How Do Spiral Galaxies Form and Why Are They Important?
The galaxy is a vast system of stars, gases, and dust that makes up the universe. It contains somewhere between 100 and 400 billion stars, depending on the type and size of the galaxy. The gas and dust in the interstellar medium are the raw materials from which stars form. The spiral galaxy is the most commonly observed galaxy shape in the universe. They can contain multiple spiral arms and are known for their clusters of new star formations. Spiral galaxies are just one of many different types of galaxies.
Some spiral galaxy examples are Pinwheel Galaxy, Andromeda Galaxy, Sunflower Galaxy, Triangulum Galaxy, and Whirlpool Galaxy.
(Image will be uploaded soon)
Spiral galaxies are a part of the Hubble sequence, and they consist of a flat disk, rotating with stars, gas, and dust. Most spiral galaxies have a bulge in their center, which is composed of stars. A halo of stars sometimes surrounds these. Spirals are branched out arms with clusters of young, hot stars in them. It is these stars that make the spiral brighter than the surrounding galactic disk. Roughly two-thirds of all spirals have a bar structure that begins at the end of the central bulge, with spiral arms branching off of it.
Characteristics of Spiral Galaxy
They have a circular, flat disc surrounded by spiral arms with dark matter concentrations and fewer stars. A spiral nebula encircles the flat disc in a spiral galaxy.
In the spiral galaxies, the dark matter movement is guided by a structured rotation path- orbital movement, unlike other galaxies that do not possess such rotations.
The disc of stars and space orbiting around the bulge creates separate spiral arms in the galaxy.
A spiral galaxy consists of millions of globular clusters apart from a disc of stars that revolve around the center of the entire galaxy.
The most common type of galaxy in the universe, and often the largest in terms of volume, is the spiral galaxy. The stars in a spiral galaxy are densest around the center and thin out as they reach the outer edge of the spiral arms.
One of the largest spiral galaxies known to man is NGC 6872. Spanning 522,000 light-years across, this galaxy is five times the size of the Milky Way.
Types of Spiral Galaxy
Broadly, there are two types of spiral galaxies that space scientists have observed to date.
Normal Spiral Galaxy- Normal Spiral Galaxy is a type of galaxy with arms protruding from the nucleus in all directions. In the center of the galactic disc, a spiral nebula surrounds the nucleus from all sides. The arms themselves have no specific points of origin when it comes to a normal spiral galaxy. This is why it's also known as just a spiral galaxy.
Barred Spiral Galaxy- A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure made up of celestial bodies. Most of the spiral galaxies are barred, where the solar system is located; the spiral galaxy milky way galaxy is named a barred spiral galaxy.
(Image will be uploaded soon)
Spiral Galaxies Classification
Spiral galaxies can be classified based on a number of factors, including how tightly they are spiralled, how lumpy their arms are, and the size of their central bulge. These galaxies can contain different amounts of gas and dust, which help portray these differences. There are currently three different kinds of classical spiral galaxies and three different kinds of barred spiral galaxies. They are as follows.
SA Spiral Galaxies- They have a big, central bulge and smooth, broad spiral arms. About 2% of their mass comes from gas and dust. This means that a relatively small number of Sa galaxies are forming stars.
SB Spiral Galaxies- SB galaxies have a medium-sized nucleus with spiralling arms around the nebula. This galaxy looks less smooth and has fewer stars than the SA spiral galaxy.
Sc Spiral Galaxies- They have a little bulge in the centre as well as narrow, well-defined arms. These galaxies contain about 15% gas and dust.
Barred Spiral Galaxy Type A(Sba)- They feature tightly bound arms.
Barred Spiral Galaxy Type B(Sbb)- They feature both tightly and loosely bound arms.
Barred Spiral Galaxy Type C(Sbc)- They have loosely bound arms.
Elliptical Galaxies
Elliptical galaxies are typically oval in shape and do not have any formation, such as spiral arms. They lack the internal structure common among spiral galaxies but still have a denser nucleus that accounts for about 20% of the galaxies in the universe.
An elliptical galaxy may contain 10⁵ to 10¹³ solar masses and has the ability to create luminosity between 3×10⁵ and 10¹¹ solar luminosities. It contains a mixture of Population I and II stars, with the exception of globular clusters.
