What is the Milky Way Galaxy? It is home to the Sol System, a solar system that plays home to Earth, which is where we live.
There is a lot to learn about the Milky Way, including a lot that we still don’t quite understand or know about. After all, it is a pretty big place and we don’t have the means or capability to observe all of it.
Allow us to shed a little light on the galaxy that our solar system calls home, starting with how big it is.
How big is the Milky Way Galaxy
In 1917, an American astronomer named Harlow Shapley made the first reliable measurement of the size of our Galaxy. He did this by studying the position of groups of stars known as globular clusters.
What is the Milky Way Galaxy? Bloody big, is what. Shapley discovered that our Galaxy is much bigger than previously thought and that the Sun is closer to the edge than the centre.
He estimated that the Galaxy is about 100,000 light-years across and that the Sun is about 30,000 light-years from the centre. (A light-year is the distance light travels in one year, which is roughly 9,460,000,000,000 km or 5,880,000,000,000 miles.)
These measurements have proven to be quite accurate over time. The Milky Way’s star-filled disk is about as big as Shapley predicted, with some elements like neutral hydrogen and dark matter spread out even more.
The farthest stars and gas clouds that we can measure are about 100,000 light-years from the centre of the Galaxy, and the Sun is now known to be about 25,000 light-years away from the centre.
What is the Milky Way Galaxy in terms of its structure?
The Milky Way Galaxy is shaped like a big spiral and is typical for large galaxies. It has six main parts:
- A nucleus, which is the very centre
- A central bulge (large round group of stars around the nucleus)
- A disk, which is flat and round like a pancake. There’s a thin one and a thick one
- Spiral arms (long curvy paths of stars that come out from the centre)
- A spherical component
- A massive halo (a large ring around the outside)
Some of these parts blend into each other at the edges. You can read more about galaxies in the Galaxy Archive (link).
The nucleus of the Milky Way Galaxy
What is the Milky Way nucleus? At the very centre of our Milky Way Galaxy, there’s a huge black hole. This black hole is surrounded by a disk of very hot gas.
We can’t see the black hole or the gas around it with our eyes because there’s a lot of dust in the Milky Way that blocks our view. But, we can detect it using radio waves. This black hole is called Sagittarius A*.
Just like in other galaxies, there’s a lot of activity happening around this black hole. It gives off infrared radiation and X-rays, and there are fast-moving gas clouds around it.
We think that the black hole is pulling in material from outside its immediate area, including some gas that’s coming in from a direction that’s at a right angle to the flat part of the galaxy.
As this gas gets closer to it, the black hole’s strong gravity squishes the gas into a disk that spins quickly. This disk stretches out for about 5 to 30 light-years from the black hole. By looking at how this disk and the stars around it move, we can tell that the black hole is more than 4 million times heavier than the Sun.
In 2020, two astronomers named Andrea Ghez and Reinhard Genzel won the Nobel Prize for Physics. They won this prize because they used infrared observations to show that Sagittarius A* is a black hole.
What is the Milky Way Galaxy? Looking at the Central Bulge
Below is an image of the Milky Way Galaxy’s centre, taken by the Infrared Astronomy Satellite (IRAS). The bulge you see is the Galaxy’s centre. The yellow and green spots are huge clouds of gas and dust in space. The blue areas are the warmest, and the red areas are colder. IRAS was launched on January 25, 1983.
Around the core, there’s a large, almost round, bulge of stars. These are mainly Population II stars, which have a lot of heavy elements. There are also groups of similar stars, called globular clusters, mixed with the stars.
Both the stars and the clusters move in nearly straight lines around the core. You can see the bulge stars where they rise above the dust of the galaxy’s flat plane.
What is the Milky Way Galaxy Disk?
If you look at the Galaxy from far away, the most noticeable part is the disk. This disk stretches from the centre of the Galaxy to about 75,000 light-years away. The Galaxy looks like other spiral galaxies, with a bright, flat layer of stars and gas clouds spread out all over it in a spiral shape.
You can think of the disk as a base of stars where the spiral arms are placed on top. The thickness of this base is about one-fifth of its diameter, but different parts of it have different thicknesses.
The thinnest part, often called the “thin disk,” has dust, gas, and the youngest stars. A thicker part, known as the “thick disk,” contains somewhat older stars.
The spiral arms of the Milky Way
The Milky Way galaxy, our home in the universe, is a barred spiral galaxy. It has a central bar and spiral arms that emanate from this centre. Here are some details about the spiral arms:
- Major Arms
The Milky Way has two major arms, the Perseus Arm and the Scutum-Centaurus Arm. These arms are characterized by the highest densities of both young and old stars.
- Minor Arms
In addition to the major arms, the Milky Way also has several minor arms or spurs. These include the Norma Arm, Sagittarius Arm, and the Local Arm or Orion Arm1243. These minor arms are primarily filled with gas and pockets of star-forming activity.
- Orion Arm
Our solar system, including the Sun, is located in the Orion Arm, which is sometimes also referred to as the Orion Spur. This arm is about 3,500 light-years wide and more than 20,000 light-years long.
- Far-3 kiloparsec Arm
There’s also a shorter arm called the “Far-3 kiloparsec arm” that lies along the bar of the galaxy.
These arms are not solid structures but are areas of higher star density within the galaxy. The spiral pattern is thought to be caused by density waves that orbit around the Milky Way. It’s important to note that our understanding of the Milky Way’s structure is still evolving as new observations are made.
The Spherical Component of the Milky Way Galaxy
The area above and below the Galaxy’s disk is filled with a sparse extension of the central bulge. This almost spherical region is home to the outer globular clusters. It also has many individual field stars, like RR Lyrae variables and dwarf stars that lack heavy elements.
