Physics Tutorial: Galaxies and Hierarchic Structures in the Universe

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In this Physics tutorial, you will learn:

  • What are galaxies? How are they organized?
  • What are the main features of our galaxy?
  • How many types of galaxies are there? What are the main features of each of them?
  • How is the Universe is organized?
  • What is cosmic radiation? What does it cause?
  • What is dark matter? Can we see it?

Introduction

Could you survive for long periods alone in a remote place like a deserted island? Why? Do you feel the need to live in a community with other people? Why?

Take a look at the sky during the night. Do all stars have the same distance from each other?

Do you know the names of horoscope signs? Where do they come from? Do you believe the horoscope? Does it have any scientific base?

In this tutorial, we will discuss these points (and many others) and explain how the universe is organized. This information will provide the foundation that will make it easier to understand the next topics of this Section.

What are Galaxies?

There are billions of stars in the universe. They are not distributed in space in a uniform way. We have explained in the previous tutorials that stars tend to get closer to each other over time because of gravitational forces that have an attractive nature. Hence, various groups of stars from the most remote to those in clusters are formed in the sky.

Astronomers realized long ago that stars are organized in groups according to certain hierarchies, similar to the hierarchic structure of humans, who are first grouped and organized in families, then in local communities (villages, towns, cities, etc.), then in nations, and ultimately in international organizations such as ONU.

Likewise, stars have their own families which are components of larger groups called galaxies. A number of galaxies that are close to each other form groups of galaxies, groups are organized in supergroups and all supergroups of galaxies taken together form the Universe. We will explain each of them but our main focus will be on galaxies as these are the most consolidated groups of stars.

By definition, a galaxy is a huge collection of gas, dust, and billions of stars and their solar systems held together by gravity.

Our Galaxy

Our galaxy is the group of stars which the Sun belongs to. It has about 200 billion stars. It represents a white belt of stars that looks resembles milk that has been poured on a table. It is this similar image that led scientists to call it the "Milky Way". The term "galaxy" itself has been used since antiquity is translated as "milky way". Galilei was the first who understood that the view of the galaxy is due to the light emitted by a very large number of stars. The figure below shows the Milky Way galaxy, where the Sun is at its periphery. We know this because scientists are able to see the centre of this galaxy from Earth. However, it is impossible to take an inclusive photo of the Milky Way. Phtographing the galaxy presents a similar challenge to trying to take a photo of an entire forest whilst being inside it. All images of the Milky Way produced by NASA and other sources are computational compositions made from a number of observations. An image of the Milky Way galaxy is shown below.

Physics Tutorials: This image provides visual information for the physics tutorial Galaxies and Hierarchic Structures in the Universe

As evident from the above image, the Milky Way has a disc shape with a spherical bulge at its centre, called the bulb. The diameter of the Milky Way is 100 000 light years while the bulb thickness is 3000-4000 light years. However, the thickness of the bulb around the centre of the galaxy is about 10000 light years. The figure below shows a lateral view of the Milky Way galaxy.

Physics Tutorials: This image provides visual information for the physics tutorial Galaxies and Hierarchic Structures in the Universe

The Milky Way has a spiral shape when viewed from above. The distribution of stars is not homogenous but they have the appearance of spiral wings. Hence the name spiral galaxy.

Physics Tutorials: This image provides visual information for the physics tutorial Galaxies and Hierarchic Structures in the Universe

If you take a closer look at the previous figure (the lateral view of Milky Way), you can spot some white dots that are too large to be individual stars. They are groups of stars close to each other and they are known as a spherical group of stars. Each group contains thousands of stars with a spherical symmetry. In addition, these groups are distributed in another (wider) spherical symmetry around the centre (bulb) of galaxy. They look like a magnificent halo of our galaxy. Indeed, this is the name by which this interesting structure is known (halo). Its groups of stars are distributed uniformly and the entire halo has a radius of 50 000 thousand light years. Its diameter is equal to that of the Milky Way galaxy itself.

From the points above, we identify three important parts of our galaxy containing a large number of stars each: disc, bulb and halo. All three parts contain groups of stars, they also contain lonely stars as well which revolve around the centre of galaxy.

As for the Sun, it is at a distance of 28 000 light years (or about 2/3) away from the centre of the Milky Way galaxy. It completes one revolution about the centre of galaxy every 200 million years. We are able to see different regions of the Milky Way due to rotation of the Earth around the the Sun, which in turn allows us see the galaxy from different points of view, just like a rotating platform on a high tower that allows tourists see different parts of the city during a day.

