Detailed solar photos are typically not taken with a DSLR and lens alone. The examples described were likely made through a telescope set up specifically for solar imaging, usually with a hydrogen-alpha (H-alpha) filter to reveal surface detail and prominences.

A typical setup would be:

• a solar telescope or refractor designed for Sun viewing/imaging
• a proper H-alpha solar filter, or a telescope with one built in
• a camera-to-telescope adapter for your Nikon F mount

Some specialized solar scopes from makers like Lunt or Coronado have the filtering built in. Another approach uses a telescope with a removable front solar filter, but the key point is that the filter must be made specifically for solar observation/imaging.

Important: do not point your DSLR at the Sun without proper certified solar filtration. That can permanently damage your eyes and camera.

For images like the ones you saw, many photographers also process the results into monochrome. Some advanced solar imagers use dedicated astronomy cameras instead of DSLRs, but your D5100 can be adapted to a suitable solar telescope with the right mount adapter.