SPIRAL GALAXIES: CHARACTERISTICS AND TYPES - ARTE - 2025

A spiral galaxy is a monumental cluster of disk-shaped stars with spiral arms, resembling the shape of a pinwheel. The shape of the arms varies a lot, but in general a condensed center is clearly distinguished surrounded by a disk from which the spirals sprout.

Almost 60% of the galaxies that are currently known are spirals with the following characteristic structures: central bulge or galactic bulge, disk, spiral arms and halo.

Figure 1. Pinwheel Galaxy in the constellation Ursa Major located 21 million light years from Earth. Source: Wikimedia Commons. ESA / Hubble.

They are galaxies of extraordinary beauty that can be located in constellations such as Eridano. All of them encoded thanks to the work of the astronomer Edwin Hubble (1889-1953).

characteristics

Two thirds of spiral galaxies have a central bar, constituting a subtype called barred spiral galaxies, to differentiate them from simple spiral galaxies. They have only two spirals coming out of the bar and winding in the same direction. Our Milky Way is an example of a barred spiral galaxy, although we cannot observe it from our position.

The central bulge is reddish in color, due to the presence of older stars. There is little gas in the nucleus itself and a black hole is usually found in the center.

The disk for its part is bluish and rich in gas and dust, with the presence of young and hotter stars that orbit in almost circular paths around the galactic nucleus, but more slowly than those of the nucleus.

As for the spirals, they come in a great variety, ranging from those that wrap tightly around the central bulge or the arms arranged more openly on it. They stand out thanks to the large number of hot, blue, young stars they contain.

There are several theories about why they are formed, which we will talk about later.

Finally, there is a spherical halo that surrounds the entire disk, poor in gas and dust, in which the oldest stars are grouped into globular star clusters, huge groups with thousands and even millions of stars that move at high speed.

Types of spiral galaxies

To classify galaxies according to their morphology (appearance as seen from Earth) the tuning fork created by Edwin Hubble in 1936 is used. This classification was later modified by other astronomers by adding subtypes and numbers to the original notation.

Hubble letter-coded the galaxies this way: E for elliptical galaxies, SO for lenticular shaped galaxies, and S for spirals.

Later two more categories were added to include SB barred spiral galaxies 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.

Figure 2. Tuning fork diagram of galaxies. Source: Wikimedia Commons. The original uploader was Cosmo0 at English Wikipedia. (Original text: None given).

Hubble believed that galaxies began their life as spherical structures of the E0 type and then developed arms and became spiral galaxies that would end up being irregular.

However, it has been shown that this is not the case. Elliptical galaxies have a much slower rotational motion that will not cause them to flatten out and generate spirals.

On the arms of the Hubble tuning fork are the spiral galaxies: S for normal spirals and SB for barred spirals. Lowercase letters indicate subtypes: "a" indicates that the coils are tightly closed around the nucleus, while "c" is used when they are looser. Also the gas proportion increases accordingly.

The Milky Way is of the SBb type, with the Sun in one of the spiral arms: the arm of Orion, so called because it also contains the stars of this constellation, one of the most striking seen from Earth.

Theories about the origin of spirals

The origin of the spiral arms is not yet known with certainty, however there are several theories that attempt to explain them. To begin with, astronomers soon observed that different structures in a spiral galaxy rotate at different speeds. This is known as differential rotation and is a characteristic of these types of galaxies.

The interior of the disk of spiral galaxies rotates much faster than the exterior, while the halo does not rotate. For this reason, at first it was believed that this was the cause of the spirals appearing and not only this, it is also the evidence of the existence of dark matter.

However, if this is the case, the spirals would be short-lived (in astronomical terms of course), because they would end up wrapping around themselves and disappearing.

Density waves and star self-propagation

A more accepted theory to explain the existence of spirals is that of density waves. This theory, created by the Swedish astronomer Bertil Lindblad (1895-1965) postulates that matter experiences variations in its concentration, which, like sound, can propagate in the galactic environment.

In this way, areas with more concentration are created, such as spirals, and others with less, which would be the spaces between them. But these areas have a limited duration, so the arms can move although their shape persists over time.

This explains why spirals are very active regions in terms of stellar production. There the gas and dust are more concentrated, so gravity intervenes so that the matter clumps together forming protostars, which will give rise to young and massive stars.

