Spiral galaxy type A — abbreviation Sa — they have a big central bulge and smooth, broad spiral arms. This means that a relatively small proportion of Sa galaxies are involved in star formation.
Spiral galaxy type B — abbreviation Sb — have a moderately-sized central bulge and moderately-well-defined spiral arms. Spiral Galaxy type C — abbreviation Sc — small central bulge as well as narrow and well-defined central arms. Barred spiral galaxy type A — abbreviation SBa — they feature tightly bound arms. Barred spiral galaxy type B — abbreviation SBb — they feature both tightly and loosely bond arms.
Barred spiral galaxy type C — abbreviation Sbc — they have loosely bound arms. Usually, spiral galaxies are believed to evolve into elliptical galaxies as they get older. Usually, with time, galaxies collided with one another and former bigger structures. Spiral Galaxies Facts Spiral galaxies are a class of galaxies originally described by astronomer Edwin Hubble in his work The Realm of the Nebulae.
Home » Galaxies » Spiral Galaxies. They are part of the Hubble sequence a morphological classification scheme for galaxies. Spiral galaxies usually consist of a rotating disk that contains stars, dust, gas, and a concentration of stars known as the bulge in the center. These bulges are often surrounded by a faint halo of stars, many of which reside in globular clusters — a spherical collection of stars that orbits a galactic core. As such, Spiral galaxies are named after their spiral structures that extend from the center into a galactic disk.
They have spiral arms — sites of ongoing star formation that are brighter than the surrounding disk due to the presence of young OB stars that inhabit them. Around two-thirds of all spiral galaxies have an additional component in the form of a bar-like structure. This bar extends from the central bulge at the ends of which spiral arms begin.
One of the largest known spiral galaxies is NGC , which is , light-years across from the tips of its outstretched spiral arms — that's about 5 times the size of the Milky Way. In , astronomers discovered an billion-year-old ancient spiral galaxy called AB Its discovery will help scientists understand how galaxies transition from "highly chaotic, turbulent discs" to more organized and thinner discs, like that of the Milky Way.
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The stars in the disks of spiral galaxies are generally younger than the majority of stars found in the bulge and halo. For this reason, disks are thought to form after the primordial collapse event responsible for the formation of the spheroidal bulge and halo, possibly through the cooling of the hot gas contained within the halo of the newly formed galaxy. However, this cannot be the whole picture, as many spiral galaxies possess two distinct disk structures a thick disk and a thin disk which vary in content thick disks are composed entirely of stars while thin disks also contain cold gas as well as thickness.
The gas and dust contained within the spiral arms of the thin disk are continually creating new stars through secular evolution processes.
Each red knot is a newly formed massive star cluster with the reddish colour the result of extinction. In the Milky Way, on average, the thick disk is older than the thin disk but younger than the bulge. Both observations and N-body modelling indicate that such an event would disrupt the thin disk and consume a significant fraction of the cold gas in a burst of new star formation, so the proposed merger event must have taken place before the thin disk had time to fully form.
Lenticular galaxies , such as the iconic Sombrero Galaxy , sit between elliptical and spiral galaxies. Like elliptical galaxies, they have little dust and interstellar matter, and they seem to form more often in densely populated regions of space. Galaxies that are not spiral, lenticular, or elliptical are called irregular galaxies. Irregular galaxies—such as the Large and Small Magellanic Clouds that flank our Milky Way—appear misshapen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by.
They are full of gas and dust, which makes them great nurseries for forming new stars. Some galaxies occur alone or in pairs, but they are more often parts of larger associations known as groups, clusters, and superclusters.
Our Milky Way, for instance, is in the Local Group , a galaxy group about 10 million light-years across that also includes the Andromeda galaxy and its satellites. The Local Group and its neighbor galaxy cluster, the Virgo Cluster , both lie within the larger Virgo Supercluster , a concentration of galaxies that stretches about million light-years across.
The Virgo Supercluster, in turn, is a limb of Laniakea, an even bigger supercluster of , galaxies that astronomers defined in Galaxies in clusters often interact and even merge together in a dynamic cosmic dance of interacting gravity.
When two galaxies collide and intermingle, gases can flow towards the galactic center, which can trigger phenomena like rapid star formation. Our own Milky Way will merge with the Andromeda galaxy in about 4. Because elliptical galaxies contain older stars and less gas than spiral galaxies, it seems that the galaxy types represent part of a natural evolution: As spiral galaxies age, interact, and merge, they lose their familiar shapes and become elliptical galaxies.
But astronomers are still working out the specifics, such as why elliptical galaxies follow certain patterns in brightness, size, and chemical composition.
The universe's first stars ignited some million years after the big bang, the explosive moment Gravity had sculpted the first galaxies into shape by the time the universe turned million years old , or less than 3 percent of its current age. Astronomers now think that nearly all galaxies— with possible exceptions —are embedded in huge haloes of dark matter.
Theoretical models also suggest that in the early universe, vast tendrils of dark matter provided normal matter the gravitational scaffold it needed to coalesce into the first galaxies. But there are still open questions about how galaxies form.
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