Giovanni Covone » 1.Introduction to the study of the physics of the interstellar medium

Preface

What is the interstellar medium?
The term “interstellar medium” (ISM) refers to the material between stars in galaxies, and around galaxies.
On a larger scale, the ” intergalactic medium” (IGM) refers to the material spread in the space between galaxies.

Why is it important to understand its properties and nature?
In the Galaxy, stars originate from the ISM, interact with it, “pollute” it with new elements.
On a cosmological scale, understanding the properties of the IGM and its link with galaxies
is crucial when addressing the problem of the formation and evolution of galaxies.

Goal of the course
We will address a few selected topics.
The aim is to be able to read and understand at least part of the current research literature.

History: the Orion Nebula

Left panel: A drawing of the Orion Nebula by the French astronomer Messier, published in 1771,"Mémoires de l'Académie Royale". This object is now known as Messier 42. Right panel: A modern view of the same object with a large telescope.

History: The Messier catalog

The page describing the object number 42, in the "Catalogue des Nébuleuses & des amas d'Étoiles.", published in 1784 by Charles Messier.

History: the nebular hypothesis

Nebular hypothesis, developed by Kant (1755) and Laplace (1796).

The model was the accepted explanation in the 19th century.

Observations by the English astronomer Herschel.

Drawing of Nebulae by William Herschell. From the "Philosophical Transactions of the Royal Society of London" vol. 101, year 1811.

Proto-planetary disks

Images of proto-planetary discs, recently discovered in the Orion Nebula by the Hubble Space Telescope.

History: the discovery of nebular emission lines

Sir William Huggins (1824 – 1910), best known for his pioneering work in astronomical spectroscopy, at his private observatory.

First photographs of the Orion Nebula

First two photographs of the Orion Nebula by Henry Draper. Credit: Harvard College Observatory.

History: The discovery of interstellar clouds

The first actual detection of cold diffuse matter in interstellar space was made by Johannes Hartmann in 1904,
by means of absorption line spectroscopy.

Hartmann took spectra of the bright star Delta Orionis, and noticed that a fraction of light was being absorbed along the line of sight.

He noticed some remarkable, stationary absorption lines, while the star itself was known to be oscillating: how could the absorption lines be stationary?

A modern application of the same technique: investigation of the IGM by mean of optical spectroscopy of distant quasars.

History: the dark clouds

Optical observations of the Milky Way shows the striking presence of completely dark regions.

Along their line of sight, the very large extinction by dust causes the obscuration of the light from background stars.

The “holes in the heavens” discovered by William Herschel.

The advent of deep photographic imaging allowed Edward Barnard to produce the first images of dark nebulae already noticed by William Herschel.
He could show their silhouette against the rich background star fields of our galaxy.

In 1919, Barnard formulated the hypothesis that these were caused by ”obscuring masses of matter in space”.