The other theory that seeks to explain spirals is that of self-propagation. The massive blue stars in the spiral arms are known to be short-lived relative to cooler, redder stars in the core.

The former usually end their lives in gigantic supernova explosions, but the material can be recycled into new stars in the same place as the previous ones: the spiral arms.

This would explain the persistence of the arms, but not the origin of them. For this reason, astronomers believe rather that they are due to a combination of factors: the same differential rotation, the existence of density waves, the self-propagation of stars and the interactions suffered with other galaxies.

All these circumstances together give rise to different types of spiral arms: thin and clearly delineated or thick and poorly defined.

Differences with elliptical galaxies

The most obvious difference is that the stars in elliptical galaxies are more evenly distributed than in spirals. In these they appear concentrated in the reddish disk and scattered in the spiral arms, bluish in color, while the distribution in elliptical galaxies is oval.

Another distinctive feature is the presence or absence of interstellar gas and dust. In elliptical galaxies, most of the matter turned into stars long ago, so they have little gas and dust.

For their part, spiral galaxies have areas where gas and dust, which give rise to new stars, is abundant.

The next notable difference is the type of stars. Astronomers distinguish two stellar populations: population I young and population II, older stars. Elliptical galaxies contain population II stars and few elements heavier than helium.

In contrast, spiral galaxies contain Populations I and II. In the disc and the arms the population I predominates, younger and with high metallicity. This means that they contain heavy elements, remnants of stars that have already disappeared, while in the halo are the oldest stars.

This is why stars continue to form in spiral galaxies, while in elliptical galaxies they do not. And it is that elliptical galaxies are probably the result of collisions between spiral and irregular galaxies, during which most of the cosmic dust disappears and with it the possibility of creating new stars.

These collisions between galaxies are frequent events, in fact it is believed that the Milky Way is in collision with small satellite galaxies: the Sagittarius elliptical dwarf galaxy SagDEG and the Canis Major dwarf galaxy.

Comparison table

Differences between elliptical and spiral galaxies. Source: Fanny Zapata.

Examples of spiral galaxies

Spiral galaxies abound in the universe. Viewed from Earth, they are objects of extraordinary beauty due to their various forms. For example, in the constellation Eridano there are five spiral galaxies of different types, including three barred. One of them is NGC 1300, shown below.

Figure 3. Barred spiral galaxy NGC 1300 in Erídano. Kevin Gill from Los Angeles, CA, United States

The milky way

It is the galaxy that houses the Solar System in one of its spiral arms. It contains between 100 and 400 billion stars with an estimated size of between 150 - 200 thousand light years. It is part of the so-called Local Group of galaxies, along with Andromeda and some 50 other galaxies, almost all dwarfs.

Andromeda

Also known as M31, it is located in the constellation Andromeda, close to that of Cassiopeia with its recognizable W shape. It can be seen with the naked eye or with good binoculars on clear, moonless nights.

Although it already appeared in the records of ancient Arab astronomers, it was not known that it was a galaxy until the beginning of the 20th century, thanks to the observations of Edwin Hubble.

Figure 4. The Andromeda galaxy. Source: Pixabay.

It is about 2.5 million light years away and is about the size of the Milky Way, although it is believed to be slightly more massive. However, recent estimates indicate that its mass is comparable to that of our own galaxy.

Andromeda is approaching us at great speed, so it is expected that in about 4.5 billion years it will collide with the Milky Way, giving rise to a gigantic elliptical galaxy.

Whirlpool Galaxy

It appears in Messier's catalog as object M51 and was discovered by Charles Messier himself in 1773. It is found in the constellation of Canes Venatici in the boreal sky, near Bootes and Leo, from where it can be seen with binoculars.

This majestic astronomical object has the typical shape of a spiral galaxy and is at an estimated distance of between 16 and 27 million light years. It has a companion galaxy clearly visible in telescope images: the galaxy NGC 5195.

Figure 5. Whirlpool Galaxy and its satellite galaxy. Source: Wikimedia Commons. NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team STScI / AURA)

References

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3. Oster, L. 1984. Modern Astronomy. Editorial Reverté.
4. Wikipedia. Formation and evolution of galaxies. Recovered from: es.wikipedia.org.
5. Wikipedia. Spiral galaxies. Recovered from: en.wikipedia.org.