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.Physics Tutorials: This image provides visual information for the physics tutorial Galaxies and Hierarchic Structures in the Universe
  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.Physics Tutorials: This image provides visual information for the Cat's Eye Galaxy
  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.Physics Tutorials: This image provides visual information for Irregular galaxies

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
    dgalaxy/ddisc = hgalaxy bulb/hdisc bulb
    Substituting the given values, we obtain for thickness of the disc:
    100 000 ly/40 cm = 4000 ly/hdisc bulb
    hdisc 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
    dgalaxy/ddisc = dgalaxy bulb/ddisc bulb
    100 000 ly/40 cm = 10 000 ly/ddisc bulb
    ddisc bulb = 10 000 ly ∙ 40 cm/100 000 ly
    = 4 cm
  3. The Sun is LSun = 28000 light years away from Milky Way centre. Thus, we have for the distance of the Sun from the centre of lab disc:
    dgalaxy/ddisc = LSun from centre of galaxy/LSun from centre of bulb
    100 000 ly/40 cm = 28 000 ly/LSun from centre of bulb
    LSun from centre of bulb = 28 000 ly ∙ 40 cm/100 000 ly
    = 11.2 cm

Hierarchic Structures in the Universe

From the information we have covered it is clear that galaxies are large families of stars that rotate around their own centre due to the effect of gravitational forces. Stars move either alone or in groups. Thus, the Sun is a lone star; no other star accompanies the Sun in its trajectory around the centre of the Milky Way except the other components of the solar system such as planets, natural satellites, asteroids, etc. This is not the case of stars located in the spherical halo groups in the Milky Way or in other galaxies; they move, altogether, in their long trajectory around the centre of the corresponding galaxy. Most of the stars in the universe move in groups.

Careful observations made by astronomers have led to the conclusion that, in most cases, the movement of stars around the centre of their corresponding galaxy occurs in pairs. They form a system of stars known as binary stars which consist of two stars orbiting around their common centre of gravity. During this movement, they follow the Kepler Laws discussed in previous sections. As for the movement around the centre of galaxy, they move as a single object. For example, the brightest star in the sky, Sirius A, is a binary star as its motion is associated with a white dwarf called Sirius B. This is shown in the figure below.

Sirius A with the white dwarf called Sirius B

In tutorial 22.1 "The Earth and the Planets" we have pointed out that if Jupiter was 10 times larger than it actual size, it would have meet the conditions to be a star of main sequence; the Solar System therefore would have had two stars. In this case, our Solar System would have been a binary star system. Due to higher gravitational effect, thermonuclear fusion reactions would have been activated in Jupiter in such conditions.

Stars also exist in groups of three, four, or even tens, hundreds and thousands. When the number of stars in a group is large, they are known as clusters. Clusters form in two seperate types: open and spherical clusters. Pleiades and Hyades are examples of open star clusters. The dimensions of open clusters may reach tens of thousands of light years. As for spherical clusters, they exist mainly in galaxy halos and contain anything between a few hundred thousand to millions of stars. Stars are much closer to each other in spherical clusters than in open clusters.

Physics Tutorials: This image provides visual information for the physics tutorial Galaxies and Hierarchic Structures in the Universe

All groups of stars share a common feature: they revolve, altogether, around the centre of their galaxy, this includes lone stars like the Sun. This means the process of creation of the solar system components is not very different from that of stars. The difference lies only in the low and insufficient dimensions and temperature of planets, which prevent them becoming stars in the future.

Hierarchic Structures of Galaxies Organization

Galaxies themselves are organized in even larger structures based on the mutual attraction effect caused by gravitational forces. These structures are known as groups of galaxies. For example, the Milky Way galaxy is part of a group that includes 31 galaxies known as the Local Group. Three of the aforementioned galaxies, namely Large Magellanic Cloud, Small Magellanic Cloud and Andromeda are members of the Local Group of galaxies. The dimensions of Local Group are a few million light years. Despite the huge dimensions of local Group, there exist other groups of galaxies that are much larger than Local Group and which include several thousand galaxies.

Groups of galaxies on the other hand, are not distributed uniformly in the space; they tend to gather in supergroups leaving enormous empty spaces between them. However, in a wider scale, the Universe is quite uniform as shown in the figure below, where the two hemispheres (Northern and Southern) of the Universe are visibly distinct from each other.

Physics Tutorials: This image provides visual information for the physics tutorial Galaxies and Hierarchic Structures in the Universe

The order of dimensions and quantity of all groups of stars are summarized in the table below.

Physics Tutorials: This image provides visual information for the physics tutorial Galaxies and Hierarchic Structures in the Universe

The long distances given in the above table are not detected immediately; the information from a given star comes to us with a delay in time equal to the distance in light years of that star from the Earth (where we observe the light). The Universe is believed to be 13.7 billion years old. This means the farthest object we can detect from Earth can be 13.7 billion light years away.

Cosmic Radiation

Cosmic radiation is an important component of the Universe. It does not belong to any of the star structures mentioned above but it fills, uniformly, the entire Universe. As a result, the absolute temperature can never be 0 K, no matter how far away a celestial object is from the closest star but its temperature is 2.73 K exactly. Cosmic radiation is isotropic (equal in all directions), and it has the same features of a black body at 2.73 K. For this reason, the temperature of the sky is taken as 2.73 K.

