Is this really a spiral galaxy?
The uncertainty of science: The picture to the right, cropped, reduced, and sharpened to post here, was taken by the Hubble Space Telescope and released on March 4, 2024 by the PR department of the European Space Agency (ESA) as part of its Hubble Picture of the Week program. It shows what the press release claims is a spiral galaxy about 55 million light years away, seen edge on.
In this image NGC 4423 appears to have quite an irregular, tubular form, so it might be surprising to find out that it is in fact a spiral galaxy. Knowing this, we can make out the denser central bulge of the galaxy, and the less crowded surrounding disc (the part that comprises the spiral arms).
If NGC 4423 were viewed face-on it would resemble the shape that we most associate with spiral galaxies: the spectacular curving arms sweeping out from a bright centre, interspersed with dimmer, darker, less populated regions. But when observing the skies we are constrained by the relative alignments between Earth and the objects that we are observing: we cannot simply reposition Earth so that we can get a better face-on view of NGC 4423!
This picture provides a great example of the amount of assumptions that are often contained in astronomical observations. Though the data strongly suggests this is spiral, we must remember this is merely an educated guess, based on that central bulge and the dust lanes visible along the galaxy’s profile. There is actually no guarantee that this is so. As the press release also notes, astronomers are constrained by our viewpoint, and cannot change that viewpoint to get a better view to confirm this guess. For all we know, a face on veiw of this flat galaxy would reveal it has no spiral arms, but instead is mottled and chaotic, a rare type that does exist.
Astronomers do the best they can, but it is important that they (and we) always recognize the limitations.
The uncertainty of science: The picture to the right, cropped, reduced, and sharpened to post here, was taken by the Hubble Space Telescope and released on March 4, 2024 by the PR department of the European Space Agency (ESA) as part of its Hubble Picture of the Week program. It shows what the press release claims is a spiral galaxy about 55 million light years away, seen edge on.
In this image NGC 4423 appears to have quite an irregular, tubular form, so it might be surprising to find out that it is in fact a spiral galaxy. Knowing this, we can make out the denser central bulge of the galaxy, and the less crowded surrounding disc (the part that comprises the spiral arms).
If NGC 4423 were viewed face-on it would resemble the shape that we most associate with spiral galaxies: the spectacular curving arms sweeping out from a bright centre, interspersed with dimmer, darker, less populated regions. But when observing the skies we are constrained by the relative alignments between Earth and the objects that we are observing: we cannot simply reposition Earth so that we can get a better face-on view of NGC 4423!
This picture provides a great example of the amount of assumptions that are often contained in astronomical observations. Though the data strongly suggests this is spiral, we must remember this is merely an educated guess, based on that central bulge and the dust lanes visible along the galaxy’s profile. There is actually no guarantee that this is so. As the press release also notes, astronomers are constrained by our viewpoint, and cannot change that viewpoint to get a better view to confirm this guess. For all we know, a face on veiw of this flat galaxy would reveal it has no spiral arms, but instead is mottled and chaotic, a rare type that does exist.
Astronomers do the best they can, but it is important that they (and we) always recognize the limitations.