Introduction to Astrophysics Lecture 12: Nebulae The hourglass nebula

Nebulae Long ago is was recognised that not all objects in the sky are pointlike, as stars are. The fuzzy diffuse images became known as nebulae. The first catalogue was created by Charles Messier in 1781, as a tool to prevent these blobs being confused for new comets. It contained 103 objects. Known as the Messier catalogue, it is still in use today. For example, M31 is the Andromeda galaxy.

What are nebulae? By the early 1900s, around 15,000 nebulae had been catalogued, but the poor image quality then available led to considerable controversy as to their nature. Were they...  Objects within our Galaxy (at that time a popular view was that our galaxy was all there was).  Objects beyond our Galaxy, and perhaps as large as our Galaxy.

Unusually for astronomy, there is no single answer. The objects in the Messier catalogue turned out to be of several distinct types. Nowadays, seen with the resolving power of the Hubble Space Telescope, they are amongst the most spectacular sights in the sky.

Types of nebulae  Supernova remnants  Planetary nebulae  Diffuse nebulae  Open star clusters  Globular star clusters  Galaxies

Supernova remnants As we’ve seen, the explosion at the end of the life of a massive star throws off piles of material into space. As the material expands and cools it can often be spectacular.

The Crab Nebula: remains of a supernova in 1054

The Vela supernova remnant

Planetary nebulae Planetary nebulae are nothing to do with planets! They are formed from the material ejected from stars at the late stages of their evolution. The name arises because when they were first studied people thought that some resembled the solar nebula and were planetary systems in the process of formation. But in fact they are much further away than people thought, and so much larger.

The Eye Nebula

The Spirograph Nebula (yes it is really called that)

The Hourglass Nebula The Turtle Nebula

Eta Carinae: a dying star

Diffuse nebulae Diffuse nebulae are basically giant clouds of dust and gas. Some may be in the process of star formation. Emission nebulae are made from hot gas which is absorbing radiation from hot nearby stars and re-emitting it. Reflection nebulae are reflecting the light from nearby stars. Dark nebulae are clouds of dust which obscure light from behind them.

The Orion emission nebula

The Horsehead nebula

Open star clusters Groups of stars, usually bright young ones, so close that they merge into a blur if seen at poor resolution. The Pleiades

Globular clusters These are groups of stars outside the main disk of our galaxy. There are around 100 associated with our own galaxy, some containing as many as a million stars. Globular cluster M10

Galaxies While all the other nebulae discussed so far are in and around our own galaxy, the remainder correspond to other galaxies distinct from our own. M31: the Andromeda galaxy

Chance line-of-sight superposition of two galaxies