Lesson 4 – What are galaxies? The basics of galaxy types, formation, and evolution.

Lesson learning outcomes:

By the end of this lesson, you should be able to:

1.   Explain the different types of galaxies and how they differ in appearance.

2.   Describe astronomers’ current understanding of how different types of galaxies have evolved and come to be.

3.   Outline how gas is recycled in galaxies.

4.   Identify the appropriate standard candle to use for objects at varying distances.

5.   Describe Hubble’s law, its significance, and the observations Hubble used to discover the law.

6.   Explain the concept of lookback time and how it relates to distance measurements.

4.1 Introduction to galaxies

You might not have realised it when you looked into the sky, but some of the spots of light you see might in fact be galaxies, not singular stars. In this lesson, we will be looking at galaxies in more detail.

You would recall the Hubble eXtreme Deep Field image that we showed earlier. This image, shown below, took 23 days of exposure time by the Hubble Telescope to capture (i.e., it took Hubble 23 days of uninterrupted capturing of light). The photos we take these days have fractions of a second exposure time. Hopefully this gives you an idea of how faint these objects are in the sky!

Figure 20.1 in the Cosmic Perspective. To visualize the size of this patch on the celestial sphere, picture on your own thumb the tiny version of the eXtreme Deep Field on the thumbnail to the right, then hold your thumb at arm’s length.

Just like people, galaxies come in many different types, categorised by their size, shape, colour, etc. To understand how a person came to be who they are, we can look at old pictures, or perhaps we watched them grow up. We can’t do this with anything in the Universe because the lifetimes of these objects are far too long, much longer than a human lifetime. Just like you might look at baby pictures of other people to understand what they looked like as they were growing up, the best we can do with astronomical objects is capture pictures of objects at various stages of their lives and try to piece together how these objects came to be.

How do we find galaxies at various life stages? Remember that when we look far away, we are looking into the past. We need to use this fact and observe galaxies at various distances to capture them at different life stages. The galaxies in the image above are billions of light years away, meaning that we are seeing them as they were billions of years in the past when they (and the Universe!) was much younger.

Astronomers classify galaxies into three broad types based upon their appearance. We will go into each of these types now.

4.2 Spiral galaxies

The most recognisable type of galaxy is the spiral. The Milky Way Galaxy is a spiral galaxy (we will learn more about the Milky Way in the next lesson). Spiral galaxies have a thin disk and a central bulge. The bulge is seamlessly connected to a halo. The bulge and disk are much brighter than the halo, so even though the halo can extend to several hundred thousand light years in diameter, the stars in the halo are much dimmer than those in the bulge and disk.

You might be thinking how curious it is that galaxies have these distinct features, and you would be right! There are generally different types of stars in galaxies that reside in each of these different features:

-     Disk: the stars in the disk following an orderly orbit around the galactic centre. Galactic disks are where all of the exciting things happen, like star formation. The disk, therefore, is home to significant amounts of gaseous interstellar medium (the stuff stars are made of! We talk about this more later) but the amounts and proportions of molecular, atomic, and ionised gas differ between galaxies.

-     Halo: this generally includes stars in both the central bulge and halo. The halo and bulge are generally round or elliptical in shape, and therefore stars in the halo and bulge have randomly orientated orbits around the galactic centre. They are also typically older than stars in the disk.

NGC 4594, the Sombrero Galaxy.

Variations on a theme

You’ll notice that when a classification system is created for astronomical objects, there are always exceptions to the rule, or variations on a very general classification. This is the case here. Within the classification of spiral galaxies, there are spirals with uniquely shaped bulges and disks.

For example, some galaxies have particularly elongated bulges, and we call these barred spirals.

NGC 1300.

Other galaxies have a spiral shape but lack definition in the spiral arms. We call these types of galaxies lenticular (“lens- shaped”). Recall earlier that we said that spiral galaxies have lots of gas in their disks, ready to be used for star formation.

Lenticular galaxies typically have less gas in their disks than other spirals (but more than ellipticals, the next type of galaxy we are looking at) . Most of the large galaxies in the Universe (75-85%) are spiral or lenticular.

The lenticular galaxy NGC 4866. ByESA/Hubble, CC BY 4.0.

