Stars and their planetary systems are very small and far away from each other in the galactic scale. Between the stars can be found clouds of interstellar matter (ISM). In our own galaxy it makes up 10% of the total (visible) mass. The rest is stars (and their planets, but those are a tiny minority).
Interstellar matter comes in two basic kinds: gas and dust. The majority of ISM is gas, with a ratio of about 100 to 1.
The gas clouds are mostly hydrogen, with some helium. These were created in the early universe. Heavier elements may also be present, these are the remains of dead stars (see previous lecture).
Dust clouds contain micrometer-sized particles (very large compared to the individual atoms of the gas clouds, about the size of tobacco smoke). They consist of various elements like silicon and carbon. They are formed by condensation in the top layers of very cool stars that are rich in heavy elements, then blown into space.
It must be remembered that these ISM clouds are very spars. A typical gas cloud might have 1–100 atoms per cubic centimetre. This is much closer to total vacuum than the best vacuums we can produce in laboratories. However, the ISM clouds are also very big, they can be light years in size. Thus they can also contain tens or hundreds of solar masses worth of material.
Our sun is only one star among the approximately hundred billion in the galaxy we call the Milky Way. Practically all the stars we see in the night sky are located in the close neighbourhood of the Sun.
The Milky Way is called such because when seen in a dark place, it is a milky white band arcing over the sky. After telescopes were invented, it was realized that all of the smooth milky whiteness is just a huge number of stars, so many that they blend into each other when seen with the naked eye.
After the idea that the stars are far away and may just be other suns gained acceptance, William Herschel was the first to systematically consider the shape of the system of stars surrounding us. He looked into different directions in the sky with his telescope and counted the stars in each direction. Assuming that the conditions are similar in every direction, the numbers of stars would tell how far the mass of stars extends. His result was that we live in the middle of a relatively flat disc of stars.
Later it was realized that the interstellar matter actually dims the light of stars shining through it. Because of this we cannot see as far in the milky way as we otherwise would. Other wavelengths beside visible light can pass through the dust more easily, allowing us to study the structure of our galaxy and the dust.
It turns out the galaxy is a large relative flat system stars with a central bulge. To quote The Galaxy Song by Monty Python (how I always remember these numbers, look it up on Spotify or Youtube):
Our galaxy itself contains a hundred billion stars, / it's a hundred thousand light years side to side. / It bulges in the middle, sixteen thousand light years thick, / but ouy by us it's just three thousand light years wide. We're thirty thousand light years from galatic central point, / we go around every two hundred million years. / And our galaxy is only one of millions of billions, / in this amazing and expanding universe.
The Milky Way is a barred spiral galaxy. It has a bar-like structure in the central bulge, and several spiral arms which contain more stars and ISM than the regions between the arms.
Most of the interstellar matter is very close to the middle plane of the galactic disc. This is due to the flattening of the galaxy as it spins, which has been going on ever since its birth more than ten billion years ago.
The centre of the galaxy contains a supermassive black hole. It contains about four million solar masses worth of matter and is much larger than the Sun.
There are lots of galaxies in the universe. More than there are stars in our galaxy.
Galaxies were at first thought to be some kind of nebulous bodies inside the Milky Way. Finally in the early 20th century it was proved conclusively that the distances to other galaxies are much greater than the size of our own galaxy.
Galaxies are classified into three basic types: Elliptical galaxies have no structure, they are simply ellipsoidal blobs of stars and matter. They range in shape from very elongated cigar-shapes to almost perfectly spherical. They very largest of galaxies are ellipsoidal; this is probably because when two galaxies merge, the resulting galaxy tends to be more ellipsoidal.
The second type is spiral galaxies, which are relatively flat discs with some number of arms that curve in a spiral, with less dense regions between.
The third type is the barred spiral, which is like a spiral but additionally has a thick bar in the centre. Our galaxy is of this type.
Galaxies were created in the early universe when the gas that existed started collapsing slowly. The process is similar to the smaller-scale process that creates stars and planetary systems inside the galaxies, but much much slower and larger.
Galaxies form groups, which generally contain some small number of larger galaxies and some smaller ones. Our local group contains two major galaxies: the Milky Way and the Andromeda Galaxy, as well as a large number of small galaxies that orbit either of these.
Several galaxy groups form supergroups. These in turn are organized into larger structures, eventually reaching the largest scales in the universe. At the large scale the universe consists of filamentary structures of galaxy groups with large voids in between.