What kind of barred spiral galaxy is the Milky Way?

Three types of barred spiral galaxies
Click for original image.

The uncertainty of science: Though astronomers have long believed that the Milky Way galaxy is a barred spiral galaxy, defined as having a major straight arm coming out in two directions from its nucleus with other spiral arms surrounding it, determining the exact structure has been difficult because of our presence within the galaxy.

The image to the right, taken from a paper just published, shows three different types of barred spirals. On the left is one where the surrounding spiral arms hardly exist. In the center the central bar is surrounded by multiple arms. On the right is a barred spiral with just one major spiral arm.

Though it has been generally accepted that the Milky Way belongs in the center category, astronomers remain unsure about the actual spiral structure. Previous work had suggested the galaxy actually had four major arms, not two as seen by most barred spirals. As noted in the paper, “If that is the case, the [Milky Way] may be an atypical galaxy in the universe.”

The research from the new paper however now proposes that the Milky Way is actually not atypical, but instead more resembles the center image, with two main arms and multiple segmented arms beyond. From the abstract:

Using the precise locations of very young objects, for the first time, we propose that our galaxy has a multiple-arm morphology that consists of two-arm symmetry (the Perseus and Norma Arms) in the inner parts and that extends to the outer parts, where there are several long, irregular arms (the Centaurus, Sagittarius, Carina, Outer, and Local Arms).

The astronomers cheerfully admit that this conclusion is uncertain, and will need many further observations for confirmation.

A barred spiral galaxy with distorted galaxy

A barred spiral galaxy
Click for full image.

Cool image time! The photo to the right, reduced and cropped to post here, was taken by the Hubble Space Telescope and shows a barred spiral galaxy about 214 million light years away.

Scientists used NASA’s Hubble Space Telescope to image NGC 6956 to study its Cepheid variable stars, which are stars that brighten and dim at regular periods. Since the period of Cepheid variable stars is a function of their brightness, scientists can measure how bright these stars appear from Earth and compare it to their actual brightness to calculate their distance. As a result, these stars are extremely useful in determining the distance of cosmic objects, which is one of the hardest pieces of information to measure for extragalactic objects.

The press release of course focuses on this magnificent barred spiral. I however want to draw your attention to the smaller galaxy in the white box.

background distorted galaxy

Since we do not know the distance to this distorted galaxy, we do not know if its distortion is caused by the barred spiral. It could be the bigger galaxy’s gravity is pulling material away.

More likely, based on the shape of this smaller galaxy, is that it is many light years farther away — which is why it looks so much smaller — and has been distorted because it is actually a collusion of two galaxies. The two bright nuclei with the red dust between strongly suggests such a collusion.

I highlight this background galaxy because it illustrates the importance when looking at any Hubble image to look at everything. To coin a phrase, there is gold in them thar hills, if you make the effort to look.