4.3 Elliptical galaxies

Elliptical galaxies are comprised only of a bulge/halo and lack a disk component. The size of elliptical galaxies varies widely, with giant ellipticals being relatively rare and smaller dwarf ellipticals being much more common. As we alluded to earlier, elliptical galaxies have very little excess gas and therefore have very few stars forming. We will learn more about stars in lessons to come, but in general, young, hot stars are bluer in colour, whereas older stars are redder. Therefore, elliptical galaxies often look redder compared to spirals, which often appear much bluer. There are always exceptions, including large ellipticals that can contain hot gas that emit x rays.

Some galaxies in our Local Group are what we call dwarf spheroidal galaxies, which are particularly small and much less bright than normal elliptical galaxies.

4.4 Irregular galaxies

So, do all galaxies fit into 2 general types? Not at all! There are always things in nature that surprise us, and this third category encompasses everything that we can’t categorise into the first two categories. We call all peculiar-looking galaxies irregular.

You might have heard of the Large Magellanic Cloud, which is considered an irregular galaxy. Irregular galaxies, however, only make up a small fraction of nearby galaxies but are much more common at great distances. Remember, if these galaxies are more common at greater distances, that means that they were more common when the Universe was younger.

4.5 Hubble’s tuning fork

Hubble’s tuning fork is a useful way to represent the different categories of galaxies. It was created by Edwin Hubble (the namesake of the Hubble Space Telescope). On the handle of the fork are elliptical galaxies (designed by the letter E), with the most “rounded” galaxies on the tip of the handle. Spherical galaxies are classed as E0, with more elliptical shapes being designated with higher numbers. The fork branches at lenticular galaxies, with S0 lenticulars having no obvious elongated bulge and SB0 having a more prominent bar. Ordinary spirals (S) are further classified with letters a, b and c, with the size of the bulge decreasing from a to c (and the amount of dust increasing). This is similar for barred spirals (SB). Irregular galaxies (Irr) are not shown.

It was traditionally thought that Hubble’s tuning fork represented galactic evolution, where galaxies would start off as ellipticals and eventually turn into spirals, but galaxy evolution is more complicated than this! The tuning fork is still a useful representation, nevertheless.

Figure 20.8 from The Cosmic Perspective. This “tuning fork” diagram illustrates Hubble’s galaxy classes.

4.6 Patterns in galaxy properties

Remember how we said earlier that spirals tend to have bluer stars and therefore appear bluer (and brighter) than elliptical galaxies, which consist of mainly older redder stars. Well, this isn’t just a trend that astronomers think they saw; there is evidence to back this up! Using data from the Sloan Digital Sky Survey, astronomers can measure many properties of galaxies, including the power output, called luminosity (think of it like intrinsic brightness), and colour. If we plot galaxy luminosity against colour, we get the following, and we can see that there are some trends.

Figure 20.9 from The Cosmic Perspective. (Data from the Sloan Digital Sky Survey.)

As you might expect, spiral galaxies that contain predominantly hot, bright, blue stars are on the bottom left-hand side of the plot, and ellipticals, consisting of mainly old, dim, red stars, are towards the red end of the plot (to the right). We can therefore infer that galaxies must start off in the blue cloud (labelled) and eventually end up in the red sequence. But how do galaxies change from spiral to elliptical?

Environment can play a role in this. Recall our Local Group. A few dozen galaxies clumped together are called a group. A cluster of galaxies can contain hundreds or thousands of galaxies. Interestingly, of the large galaxies that make up the centre of galactic clusters, about half are elliptical. Ellipticals though, in general, only make up about 15% of large galaxies not in clusters . From this we infer that since large ellipticals tend to exist where there are lots of other galaxies, the way to form a large elliptical is through collisions between spirals.

This may also help to explain another curious fact about the figure above. Do you notice that although ellipticals are full of dim, old stars, that the large ellipticals are brighter than spirals? Why is this the case? The large ellipticals are so luminous because they contain many, many stars. Consider how much brighter 1 million dim light bulbs are compared to 1 bright light bulb! So, if we know that large ellipticals have lots of stars and they are close to the centre of galactic clusters, this also suggests that large ellipticals have probably become so large by consuming smaller galaxies.

