such as Canada's CANDU design, via neutron capture on deuterium (²H + n → ³H) or, more efficiently, on lithium-6 targets (⁶Li + n → ⁴He + ³H), yielding up to several grams per year per facility for research and applications.[21] In chemical research, tritium functions as a tracer isotope due to its chemical similarity to protium and deuterium, enabling precise tracking of hydrogen atoms in reactions without perturbing kinetics significantly; for instance, tritiated water (HTO) or organic compounds labeled with ³H reveal mechanisms in hydrogen transfer processes, such as enzyme-catalyzed reductions.[22] [23] Tritium labeling is integral to pharmaceutical chemistry, where ³H-tagged molecules facilitate autoradiography, scintillation counting, and metabolic pathway elucidation in drug discovery, with over 10,000 such compounds synthesized annually for binding affinity assays and pharmacokinetics studies.[24] In analytical chemistry, tritium's beta emissions support liquid scintillation spectrometry for quantifying low-level hydrogen fluxes or isotopic exchange rates in catalytic reactions.[23] A key application lies in nuclear fusion science, where tritium pairs with deuterium in the DT reaction (D + T → ⁴He + n + 17.6 MeV), offering the highest fusion cross-section at plasma temperatures around 100 million K achievable in tokamaks like ITER; this reaction's neutron output drives blanket breeding of further tritium from lithium, enabling self-sustaining fuel cycles for potential energy production.[25] Experimental facilities, such as the JET tokamak, have demonstrated DT yields exceeding 16 MW of fusion power in 2021 pulses, validating tritium's role despite handling challenges from its radioactivity and inventory limits.[25] In geochemical and hydrological studies, environmental tritium serves as a transient tracer for groundwater dating and recharge rates, with pre-1950s baselines (from cosmic production) distinguishing modern anthropogenic inputs from nuclear testing peaks in the 1960s.[26] Safety protocols emphasize containment to prevent tritiation of air or water, as organically bound tritium (e.g., in biomolecules) exhibits higher biological impact than tritiated water due to prolonged retention.[20]Triatomic hydrogen (H₃)
Triatomic hydrogen, denoted H₃, is a neutral triatomic molecule composed solely of three hydrogen atoms, representing the simplest neutral polyatomic radical.[27] Unlike the stable trihydrogen cation H₃⁺, the neutral species is highly unstable, with its ground electronic state featuring a shallow potential energy surface that leads to rapid dissociation into H₂ and H.[28] Theoretical calculations from the 1930s indicate that the lowest-energy configuration of H₃ adopts a linear geometry, contrasting with the equilateral triangular structure of H₃⁺, as the neutral molecule lacks the electrostatic stabilization provided by the positive charge.[29] The concept of H₃ emerged in the early 1910s amid J. J. Thomson's investigations of positive rays, where mass spectrometry-like observations suggested the presence of triatomic hydrogen species, though initial interpretations conflated neutral and ionic forms.[27] Early experimental claims, such as volume contraction of hydrogen gas under alpha particle irradiation reported by Wendt and Duane in 1917, were later attributed to other effects rather than stable H₃ formation.[30] Definitive transient observation of neutral H₃ came in the 1980s through neutralization-reionization techniques and fast beam experiments, where H₃⁺ ions undergo charge exchange to form H₃, followed by reionization to confirm its survival over short distances in collisional sequences like H₃⁺ → H₃ → H₃⁺ in argon targets. These methods demonstrated H₃'s fleeting existence, with formation also achievable in low-pressure gas discharges or via alkali vapor neutralization of H₃⁺ beams.[31] Spectroscopic studies have characterized excited and metastable states of H₃. An emission spectrum featuring diffuse rotational structure near 5600 Å was first reported in 1980, attributed to electronic transitions in H₃ and its deuterated analog D₃.[32] Metastable Rydberg states, accessed via charge exchange or photoexcitation, exhibit lifetimes of approximately 640 ns for the ground vibrational level and 740 ns for the symmetric stretch-excited level, limited by predissociation mechanisms including spin-orbit coupling.[33] These short lifetimes underscore H₃'s reactivity, with dissociation barriers low enough (on the order of electronvolts or less in excited manifolds) to preclude stable isolation under standard conditions.