Spiral Galaxy: Meaning, Comparison, Formation, Discovery
Spiral galaxies are flattened stellar systems fashioned like spirals and named by their spiral structures. These twisted collections of stars and gas contain a flat, rotating disk of stars, a central bulge of tightly packed stars, and surrounding sparsely populated halos. Their distinctive spiral arms, made up of hot young stars, mark regions where star formation occurs slowly over the entire lifetime of the galaxy, making spirals the most common and fundamental objects in the universe.
What is a spiral galaxy?
A spiral galaxy is a galaxy with curved arms and higher luminosity, which forms a class of galaxy exhibiting a central nucleus and a central concentration of stars known as a bulge. Spiral galaxies are twisted collections of stars and gas, and they may exhibit a barred structure. The Milky Way is an example of a spiral galaxy.
A spiral galaxy is a galaxy that is fashioned like a spiral with a bulge in the center and spiral arms extending into the galactic disc. Astronomers name galaxies as spiral galaxies because of this distinctive shape. Edwin Hubble originally described spiral galaxies within the Hubble sequence, classifying galaxies that feature prominent regular arms as “grand design” spiral galaxies while patchier versions are termed irregular spiral galaxies. A barred spiral galaxy is one in which a straight bar of stars bisects the nucleus, and the arms depart from the ends of this central bulge before sweeping outward. Spiral galaxies make up roughly 72 percent of the galaxies observed according to a 2010 Hubble Space Telescope survey. Most galaxies discovered so far are spiral galaxies. The Milky Way is a grand-design barred spiral galaxy.
What are the types of spiral galaxies?
Spiral galaxies are divided into normal and barred spirals. The types of spiral galaxies are outlined below.
- Normal Spiral Galaxy: These galaxies have arms that emanate from the nucleus.
- Barred Spiral Galaxy: These galaxies have a bright linear feature called a bar that straddles the nucleus and the arms unwind from the ends of the bar.
- Sa Spiral Galaxy: These galaxies have a big central bulge with narrow, tightly wound arms, which are visible due to the presence of interstellar dust and bright stars.
- Sb Spiral Galaxy: These galaxies have more widely spread arms that appear less smooth with a moderately sized central bulge.
- Sc Spiral Galaxy: These galaxies have a very small nucleus and multiple spiral arms that are open with large pitch angles.
What are the components of a spiral galaxy?
The components of a spiral galaxy are outlined below.
- Spiral galaxies consist of thin and thick disk components with different kinematics.
- Spiral galaxies consist of a central bulge containing older stars.
- Spiral galaxies have a flat, rotating disk containing stars, gas, and dust.
- Spiral galaxies have an interstellar medium that includes HI, CO, Hα, and radio continuum emission.
- Spiral galaxies have massive halos composed of unseen material extending beyond the visible disk.
- Spiral galaxies contain spiral arms that extend from the center or bar.
- Spiral galaxies include a near-spherical halo of stars, including many in globular clusters.
- Spiral galaxies are surrounded by halos dominated by dark matter containing HI gas.
- Spiral galaxies may have a bar-crafted distribution of stars.
The most luminous component is a flat, rotating disk that contains stars, gas and dust. Within this disk, thin and thick components coexist, each with its own kinematics and stellar age. Spiral arms, coherent, trailing features that curve outward from the centre, are embedded in the disk and mark the cold, dense sites where new stars form. Bars, straight bar-like distributions of stars, often connect the inner arms to a compact nucleus. Rising from the mid-plane, the disk gives way to a near-spherical halo. This halo is dominated by dark matter and holds HI gas, field stars, globular clusters and diffuse hot gas that extends to at least 100 kpc. At the very centre sits the bulge, a dense, flattened concentration of older, metal-poor stars that often hosts a supermassive black hole. Random stellar motions rather than rotation support this region.
Does a spiral galaxy contain young stars?
Yes. Spiral galaxies contain lots of young stars. Spiral galaxies are made up of young stars, gas, and dust. Spiral galaxies are sites of ongoing star-formation. Young, hot OB stars inhabit spiral arms. Bright nebulae and young blue stars formed from gas and dust trace out the spiral arms within the disk. Spiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than the rest of the galaxy. Youngest stars form in gas-rich arms.
How do spiral galaxies differ from barred spiral, lenticular, irregular, and disk galaxies?
