Spiral galaxy

  • Spiral galaxies are disk-shaped with spiral arms emerging from a central core.
  • 60% of known galaxies are spiral, including the Milky Way.
  • Spiral galaxies are classified as barred and non-barred, depending on their internal structure.
  • The main difference with elliptical galaxies is the distribution of stars and the presence of gas and dust.

spiral galaxy features

Throughout the known universe we have numerous types of galaxies. One of them is the spiral galaxy. It is a huge action group of disk-shaped stars that have spiral arms and reminiscent of the shape of a windmill. The shape of the arms varies in many ways, but they are usually distinguished in all a condensed center in which the spirals sprout. Since almost 60% of known galaxies are spirals, we are going to focus this article on explaining it to you.

In this article we are going to tell you everything you need to know about the spiral galaxy and its characteristics.

Key features

spiral arms

Two-thirds of spiral galaxies have a central bar that is made up of one of its kind known as a barred spiral galaxy. It is called like this to be able to differentiate them from the simple ones. It has only two spirals that come out from the bar and wind 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 a reddish color due to the presence of stars that are older. At the core of the galaxy there is a small amount of gas and a black hole is usually placed in the center.

The disks that form the arms of the spiral galaxy are bluish in color and rich in gas and dust. Most of these arms are filled with young, hotter stars that continually orbit in near-circular paths. As for spirals, there are different types, ranging from those that coil around the central bulge to those with more openly arranged arms. Most of them are notable for having large number of young stars, blue and with high temperatures.

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

middle galaxy

As we have mentioned before, there are different types of spiral galaxy depending on the shape of the arms and the composition of the interior. To classify these galaxies according to their morphology, the tuning fork created by Edwin hubble. This classification has been later modified by other astronomers by adding new characteristics and new types.

Hubble letter-coded the galaxies this way: E for elliptical galaxies, SO galaxies with lenticular shape and with S for spirals. As the information on these types of galaxies has increased, other categories have been added, such as barred spiral galaxies, those with SB, and galaxies whose shape does not follow a pattern and are irregular: Irr. About 90% of all observed galaxies are elliptical or spiral. Only 10% are in the Irr category.

Our galaxy, the Milky Way it is of type SBb. The sun is in one of the spiral arms known by the name of Orion. The arm of Orion is so called because the stars of this constellation are found. The constellation of Orion is one of the most striking that can be seen from our planet.

Origin of the spiral galaxy

spiral galaxy

The origin of the spiral galaxy is not known for sure, but there are some theories about it. 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 it is a unique characteristic of this type of galaxy. Inside the disk, the spirals rotate much faster than outside, while in the spherical halo region they do not rotate. For this reason, it has been thought that this was the cause of the appearance of spirals. Currently, this is evidence of the existence of dark matter.

If so, the spirals would be short-lived in astronomical terms. And it is that these spirals would wind up on themselves and would end up disappearing.

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 feature that we can look at to differentiate these galaxies is the notable difference that exists in the number of stars. Astronomers distinguish stellar populations according to whether they are young or older. Elliptical galaxies contain more ancient stars and few elements heavier than helium. On the other hand, if we analyze the spiral galaxies we see that they contain populations of both younger stars and older stars. However, in the part of the disk and the arms the younger population predominates and with a high degree of metallicity. Eta means that they contain a higher concentration of heavy elements and remnants of stars that have already disappeared. On the other hand, in the spherical halo are the oldest stars.


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