I'm guessing that the images are due to the effect of the heat on the chemicals that make up the 'phosphor'. As to the glass - it should melt before it cracks, presuming that it heats fairly evenly and isn't subjected to a sudden partial quenching. Because the tube is in vacuum, the most pressure ever exerted is 1 atmosphere, as a crushing pressure. Since the glass is at its most brittle when cold, if it handles 1atm when cold, it should have no problem when hot (until the glass starts to melt anyway ) . If it were a sealed bottle with air inside it, then the air would expand creating a far higher pressure than 1 atm. That, and the fact that glass is stronger under compression than under tension, would cause the bottle to explode.