4.7 Galactic distances

In the previous section, we showed a plot of galaxy luminosity versus colour. Luminosity, remember, is the total output of the stars in a galaxy. When we look at stars and galaxies in the night sky, we observe that they have a particular brightness, but this is not the same as luminosity. How do we work out luminosity? What’s the difference between luminosity and brightness? Let’s consider this with an example.

Think about a scenario where you are looking at a light bulb from an unknown distance. What is the power output of the bulb? As you look at it, you notice it has a particular brightness but you’re not sure of the power. This is because if we were to move the bulb to different distances, you would notice that as you move the bulb further away, the bulb appears dimmer, and vice versa. But what if we put a lower wattage bulb closer to you so that the two bulbs appear to be the same brightness? This is the problem with observing brightness only and not luminosity. Unless we know the distance to the two bulbs, or the two galaxies, then we cannot know what their power, or luminosity, is. How do we work out the distance to galaxies?

Standard candles

Think about our bulb example from before. If you took a third bulb, one that you knew the power of, and put it at the same distance as each of the bulbs from earlier, you could measure the brightness of that third bulb, and since you knew it’s power, you could calculate the distance to the object. Astronomers use a similar concept that we call standard candles. This simply means that astronomers try to locate objects with known luminosities in the galaxies that they are trying to work out the distances to, and do a similar comparison as described above.

It uses a concept known as the inverse square law for light intensity . The inverse square law says that if we double the distance, the brightness of the object will reduce by a factor of 2^2 = 4. This technique depends on whether the object that we think we know the luminosity of, actually does have the luminosity that we assume. It’s not a perfect system, but it works very well for distance approximations of objects where standard candles can be observed.

Cepheid variables

Cepheid variable stars, or Cepheids for short, are a classic example of a standard candle. Cepheids are very bright     stars that pulsate in size. As they change size, they also change in luminosity (and therefore brightness). The timeline over which a Cepheid pulsates, or its pulsation period, is closely linked to its luminosity . Astronomers just have to        measure the variation of a Cepheids brightness over time to determine its pulsation period, which they can then use to calculate its luminosity and therefore its distance.

This special relation is called the Cepheid period-luminosity relation and was first discovered by Henrietta Leavitt in  1912. It’s often called Leavitt’s law to commemorate Leavitt’s ground-breaking work in this field, which helped us not only to determine the relation between a Cepheid’s period and luminosity, but also helped us to determine the          distance to the Large Magellanic Cloud.

Figure 20.14 from The Cosmic Perspective. The data (red dots) on this graph show that the period of a Cepheid variable star is very closely related to its luminosity, leading to the relationship (red curve) known as Leavitt’s law.

Interestingly, Cepheids are only bright enough to help us determine the distance to galaxies that are around 100 million light years away (this is relatively short in terms of galactic distances). We need another, far brighter standard candle to help us with galaxies further away than this.

Supernovae

You will learn more about supernovae in lessons to come, but basically, supernovae are explosions that occur when certain stars reach the end of their lives. There is a special type of supernovae, called white dwarf supernovae, that are used as standard candles. White dwarfs are a particular type of star (you’ll learn more about this later on too!) and when they reach a certain mass, they explode. Since this mass limit for exploding doesn’t change, white dwarf supernovae all have roughly the same luminosity. You might ask how we know that an observed supernova is a white dwarf supernova? The answer is in the way that the light becomes brighter and decreases over time, and the chemical elements that are present in the explosion.

White dwarf supernovae are very bright, around 10 billion times the brightness of the Sun! This enables us to use them as a measurement of distance to far away galaxies .

Light seconds, light years, light centuries: How to measure extreme distances in thisTED-EdAnimation!

Another measurement of cosmic distances (in the absence of supernovae!) is called the Tully-Fisher relation. The Tully- Fisher relation relates a spiral galaxy’s luminosity (intrinsic brightness) to its rotation. We can use the Doppler method to calculate how fast a spiral galaxy is rotating, and the bigger (brighter) a galaxy is, the faster it is rotating. By comparing the galaxy’s luminosity to how bright it appears in our skies, we can work out the distance to it.

