Spiral galaxies fascinate both professional and amateur astronomers thanks to their distinctive shape and visual beauty. Their structure, dynamics, and diversity have motivated centuries of study and thousands of observations, revealing surprising details about how the universe evolves and what role these colossal stellar systems play in the formation of new stars and the dispersal of matter and energy.
Understanding the characteristics and classification of spiral galaxies allows you to delve into the heart of modern astronomy. Throughout this article, you'll discover everything relevant, from their internal components and the differences between their types to how their distinctive arms form and what theories attempt to explain their complex structure. Let's delve together into the amazing world of spiral galaxies, their secrets, their most famous examples, and their importance for understanding the cosmos.
What is a spiral galaxy?
A spiral galaxy is a large grouping of stars, gas, interstellar dust, and, of course, dark matter, organized in a rotating flat disk with one or more spiral arms radiating from a central nucleus (the galactic bulge). At first glance, the image that most resembles a windmill or even a whirlpool. These structures are not exclusive, since Our own galaxy, the Milky Way, is a stunning example of a barred spiral galaxy. In fact, Approximately 60% of the galaxies that have been identified in the nearby universe have a spiral morphology..
Spiral galaxies are not only remarkable for their appearance; their internal structure is complex and diverse. They are composed of several fundamental elements: the disk (where the arms are located), the central bulge, the bar of stars (in many cases), the spherical halo that surrounds them, and, in most cases, a supermassive black hole at the core. Each of these components plays a key role in the dynamics, appearance, and evolution of the galaxy.
The beauty of spiral galaxies is evident in photographs taken with modern telescopes and in observations from Earth. Visually, they can resemble hurricanes, water ripples, or even cosmic fireworks, given the number of bright young stars in their arms.
Structure and components of a spiral galaxy
Spiral galaxies may seem simple at first glance, but their structure is the result of a delicate balance between gravity, rotation, star formation, and cosmic evolution. The main components of a spiral galaxy are:
- Galactic disk: This is the most characteristic and visible region, where most of the young stars, clouds of gas and dust, and the well-known spiral arms are found. In this disk, stars follow almost circular paths around the galactic nucleus, and much of the star formation occurs here thanks to the high concentration of raw material.
- Spiral arms: These prominent structures extend outward from the center, coiling tightly or more loosely depending on the galaxy subtype. They are regions that stand out for their brightness and blue color due to the presence of young and hot stars., as well as large clouds of gas and dust. Here, the rate of star formation is very high.
- Galactic bulge (or protuberance): It is located in the center and is a spherical or ellipsoidal concentration of old stars (called Population II, reddish and with low metallicity). The bulge often houses a supermassive black hole at its core. The size of the bulge varies depending on the type of spiral galaxy.
- Central star bar: Approximately two-thirds of spiral galaxies have a bar of stars running through the central bulge. This bar acts as a transit route for gas and stars toward the nucleus and is often the site of the birth of two well-defined main arms.
- Spherical halo: This component surrounds the entire disk and is poor in gas and dust. In the halo, old stars are located grouped in globular clusters., which can contain thousands to millions of stars. Furthermore, the halo is the main reservoir of dark matter, invisible but essential for gravitational balance.
The combination of these components and the differences in their proportions give rise to the great diversity of spiral galaxies observed in the universe.
Main characteristics of spiral galaxies
Spiral galaxies are notable for both their internal dynamics and their stellar and chemical composition. Among its most important features are:
- Distribution of stars: The bulge is dominated by older, reddish stars, while the disk and arms are populated by younger, bluer, and hotter stars. This explains the contrast in color and brightness between the core and the arms.
- Star formation is especially intense in the arms: Here, the gas and dust are denser, allowing the formation of new, high-mass stars that evolve rapidly and brilliantly, seeding the environment with supernovae and heavy elements.
- The disk is often dotted with open clusters and nebula regions: Unlike globular clusters in the halo, open clusters contain young, recently formed stars.
- Rotation differences: The disk experiences what's called "differential rotation," meaning the interior spins much faster than the periphery. This difference in speed is key to the design and durability of the spiral arms.
- Dark matter: The observed rotation curves suggest that there is up to 90% invisible (dark) matter in some spiral galaxies, essential to explaining their stability and high rotation speeds.
- Frequent presence of a supermassive black hole: Observations have shown that at the center of most spiral galaxies lies a black hole of millions of solar masses, as is the case in the Milky Way.
These characteristics make spiral galaxies true cosmic laboratories, where the life of stars and the evolution of elements are displayed in all their splendor.
Classification of spiral galaxies: Hubble sequence and variants
The detailed classification of spiral galaxies was initially developed by Edwin Hubble in 1936, giving rise to the so-called Hubble tuning fork diagram. This system is based on the morphology visible from Earth, identifying three main groups and several subtypes:
- Normal spiral galaxies (S): They have arms that originate directly from the central protuberance.
- Barred spiral galaxies (SB): They have a central bar of stars from which the arms emerge.
