One of the first lessons of astronomy is that the Earth is located in the Milky Way, and this is a spiral-shaped galaxy. However, there are billions of galaxies in the Universe And not all galaxies are spiral-shaped. Galaxies can take on different forms, which has led scientists to classify them into several types. Throughout this article, we will analyze the main classes of galaxies based on their morphological characteristics, composition, and evolution. Galaxies are divided into four main types: spiral galaxies, elliptical galaxies, lenticular galaxies y irregular galaxies. In addition, some galaxies exhibit particular behaviors that make them active galaxies, which we will also discuss. Although the most well-known images of galaxies come from the Milky Way and other spirals, the diversity of shapes and sizes in the universe is much greater than we can imagine. Below, we will delve into each of these fascinating types of galaxies.
Spiral galaxy
The spiral galaxies Spiral galaxies are perhaps the most visually striking type of galaxy in the universe and represent approximately 60% of the galaxies near Earth. These galaxies typically resemble cosmic swirls composed of a bright central nucleus and arms that curl outward. The spiral arms are filled with young stars, gas, and dust, making them luminous and dynamic entities. Constant star formation occurs in these spiral arms, as gas and dust are the essential ingredients for this process. A prime example is our own galaxy, the Milky Way, which is a barred spiral galaxy. This means that it has a bar of stars running through the central core. From the core, arms extend outwards, formed by young stars, and in them are found essential elements that allow star formation. These elements include gas and dust clouds which are found in the arms and enable the creation of new stars. Spiral galaxies can be divided into two main subtypes: normal spirals and barred spirals. The Milky Way It belongs to this second category, where the spiral arms do not emerge directly from the nucleus, but from the ends of a bar of stars. This bar may be a transitional state in the galaxy’s evolution, a phenomenon still being studied by astronomers. The typical structure of a spiral galaxy includes several parts:
- Galactic core: a compact, bright region in the center, containing a high density of ancient stars.
- Galactic disk: region that includes the spiral arms, where star formation predominates.
- Halo: a region around the disk composed mainly of very old stars and globular clusters.
Elliptical galaxy
Unlike spirals, elliptical galaxies They have no arms or spiral structures. These galaxies, which can be nearly spherical or highly elongated in the shape of an ellipse, contain mainly old stars, so they tend to be less bright than spirals. Furthermore, these galaxies lack large amounts of gas and dust, which implies that new stars are not actively forming. A distinctive feature of elliptical galaxies is that they are, on average, the most massive galaxies in the universe, reaching enormous sizes. Some of these galaxies, known as elliptical giants, contain up to billions of stars and are often found at the centre of galaxy clusters. Many giant elliptical galaxies are thought to have formed through Merger of other galaxieswhich gives them their current shape and composition. Within this classification, elliptical galaxies are divided into subtypes, depending on how elongated they are:
- E0: practically spherical.
- E7: extremely elongated.
The absence of gas and dust in elliptical galaxies also indicates that these galaxies are “low-energy” in terms of current star formation. One consequence of this is that elliptical galaxies do not shine as brightly as their spiral counterparts, despite containing many more stars. Stars within elliptical galaxies are often of evolved types, such as red giants, which gives them a reddish hue.
Irregular galaxy
[Caption id = “attachment_34094” align = “alignnone” width = “2048”]
Image – Flickr/NASA Hubble[/caption] The irregular galaxies Irregular galaxies are those that lack a defined shape. Their appearance is chaotic, without a clear structure like spirals or ellipses, and many of them have likely been deformed by collisions with other galaxies or by gravitational interactions. Although they may appear disordered, irregular galaxies contain large amounts of gas and dust, making them places where intense star formation occurs. Some notable examples of irregular galaxies are the Magellanic Clouds, which are satellites of the Milky Way. Irregular galaxies are divided into two main types:
- Irr I: show some structure, such as traces of spiral arms or similar features.
- Irr II: completely amorphous with no discernible pattern.
The presence of abundant interstellar gas in irregular galaxies allows many young stars to form. These stars give these galaxies their brightness, which can stand out in areas of the universe where elliptical and spiral galaxies can be fainter.
lenticular galaxy
The lenticular galaxies Lenticular galaxies lie between spiral and elliptical galaxies in terms of their morphology and characteristics. They have a prominent nucleus and disk, but lack the spiral arms of regular spirals. In appearance, lenticular galaxies can resemble elliptical galaxies, as they contain primarily old stars and have little gas and dust. These galaxies are often viewed as an evolutionary transition between spiral and elliptical galaxies. Some theories suggest that lenticular galaxies may have been spiral galaxies in the past, but after exhausting the gas and dust in their disks, they lost their ability to form new stars. This left them with an aging stellar population and no prominent spiral arms.
active galaxies
The active galaxies are not defined by their morphology, but by their behavior. In these galaxies, the nucleus releases exceptional amounts of energy, often more than the rest of the galaxy combined. The source of this energy is thought to be a supermassive black hole at the center of the galaxy that is accreting matter at an accelerating rate. Notable examples of active galaxies include the quasars, Blazars y Seyfert galaxiesThis type of activity is also observed in radio galaxies, which emit enormous amounts of energy in the form of radio waves. quasars They are the brightest and most energetic objects in the universe, and can be observed from extremely large distances, providing scientists with information about the state of the early universe. In general, active galaxies offer us a fascinating glimpse into how supermassive black holes can shape and affect the overall properties of a galaxy. The universe contains a vast number of galaxies, each with its own history, characteristics, and internal dynamics. From majestic spiral galaxies to the most chaotic irregular galaxies, all play a crucial role in our understanding of the cosmos and how it has evolved. Galaxies are not merely collections of stars, but essential ingredients for studying the history of the universe.