4.8 Beyond the Milky Way: The Great Debate

We are  now going to consider the amazing scientific  achievements of Edwin  Hubble.  Back  in the early  1900’s, astronomers still didn’t know whether anything existed beyond the Milky Way, or whether the Milky Way was the Universe. Astronomers knew that spiral-shaped objects existed, but were they clouds of dust within the Milky Way or groups of stars far beyond?

As advancements were made in astronomical observing and data collection, these so-called “spiral nebulae” became increasingly more confusing. The Great Debate regarding spiral nebulae was held on April 26, 1920 between two astronomers, Harlow Shapley of the Harvard Observatory and Heber Curtis of the Lick Observatory. Shapley rooted for team Milky Way gas clouds, whereas Curtis rooted for groups of stars. Unfortunately, no definitive conclusion came from The Debate given the lack of accurate distance measurements to these objects .

A year before this debate, Edwin Hubble joined the staff at the Mount Wilson Observatory in Pasadena, California. Hubble decided to study these spiral nebulae. With the impressive 100-inch telescope at the observatory, Hubble could observe individual stars in the Andromeda Galaxy . Even more excitingly, Hubble observed Cepheids in Andromeda, and using Leavitt’s law, could determine their luminosities and the distance to Andromeda.

Figure 20.18 from The Cosmic Perspective. Edwin Hubble at the Mount Wilson Observatory.

Even though Hubble’s calculations lead him to underestimate the distance to Andromeda*, he could now prove that it was beyond the Milky Way.

*This was because even though Hubble used Leavitt’s law, there are two types of Cepheids, Type I and Type II, which differ in their heavy element content . They also differ in their brightness/period relation, which is what cause Hubble’s miscalculation.

4.9 Hubble’s law

Hubble and his coworkers continued to estimate the distance to more galaxies, which is when Hubble made one of the most significant scientific discoveries.

This famous discovery starts with a topic that we have look at previously, which is the Doppler effect. It was known to astronomers at the time that the spectra of all galaxies, aside from nearby galaxies, show a redshift. Recall that a redshift means that the object is moving away from us along our line of sight.

After observing many galaxies and determining the distances to those galaxies, Hubble discovered that the more distant a galaxy, the greater its redshift. Remember, the size of the redshift is related to the speed at which the object is moving away from us, therefore Hubble’s results mean the more distant a galaxy, the faster is it moving away from us . Hubble therefore announced his conclusion in 1929, which is that the entire Universe is expanding.

Figure 20.20 from The Cosmic Perspective. Hubble’s original velocity-distance diagram.

You’ll notice that the graph above is linear (straight lined). We can write Hubble’s law as a formula, which says: V = H0 x d

Where H0 is Hubble’s constant, v is velocity, and d is distant. You’ll notice that on the plot above, v is on the y-axis and d is on the x-axis, which means that Hubble’s constant is the gradient of the line in the plot .

In reality, we usually measure the redshift of a galaxy, from which we can calculate its distance. Then, using Hubble’s constant, we can determine the galaxy’s distance from us. Hubble’s law isn’t perfect though! This is for a couple of reasons, being that the accuracy of our calculation is dependent on the accuracy of Hubble’s constant, and Hubble’s law only takes into account the expansion of the Universe. What we mean by the second reason is that some galaxies exist in groups and clusters, and galaxies closest to us, such as those in our Local Group, are bound to each other gravitationally and do not follow Hubble’s law.

4.10 The age of the Universe

Earlier, we looked at the “raisin cake analogy” for representing the expansion of the Universe. We can use dots on the surface of a balloon to also describe this, and both are good analogies for visualising Hubble’s law. The surface of the balloon in the balloon analogy is the Universe (inside the balloon is not part of the representation so ignore it!). There is

no edge or centre of the balloon’s surface (just like there is no centre of the Earth’s surface).

Is there a centre of the universe? Watch thisTED-Ed Animation.

Figure 20.23 from The Cosmic Perspective. As the balloon expands, the dots move apart in the same way that

galaxies move apart in our expanding universe.

As the balloon expands, we can pick a specific dot to concentrate on, but this analysis works for any dot that we could choose. In the figure above, we choose dot B. Suppose that 3 seconds after the balloon begins to expand, we measure the following:

Dot A is 3 cm away and moving at 1 cm per second.

Dot C is 3 cm away and moving at 1 cm per second.

