Here's why:

* Alpheratz is a main sequence star: It's classified as an A0 spectral type, meaning it's a hot, bright, and relatively young star that's currently fusing hydrogen into helium in its core.

* White dwarfs are remnants: White dwarfs are the dense, leftover cores of stars like our sun after they've exhausted their nuclear fuel and shed their outer layers.

* Alpheratz is massive: Alpheratz is significantly more massive than our sun, and white dwarfs are much smaller than the original stars from which they formed.

Therefore, Alpheratz is a large, luminous, and still-active star, quite different from a white dwarf.