Unit 1:  Our place in the Universe                                          1   2   3   4  

3.3. Nebulae

Nebulae are gaseous clouds of interstellar gas (hydrogen and helium) and cosmic dust (made up of heavier chemical elements including some organic compounds).

They were formed by different causes and can be of several kinds:

•   Emission nebulae

They emit radiation which origin is the energy they receive from close stars.

They can be:

•   Extinct stars reminders (planetary nebulae and supernovas remnants)

•   Places where stars are born. This type is related to very young stars

   or proto-stars.

•    Reflexion nebulae

They reflect the light they receive from a close star.

This energy is not enough to make the nebula dust and gas emit radiation.

They are reminders of the gas that give birth to the star.

•    Absorption nebulae

They are not related with stars or they are very far away of them.

They are only visible when they interpose against the stellar background.

This is because they neither emit nor reflect light, although they absorb it.

Animation: Assorted nebulae

Animation: Planetary nebulae

3.5. Answer the questions:

a. Why are any nebulae called "star's nursery"?

b. Explain which difference exists among absorption, emission and reflexion nebulae.