- Lenticular galaxies (S0): Considered a transition between elliptical and spiral, they have a disk but without visible arms or with extremely coiled arms.
Within the subtypes of spiral galaxies, the Hubble classification uses lowercase letters to indicate how coiled the arms are and how prominent the central bulge is:
- Type “a”: Very tight arms, large bulge, little gas and low star formation.
- Type “b”: Moderately coiled arms, intermediate bulge, more gas and greater star formation.
- Type “c”: Very loose arms, small bulge, abundant gas and intense star formation.
- Additional types such as “d” or “m”: Traditionally, the letter “d” or “m” has been added to indicate extremely loose arms and galaxies with low surface brightness.
In the case of bars, the scheme is exactly the same: SBa, SBb, SBc, and so on.
In addition to the Hubble sequence, scientists such as Debra Melloy Elmegreen and Bruce G. Elmegreen have proposed systems based on the appearance and development of the arms, with 12 phases ranging from galaxies with poorly defined, chaotic arms to the highly prominent, double-armed “grand design spirals.”
Sydney Van den Bergh introduced another category based on the rate of star formation, distinguishing between normal spiral galaxies and anemic galaxies with poorly defined arms and low stellar activity. These types of galaxies are usually found in rich clusters and a transition to “passive spiral galaxies” with hardly any young stars is often observed.
Comparison with other types of galaxies
Spiral galaxies share the universe with other, no less amazing forms:
- Elliptical galaxies: They are shaped like spheres or ellipses and lack an arm structure. They are usually dominated by older stars and show a low incidence of star formation, as well as little gas and dust. They are extremely luminous but less visually spectacular than spirals.
- Irregular galaxies: They have no defined shape and are usually filled with young stars, gas, and dust. Their chaotic morphology is often the result of interactions or collisions with other galaxies.
- Lenticular galaxies: They represent a midpoint between elliptical and spiral, with a defined disk but no visible arms and very low star formation.
The main difference between spirals and ellipticals lies in the abundance of gas and stellar activity: In spirals, stars continue to form actively thanks to the presence of raw material, while in ellipticals, the material needed to create new stars has long since been consumed.
Morphology and subtypes of spiral galaxies: emblematic examples
The spiral galaxy typology includes outstanding examples known and visible from Earth, both with telescopes and, in some cases, with the naked eye in dark skies.
- The Milky Way: It's our galaxy, a type SBb. It hosts between 100 and 400 billion stars, and its disk measures about 150-200 light-years across. From our position, we can only glimpse its structure from indirect observations and mathematical models. The Sun is estimated to be located in the well-known Orion Arm, a region with abundant star formation.
- Andromeda (M31): The Milky Way's largest neighbor, visible to the naked eye in clear skies. Its structure is also barred spiral, and it is expected to collide with the Milky Way in several billion years, merging into a gigantic elliptical galaxy.
- Whirlpool Galaxy (M51): An example of a “grand design spiral,” with imposing, well-defined arms, accompanied by a small satellite galaxy (NGC 5195) that has slightly altered its shape.
- NGC 1300: Typical barred spiral galaxy located in the constellation of Eridanus, famous for its symmetry and visual beauty.
- NGC 2841: Example of a “flocculent” galaxy, where the arms are not clearly visible and appear fragmented into several segments.
These examples illustrate the enormous diversity within the same morphological group and help us understand the wealth of forms that spiral galaxies can take in the universe.
Stellar and chemical composition in spiral galaxies
The study of the composition of spiral galaxies has allowed astronomers to identify two large “populations” of stars:
- Population I: Young, hot, blue stars rich in heavy elements (known as "high metallicity"). They are usually found in the disk and especially in the arms, where star formation occurs. These stars have short lives and, eventually, explode in supernovae, recycling material that will give rise to new generations of stars or even planets.
- Population II: Much older, cooler, and redder stars with very low metallicity because they formed in times when there were few elements other than hydrogen and helium. They populate the central bulge and halo of the galaxy, including globular clusters.
This difference in the chemical composition and age of stars allows us to trace how galaxies form and evolve, providing information about the processes of merger, gas acquisition, and differential rotation.
Galactic dynamics and rotation: the mystery of dark matter
The rotation of spiral galaxies has been one of the great enigmas of modern astrophysics. The expected behavior (following a Keplerian rotation curve like that of the planets around the Sun) does not correspond to what is observed in reality: instead of slowing toward the edges, the rotation speed remains high even in regions where there is little visible light. This anomaly led to the discovery of the concept of dark matter.
The data indicate that:
- The peak rotation speed is usually between 150 and 300 km/s.
- The most massive galaxies rotate the fastest.
- The Sa and Sb galaxies show much sharper velocity increases than the Sd and Sm galaxies.
- Galaxies with low surface brightness rotate at lower speeds.
- The estimated proportion of dark matter is 50% in Sa and Sb galaxies and reaches 90% in Sd and Sm galaxies.
The study of these rotation curves has also made it possible to calculate galactic distances and construct empirical relationships such as the Tully-Fisher relationship, which links a galaxy's luminosity to its rotation speed.
