They consist of a central bulge, a halo, a disk, and spiral arms. Interstellar material is usually spread throughout the disks of spiral galaxies. Bright emission nebulae and hot, young stars are present, especially in the spiral arms, showing that new star formation is still occurring.
Spiral galaxies are classified as Sa/SBa, Sb/SBb or Sc/SBc (classic/barred) according to the tightness of their spiral, the clumpiness of their spiral arms, and the size of their central bulge. These differences can be traced back to the relative amounts of gas and dust contained within the galaxies.
Astronomers believe that a galaxy's spiral structure originates as a density wave emanating from the galactic center. The idea is that the entire disk of a galaxy is filled with material. The spiral arms of a galaxy mark where in the galaxy the density wave recently passed, causing new stars to form and burn brightly.
1) When you look toward the galactic center with your eye, you see a long, thin strip. This suggests a disk seen edge-on, rather than a ellipsoid or another shape. We can also detect the bulge at the center. Since we see spiral galaxies which are disks with central bulges, this is a bit of a tipoff.
As dust and gas is compressed in the spirals, it is heated up and results in the formation of new stars. Astronomers have determined that the Milky Way is a spiral galaxy by mapping the movements of stars and hydrogen clouds in the disk. The Milky Way is far from being the only spiral galaxy in the Universe.
Both the arms and the disk of a spiral system are blue in color, whereas its central areas are red like an elliptical galaxy. The Hubble sequence classifies spirals on the amount of winding in their spiral arms, tightly wound spirals are type Sa, more open spirals are classed Sc.
The arms of a spiral galaxy have lots of gas and dust, and they are often areas where new stars are constantly forming. The bulge of a spiral galaxy is composed primarily of old, red stars.
Spiral galaxies get their name from the shape of their disks. In a spiral galaxy, the stars, gas and dust are gathered in spiral arms that spread outward from the galaxy's center.
"Some galaxies have no long spiral arms at all, but only numerous, short and non-symmetric arms, as in the Sculptor group galaxy NGC 7793. These arms are probably not density waves at all, but are short-lived star-forming regions that are sheared into spiral-like pieces by differential rotation of the galaxy.
This classification system is known as the Hubble Sequence. It divides galaxies into three main classes with a few variations. Today, galaxies are divided into four main groups: spiral, barred spiral, elliptical, and irregular.
The Milky Way – the galaxy that includes Earth and our solar system – is an example of a spiral galaxy. Approximately two-thirds of spiral galaxies also contain a bar structure through their center, as does the Milky Way. The disk of stars orbiting the bulge separates into arms that circle the galaxy.
Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.
The bulge and halo consist mainly of older stars, where spiral arms have more gas, dust and younger stars. Our Milky Way Galaxy is a spiral galaxy. Some spiral galaxies are what we call "barred spirals" because the central bulge looks elongated – like a bar. The stars in ellipticals tend to be older.
Originally, Edwin Hubble hypothesized that elliptical galaxies evolved into spiral galaxies, which was later discovered to be false, although the accretion of gas and smaller galaxies may build a disk around a pre-existing ellipsoidal structure.
Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. They are mostly found in low-density regions and are rare in the centers of galaxy clusters.
This leaves these galaxies dominated by their large bulges of old stars, and their disks relatively small containing faint, smooth, tightly wound arms. In contrast, Sc spirals contain about 15% gas and dust meaning that a relatively high proportion of the mass of the galaxy is involved in star formation.
The spiral arms are a wave of star formation caused by wave of density propagating outward through the disk of the galaxy. They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.
Nearly 70 percent of the galaxies closest to the Milky Way are spirals. New research finds that spiral arms are self-perpetuating, persistent, and surprisingly long lived. Because elliptical galaxies are made up of older, dimmer stars, they are more challenging to spot.
Key Facts & Summary
- There are four main categories of galaxies: elliptical, spiral, barred spiral, and irregular.
- The most common type of galaxy found throughout the universe is the spiral galaxy.
- Around two-thirds of all spiral galaxies have a bar-like structure – thus they are classified as barred spiral galaxies.
Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3000 to 300,000 light years) and separated by distances on the order of millions of parsecs (or megaparsecs).
Spiral arms are caused by spiral density waves - a spiral shaped disturbances traveling through the disk of the galaxy that compressed gas and triggers star formation. Why are the arms of spiral galaxies typically blue in color? Stars are forming in the spiral arms so there are high mass, hot, blue stars in the arms.
Elliptical galaxies typically appear yellow-red, which is in contrast to the distinct blue tinge of most spiral galaxies. Very little star formation is thought to occur in elliptical galaxies, because of their lack of gas compared to spiral or irregular galaxies.