Differences between spiral galaxies and barred spiral, lenticular, irregular, and disk galaxies are explained in the table below.
| Feature | Spiral Galaxies | Barred Spiral Galaxies | Lenticular Galaxies | Irregular Galaxies |
| Shape | Have more tightly wound or loosely extended arms; Arms appear to trail | Similar to spiral galaxies but with a bar; Range of different shapes | Central bulge and disk, no arms; Lens shaped | Unusual shapes like toothpicks, rings, etc. Can have any number of shapes |
| Structure | Central bulge and disk | Central bulge, disk, and bar | Central bulge and disk | No defined structure |
| Classification | Classified by size of central bulge and texture of arms | Classified similarly to spirals | Disc galaxies (S0, SB0) | Neither spiral nor elliptical |
| Features | Some have bright line or bar Arms emerge from ends of bar | Bars run through centers Ribbons of stars cut across centers | Older spirals whose arms have faded Consist of a disc and smaller bulge | Wide variety of shapes and characteristics |
| Examples | Whirlpool | Nearby galaxies have boxy or peanut-shaped bars | S0 and SB0 types | Irregulars as a class |
Barred spiral galaxies are similar to spiral galaxies but differ in shape slightly because a bar of matter runs through their centers. Arms emerge from the ends of this bar and extend out from the galactic nucleus. Edwin Hubble classified both spiral and barred spiral galaxies further according to the size of their central bulge and the texture of their arms.
Lenticular galaxies have the central bulge and disk common to spiral galaxies but no spiral arms. They are older spirals whose arms have faded and are lens-fashioned. Disc galaxies (S0, SB0) are called lenticular galaxies and do not have any spiral arms. Barred lenticular galaxies are denoted SB0.
Irregular galaxies do not fit into spiral classes and are neither spiral nor elliptical. They have a wide variety of shapes and characteristics, appearing as toothpicks, rings, or little groupings of stars.
Is an elliptical galaxy different from a spiral galaxy?
Elliptical galaxies differ from spiral galaxies. Elliptical galaxies do not have detailed spiral structure and show very little organization or structure. Instead of a flat disk with rotating arms, an elliptical galaxy is characterized by a spherical or cucumber-like shape. The stars orbit the center in all planes and in all different directions without coherent rotation. In spiral galaxies, stars rotate around the center in a coherent manner. These galaxies have a detailed spiral structure.
Where a spiral galaxy keeps bright clumps of young stars and plentiful gas and dust for new star birth, an elliptical galaxy usually contains little gas and dust and is known for its dearth of star-forming material. The stars inside it are older and dimmer, giving the whole system a smooth, reddish appearance.
Scientists think elliptical galaxies originate from collisions and mergers with spirals. Two colliding spiral galaxies form an elliptical galaxy rather than one larger spiral galaxy, and once the merger is complete the elliptical galaxy never again settles into a spiral shape and remains puffed up and smooth. Because of this origin, ellipticals are less common than spiral galaxies and are deemed older systems, while spirals continue to organize and form stars along their arms.
What is different about the center of a quasar when contrasted with a spiral galaxy?
Differences between the center of a quasar and a spiral galaxy are explained in the table below.
| Center of a Quasar | Center of a Spiral Galaxy |
| Quasars occur when immense amounts of matter fall into central supermassive black hole | All spirals with nuclear bulges have black holes at their centers |
| Quasars lie at the centers of galaxies | Spirals can harbor quasars |
| Quasars have extremely bright core and are about the size of our solar system | Seyfert 1 nucleus shows galactic envelope whereas quasar is generally star-like without nebulosity |
| Quasars can radiate thousands of times the energy emitted by a galaxy like the Milky Way | Milky Way has a black hole in its center |
| Quasars derive their energy from material falling toward massive black hole | Mechanism that powers quasars is similar to the mechanism that powers the Milky Way black hole |
| Material forms a hot accretion disk around massive black hole | Matter falls into supermassive black hole spiraling around it in form of accretion disk |
| Quasars have long jets that glow with radio radiation | Jets extend far beyond host galaxies |
| Quasars emit light across the electromagnetic spectrum and have the same redshift as host galaxy | Jets are powerful sources of radio and gamma‑ray radiation |
The center of a quasar differs from the center of an ordinary spiral galaxy because matter spirals around a supermassive black hole in the form of a disk, releasing energy across the electromagnetic spectrum. In a spiral the central black hole is quiet. In a quasar the same black hole becomes an engine that can radiate thousands of times the energy emitted by the Milky Way. Quasars derive their energy from material falling toward a massive black hole, forming a hot accretion disk that glows far brighter than any normal galactic bulge. The distinction results from different viewing angles into centers of galaxies, yet the core phenomenon is the same. Matter falls into a supermassive black hole, and during the quasar phase that inflow is so rapid that the nucleus outshines the entire host.