Dot D is 6 cm away and moving at 2 cm per second.

These observations are exactly what Hubble’s law tells us: more distant dots are moving away faster. In this example, Hubble’s constant would be equal to:

V = H0 x d → H0 = v / d = 1 cm/s / 3 cm = 1 / (3 s)

The inverse of the Hubble constant would be the time that the balloon has been expanding, so, 3 s in this case. This is the same for the Hubble constant for our Universe! A simple estimate of the inverse of our Hubble constant puts the age of the Universe at just under 14 billion years. The accuracy of this value depends on whether the expansion has been constant over the age of the Universe. We will discuss this in more detail towards the end of the course.

4.11 Lookback time

Remember earlier that we were discussing determining the distances to distant galaxies. Now, however, we have just said that the more distant a galaxy, the faster it is moving away from us. How does this expansion affect our distance calculations for these distant galaxies?

Consider the following scenario. You are trying to observe a supernova in a galaxy. The supernova exploded in the galaxy 1 billion years ago and the photons have therefore travelled a distance of 1 billion light years at the speed of light. But what is this  1 billion light years distance? When the light left the supernova, the distance between the supernova and our Galaxy was smaller than 1 billion light years. By the time the light reaches us, the distance has increased and is now more than 1 billion light years. We use the concept of lookback time to describe distance, which is the time it took for the light to reach us.

Since the space between the two galaxies is expanding, we also have a neat way of explaining the Doppler shift, which we also call cosmological redshift when talking about distant galaxies. As the Universe expands, photons stretch, and their wavelengths become longer. Longer wavelengths mean that the colour of photons will move towards the red end of the visible spectrum (become redshifted).

4.12 The observable Universe

It’s important to note that the Universe does not have an edge. However, the concept of the observable Universe or a cosmological horizon, which we touched on earlier, means that we cannot see to lookback times that are greater than the age of the Universe. This doesn’t mean that we can say that the cosmological horizon is 14 billion light years away, but instead it means that we can only see photons that have travelled for up to 14 billion years. The expansion of the Universe suggests that the space between us and the light near the horizon has stretched to beyond this 14 billion light year distance (in fact, Einstein’s theory of relativity suggests that the matter and photons near the horizon are probably around 47 billion light years away).

4.13 Galaxy evolution

Now that we have learned about galaxies, let’s take a closer look at how they evolve.

Earlier, we mentioned the concepts of lookback time and galaxy ages. We can now merge the two ideas together. For example, if a galaxy has a lookback time of 13 billion years, and the age of the Universe is approximately 14 billion years, then the galaxy must have an age that is around 1 billion years. Therefore, as we have said earlier, by observing galaxies with different lookback times (distances), then we will have observed a range of galaxies across different ages.

To study galaxy formation, we need to look further back in time, to before the first galaxies were fully formed. To do this, we need better telescopes with greater sensitivity. This is the primary focus of the James Webb Space Telescope (JWST), which is an orbiting infrared observatory that will complement and extend the discoveries of the Hubble Space Telescope. For more information on JWST,read here.

JWST launched in December 2021. Until JWST starts giving astronomers data, we must rely on computer simulations to help us understand the early Universe. Assuming that hydrogen, helium, trace elements, and dark matter (this comes up later in the course!) filled the Universe almost uniformly, and that matter wasn’t exactly uniformly distributed, and incorporating these assumptions into our computer models yields a convincing story of how galaxies may have formed.

4.14 A galaxy’s birth

We talked about the different types of galaxies earlier: elliptical, spiral, lenticular, and irregular. Clearly, their formation and/or evolution histories must be different . Like all fields of astronomical research, there is still much to learn, and the field of galaxy evolution is still being actively researched.

So, which was it? Do galaxies all start form the same way and become different through interactions during their lives, or do galaxies differ because of different birth conditions?

Birth conditions

If we start off by looking at the birth conditions, this is influenced by a few factors. The first factor is protogalactic rotation, and the second is protogalactic density. Let’s look at these in more detail in turn.

A galaxy’s type may be influenced by the rotation of the protogalactic cloud in which the galaxy formed from. If the protogalactic cloud had a significant amount of angular momentum during collapse, then it would have collapsed as a disk and produced a spiral galaxy. If the protogalactic cloud had little or no angular momentum, then the collapsing cloud may not have formed a disk and would have produced an elliptical galaxy.

