Throughout the known universe, we find numerous types of galaxies. One of them is the spiral galaxy . It is an enormous, disk-shaped group of stars with spiral arms, reminiscent of a windmill. The shape of the arms varies greatly, but they all typically share a central, condensed region from which the spirals radiate. Since almost 60% of known galaxies are spirals, this article will focus on explaining them.
In this article we are going to tell you everything you need to know about the spiral galaxy and its characteristics.
Key features
Two-thirds of spiral galaxies have a central bar, which is a type known as a barred spiral galaxy. It is called this to distinguish it from simple spiral galaxies. It has only two spiral arms that extend from the bar and twist in the same direction. An example of this type of spiral galaxy is the Milky Way. The central bulge of this type of galaxy is reddish in color due to the presence of older stars. The nucleus of the galaxy contains a small amount of gas, and a supermassive black hole is usually located at its center.
The disks that form the arms of a spiral galaxy are bluish and rich in gas and dust. Most of these arms are filled with young, hot stars that continuously orbit in nearly circular paths. As for the spirals themselves, there are different types, ranging from those that coil around a central bulge to those with more open arms. Most are characterized by a large number of young, blue, and hot stars.
We also have in the spiral galaxy a spherical halo that surrounds the entire disk in its entirety that is composed of little amount of gas and dust. In this spherical halo are old stars that are grouped in globular star clusters. These globular star clusters are nothing more than huge clusters of stars containing up to billions of stars and moving at high speed.
Types of spiral galaxy
As we mentioned earlier, there are different types of spiral galaxies depending on the shape of their arms and the composition of their interior. To classify these galaxies according to their morphology, the tuning fork system created by Edwin Hubble was used . This classification has subsequently been modified by other astronomers who have added new characteristics and new types.
Hubble coded galaxies with letters as follows: E for elliptical galaxies, SO for lenticular galaxies, and S for spirals. As information about these types of galaxies has increased, other categories have been added, such as barred spiral galaxies (SB) and irregular galaxies (Irr). Approximately 90% of all observed galaxies are elliptical or spiral. Only 10% fall into the Irr category.
Our galaxy, the Milky Way , is a type SBb galaxy. The Sun is located in one of its spiral arms, known as the Orion Arm. The Orion Arm is named after the constellation Orion, which contains the stars of that constellation. The Orion constellation is one of the most striking constellations visible from Earth.
Origin of the spiral galaxy
The origin of spiral galaxies is not fully understood, but several theories exist. To begin with, astronomers observed that the different structures that make up a spiral galaxy rotate at different speeds. This rotation is called differential rotation and is a unique characteristic of this type of galaxy. Within the disk, the spirals rotate much faster than those on the exterior, while in the spherical halo region, they do not rotate at all. For this reason, it was thought that this was the cause of the appearance of spirals. Currently, this is considered evidence for the existence of dark matter.
If so, the spirals would be short-lived in astronomical terms. These spirals would eventually coil back on themselves and disappear.
Differences with the elliptical galaxy
It is easy to confuse the spiral galaxy with the elliptical galaxy. The most obvious difference between them is that the stars in the elliptical galaxy are distributed more evenly than in spirals. In this type of spiral galaxy, the stars appear more concentrated in the reddish discs and scattered in the spiral arms. On the other hand, if we analyze the distribution of the stars in the elliptical galaxy, we see that it has an oval shape.
Another feature that helps distinguish the two types of galaxy is the presence or absence of interstellar gas and dust. If we go to elliptical galaxies we see that most of the matter has been transformed into stars and therefore they have little gas and dust. In the spiral galaxy we have areas where gas and dust give rise to new stars. These areas are more abundant.
Another characteristic we can look at to differentiate these galaxies is the notable difference in the number of stars. Astronomers distinguish stellar populations based on whether they are young or old. Elliptical galaxies contain a greater number of old stars and few elements heavier than helium. In contrast, spiral galaxies contain populations of both younger and older stars. However, the younger population, with a high degree of metallicity, predominates in the disk and spiral arms. The "Eta" indicates a higher concentration of heavy elements and remnants of stars that have already died. On the other hand, the oldest stars are found in the spherical halo.