The object that produces a shimmering tail that lasts for a few days when it approaches the sun is a comet.

Here's why:

* Composition: Comets are icy bodies composed of dust, rock, and frozen gases like water, carbon dioxide, and methane.

* Solar Radiation: As a comet nears the sun, the solar radiation heats the comet's surface, causing the frozen gases to sublimate (turn directly from solid to gas).

* Dust and Gas Tail: This sublimation process releases a stream of gas and dust, creating the characteristic tails. The gas tail, also called the ion tail, is pushed away from the sun by the solar wind and glows with a blue color. The dust tail, reflecting sunlight, appears white or yellowish and is usually curved.

* Duration: The tails can persist for days, weeks, or even months after the comet has passed its closest point to the sun (perihelion), depending on the comet's size and composition.

So, the shimmering tail you're describing is a telltale sign of a comet's presence!