Final answer:
The Oort cloud is a region around the Sun that contains ancient icy objects and gives rise to long-period comets. When a comet from the Oort cloud approaches the Sun, the heat causes the ices within the comet to vaporize, creating a glowing coma and a tail that points away from the Sun.
Step-by-step explanation:
The Oort cloud is a large spherical region around the Sun that contains ancient icy objects, from which long-period comets are derived. It extends out to about 50,000 AU, near the limit of the Sun's gravitational sphere of influence. When a comet from the Oort cloud approaches the Sun, the heat causes the frozen ice and gases within the comet to vaporize, creating a glowing coma and a tail that points away from the Sun.
For example, when a comet from the Oort cloud, like Comet Hale-Bopp, gets closer to the Sun, the heat causes the ices within the comet to sublimate (change directly from solid to gas), creating a glowing coma and a tail. The solar wind and radiation pressure push against the coma and tail, causing them to point away from the Sun due to the Sun's gravitational influence. This is why comets appear to have a bright head and a tail that always points away from the Sun when they are in proximity to it.