It's also a really good example of how captions (this is a caption really, not exactly alt text) are context-dependent.

The caption you mention continues:

> The maser observed is almost directly opposite the Sun from the center in the S-C arm, 65,000 light years away.

...which makes no sense (there's no maser visible in the image or mentioned in the article) until you realise that this image was uploaded for another article and is being reused here.

Evidently the website's content management system asks for caption text when uploading the image, rather than when selecting it for use in the article. The latter is the right way to do it.