[31] In interstellar environments, neutral H₃ forms transiently through reactions involving H₃⁺ and electrons or neutrals, contributing to hydrogen chemistry despite its instability, as the abundance of H₃⁺ drives repeated formation-destruction cycles.[30] Advanced theoretical models, including ab initio potential energy surfaces, continue to refine predictions of its dynamics near conical intersections and in roaming mechanisms, aiding understanding of dissociation pathways.[34] Experimental confirmation relies on indirect techniques due to the molecule's evanescence, with no evidence for thermal stability at room temperature or above.[35]Histone H3
Histone H3 is one of the four core histone proteins that form the octameric nucleosome core particle, the fundamental unit of chromatin in eukaryotic cells, where two copies each of H2A, H2B, H3, and H4 wrap approximately 147 base pairs of DNA.[36] This structure compacts DNA while regulating access for processes like transcription, replication, and repair. The protein consists of a globular domain that mediates histone-histone and histone-DNA interactions, and an unstructured N-terminal tail protruding from the nucleosome, which is the primary site for post-translational modifications influencing chromatin dynamics.[37] The primary amino acid sequence of human histone H3 comprises 136 residues, with high conservation across eukaryotes; for instance, calf thymus H3 has 135 residues, a molecular weight of 15,324 Da, and alanine at both termini.[38] Key structural features include alpha-helical segments in the globular domain that facilitate dimerization with H4 and tetramer formation, enabling the wrapping of DNA in a left-handed superhelix around the histone octamer.[39] Variants of H3, such as the canonical replication-coupled H3.1 and H3.2, differ minimally from the replication-independent H3.3 by 4-5 amino acids, primarily in the globular domain, which alters nucleosome stability and incorporation preferences.[36] H3.3, chaperoned by HIRA or DAXX/ATRX complexes, preferentially deposits at active euchromatin and regulatory elements, while centromeric variant CENP-A (H3-like) ensures kinetochore assembly.[40] Post-translational modifications on H3 tails and globular domain serve as epigenetic signals, with lysine residues being hotspots for acetylation (e.g., K9ac, K14ac promoting open chromatin) and methylation (mono-, di-, or tri- states).[41] Notable marks include H3K4 methylation (H3K4me3) at promoters of active genes, mediated by SET1/COMPASS complexes, which recruits transcription machinery; H3K9me2/3 for heterochromatin formation via HP1 binding; and H3K27me3 by PRC2 for gene repression.[42] Phosphorylation at S10 occurs during mitosis, aiding chromosome condensation, while ubiquitination at K119 influences Polycomb-mediated silencing.[43] These modifications are dynamically added or removed by writers (e.g., methyltransferases like EZH2) and erasers (e.g., demethylases like JMJD2), with combinatorial patterns dictating chromatin states without altering the DNA sequence.[44] In epigenetics and chromatin biology, H3 variants and modifications enable heritable yet reversible control of gene expression; for example, H3.3 incorporation correlates with transcriptional activity and is enriched at telomeres and bivalent domains in development.[45] Disruptions, such as mutations in H3.3 (e.g., K27M in pediatric gliomas), alter PTM landscapes and drive oncogenesis by globally reducing H3K27me3.[45] Empirical studies using mass spectrometry and chromatin immunoprecipitation confirm site-specific PTMs' causal roles in processes like learning (via H3K4 methylation) and DNA methylation maintenance.[46] Chaperone-mediated deposition ensures variant-specific functions, with HIRA directing H3.3 to promoters and DAXX to pericentromeric heterochromatin.[40]Biological uses
Histone H3 variants and modifications
