PHYS1160 Lesson Plan

Lesson 5 – What’s so special about The Milky Way? A look into our own home galaxy.

Lesson learning outcomes:

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

1.   Describe the structure of the Milky Way, including the different types of stars that exist in the Milky Way.

2.   Outline the processes of galactic recycling in the Milky Way and how that influences chemical compositions and star formation.

3.   Explain how astronomers think the Milky Way formed, and the evidence that supports that idea.

4.   Discuss the evidence that a black hole exists in the centre of the Galaxy.

5.1 Introduction to the Milky Way

Figure: The Milky Way from different locations near Sydney, Australia.Photos by Luke Tscharke. Left: Barrenjoey

Headland. Middle: Mount Banks, Blue Mountains. Right: Breeza, NSW.

Figure: The Milky Way over Uluru.Luke Tscharke.

We are very fortunate that we can see the Milky Way in our sky, and seeing the Milky Way, it is clear to see why we call it that: it looks like a milky band across the sky. The challenge remains that because we live in the Milky Way, it is difficult to study the true size and shape of the Milky Way.

By comparing our Galaxy to other galaxies, and through careful observation, we have determined many properties of our Milky Way.

5.2 The structure of the Milky Way

We estimate that our Galaxy holds more than 100 billion stars (and we are one in 100 billion galaxies!). The Milky Way is a spiral galaxy, as evidenced by our spiral arms that consist of bright stars. Our Galaxy is quite flat, and like other spirals that we have learned about, we have a brighter central bulge surrounding by a dimmer halo. Another key feature of our Galaxy is the presence of globular clusters. These are clusters of stars that reside in the halo, and you’ll learn more about these later on.

Figure 19.1 from The Cosmic Perspective. Artist’s conception of the Milky Way.

If you notice the geometry of the Galaxy, you can see that while our Galaxy is around 100,000 light years across, the width of the disk is only 1,000 light years. Also, you can see that the location of the Sun is 27,000 light years from the centre, so we are quite far away from the bulge.

The disk is filled with gas and dust that we call the interstellar medium (the stuff between stars). The interstellar medium obscures our view of the majority of the Galaxy when observing visible light . Since we couldn’t see most of the Galaxy, we naturally thought that we were near the middle. It wasn’t until Harlow Shapley observed globular clusters in 1920 and realised that they were in fact near the centre of the Galaxy that we were put in our place in the Galaxy, literally!

5.3 Stars in the Milky Way

We’ve already touched on this, but we will describe in a bit more detail the orbits of stars in the Milky Way, particularly with regards to each population.

It may surprise you to learn that the Milky Way does not rotate as a solid structure; each star follows its own orbital path around the centre of the Galaxy. If this is the case, you may think that it should eventually “wind itself up” . Shouldn’t stars closer to the galactic centre move much faster than those on the edges, like the planets in our Solar System? We will talk more about this later on in the course, but first, let’s look at the paths that different types of stars take.

Figure 19.2 from The Cosmic Perspective. Characteristic orbits of disk stars (yellow), bulge stars (red), and halo stars

(green) around the galactic centre. (The yellow path exaggerates the up-and-down motion of the disk star orbits.)

Disk stars orbit the galactic centre in nearly the same plane and in the same direction. Interestingly though, the plane is somewhat bumpy, as shown above. Think of a merry-go-round and the horses that bob up and down. The bobbing up and down in the Galaxy is due to the gravity of the disk pulling stars back that stray either too far above or below the disk. It’s this bobbing motion that gives the disk its thickness of 1,000 light years.

Halo stars orbit the disk on random orbits. Their orbits take them far above and below the disk while still orbiting the galactic centre.

Bulge stars are a bit more difficult to observe, given that they are concentrated in the centre of the Galaxy, unlike halo and disk stars. Observations of bulge stars show that they are somewhat a mixture of halo-like and disk-like orbits: some bulge stars have more random orbits like halo stars, while some have more organised, elongated orbits like disk stars.

