Galaxies #1 - Spiral Galaxies
Galaxies are mainly of three types - Spiral, Elliptical and Irregular. They are subdivided into classes and designated a specific type.
Spiral Galaxies :
They have three main features namely a bulge, a halo and a disc. The bulge is the central portion of the spiral which is significantly brighter than the rest of the galaxy. The bulge of most galaxies is known to contain a super massive black hole (SMBH) in it. The disc consists of the spiral arms around the bulge. The halo is a faint enclosure around the galaxy. The halo mainly contains globular clusters which contain old stars.
Spiral galaxies are categorised into two varieties. The spiral galaxies, whose arms extend right from the bulge are termed as ordinary spiral galaxies and designated as SA. On the other hand, some spiral galaxies have spiral arms which don't extend from the bulge, but extend from an elongation of the bulge. Theses galaxies are known as Barred Spiral Galaxies and are designated as SB.
These classes are further subdivided into subclasses based on the tighness of their spiral arms . The spirals with the tightest arms are designated as a and more looser arms are designated b,c,d and so on.Some galaxies which have a tightness in between two subclasses, like in between b and c is termed as bc.
Galaxies with ring like structures are termed as (r) and those without it are termed as (s).
For example, Andromeda, (shown in pic ) the biggest galaxy in the Local Group, is designated as SA(s)b. So we can conclude that Andromeda is a ordinary spiral galaxy with a slightly lower arm tightness and no rings.
Spiral Galaxies :
They have three main features namely a bulge, a halo and a disc. The bulge is the central portion of the spiral which is significantly brighter than the rest of the galaxy. The bulge of most galaxies is known to contain a super massive black hole (SMBH) in it. The disc consists of the spiral arms around the bulge. The halo is a faint enclosure around the galaxy. The halo mainly contains globular clusters which contain old stars.Spiral galaxies are categorised into two varieties. The spiral galaxies, whose arms extend right from the bulge are termed as ordinary spiral galaxies and designated as SA. On the other hand, some spiral galaxies have spiral arms which don't extend from the bulge, but extend from an elongation of the bulge. Theses galaxies are known as Barred Spiral Galaxies and are designated as SB.
These classes are further subdivided into subclasses based on the tighness of their spiral arms . The spirals with the tightest arms are designated as a and more looser arms are designated b,c,d and so on.Some galaxies which have a tightness in between two subclasses, like in between b and c is termed as bc.
Galaxies with ring like structures are termed as (r) and those without it are termed as (s).
For example, Andromeda, (shown in pic ) the biggest galaxy in the Local Group, is designated as SA(s)b. So we can conclude that Andromeda is a ordinary spiral galaxy with a slightly lower arm tightness and no rings.
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