4. Give reason a. Nothing is permanently at rest in the universe. b. All the gaseous masses in the universe do not change into stars, c. Black holes cannot be observed directly like other celestial bodies, d. The sun will not change into a black hole. e. Astronomical unit and lightyear are used as unit for measurement in astronomy.

Below are the reasons for each statement: a. Nothing is permanently at rest in the universe.  Reason: In the universe, every object is influenced by gravitational forces from other objects and by the overall expansion of space. Even if an object appears to be at rest from one perspective, relative motion exists when viewed from another reference frame. In accordance with Newton's first law, an object remains in uniform motion unless acted upon by an external force; given the constant interactions (both gravitational and due to cosmic expansion), “absolute rest” is never achieved. b. All the gaseous masses in the universe do not change into stars.  Reason: For a gaseous mass to become a star, specific conditions must be met—such as sufficient mass (exceeding a critical threshold), appropriate density, and the ability to cool and collapse under gravity. Many gas clouds are too diffuse, fail to overcome internal pressure or turbulence, and may dissipate or form other structures (like planets or brown dwarfs) instead of igniting nuclear fusion to become stars. c. Black holes cannot be observed directly like other celestial bodies.  Reason: A black hole’s gravitational pull is so intense that not even light can escape from within its event horizon. This absence of emitted or reflected light means we cannot directly image black holes using conventional telescopes. Instead, their presence is inferred by observing the gravitational effects on nearby matter, radiation emitted from accretion disks, or the motion of stars near the black hole. d. The sun will not change into a black hole.  Reason: The formation of a black hole typically requires the collapse of a very massive star (usually several times the mass of the Sun) after it has exhausted its nuclear fuel, often after a supernova explosion. The Sun’s mass is insufficient to trigger this process. Instead, it will eventually expand into a red giant and ultimately shed its outer layers, leaving behind a white dwarf. e. Astronomical unit and lightyear are used as units for measurement in astronomy.  Reason: Distances in space are enormous compared to everyday scales, so it is practical to use units that simplify these measurements. An astronomical unit (AU), defined as the average distance between the Earth and the Sun, is useful for expressing distances within our solar system. A lightyear—the distance light travels in one year—is convenient for expressing vast interstellar or intergalactic distances. These units make it easier to communicate and comprehend the enormous scales found in the universe.