Lorem ipsum dolor sit Nunc varius massa ut tristique tincidunt. Vivamus quis metus ut arcu condimentum dictum. Vestibulum porttitor velit sollicitudin eros malesuada, ut pretium sem varius. Aenean vel mattis dolor. Sed vitae vestibulum mi. Etiam id lorem sed justo dignissim convallis. Maecenas vehicula consectetur enim pellentesque porta. Praesent quis dolor vulputate velit venenatis faucibus. Phasellus quis erat dignissim, lacinia tortor eget, fermentum lacus. Donec euismod vestibulum metus. Curabitur fringilla porttitor sapien. Proin commodo fermentum mi, at faucibus ipsum laoreet sed. Vivamus sed dapibus augue. Quisque nisi metus, pulvinar vel justo vitae, facilisis pretium dui. Vestibulum ac mi fringilla, vehicula dui in, fringilla risus. Fusce congue pretium eros, vitae hendrerit augue eleifend quis. In vulputate urna vel enim egestas vehicula. Duis dignissim ex nec purus vestibulum lobortis. Proin lobortis molestie sapien, et commodo risus viverra in. Nullam a volutpat arcu.

Globular Clusters

Globular clusters are among the oldest objects in our Universe; in fact, the average age of these objects exceeds 10 billion years. In the following paragraphs, we will understand what these objects are and discuss their main characteristics.

What is a globular cluster?

Our Sun is a solitary star. However, many stars are grouped in pairs, known as binary stars, with both orbiting around their common center of mass. Larger groups of stars are also relatively common. This is the case with globular clusters.
A globular cluster is a very large collection of stars. A typical Globular Cluster can contain, in orders of magnitude, between one hundred thousand and one million stars. In Figure 1, there is a photo of one of these objects; the subject of the photo is M13, also known as the Hercules Globular Cluster because it is located in the constellation Hercules, a constellation visible from Italian latitudes only during the summer.
The stars that make up these objects are characterized by their great age. As mentioned earlier, the age of the stars in globular clusters exceeds 10 billion years. In the case of M13, the age of its stars is estimated to be around 12 billion years. The stars that form globular clusters belong to a category that astrophysicists have named Population II stars. This type of star is not only found in globular clusters but can also be found, for example, near the central region of our galaxy. Population II stars are characterized by a low metallicity. The term "metallicity" refers to the percentage or mass content of a star's constituents other than Hydrogen and Helium. Therefore, when a star has low metallicity, it means that it has much fewer chemical elements other than Hydrogen and Helium compared to other stars, particularly compared to Population I stars, which include our Sun. This characteristic of low metallicity in Population II stars allows us, through simple reasoning that we won't explore in detail at this time, to conclude that these stars are very old.
As we will see in detail shortly, these objects are part of our galaxy, and current knowledge about them allows us to conclude that they formed along with our galaxy. Therefore, the estimated age of the stars in these objects also provides an estimate of the age of our galaxy!

Morphology of globular clusters

The photo in Figure 1 gives us an idea of the shape of globular clusters. These celestial objects are spherical in shape. The main reason for this shape is the same reason these groups of stars remain bound together and do not disperse in all directions: Gravity. The gravitational interaction that the stars within the cluster exert on each other holds the cluster together and gives it a spherical shape. This same gravitational interaction among the stars in the cluster is also responsible for another phenomenon that astrophysicists have named mass segregation. Without delving too deeply into this phenomenon, what is observed is that the heavier stars in these clusters tend to accumulate in the central regions, while the lighter ones are found in the outer regions.
A very "scenic" effect caused by mass segregation is the disintegration of the cluster itself. In fact, due to this distribution of stars based on their mass, the heavier stars in the central areas of the cluster are more tightly bound to the cluster, while the lighter stars in the peripheral areas are less bound. This phenomenon, combined with the fact that globular clusters rotate around the center of our galaxy, in the same way that the planets in the solar system orbit the Sun, causes globular clusters to leave behind a trail of stars along their trajectory around the galactic center. As they orbit around the galactic center, they must also pass through regions of the galactic disk. When this happens, the objects in the galactic disk exert sufficient gravitational forces on the globular cluster to strip away the less tightly bound stars, dispersing them along its trajectory.

Hunting for globular clusters

Where are globular clusters found? As mentioned earlier, globular clusters are located near our galaxy, specifically in the region that astrophysicists have called the galactic halo (or halo in English). To understand where this region is, let's briefly review the morphological structure of our galaxy and, more generally, of spiral galaxies.

The structure of spiral galaxies

If you've had the fortune of observing the night sky in a region with very low light pollution during a time of year when the Milky Way is clearly visible, what you would have seen is a spectacular scene similar to the one shown in Figure 2.

But what is the Milky Way? What is this sort of milky trail that traverses the night sky? The answer is that the Milky Way is our galaxy, and we are observing it from within, but let's clarify this.
What appears to be a milky trail is the collection of stars, gas, and interstellar dust that forms the disk of our galaxy, the Milky Way. Surely, you have come across photos of spiral galaxies on the internet, like those shown in Figure 3.

As can be distinguished from the photos above, spiral galaxies have a very flattened and circular, disk-like shape. This is a characteristic of all spiral galaxies. Another particularly evident feature from the photos of spiral galaxies is the presence of arms, like those seen in the left-hand photo above, but which we are not interested in discussing further at this moment. In general, the structure of spiral galaxies can be schematized in Figure 4.

With reference to Figure 4, let's list and briefly comment on the components of the structure of a spiral galaxy:

Finally, in Figure 4, many "dots" are shown; these are the globular clusters belonging to the galaxy. As seen in the figure, they are randomly dispersed in the region of the galactic halo. As we mentioned in the previous paragraph, globular clusters orbit around the center of the galaxy. This important characteristic of globular clusters allowed Harlow Shapley in 1918 to establish that the center of our galaxy is located in the direction of the constellation Sagittarius. With today's knowledge, we know that the center of our galaxy is indeed in the direction identified by Shapley and is located 26,000 light-years away from the Sun.
To conclude this chapter, let's clarify how all the characteristics of an elliptical galaxy can be seen in the milky stripe in Figure 2. At least regarding the various parts of the disk of the Milky Way, Figure 5 can immediately clarify how what we see as a milky stripe is indeed the disk of our galaxy.

What we see in Figure 5 is that when we look at the milky stripe, we are indeed looking at the disk of our galaxy. The milky effect is due to the high density of stars, gas, and dust populating the galaxy's disk compared to the galactic halo, where there are far fewer objects to observe.