Histone H3 exists in multiple variants that differ in amino acid sequence, deposition mechanisms, and functional roles in chromatin dynamics. The canonical variants H3.1 and H3.2 are synthesized primarily during S-phase and incorporated into nucleosomes in a replication-coupled manner by chaperones such as CAF-1, maintaining chromatin structure during DNA duplication.[47][48] In mammals, H3.1 and H3.2 differ from each other by a single amino acid at position 31 (serine in H3.1, alanine in H3.2), but both contrast with the replication-independent variant H3.3, which shares only 96-97% sequence identity and is deposited throughout the cell cycle at transcriptionally active loci via chaperones like HIRA or DAXX/ATRX.[45][40] H3.3 facilitates epigenetic memory at promoters and enhancers, with its incorporation enriched in euchromatin and poised for gene activation.[49] Specialized variants include CENP-A, which replaces H3 in centromeric nucleosomes to direct kinetochore assembly; testis-specific H3t, linked to spermatogenesis; and H3.5, involved in sperm cell differentiation.[50][51] Post-translational modifications (PTMs) of H3, predominantly on its flexible N-terminal tail but also in the globular core, regulate nucleosome stability, chromatin compaction, and protein recruitment, influencing processes like transcription, replication, and repair. Acetylation neutralizes lysine residues' positive charge, promoting open chromatin; for instance, H3K9ac and H3K14ac correlate with active transcription by recruiting bromodomain proteins, while H3K56ac in the core domain enhances nucleosome mobility and DNA accessibility during replication.[41][44] Methylation is context-dependent: H3K4me3 marks active promoters and is catalyzed by SET/MLL complexes, H3K36me3 signals transcriptional elongation along gene bodies via NSD enzymes, whereas H3K9me2/3 and H3K27me3 enforce repression through heterochromatin formation and Polycomb group proteins, respectively.[52][41] Phosphorylation events, such as H3S10ph during mitosis or H3T3ph in heterochromatin, alter tail interactions and facilitate chromosomal condensation.[43] These PTMs often combine in combinatorial patterns, interpreted by reader domains (e.g., chromodomains for methylated lysines), though their precise causality in outcomes like gene activation remains under study due to interdependent "writer," "eraser," and "reader" activities.[53] Variant-specific PTMs, such as enhanced H3.3 S31 phosphorylation promoting K27 acetylation in development, further diversify functions.[54] Aberrant H3 modifications, including oncogenic mutations like H3K27M in pediatric gliomas, disrupt these codes and drive tumorigenesis by globally impairing methylation.[55]Computing and web technologies
HTML heading element (
)
The<h3> element in HTML represents the third level of a section heading, positioned hierarchically below <h2> and above <h4> in a document's outline structure. It is one of six heading levels (<h1> to <h6>), with <h1> denoting the highest rank for top-level sections and decreasing importance thereafter, enabling browsers and assistive technologies to generate a table of contents or navigate content semantically.[56] This element contributes to the document's heading hierarchy, where proper nesting—such as using <h3> for subsections under an <h2>—facilitates logical content organization without relying on visual styling alone.
The syntax for <h3> requires both opening and closing tags, enclosing phrasing content such as text, images, or inline elements, but excluding block-level elements like paragraphs or other headings. Permitted contexts include flow content within sections, articles, or other containers that accept headings, but it must not nest within table headers (<th>) or address elements.[57] The element supports only global HTML attributes, including id for unique identification, class for styling hooks, lang for language specification, and title for advisory text; the obsolete align attribute, once used for horizontal alignment, has been deprecated in favor of CSS properties like text-align. No ARIA roles are needed, as <h3> implicitly conveys a "heading" role with level 3 semantics.[58]
In practice, <h3> is employed for sub-subsections, such as detailing components within a major topic outlined by <h2>, enhancing readability and search engine crawling by signaling content depth.[59] For accessibility, it allows screen readers to announce heading levels, aiding navigation for users with disabilities; skipping levels (e.g., <h1> directly to <h3>) can confuse this outline, though HTML5 permits it without invalidating the document.[58] Styling defaults to a medium font size and bold weight in user agents, but authors should override via CSS for consistency across devices, avoiding reliance on heading tags for mere bold text, which undermines semantic value. The element traces to early HTML specifications, with standardized behavior refined in HTML5 to emphasize outline algorithms over strict rank enforcement.