Dark Matter

Scientists believe that a large deal of matter in the universe is invisible and undetectable through actual devices (that is, devices within our current technological knowledge). This "dark matter" is supposed to exist inside the galaxies, predominantly in halos but also in the space between galaxies, as a component of groups of galaxies. The only indicator of dark matters' existence is the difference resulting in the total gravitational force existing in the universe and the theoretical gravitational force if the existence of this dark matter is not considered.

It is probable that the amount of dark matter in the universe is 10-100 times greater than that of the actually detectable matter. If the existence of dark matter is confirmed, this will be a huge achievement in science, as it will allow scientists to understand the future of Universe.

Example 2

What is the characteristic wavelength of dark matter (if identified and everything discovered about it is in accordance with the actual theory)?

Solution 2

Using Wien's Law equation

λm ∙ T = b

where b = 2.898 × 10-3 m · K is the Wien's constant and λm is the characteristic wavelength of radiation. Since the temperature of the universe is T = 2.73 K, we have for the characteristic wavelength of dark matter:

λm = b/T
= 2.898 × 10-3 m ∙ K/2.73 K
= 1.06 × 10-3 m

This is a kind of microwave (the range of wavelengths for microwaves is 10-5 m to 10-1 m).

Summary

A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems held together by gravity. The Milky Way is the galaxy our Solar System belong to. It has about 200 billion stars.

The Milky Way has a disc shape with a spherical bulge at its centre, called the bulb. The diameter of the Milky Way is 100 000 light years while the bulb thickness is 3000-4000 light years. However, the thickness of the bulb around the centre of galaxy is about 10000 light years. The Milky Way is a spiral galaxy.

We can identify three important parts of our galaxy containing a large number of stars each: disc, bulb and halo. All these three parts contain groups of stars and lonely stars that revolve around the centre of galaxy.

The Sun is 28 000 light years (or about 2/3) away from the centre of the Milky Way galaxy. It completes one revolution about the centre of galaxy every 200 million years. We are able to see different regions of the Milky Way due to rotation of the Earth around the Sun.

Galaxies are classified in three major categories determined by their shape: spiral, elliptical and irregular. Spiral galaxies are similar to the Milky Way, only the dimensions vary. 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. Irregular galaxies represents collections of stars without any specific shape.

Stars may exist alone (such as the case of our Sun), in pairs, in groups of three, four, or even tens, hundreds and thousands. When the number of stars in a group is large, they are known as clusters. Clusters may be of two types: open and spherical clusters. The dimensions of open clusters reach tens of thousands of light years. As for spherical clusters, they exist mainly in galaxy halos and contain stars ranging from a few hundred thousand to millions in number. In spherical clusters, stars are much closer to each other than in open clusters.

All groups of stars share a common feature: they revolve altogether around the centre of their galaxy.

Galaxies themselves are organized in even larger structures based on the mutual attraction effect caused by gravitational forces. These structures are known as groups of galaxies. For example, the Milky Way galaxy is part of a group that includes 31 galaxies known as the Local Group.

Groups of galaxies on the other hand, are not distributed uniformly in space; they tend to gather in supergroups leaving enormous empty spaces between them.

Cosmic radiation is an important component of the Universe. It does not belong to any of star structures mentioned above but it fills uniformly the entire Universe. Cosmic radiation is isotropic (equal in all directions), and it has the same features of a black body at temperature 2.73 K. For this reason, the temperature of sky is taken as 2.73 K.

Dark matter is, potentially, a kind of invisible matter that fills the entire universe. The only indicator that dark matter exists is the mathematical difference resulting in the total gravitational force existing in the universe and the theoretical gravitational force if the existence of this dark matter is not considered. It is probable that the amount of dark matter in the universe is 10-100 times greater than that of the actually detectable matter.

Galaxies and Hierarchic Structures in the Universe Revision Questions

1. What is the revolution speed of Sun around the centre of Milky Way galaxy? Take 1 light year = 9.46 × 1012 km.

  1. 8315 km/h
  2. 260 km/h
  3. 948.5 km/h
  4. 8766 km/h

Correct Answer: C

2. The diameter of Local Group is about 10 million light years. What part of diameter would the galaxies contained in this group occupy if all galaxies had the dimensions of Milky Way and all of them were aligned in a row?

  1. 6 900 000 light years
  2. 3 100 000 light years
  3. 9 900 000 light years
  4. 100 000 light years

Correct Answer: A

3. How far from the Sun is the closest periphery of Milky Way bulb?

  1. 28 000 light years
  2. 72 000 light years
  3. 18 000 light years
  4. 23 000 light years

Correct Answer: D

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