Sam was gazing through a telescope and notice a bright spot moving fast through the sky.

Sam most likely observed a comet.

Sam most likely observed a comet.

To determine what Sam most likely observed, we can look at the characteristics described in the question. Sam noticed a bright spot moving fast through the sky, followed by a stream of dust.

Based on these observations, it is most likely that Sam observed a comet. Comets are celestial objects composed of ice, dust, and rocky material. As a comet gets closer to the Sun, the heat causes the icy material to vaporize, leaving behind a glowing coma (the bright spot) and a tail of dust and gas (the stream of dust). Comets often appear as transient objects in our sky and can move relatively fast.

Therefore, the most likely answer is that Sam observed a comet.