Physics Lesson 22.6.3 - Other Galaxies

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Welcome to our Physics lesson on Other Galaxies, this is the third lesson of our suite of physics lessons covering the topic of Galaxies and Hierarchic Structures in the Universe, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.

Other Galaxies

There are billions of galaxies in the universe in addition to the Milky Way. They are grouped in large families where all stars in the universe are included. Galaxies come in a variety of shapes; some are similar to Milky Way and some have other shapes. Galaxies have clear borders determined by the large empty spaces between them. These spaces are much larger than the dimensions of galaxies, no matter how big they are.

Galaxies are classified in three major categories determined by their shape: spiral, elliptical and irregular. Let's take a look at them in more detail.

1. Spiral galaxies are similar to the Milky Way, only the dimensions may vary. All of them include a bulb around the centre, the disc that contains the spiral wings and the halo with the corresponding groups of stars it contain. To illustrate this example of spiral galaxies, we take Andromeda (shown in the figure below), which is very similar to the Milky Way and lies 2 million light year from us.
2. Elliptic galaxies. Elliptic galaxies have 3-dimensional elliptic shapes (like eggs). They are common in the universe but are less visible because the brightness they produce is not very high.
   The image below is of an elliptic galaxy called M94. It is also known as "the Cat's Eye Galaxy" because of its appearance.
3. Irregular galaxies. They represents collections of stars without any specific shape. The two closest irregular galaxies to us are the Large Magellanic Cloud (LMC) (which is a satellite galaxy of the Milky Way) and the Small Magellanic Cloud (SMC), which is a dwarf galaxy near the Milky Way.

Example 1

Imagine there is a disc similar in shape to the Milky Way in a school lab. The radius of this disc is 20 cm. Calculate:

1. The height (thickness) of bulb at centre of the disc
2. The horizontal diameter of bulb
3. The distance corresponding to the position of Sun from the centre of disc

Solution 1

1. This situation is similar to when we want to convert distances in a map to real distances. In this case, 100 light years correspond to 20 cm + 20 cm = 40 cm (we have the radius of disc given not the diameter). Hence, if we take the "height" of the bulb in the Milky Way equal to 4000 light years, we have
   d_galaxy/d_disc = h_{galaxy bulb}/h_{disc bulb}
   Substituting the given values, we obtain for thickness of the disc:
   100 000 ly/40 cm = 4000 ly/h_{disc bulb}
   h_{disc bulb} = 4000 ly ∙ 40 cm/100 000 ly
   = 1.6 cm
2. The horizontal diameter of Milky Way bulb is 10000 ly. Using the same approach as in (a), we obtain
   d_galaxy/d_disc = d_{galaxy bulb}/d_{disc bulb}
   100 000 ly/40 cm = 10 000 ly/d_{disc bulb}
   d_{disc bulb} = 10 000 ly ∙ 40 cm/100 000 ly
   = 4 cm
3. The Sun is L_Sun = 28000 light years away from Milky Way centre. Thus, we have for the distance of the Sun from the centre of lab disc:
   d_galaxy/d_disc = L_{Sun from centre of galaxy}/L_{Sun from centre of bulb}
   100 000 ly/40 cm = 28 000 ly/L_{Sun from centre of bulb}
   L_{Sun from centre of bulb} = 28 000 ly ∙ 40 cm/100 000 ly
   = 11.2 cm

You have reached the end of Physics lesson 22.6.3 Other Galaxies. There are 7 lessons in this physics tutorial covering Galaxies and Hierarchic Structures in the Universe, you can access all the lessons from this tutorial below.

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