It is the term with several applications in astronomy but used most generally to describe any fuzzy patch seen in the sky. Nebulae (the plural form of the term) are often (though not always) vast clouds of dust and gas.Solar nebula is the vast primordial cloud of gas and dust from which (it has been theorized) the Sun and solar system were formed.
It is the star that suddenly and very dramatically brightens, resulting from the triggering of nuclear fusion caused by the accretion of material from a binary companion star.
The explosion accompanying the death of a massive star as its core collapses.
Variable star :
It is the star that periodically changes in brightness. A cataclysmic variable is a star, such as a nova or supernova, that changes in brightness suddenly and dramatically as a result of interaction with a binary companion star, while an intrinsic variable changes brightness because of rapid changes in its diameter. Pulsating variables are intrinsic variables that vary in brightness in a fixed period or span of time and are useful distance indicators.