How does a spiral galaxy form?
Spiral galaxies are bulges and halos formed through primordial collapse of individual gas clouds early in the history of the universe. Spirals form when compressional waves propagate through the disk, growing in length and amplitude because of self-gravitational forces. The spiral structure arises from differential rotation of the galaxy’s disk. Gravity effects of small galaxies create a spiral structure in the disk. The galaxy accretes enough gas from its surroundings, known as the Intergalactic Medium. Accretion of gas continues, establishing disk instability. Thin kinematically cold disks are unstable to non-axisymmetric instabilities which give rise to spiral arms and bars. Galactic bars develop when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. Bar becomes even more pronounced as it collects more stars in elliptical orbits. Secular evolution occurs through actions of spiral arms and bars. Formation of spiral galaxies has on-going star formation evident in thin disks.
Where is the location of a spiral galaxy?
A spiral galaxy is a structure that extends through space rather than occupying a single point. The Sunflower Galaxy lies in the constellation Canes Venatici, the Whirlpool Galaxy is found in the same constellation, and the Pinwheel Galaxy resides in Ursa Major. Each spiral galaxy is located within a specific constellation as seen from Earth.
On larger scales, barred spiral galaxy 2607238 is part of the Cosmic Evolution Survey field at redshift distance 6.4 billion light-years, NGC 2835 lies 35 million light-years away in Hydra, and the extremely old spiral A1689B11 sits inside the Abell 1689 galaxy cluster in Virgo. Spiral galaxies populate both nearby groups and distant clusters.
Within any given spiral galaxy, the bulk of the stars are located close to the galactic plane, while a fraction resides in the spheroidal galactic bulge around the galactic core, and all stars move in a more or less conventional circular orbits around the Galactic Center.
When was the first spiral galaxy discovered?
The first spiral galaxy, the Whirlpool Galaxy, was discovered by Charles Messier on October 13, 1773. A spiral structure discovered by Lord Rosse in 1845 was presented at the 1845 meeting of the British Association for the Advancement of Science. The Whirlpool was the first nebula to be known to have a spiral structure.
Do spiral galaxies resemble hurricanes?
Pictures of spiral galaxies resemble those of hurricanes. Many morphological characteristics of hurricanes bear an uncanny resemblance to those of spiral galaxies. Hurricane Fabian looks very similar to Whirlpool Galaxy M51, whose diameter is 98 thousand light years.
Galaxies and hurricanes obey the same laws of physics. Differential rotation pattern, pancake shape, and distribution of energy resemble spiral galaxies. Winds spiral inward in hurricanes. Time dilation produces a whirlpool of star gas matter.
What determines the velocity of a spiral galaxy?
The velocity of a spiral galaxy is governed by the balance between gravity and centrifugal force. The magnitude is set by how the objects and their masses are distributed. Close to the centre the velocity increases linearly with radius, matching the rigid-disk law v ∝ r, while farther out most galaxies exhibit a flat rotation curve V(R)=constant. This flattening is not reproduced by the visible stars alone. The outer regions lack enough luminous mass to cause the measured velocities, so a dark-matter halo distributed spherically around the disk is inferred.
For any galaxy the circular velocity at each radius gives the total mass within that radius. Empirically, the same velocity is uniquely related to the total luminosity of the system through the Tully-Fisher relation. Measuring the bolometric luminosity predicts the rotational velocity. The relation is read directly from the galaxy’s location on the Tully-Fisher diagram, and it supplies a distance estimate once apparent brightness and velocity width are known.
Neutral-hydrogen (HI) line observations provide a unique tracer for the velocity field in spiral galaxies, allowing the rotation curve to be reconstructed even for nearly face-on systems. In barred or spiral-arm regions the spiral density wave causes systematic non-circular motions that appear as deviations in the line-of-sight velocity field. Fourier decomposition of this field isolates the harmonics and yields the pattern speed. Where the curve remains flat, the angular velocity Ω(R)=V/R falls off as ~1/R, preserving the differential rotation required to sustain trailing spiral arms whose pitch angle slowly decreases as the pattern rotates.