How do we work out the orbits of stars? We need to know their trajectory in space, meaning that we need both their positions and velocities (position, speed, and direction). Using the Doppler method described earlier, we can obtain their velocity along our line of sight but not across our line of sight. To do this, we need to compare the precise position of objects observed across long timescales. The European Space Agency’sGAIA missionhas measured what we call the “proper motion” (velocity) of 1 billion stars in the Galaxy.

5.4 The mass of the Galaxy

Now we are going to go back to what we alluded to earlier. Why doesn’t the Milky Way wind itself up? The answer lies in the mass of the Galaxy.

We touched on the physics related to orbits in an earlier lesson. Remember Newton’s and Kepler’s laws, which relates the mass of a central object in a system to the orbital speed and distance of orbiting bodies around it. We can use exactly the same principle here, except that there is no “central body” in this case. Instead, we can use the laws to calculate the mass within a particular orbital distance. You might wonder how we can suddenly change the formula, but we aren’t.

Consider the Earth-Sun system. All of the mass “within the orbit of the Earth” is essentially the mass of the Sun. Yes, there are Mercury and Venus, but their masses are insignificant compared to the Sun’s mass. Also consider the effects of gravity on the Sun in the Galaxy. All matter is attracted towards the centre of the Galaxy. Outside of the Sun’s orbit, the matter is trying to “pull” the Sun away from the Galactic centre, but this cancelled out by forces acting in the other direction. The gravitational effect of objects outside of the Sun’s orbit is negligible, and we can therefore calculate the mass within the Sun’s orbit based on the velocity of the Sun through the Galaxy and its distance from the centre. When we do that, we get a value that is around 100 billion times the mass of the Sun.

Observations of other spirals indicate that most of the mass (stars, black holes, etc.) should be concentrated near the centre. If this were the case, we expect the stars to orbit the galactic centre in a way that is similar to planets: slower further out, faster closer in. This is what we call Keplerian orbits, based on Kepler’s laws. Interestingly, this is not what we see when we look at the orbital velocities of stars in the Galaxy! What we find, in fact, when we look at our Galaxy and other galaxies is that stars towards the outer edge travel at similar velocities to those closer in. Astronomers conclude that there must be large amounts of invisible matter in the halo, and this matter is typically referred to as dark matter. We will cover this in more detail towards the end of the course.

This has implications for the shape of the Galaxy. If the stars are generally travelling at the same speed around the galactic centre, stars in the centre will complete their orbits much faster than stars further out, and the spiral arms will wind up. We don’t see this occurring though. Therefore, we conclude that the spiral arms are more like swirling ripples around the centre than rigid structures like the fins of a pinwheel.

5.5 Recycling of gas in the Milky Way

We are going to learn all about stars and the chemical elements that they produce in later lessons, but for now, it is sufficient for you to know that stars produce elements heavier than hydrogen and helium, and that those heavy elements are (usually) returned to the interstellar medium when stars die (we touched on this earlier). Thanks to over 10 billion years of recycling of gas in the Galaxy, elements heavier than helium make up around 2% of the galaxy’s gaseous content by mass. The remaining 98% consists of hydrogen (71%) and helium (27%).

Now we are going to consider the importance of recycling gas in the Milky Way and how this occurs, because without it, the Earth and our Solar System might not exist (this is because the Earth and rocky planets require heavy elements to form!). Remember the supernova explosions that we talked about earlier? You might recall that they can effectively blast gas (and key elements) out of the Galaxy, so how do we still have heavy elements in the Galaxy? We brought this up briefly last lesson, but let’s go into more detail now.

A phrase you will hear often in astronomy is the star-gas-star cycle. It underpins how stars form, live, die, return some of their gas to the interstellar medium, and then the process repeats. When we talk about stars later on, we will discuss the formation, lives, and deaths of stars. For now, though, let’s just consider what happens when stars die, because the question we are asking ourselves now is, once a star has died, how does the gas it ejects become usable for the next generation?