Figure 21.3 from The Cosmic Perspective. These diagrams show two ways in which a galaxy’s birth conditions may

have determined whether it ended up spiral or elliptical.

The density of the protogalactic cloud in which the galaxy formed may also influence a galaxy’s type. If the gas density was high, energy can be radiated away from the cloud more effectively. This causes the gas cloud to cool quicker, and stars are able to form sooner (you need cold gas to form stars; hot gas moves around too much and won’t collapse into a star!). If star formation used all of the gas in the cloud quickly enough, then the cloud may not have had time to settle into a disk, and the galaxy would be elliptical. If the gas density was low, energy cannot be radiated away from the cloud as effectively. The cloud would not have cooled as quickly, star formation would proceed slower, and the cloud may have had the chance to settle into a disk (therefore producing a spiral galaxy).

Observations of some giant, distant elliptical galaxies shows that even though the galaxies are young, there are few young, hot stars, indicating that star formation is no longer occurring. This provides some evidence that perhaps the gas cloud that the galaxy formed in was dense.

Interactions

Now we will consider how interactions play a role in a galaxy’s type. Interactions are very important when it comes to galaxy evolution.

To visualise this, we need to think about the scale of the Universe. Relative to their size, the distance between stars is huge, and therefore stars rarely interact . Galaxies, however, relative to their size, are much closer, therefore galaxy interactions are much more common than star-star interactions. You’ll learn more about the fate of the Milky Way and its current interactions in the next lesson.

These interactions occur over millions of years, and we are lucky to have captured a few in the process. In the early Universe when galaxies were much closer together, interactions were much more common , and observational evidence confirms this. Computer simulations also show that colliding spirals can create an elliptical galaxy due to the disks being torn apart and the orbits of stars being randomised. Galactic cannibalism occurs when giant galaxies engulf other galaxies, and this is probably how the giant galaxies found at the centre of galactic clusters formed.

We can link this back to the galaxy colour-luminosity plot that we showed earlier. Our theories about galaxy formation fit with the idea that galaxies start in the blue cloud because they have young, blue stars. Eventually, they transition to the red sequence where large, red elliptical galaxies reside, through collisions and mergers.

4.15 Gas recycling in galaxies

You might think that the rate at which stars form in spiral galaxies, or a spiral galaxy’s star formation rate, is relatively stable. In reality, the rate is quite variable! We know that stars form from cold gas, and the amount available for star formation dictates the rate at which stars will form.

There is a special class of galaxies, called starburst galaxies, in which the star formation rate is incredibly high.

The Milky Way Galaxy is still forming stars. Have you ever wondered about the rate at which stars are produced in the Milky Way? It’s about one star per year! In contrast, starburst galaxies can form over 100 new stars per year. In a “normal” galaxy, this could use all of the available gas in a galaxy in around 100 million years, but we know that most galaxies are far older than 100 million years old, so we infer that the burst of star formation seen in starburst galaxies is a temporary state.

With all of those new stars forming, supernova explosions (the ones that we talked about briefly earlier and will touch on more later) occur at a very high rate. A supernova explosion, being so violent, will create a bubble of hot gas . The combination of several supernova bubbles in close proximity will create a superbubble . As more supernova explosions occur, the superbubble gains kinetic and thermal energy, leading to a galactic wind. Even though galactic winds are low density, the temperature is extremely high (10-100 million Kelvin) and therefore produce x rays.

This ejection of gas doesn’t necessarily spell the end of star formation in the galaxy though. Some of the gas may cool and fall back onto the galaxy, creating new episodes of star formation. This constant recycling of gas is considered to be very important in galaxy evolution and the maintenance of star formation in galaxies .

4.16 Supermassive black holes

We are going to talk about black holes in more detail later on, but it is still useful to talk about them here in the context of galaxies.

Some galaxies emit massive amounts of radiation from their central region that can often be accompanied by jets of material travelling at near the speed of light . Galaxies that have unusually bright centres are called active galaxies (or Seyfert galaxies, named after Carl Seyfert, who categorised active galaxies in 1943), and their bright central regions are called active galactic nuclei.