APOD 1.6

The picture above shows the remains of a star that had exploded. Considering the word "had", this star had exploded around 11 thousand years ago (the time just after the last Ice Age), yet its remains are still in space reminding us of its former presence.

This star is located in the constellation Vela (the Sails).

What causes a star to explode?
Well, a star is composed of elements such as hydrogen and  helium. In the star, there is gravity which pulls in the elements, but are repelled by the heat of the nuclear reaction. The nuclear reaction, however, is continuous, which is why and how our own Sun is here and still ever bright. In the core, there is the fusion of hydrogen into helium into bigger and heavier elements such as iron, which is not as easily fused. Soon (after billions of years, depending on how big the star is), the iron builds up to a point where anymore reaction is impossible and gravity causes the star to cave inwards and explode.

This is what happened to this star.

All these beautiful colors you see in the remains of the star are the gas particles that have flown away from the star. What causes these colors is the reaction of the gas with the interstellar medium.

What is an interstellar medium?
This is basically the stuff in between the stars in space. Yes, there actually is matter up in space. This matter mostly consists of hydrogen and helium and a little bit of dust from some of the nearby stars. This medium is pretty thin, however, with enough of it packed together, some light from stars can be blocked.

Going back to the photo above, the gas decays and reacts with the medium producing these colors.

Observing how Space and its contents work just comes to show the complexity of everything and reminds us of how little we know of everything around us.