Aerospace and space exploration
H3 rocket
The H3 launch vehicle is a heavy-lift expendable rocket developed by the Japan Aerospace Exploration Agency (JAXA) in partnership with Mitsubishi Heavy Industries (MHI) as the successor to the H-IIA rocket, aiming to deliver payloads of 2 to 7 metric tons to geostationary transfer orbit (GTO) while emphasizing flexibility through modular configurations, high reliability via proven expander-cycle engines, and reduced costs compared to predecessors.[60][61] Development commenced in 2013 to address the need for a more competitive launcher amid growing international demand for satellite deployments, with the rocket's first stage powered by three LE-9 liquid engines using liquid oxygen and hydrogen propellants, providing greater thrust than prior Japanese liquid engines.[61][62] The maiden flight (H3 F1) on March 7, 2023, from Tanegashima Space Center ended in failure when the second-stage engine failed to ignite, leading to a destruct command and loss of the ALOS-3 payload; subsequent investigations identified a faulty electrical component in the ignition system, prompting design refinements.[61] Following this, H3 achieved six consecutive successful launches, including H3 F5 deploying the QZS-6 quasi-zenith satellite in February 2025 and H3 F7 on October 26, 2025 (JST), which carried the HTV-X1 uncrewed cargo spacecraft to rendezvous with the International Space Station, marking the rocket's role in resupply missions.[63][64] These successes have positioned H3 as Japan's flagship launcher, phasing out H-IIA operations and enabling commercial ventures through entities like Space One.[65] H3 variants adapt to payload masses via strap-on solid rocket boosters (SRBs) and core stage engine counts: the baseline H3-30L configuration includes two large SRBs and three LE-9 first-stage engines for up to 6.5 tons to GTO, while the minimal H3-30S omits boosters for lighter loads around 4 tons to low Earth orbit (LEO), with the second stage using a single LE-5B-3 engine restartable for precise orbit insertions.[66][61] The rocket stands 63 meters tall with a fairing diameter of 5.2 meters, and its modular design allows cost savings estimated at 30% over H-IIA by reusing components and simplifying manufacturing.[66][67]| Variant | SRBs | First-Stage Engines | Payload to LEO (kg) | Payload to GTO (kg) |
|---|---|---|---|---|
| H3-30S | 0 | 3 LE-9 | ~4,000 | ~2,100 |
| H3-22L | 2 small | 2 LE-9 | ~6,000 | ~4,000 |
| H3-30L | 2 large | 3 LE-9 | ~6,500 | ~6,500 |
Automotive and vehicles
Hummer H3
The Hummer H3 is a mid-size SUV produced by General Motors under the Hummer brand for model years 2006 through 2010. Introduced as a more maneuverable alternative to the full-size H1 and H2, it measured 16.9 inches shorter and 6 inches lower than the H2 while retaining Hummer's rugged styling and off-road focus.[69] The H3 was assembled at GM's Ramos Arizpe plant in Mexico and targeted consumers seeking compact dimensions with substantial ground clearance and towing capacity.[70] Developed on the GMT355 body-on-frame platform shared with the Chevrolet Colorado and GMC Canyon compact trucks, the H3 featured a 111.9-inch wheelbase and independent front suspension for improved on-road handling compared to its predecessors.[71] Standard off-road equipment included Torque On Demand 4WD, front and rear locking differentials, 16-inch off-road tires, and 9.7 inches of ground clearance, enabling a 60-degree approach angle and 7,700-pound towing capacity.[72] Initial base pricing started at approximately $29,405 for the 2007 model.[73] Powertrain options evolved across production. The 2006 model used a 3.5-liter inline-5 engine producing 220 horsepower and 225 lb-ft of torque, paired with a five-speed manual or four-speed automatic transmission. From 2007 onward, this was upgraded to a 3.7-liter inline-5 delivering 242 horsepower and 242 lb-ft, improving acceleration to 0-60 mph in about 9.7 seconds for automatic-equipped models.[74] [75] The H3 Alpha variant, introduced in 2007, substituted a 5.3-liter V8 engine generating 300-305 horsepower and 320 lb-ft, boosting top speed to 99 mph but reducing fuel efficiency to 14 mpg city/18 mpg highway.[76] All models achieved EPA ratings around 15 mpg city/19 mpg highway for inline-5 variants, reflecting their 4,690- to 4,700-pound curb weights and emphasis on low-end torque over efficiency.[77]| Model Year | Engine | Horsepower | Torque (lb-ft) | Transmission Options |
|---|---|---|---|---|
| 2006 | 3.5L I5 | 220 | 225 | 5-speed manual / 4-speed auto |
| 2007-2010 | 3.7L I5 | 242 | 242 | 5-speed manual / 4-speed auto |
| 2007-2010 (Alpha) | 5.3L V8 | 300-305 | 320 | 4-speed auto |