Radial arms of the SBa, Sbb and Sbc galaxies - why are they radial?

[lenfromkits]

When looking at the classifications of galaxies in the attached image,

how do we explain the SB types - especially the SBc which has long arms that reach out radially before beginning to spiral?

If these arms are spinning, then regardless of the speeds we measure, along the straight portion of the arms, the speeds appear to double as distance doubles (like the speeds of sections of a rope spinning around you). It's almost as if they are held together by gravity and are pulled outward by centrifugal force (like a rope spinning around you).

How do we explain these arms that reach 'straight' out?

Thanks!

 

[AndyW]

Thank you for your question regarding the SB types of galaxies and their unique spiral structures. The SB classification stands for "barred spiral" and refers to a type of spiral galaxy with a central bar of stars that extends through the center of the galaxy. This bar structure can be seen in the attached image as a thick, elongated feature in the center of the galaxy.

The SBc type, in particular, is known for its long, extended spiral arms that reach out radially before curving back in towards the center of the galaxy. This is due to the strong gravitational forces acting on the stars and gas within the galaxy.

The arms of a spiral galaxy are not static, but rather they are constantly in motion. As you mentioned, the stars in the arms are spinning around the center of the galaxy, much like sections of a rope spinning around a central point. This rotation creates a centrifugal force that pulls the stars outward, causing the arms to appear "straight" or extended.

However, it is important to note that the gravity from the rest of the galaxy and neighboring galaxies also play a role in shaping the spiral arms. The gravitational pull from these sources can cause the arms to curve or twist, creating the unique and intricate structures we observe in different galaxies.

In summary, the arms of a spiral galaxy are a result of a delicate balance between gravitational forces and the rotation of stars and gas. The SBc type, with its long, extended arms, is a prime example of this fascinating interplay between gravity and motion.

I hope this explanation helps to clarify the nature of spiral galaxy arms and their formation. Thank you for your interest in this topic and for your question.