Barred spiral galaxy
A
barred spiral galaxy is a
spiral galaxy with a band of bright
stars emerging from the center and running across the middle of the
galaxy.
Spiral arms appear to emerge from the ends of the "bar" in these galaxies, whereas they appear to emerge directly from the core in ordinary spiral galaxies.
Edwin Hubble classified these types of spiral galaxies as "SB" ("Spiral", "Barred") in his
Hubble sequence, and arranged them into three sub-categories based on how open the arms of the spiral are. SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb type galaxies lie in between. A fourth type, SBm, was subsequently created to describe somewhat
irregular barred spirals, such as the
Magellanic Cloud galaxies, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures.
In
2005, observations by the
Spitzer Space Telescope backed up previously collected evidence that suggested the
Milky Way is a barred spiral galaxy. Observations by
radio telescopes had for
years suggested our galaxy to be barred, but
Spitzer's vision in the
infrared region of the spectrum has provided a more definite
calculation.
Barred spiral galaxies are relatively common, with surveys showing that up to two-thirds of all spiral galaxies contain a bar. The current
hypothesis is that the bar structure acts as a type of
stellar nursery, fueling
star birth at their centers. The bar is thought to act as a mechanism that channels
gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is also thought to explain why many barred spiral galaxies have
active galactic nuclei, such as that seen in the
Southern Pinwheel Galaxy.
The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure. Another possible culprit in bar creation are tidal disruptions between galaxies.
Bars are thought to be a temporary phenomenon in the life of spiral galaxies, the bar structure decaying over time, transforming the galaxy from a barred spiral to a "regular" spiral pattern. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have short, stubby bars[
1]. Since so many spiral galaxies have a bar structure, it is likely that it is a recurring phenomenon in spiral galaxy development.
Studying the core of the Milky Way, scientists found out that the Milky Way's
bulge was
peanut-shaped. This led to the conclusion that all barred spiral galaxies have a peanut shaped bulge. When observing a distant spiral galaxy with a rotational axis perpendicular to the line of sight, or one that appears "edge-on" to the observer, the shape of the bulge can be easily observed, and therefore quickly classified as either a barred spiral or a regular spiral. Galaxy
NGC 4565 has been classified as a barred spiral galaxy using this method.
*
Active galaxy*
Dwarf galaxy*
Dwarf elliptical galaxy*
Dwarf spheroidal galaxy*
Elliptical galaxy*
Galaxy classification*
Galaxy formation and evolution*
Groups and clusters of galaxies*
Irregular galaxy*
Lenticular galaxy*
List of galaxies*
List of nearest galaxies*
Ring galaxy*
Spiral galaxy*
Starburst galaxy*
Seyfert galaxy*
Timeline of galaxies, clusters of galaxies, and large scale structure*Britt, Robert Roy.
"Milky Way's Central Structure Seen with Fresh Clarity." SPACE.com 16 August,
2005.
**An article about the Spitzer Space Telescope's Milky Way discovery
*Devitt, Terry.
"Galactic survey reveals a new look for the Milky Way." 16 August,
2005.
**The original press release regarding the article above, from the
Univ. of Wisconsin*SPACE.com staff writers.
"'Barred' Spiral Galaxy Pic Highlights Stellar Birth." SPACE.com 2 March,
2001.
*Hastings, George and Jane Hastings.
Classifying Galaxies: Barred Spirals,
1995.
*Buta, Ronald, D. A. Crocker, and G. G. Byrd.
"Astronomers Find Multiple Generations of Star Formation in Central Starburst Ring of a Barred Spiral Galaxy." January 15,
2000.
**A
press release concerning NGC 1326
*
Barred spirals come and go Sky & Telescope April
2002.
*
"ESO Provides An Infrared Portrait of the Barred Spiral Galaxy Messier 83." November 29,
2001.
**A press release from the
European Southern Observatory.
