Galaxies

1. Galaxies The different types and how they form.

3. Spiral Galaxies • The gas has time to stabilize into a disk before it is all used up • Most of the bright galaxies in the neighborhood of the Milky Way are spiral galaxies (although irregular galaxies are actually the most common). Spirals have the smallest range of masses and sizes. Typically, these objects can contain between 10,000,000,000 and 400,000,000,000 times the mass of our Sun, and diameters range from 16,300 to 163,000 light years. Our own Milky Way is close to this upper value. • Each spiral galaxy is classified according to its appearance. All spiral galaxy labels begin with the prefix "S", followed by a lower case letter, either 0, a, b or c, determined by the arrangement of the spiral arms, and of the bright central region called the nucleus. The appearance of a galaxy depends on its orientation. This means that the angle at which we view galaxies can affect the classification they are given. • The spiral arms are the regions where stars are formed. Here we find the hottest, youngest and brightest stars, and this is why we can see the arms so clearly. Along with fully formed stars, we find sites of stellar formation, where hot glowing clouds of gas and dust called nebulae form the "stellar nurseries".

4. Barred Spiral Galaxies • The diversity of spiral galaxies does not end with the simple a,b,c classification. There is also a sub-branch known as barred spirals, in which the arms emerge from a bar passing through the galactic centre, rather than coming directly from the centre. Such galaxies are given the prefix SB instead of the simple S, and astronomers believe that the Milky Way belongs to this class. • The spiral arms are the regions where stars are formed. Here we find the hottest, youngest and brightest stars, and this is why we can see the arms so clearly. Along with fully formed stars, we find sites of stellar formation, where hot glowing clouds of gas and dust called nebulae form the "stellar nurseries".

5. Elliptical Galaxies • These galaxies have few distinguishing features. They range from objects that are almost spherical, to those that are almost flat. Spherical ellipticals are classed as E0 galaxies, while flat ones are classed as E7, with the types in between being given the numbers E1 to E6. In general, an elliptical galaxy is given the label En, where the number n is determined by the galaxy shape. • There is very little structure visible in elliptical galaxies, unlike the magnificent forms we see in the spirals. Elliptical galaxies are simply vast collections of stars that remain relatively close together because of the gravitational attraction between them. Unlike the spirals there is no well defined rotation axis, so the stars in the galaxy show a wide range of orbital paths around the centre. The motions of stars are not entirely random, however. There is a degree of uniformity in the direction of these motions, and this determines the overall shape of the galaxy.

6. Irregular Galaxies • As the name suggests, "irregular" galaxies have no specific form, and so the group contains a very diverse selection of objects. In fact, there are two types of irregular galaxy. Type I's are usually single galaxies of peculiar appearance. They contain a large fraction of young stars, and show the luminous nebulae that are also visible in spiral galaxies. Type II irregulars include the group known as interacting or disrupting galaxies, in which the strange appearance is due to two or more galaxies colliding, merging or otherwise interacting gravitationally. Type II's appear to contain a large amount of dust. • These galaxies can be regarded as "primitive" as they are relatively poor in "heavy" elements (i.e. elements higher up the periodic table than Helium). In contrast, galaxies like the Milky Way are rich in these elements, which have been manufactured by stars in a process called nucleosynthesis (see the section on stellar evolution). Irregulars are also very rich in clouds of hydrogen, which, when heated by nearby stars, glow to form the luminous nebulae.

7. The Milky Way • Our own galaxy is called the Milky Way. It contains about 200 billion stars and can be seen on a very dark night as a bright band stretching across the sky. It is a spiral galaxy, with a diameter of 100 000 light years and a thickness of about 5000 light years. Our Sun is located in one of the spiral arms, called the Orion arm, at a distance of about 30 000 light years from the galactic centre and orbits every 200 million years. The centre of the Milky Way lies in the constellation of Sagittarius. When observations were made of the stellar velocities in the area, they were found to be particularly fast, which suggests that there is a high gravitational force acting on the stars, and therefore a high mass object in the centre. This is now believed to be a black hole.

8. The Local Group Cluster of Galaxies • A collection of more than 40 galaxies, spread across a volume of space some 10 million light-years in diameter, of which our own Milky Way Galaxy and the Andromeda Galaxy are the dominant and central members. Both these two giant spirals have retinues of satellite galaxies, which together account for most of the membership of the Local Group.