The diameters of elliptical galaxies vary from 1 kiloparsec to 200 kiloparsecs, with most galaxies hovering around 100 kiloparsecs.
Elliptical Galaxy vs Spiral Galaxy
FAQs on Spiral Galaxy: Definition, Types & Structure
1. What is a spiral galaxy and what are its main features?
A spiral galaxy is a type of galaxy characterized by a flat, rotating disk of stars, gas, and dust, with prominent spiral arms winding out from a dense central region called a galactic bulge. These galaxies are among the most common in the universe. Key features include active star formation within the arms, a central supermassive black hole, and a surrounding halo of older stars.
2. What is the basic structure of a spiral galaxy?
The structure of a typical spiral galaxy is composed of several distinct components:
- The Bulge: A dense, spherical concentration of older stars at the galaxy's center.
- The Disk: A flat, rotating component containing young stars, interstellar gas, and dust. This is where the spiral arms are located.
- The Spiral Arms: Bright, winding arms that are active sites of new star formation, making them appear bluer and more luminous than the rest of the disk.
- The Stellar Halo: A large, faint, spherical region surrounding the galaxy, containing old stars and globular clusters.
- The Supermassive Black Hole: Found at the very core of the central bulge.
3. How are the different types of spiral galaxies classified?
Spiral galaxies are primarily classified using the Hubble sequence, which categorizes them based on the tightness of their spiral arms and the size of their central bulge. The main types are:
- Unbarred Spiral Galaxies (S): These are classified from Sa (large bulge, tightly wound arms) to Sc (small bulge, open, loosely wound arms).
- Barred Spiral Galaxies (SB): These galaxies feature a prominent bar-shaped structure of stars running through their center. They are similarly classified from SBa (large bulge, tight arms) to SBc (small bulge, loose arms). Our own Milky Way is considered a barred spiral galaxy.
4. What are some well-known examples of spiral galaxies?
Some of the most famous examples of spiral galaxies that astronomers study include:
- The Milky Way Galaxy: Our home galaxy, which is a barred spiral galaxy.
- The Andromeda Galaxy (M31): The nearest major spiral galaxy to the Milky Way and the largest in our Local Group of galaxies.
- The Triangulum Galaxy (M33): A smaller spiral galaxy in the Local Group, known for its high rate of star formation.
- The Whirlpool Galaxy (M51): A classic example of a 'grand design' spiral galaxy with well-defined, prominent arms.
5. How do the spiral arms in a galaxy form and persist without winding up tightly?
This addresses a classic puzzle known as the 'winding problem'. The spiral arms are not fixed structures composed of the same group of stars. Instead, they are best explained by the spiral density wave theory. This theory proposes that the arms are areas of higher density (like a cosmic traffic jam) that move through the galaxy's disk. Stars and gas clouds pass through these density waves, get compressed, and trigger a burst of new star formation, which makes the arms appear bright and distinct. The wave pattern rotates at a different speed than the stars, allowing it to persist for billions of years without winding up.
6. What is the fundamental difference between a spiral galaxy and an elliptical galaxy?
The fundamental differences between spiral and elliptical galaxies lie in their structure, composition, and star formation activity:
- Structure: Spiral galaxies have a flat, rotating disk with spiral arms and a central bulge. Elliptical galaxies are smooth, featureless, and oval-shaped (ellipsoidal) with no disk or arms.
- Composition: Spiral galaxies are rich in cold gas and dust, the fuel for new stars. Elliptical galaxies have very little gas and dust.
- Star Formation: Spiral galaxies are sites of active, ongoing star formation, especially in their arms, and contain many young, blue stars. Elliptical galaxies are composed almost entirely of old, red stars, with virtually no new star formation.
7. What role does the supermassive black hole play at the center of a spiral galaxy?
The supermassive black hole at the center of a spiral galaxy acts as a powerful gravitational anchor for the stars in the galactic bulge. While its gravity is immense, it does not control the rotation of the entire galaxy; that is primarily influenced by dark matter. However, the black hole's growth and activity can significantly impact the galaxy's evolution. By accreting matter, it can release enormous amounts of energy (as seen in quasars), which can heat or expel gas from the galaxy's center, thereby regulating or even shutting down star formation in the central regions.