These stars are part of an older group known as Population II. In terms of structure, this spherical part is similar to an elliptical galaxy. It follows the same basic mathematical rule that describes how density changes with distance from the centre.
The Massive Halo of the Milky Way Galaxy
The part of the Galaxy that we understand the least is the huge halo that lies outside the entire visible part. We know the halo exists because of its effect on the Galaxy’s outer rotation curve.
What we can say for sure is that the halo stretches far beyond 100,000 light-years from the center and its mass is several times larger than the combined mass of the rest of the Galaxy.
However, we don’t know what shape it is, what it’s made of, or how far it extends into space between galaxies.
What is the Milky Way’s Magnetic Field
People used to think that a strong magnetic field controlled the spiral shape of galaxies. However, when scientists used radio techniques to detect the general magnetic field, they found it was too weak to affect the large-scale structure of galaxies.
The strength of this galactic field is only about 0.000001 times the strength of Earth’s field at its surface. This is too low to affect the gas between stars in a way that could explain the spiral-arm structure.
However, it’s strong enough to align the dust grains in space between stars. This alignment is detected by measuring the polarisation of starlight.
According to the current model, these dust particles spin rapidly and contain a bit of metal (likely iron), but are mainly made of ice and carbon. The Galaxy’s magnetic field can slowly act on these particles and line up their spinning axes. This makes their short axes parallel to the field’s direction.
The field is aligned along the Milky Way band, so the particles’ short axes also align along the plane of the galaxy. This pattern is confirmed by polarization measurements of stars at low galactic latitudes.
What is the Milky Way’s Rotation?
The way stars move in the local star area can be explained by two types of stars. One type moves in circular paths around the far-off centre of the galaxy. The other type has more oval-shaped paths and seems to move fast from Earth’s perspective as Earth and the Sun move in their circular path.
The general spinning of the disk stars was first noticed in studies from the 1920s. The Swedish astronomer Bertil Lindblad was notable for correctly explaining the uneven star movements as a result of these different star path characteristics.
The disk part of the Galaxy spins around the centre in a way that’s similar to how the planets in our solar system move in almost circular paths around the Sun. Because the spinning speed is different at different distances from the Galaxy’s centre, the speeds of disk stars measured in different directions along the Milky Way show different patterns.
Dutch astronomer Jan H. Oort was the first to explain this effect in terms of the spinning motions of the Galaxy, using the radial speeds and proper motions of stars. He showed that different rotation speeds cause a regular change in the radial speeds of stars with galactic longitude. This follows the mathematical formula:
radial velocity=Arsin2l
Where A is known as Oort’s constant and is about 15 km/sec/kiloparsec (1 kiloparsec equals 3,260 light-years), r is the distance to the star, and l is the galactic longitude.
You can also get a similar formula for the measured proper movements of stars. The fact that observed data matched Oort’s formulas was a key proof that Lindblad’s ideas about how stars move were right.
This led to our current understanding of the Galaxy. We now know it’s made up of a huge spinning disk with other parts that are more round and spin more slowly added on top.
The Mass of the Milky Way
In the 1960s, people thought they knew the total mass of the Galaxy. But now, it’s not so clear. To measure the mass, you look at the farthest large hydrogen clouds.
You need to know the speeds and positions of neutral hydrogen gas and assume the gas is moving in almost circular paths around the Galaxy’s centre.
You then make a rotation curve, which shows the gas’s circular speed about its distance from the centre. The shape of this curve and its values are set by how much gravitational pull the Galaxy has on the gas.
Speeds are low in the centre because there’s not much mass inside the gas’s path. Most of the Galaxy is outside it and doesn’t pull it inward. Speeds are high at middle distances because most of the mass is inside the gas clouds’ path and the inward pull is strongest.
At the farthest distances, the speeds go down because almost all the mass is inside the clouds. This part of the Galaxy is said to have Keplerian orbits, named after the German astronomer Johannes Kepler. He found that planets in the solar system move in a similar way, where almost all the mass is inside the paths of the orbiting bodies.
What is the Milky Way’s mass? To find the total mass of the Galaxy, you make mathematical models of the system with different amounts of material spread out in different ways. You then compare the resulting speed curves with the observed one. Using this method in the 1960s, it was estimated that the total mass of the Galaxy was about 200 billion times the mass of the Sun.
Refinement of the velocity curve
In the 1980s, improvements in how the velocity curve was determined started to question the earlier findings. The previous belief that velocities were lower in the outer parts of the Galaxy was found to be wrong.
Instead, the curve stayed almost the same, showing that there was still a lot of matter outside the measured hydrogen gas. This suggests that there’s some unseen material out there that wasn’t expected.
It must go far beyond the previously accepted edges of the Galaxy, and it must be dark at almost all wavelengths, as it hasn’t been detected even with radio, X-ray, ultraviolet, infrared, and optical telescopes.
Until this dark matter is found and its distribution is figured out, we can’t measure the total mass of the Galaxy. All we can say is that the mass is at least several hundred billion, and maybe even one trillion, times the mass of the Sun.
What the dark matter in the Galaxy is remains a big question in the study of galaxies. Many other galaxies also seem to have this unseen matter. In the 1990s, astronomers did extensive lensing experiments.
They studied millions of stars in the centre of the Galaxy and in the Magellanic Clouds to look for dark objects. These objects’ masses would cause background stars to brighten due to lensing. Some lensing events were found, but the number of dark objects suggested isn’t enough to fully explain the dark matter in galaxies and clusters of galaxies.
The dark matter is likely made up of some particle that hasn’t been discovered yet, like a WIMP (weakly interacting massive particle).
Look out for more on the nature of dark matter in future posts.
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