A star is born in a nebula. Gravity causes a region of the nebula to contract — this collapsing cloud is called a protostar. As the protostar contracts, it heats up. When core temperature reaches ~10 million K, hydrogen nuclei fuse to form helium (nuclear fusion begins). The outward radiation pressure balances gravity, and the star reaches a stable phase called the main sequence, where it spends most of its life.

A Red Giant is a large, cool, luminous star formed when a medium-mass star (like our Sun) exhausts hydrogen in its core. The core contracts and heats up, while outer layers expand enormously (up to 100× the original size) and cool, giving a red colour. The star now fuses helium into carbon in the core.

Nova: Occurs in a binary star system. A white dwarf accretes hydrogen from a companion star. Accumulated hydrogen undergoes runaway nuclear fusion on the surface, causing a sudden brightness increase. The white dwarf itself survives.

Supernova: A far more powerful explosion. Either a massive star collapses under gravity when fusion stops, or a white dwarf exceeds the Chandrasekhar limit (1.4 solar masses). The explosion destroys or significantly disrupts the star, releasing tremendous energy and creating a neutron star or black hole.

The primary factor is the initial mass of the star:
• Low-mass stars (≤ 8 solar masses, like our Sun) → Red Giant → White Dwarf → Black Dwarf
• High-mass stars (8–20 solar masses) → Supergiant → Supernova → Neutron Star
• Very high-mass stars (>20 solar masses) → Supernova → Black Hole

NASA (USA): Landed 12 humans on the Moon during the Apollo programme (1969–1972) and launched the James Webb Space Telescope (2021).

ESA (Europe): The Rosetta mission successfully landed a probe (Philae) on Comet 67P/Churyumov–Gerasimenko in 2014.

CNSA (China): Chang’e 5 mission (2020) returned the first lunar samples in over 40 years.

ISRO (Indian Space Research Organisation), founded in 1969, is India’s national space agency. Major contributions include: Chandrayaan-1 (confirmed water ice on Moon, 2008), Mangalyaan (India’s Mars Orbiter Mission, cost-effective at $74 million, 2014), Chandrayaan-3 (first soft landing on lunar south pole, 2023), and Aditya-L1 (first Indian solar mission, 2023).

Nepal’s space activities are overseen by NAST (Nepal Academy of Science and Technology). In April 2019, Nepal launched its first satellite, NepaliSat-1, from the Wallops Flight Facility in the USA aboard a Cygnus cargo spacecraft. The satellite is a small 1U CubeSat designed for Earth observation. Nepal also has growing interest in remote sensing for disaster management and environmental monitoring, and university programmes are gradually introducing space science curricula.

Space exploration requires vast financial resources, advanced technology, and diverse expertise that no single country can provide alone. International cooperation allows: (1) cost-sharing on expensive missions, (2) pooling of scientific talent across nations, (3) sharing of data for greater scientific output, (4) avoiding duplication of efforts, and (5) promoting peace and diplomacy. The International Space Station (ISS) is the best example — operated jointly by NASA, Roscosmos, ESA, JAXA, and CSA.