Origin and formation of the spiral arms
The origin and persistence of arms in spiral galaxies is another fascinating topic that has generated different theories:
- Differential rotation theory: It was observed that different parts of the disk rotate at different speeds, which could cause the material to coil into spirals. However, this effect alone cannot explain the long-term persistence of these arms.
- Density wave theory: Proposed by Bertil Lindblad, it suggests that spiral arms are high-density regions that move like waves through the disk, periodically concentrating gas and forming stars. It is the most widely accepted theory today.
- Stellar self-propagation: He explains that supernovae and explosions of massive stars can trigger the birth of new stars in nearby regions, fueling the persistence of the arms.
- Gravitational interactions and collisions: Galaxies that pass close to each other, or even collide, can experience gravitational distortions and tidal waves that create or strengthen well-defined spiral arms.
The structure of each spiral galaxy is most likely due to a combination of these mechanisms, along with the influence of dark matter and the cosmic environment in which it is located.
Galactic interactions and evolution of spiral galaxies
Galaxies are not alone in the universe; they often live in companies of tens, hundreds, or thousands, grouped into clusters or superclusters. Gravitational interactions between them generate collisions that can transform the shape and type of galaxy over millions of years.
For example:
- The collision of two spiral galaxies can lead to the formation of a much more massive elliptical galaxy.
- Small dwarf galaxies can be absorbed and assimilated by a larger spiral, enriching it with gas, stars, and the possibility of forming new planetary systems.
- Collisions can shatter the structure of the arms, deform the disk, and even trigger massive star formation through explosions and shock waves.
Computer simulations and modern observations have confirmed that these interactions have been fundamental to the evolution of many galaxies, including the Milky Way, which has merged and absorbed several dwarf galaxies throughout its history.
The role of the supermassive black hole in spiral galaxies
At the heart of most spiral and elliptical galaxies lies a supermassive black hole with a mass millions to billions of times that of the Sun.
Some of its most important functions in galactic life are:
- Regulation of star formation: An active black hole can emit energy and winds that heat the gas and limit the formation of new stars, stabilizing the growth of the galaxy.
- Influence on central dynamics: Its powerful gravity directs the movement of stars and gas in the core and can trigger active galactic nuclei (AGN), with extremely energetic emissions.
- Axis of symmetry and source of dark matter: Although it is not the only source of invisible mass, its influence is crucial to understanding the dynamics of the bulges and the inner disk.
In our Milky Way galaxy, the object Sagittarius A* is the most convincing candidate for this supermassive nucleus. Dynamical observations and the detection of extremely rapid stellar motions in the center support this.
Stars and clusters in the halo: origin and peculiarities
The galactic halo, although diffuse and barely visible, is home to some of the oldest stars in the universe.
- These stars often have eccentric and unconventional orbits., often inclined or even retrograde with respect to the galactic disk.
- The low metallicity and advanced age of these stars are reminiscent of those found in the central bulge. and globular clusters, which are true cosmic fossils.
- Some of the halo stars may have been captured during mergers with dwarf galaxies, as is the case with the Sagittarius Dwarf Elliptical and the Milky Way.
The halo also acts as an occasional transit for stars crossing the disk, and its contribution to the total mass of the galaxy, thanks to abundant dark matter, is significant.
Phenomena and objects associated with spiral galaxies
Spiral galaxies are not only scenes of star formation, but can also host extreme phenomena and curious objects:
- Active galaxies: Some spirals display very luminous nuclei, called Seyfert galaxies, which can be subdivided according to their spectral lines and energetic activity.
- Radio galaxies: Although more common in ellipticals, spirals can also emit intense radio emissions if they have active nuclei or jets of particles associated with the central black hole.
- Quasars and blazars: Extremely energetic objects anchored in the nuclei of distant galaxies, identified by their brightness and broad emission lines. In the case of quasars, they are thought to be the nuclei of very distant, active galaxies.
The future of spiral galaxies and cosmic evolution
The life of a spiral galaxy is very dynamic and subject to change over billions of years:
- Generations of stars continue until the available gas and dust are exhausted, leading, over time, to a decline in star formation.
- Collisions and mergers with other galaxies can transform a spiral into a giant elliptical, completely changing its appearance and composition.
- In the distant future, as the era of star formation declines, galaxies will be composed primarily of compact objects: red dwarfs, white dwarfs, neutron stars, and black holes, as well as large reservoirs of dark matter.
Computer simulations and observations of the deep Universe indicate that spiral galaxies, such as the Milky Way and Andromeda, will eventually merge into a large elliptical galaxy in about 4.500 billion years.
Spiral galaxies represent one of the greatest achievements of the natural organization of the cosmos. Their varied structure, the diversity of their components, and the dynamic cycle of stellar birth and death tell a fascinating story about the origin and fate of matter in the universe. From the delicate internal balance between immense quantities of dark matter to the processes that generate new generations of stars and planets, exploring spiral galaxies brings us closer to a more complete and astonishing view of universal evolution and our own place within the Milky Way.