Galaxies and Star Clusters

Hello everyone!

Continuing the theme of stars, in this post we will talk about Galaxies and Star Clusters. So what is the difference between Galaxies and Star Clusters? Don't they both contain a lot of stars? 

Galaxies are huge clusters of stars, dust and gas. They usually contain several million to over a trillion stars and can range in size from a few thousand to several hundred thousand light-years across. There are hundreds of billions of galaxies in the Universe. Galaxies come in many different sizes, shapes and brightness’s and, like stars, are found alone, in pairs in in larger groups called clusters. Galaxies are divided into three basic types: spirals, ellipticals and irregulars. Figure 1 shows the different types of galaxies and their classifications.

Figure 1

Star clusters are either one of two general types of stellar assemblages held together by the mutual gravitational attraction of its members, which are physically related through common origin. The two types are open (formerly called galactic) clusters and globular clusters. Open clusters contain from a dozen to many hundreds of stars, usually in an unsymmetrical arrangement. By contrast, globular clusters are old systems containing thousands to hundreds of thousands of stars closely packed in a symmetrical, roughly spherical form. In addition, groups called associations, made up of a few dozen to hundreds of stars of similar type and common origin whose density in space is less than that of the surrounding field, are also recognised.

One of the most famous star clusters is Pleiades. In Greek mythology, the Pleiades were seven sisters: Maia, Electra, Alcyone, Taygete, Asterope, Celaeno and Merope. Their parents were Atlas, a Titan who held up the sky, and the oceanid Pleione, the protectress of sailing. After a chance meeting with hunter Orion, the Pleiades and their mother because the object of his pursuit. Enamoured with the young women he pursued them over the face of the Earth. In pity for their plight, Zeus changed them into a flock of doves, which he set in the heavens. Thus the Olympian added the penalty of the absence of his wife and family to the Titan’s original punishment of eternally supporting the heavens from the Earth.

 

Figure 2: The Pleiades

Only six stars are distinctly visible to the naked eye (figure 2). The ancient Greeks explained the sudden disappearance of the seventh star in various narratives. According to one, all the Pleiades were consorts to the gods, with the exception of Merope. She deserted her sisters in shame, having taken a mortal husband, Sisyphus, the King of Corinth. Another explanation for the “lost” star related to the myth of Electra, an ancestress of the royal house of Troy. After the destruction of Troy, the grief stricken Electra abandoned her sisters and was transformed into a comet – to be a sign of impending doom.

The Greek legends of the disappearing star are echoed in Jewish, Hindu and Mongolian folklore: their basis is an actual event seems to be corroborated by astronomical evidence that a clearly visible star in the cluster becomes extinct towards the end of the second millennium BC.

The Milky Way is spinning and is also moving through the universe. Despite how empty space may seem in the movies, it is filled with dust and gas – and other galaxies. The massive collections of stars are constantly crashing into one another, and the Milky Way is not immune. In about four billion years, the Milky Way will collide with its nearest neighbour, the Andromeda Galaxy. The two are rushing towards each other at about 112km per second. When they collide they will provide a fresh influx of material that will kick start star formation.

Figure 3: Andromeda Galaxy

The Andromeda Galaxy (figure 3) is obviously not the most careful drivers. It show signs of having already crashed into another galaxy in the past. Although it is the same age as the Milky Way, it hosts a larger ring of dust in its centre, and several older stars. Of course, the imminent collision shouldn’t be a problem for inhabitants of Earth. By the time the two galaxies ram headlong, the sun will already have ballooned into a red giant, making our planet uninhabitable.

This prac will help with the understanding of how galaxies swirl for all ages. You need a round glass or aluminium pie-pan, a coin and a sprinkle of dots from paper punch.

1. Place the coin under the centre of the pie-pan on a kitchen counter, so the pan turns easily.
2. Pour about 2 cm of tap water into the pan.
3. Carefully sprinkle the paper-punch dots in the centre of the pan so that they float.
4. Spin the pan slowly. Notice how the dots stream into spiral arms.

The local group is the name Edwin Hubble gave to the galaxies nearest to the Milky Way. Today, astronomers know of about 35 Local Group galaxies spread across roughly 8 million light-years of space. This is not a random collection of galaxies that just happen to lie nearby. Linked by the pull of gravity, the Local Group members form a cluster of galaxies, just as the Pleiades forms a cluster of stars.

Two big galaxies dominate the Local Group - The Milky Way and the Andromeda Galaxy, which is slightly larger. Each has attracted a collection of smaller Local Group galaxies. Belonging to the Andromeda Galaxy are M32, NCG 147, MGC 185, NCG 205 and four dwarf galaxies. The Pinwheel (M33), the third-largest galaxy in the group, also lies near Andromeda. The Milky Way's satellite groups are the Large and Small Magellanic Clouds and several dwarf galaxies. The rest of the Local Group galaxies appear to stand alone.

Answers to previous questions:

1. 3, 440, 337, 033
2. 12, 865, 416, 906
3. 7, 031, 099, 937
4. 13, 763, 004, 132
5. 1, 795, 174, 452
6. Voyager 1
7. A planet with 11 times Jupiter's mass that orbits the star HD 75289
8. Study small pulsations in stars and search for planets.
9. A planet with 0.4 times Jupiter's mass that orbits the star HD 114762

Research Questions:

1. What are some famous elliptical galaxies
2. Name 3 more Star cluster and whether they are globular or open.
3. Find 5 things that the Andromeda Galaxy Milky Way have in common.
4. What are the names of 5 of the dwarf galaxies (and what type are they) in our Local Group?

Q: How do you organise a space party?
A: You planet!

Enjoy :)

Emily.