When stars die and return some of their gas to the interstellar medium, there are a few ways that this can happen. For low-mass stars, similar to our Sun, they die quietly as planetary nebulae. Around half of the initial mass of a low-mass star can be returned to the interstellar medium via planetary nebulae. Planetary nebulae are a gentle ejection of the outer gas of the star through stellar winds (the details aren’t important here; we cover them later on). High-mass stars, on the other hand, die violently as supernovae. Supernova explosions released large amounts of photons and even x- rays, which is why they can be used as standard candles for distance measurements to distant galaxies.

In fact, supernova explosions are so violent that they create shock fronts, which we can see in the photos below. This occurs when all of the high-velocity gas travels as a single high-pressure “wall” at speeds faster than the speed of sound. The shock front will sweep up more gas and ionises atoms as it travels (recall what ionisation is), but will eventually radiate its energy away, typically as x-rays.

In fact, x-ray observations show that our Solar System and neighbouring stars live inside a Local Bubble, which is a bubble of hot gas created by one or more supernovae.

A single supernova bubble remnant may be around 100 light years in diameter. What happens when a cluster of stars happen to explode at similar times (“similar” on cosmological timescales: hundreds of thousands of years!). The combined shock front from the supernovae creates a superbubble, which can extend over 3,000 light years in diameter. Recall the thickness of the galactic disk earlier (only around 1,000 light years). That means that these superbubbles can extend far wider than the width of the galactic disk. In fact, when this happens, it’s called a blowout. Theoretical modelling of these events suggests that after a blowout, the gas can cool and eventually fall back onto the disk thanks to the gravity of the disk. This can recycle gas throughout the disk and between the halo and the disk.

5.6 From hot to cool gas

We have just learned about the gas from supernovae and planetary nebulae. This gas is hot! It’s so hot in fact that the hydrogen atoms in the gas are ionised (recall that hydrogen makes up most of the Galaxy’s gas (71%), followed by helium (27%) and heavier elements (2%)). This isn’t how hydrogen tends to exist in the Galaxy though. Although much of the gas in the Milky Way is hot (10,000 K), this is still not hot enough for hydrogen to be ionised. When hydrogen exists in its neutral state, we call it atomic hydrogen. We’ve mapped out how much atomic hydrogen exists in the Galaxy and in other galaxies by observing the radio band of the electromagnetic spectrum. We do this because atomic hydrogen emits a spectral line with a wavelength of 21 cm (radio waves!). From these measurements, astronomers estimate that around 5 billion solar masses of atomic hydrogen exist in the Galaxy (this is around 10% of the Galaxy’s star mass (that is, how much of the Galaxy actually consists of stars)).

It is natural to think that because stars are hot that we need hot gas for stars to form. This isn’t the case! Atomic hydrogen exists in two forms: the larger, less dense, warm (10,000 K) clouds that we mentioned earlier, and smaller, dense, cool (100 K) clouds. In the warm regions, the kinetic energy of the atoms is higher, the atoms travel faster and are typically further away from each other. The density in the warm regions is therefore around 1 atom per cubic centimetre. The cool regions, however, are much denser at around 100 atoms per cubic centimetre thanks to the lower kinetic energy of the atoms.

There are a couple more steps before we start forming stars. We aren’t going to go into star formation in detail here, because we will cover that in later lessons, but we will cover how we get to the star formation point here in the Galaxy. As you can probably guess, the regions of higher density, the cool clouds, are the seeds of star formation. In the centre of these cool clouds, atomic hydrogen becomes molecular hydrogen by combining to form the H2 molecule. Once this happens, a molecular cloud is forming. The cloud temperature at this point is probably only a few degrees Kelvin above absolute zero, despite being very massive. Molecular clouds can often combine to form giant molecular clouds, which can be a few million solar masses in mass! Molecular clouds in the Milky Way, being massive and dense, typically lie in the disk of the Milky Way and become the patches of darkness seen in the luminous band of the spiral arm. It is in these dense molecular clouds where stars begin to form.

