Blue Snowball

Andromeda is best known for the Andromeda Galaxy, M31. But that’s not the constellation’s only good “deep-sky” object. One of them is inside our own galaxy: a dying star known as the Blue Snowball Nebula.
Even though it’s much closer than the Andromeda Galaxy, the Snowball is still a good jaunt from Earth – about 5500 light-years. It’s a bubble of gas and dust expelled by a star at the end of its life.
The star’s nuclear reactor – its hot core – is switching off. Radiation from the core is blowing away the star’s outer layers. That bubble spans close to a light-year, and it’s expanding at about 60,000 miles per hour. The gas is “energized” by ultraviolet light from the core, causing it to glow. Seen through a telescope, it looks like a blue snowball.
With the outer layers barreling away from it, the core is exposed to view. It’s tiny – only about 10 percent the diameter of the Sun. But it’s much, much hotter than the surface of the Sun, so it shines thousands of times brighter.
Before long, though, the core’s nuclear fire will be extinguished. The core will continue to shine, but at a tiny fraction of the Sun’s brilliance. With no energy source to light it up, the blue snowball will melt away.
Andromeda is low in the east and northeast as night falls. The nebula is at the right edge of the constellation, not far to the left of the Great Square of Pegasus.
Tomorrow: a faint eclipse.
Script by Damond Benningfield