Final answer:
The accretion disc spectrum of a supermassive black hole is dramatically more luminous and emits more powerful X-rays compared to a stellar-mass black hole, owing to its much greater mass and the intense gravitational forces at play.
Step-by-step explanation:
The accretion disc spectrum of a supermassive black hole differs significantly from that of a stellar-mass black hole primarily due to the vast difference in mass and the resultant gravitational influence each has. Supermassive black holes, often found at the centers of galaxies, have masses ranging from millions to billions of solar masses. These enormous black holes are capable of heating the accreting gas to temperatures so high that they can emit radiation strongly in the X-ray part of the spectrum and are often visible as quasars when feeding. Moreover, supermassive black holes can produce large, energetic jets perpendicular to their accretion disk, which can eject more material than that which falls into the black hole itself.
In contrast, stellar-mass black holes typically result from the gravitational collapse of massive stars and contain about 3 to several tens of solar masses. Evidence for these black holes is often found in binary systems where they accrete material from a companion star. The accretion discs of stellar-mass black holes also emit X-rays, but due to their smaller mass, the gravitational pull and resultant heating is less intense compared to their supermassive counterparts. As a result, the accretion disc spectrum is less luminous and the accretion disk itself is generally smaller.