5.7 Star formation in the Galaxy

We often see stars forming in the middle of glowing blobs of gas called ionisation nebulae. You can probably guess why they are called that: the atoms in the nebulae are generally ionised because hot stars release high-energy ultraviolet photons. We also called them emission nebulae or HII regions (HII, using Roman numerals, spoken “H two”, is ionised hydrogen, whereas HI is neutral hydrogen). Why do you think that ionisation nebulae often exhibit beautiful colours? You can probably guess, but this is because the atoms in the gas undergo different energy transitions based upon the wavelength (energy) of the photons. We covered this earlier, and you can see the spectrum of a nebula below.

Figure 19.13 from The Cosmic Perspective. The Orion Nebula.

You might not realise that some nebulae appear blue for the same reason that our sky appears blue. That’s right, dust particles in star forming regions can reflect light, and because blue light scatters more than red light, some parts of nebulae appear blue.

The spiral arms of the Milky Way show clear signs of star formation. Recall earlier how we said that the spiral arms are more like swirling ripples than fins of a pinwheel? Well, theoretical models suggest that the spiral arms are due to spiral density waves that move through the disk. As it moves, it can change the distance (and therefore gravitational force between) stars and gas clouds. When gas clouds are close enough, star formation can occur.

5.8 Formation of the Milky Way

To get the most accurate picture of how we think the Milky Way formed, we need to have a detailed look at the stars that make up the Milky Way. Remember earlier how we said that there were different types of stars in the Milky Way, disk and halo/bulge? Looking closely at the differences between these two types will give us a better idea of how the Galaxy may have formed.

We talked about globular clusters earlier, which exist in the halo. Observations of globular clusters indicate that they are very old, which suggests that halo stars are very old. Disk stars, on the other hand, given that there is ongoing star formation in the disk, vary in age. Another key observation is the spectra of halo stars versus disk stars . Halo stars have much fewer heavy elements (heavier than hydrogen and helium) than disk stars. Recall back to when we talked about recycling in the Galaxy and Universe, and how the amounts of heavy elements increases as the Universes ages thanks to the feedback from stars. We actually expect older stars to have a lot fewer heavy elements than younger stars, so this makes sense. Halo stars have around 1% of the heavy elements that disk stars have (recall disk stars have around 2% heavy elements, so halo stars have around 0.02% heavy elements).

Astronomers therefore divide the two populations based upon their ages, heavy element content, and orbits:

•    Population  I: disk stars. They follow an orderly orbit around the Galactic centre, have around 2% heavy elements, and are a mixture of young and old stars.

•    Population II: halo stars. They orbit the Galactic centre with random orientations, have around 0.02% or less heavy elements, and are old stars.

So, what does all of this tell us about the formation of the Galaxy? It means that halo stars formed first, and the remaining gas to be used to form stars settled in the disk.

Our Galaxy must have started out as a protogalactic cloud. Gas would have been collapsing from all directions, meaning that the initial cloud would have had no organised rotation. Consequently, the stars that formed from that cloud would have initially disorganised orbits. The remaining gas would have continued to contract due to gravity and would have eventually settled into a flattened disk (this is thanks to the law of conservation of angular momentum: rotating spheres flatten out into a disk). Since the gas in the disk had an orderly orbit, so did the stars that formed in it.

This sounds like a plausible story, however there is one fact that it does not explain . As the initial gas cloud collapses, we expect that the outermost halo stars would have the lowest heavy element content, and that the stars closer to the centre would have a higher amount, given that supernova explosions and nebulae would have occurred during the collapse (don’t forget that the collapse would have occurred over very long timescales!). We don’t see this pattern though. Instead, we see a spread in the heavy element content. It has been suggested that instead of the Milky Way forming from one cloud, that it formed from the combination of several smaller clouds.