Remove tick marks but keep labels for 3D plot

Question

I have the following image

where I have used Ticks -> {{{-0.75, "\[Pi]/4"}, 6, {-8, "Sun"}, 10}, {}, {}, {}}, to mark important points on the x-axis.

I would like to remove the tick marks under the Pi/4 and Sun labels, but don't know how to do this while retaining the labels.

If there is no way to do this, is there instead a way to label the x-axis without using Ticks as I have done?

For reference here is the code that establishes the axes:

shells = SphericalPlot3D[{6, 10}, {\[Theta], 0, Pi}, {\[Phi], 0,   
   2 Pi},  Mesh -> None, PlotPoints -> 50, ImageSize -> Large, 
  Boxed -> False, AxesOrigin -> {0, 0, 0},
  PlotStyle -> {Directive[Gray, Opacity[0.05]], 
    Directive[Gray, Opacity[0.05]]},
  PlotRange -> {{-14, 14}, {-14, 14}, {-14, 14}},
  AxesLabel -> {"\!\(\*SubscriptBox[\(x\), \(g\)]\) [kpc]", 
    "\!\(\*SubscriptBox[\(y\), \(g\)]\) [kpc]", 
    "\!\(\*SubscriptBox[\(z\), \(g\)]\) [kpc]"},
  AxesStyle -> Directive[Black, Bold, 15],
  LabelStyle -> Directive[Black, Bold, 15],
  Ticks -> {{{-0.75, "\[Pi]/4"}, 6, {-8, "Sun"}, 10}, {}, {}, {}},
  TicksStyle -> Directive[{Opacity[1]}]
  ]

Note: FrameStyle is not an option for SphericalPlot3D.

Answer 1

Specify a custom length of zero for those tick marks you want to hide, as in {-0.75, "\[Pi]/4", 0}, for example:

SphericalPlot3D[
 {6, 10}, {\[Theta], 0, Pi}, {\[Phi], 0, 2 Pi},
 Mesh -> None, PlotPoints -> 50, ImageSize -> Large,
 Boxed -> False, AxesOrigin -> {0, 0, 0},
 PlotStyle -> {Directive[Gray, Opacity[0.05]], 
   Directive[Gray, Opacity[0.05]]},
 PlotRange -> {{-14, 14}, {-14, 14}, {-14, 14}},
 LabelStyle -> Directive[Black, Bold, 15],
 
 Ticks -> {{{-0.75, "\[Pi]/4", 0}, 6, {-8, "Sun", 0}, 10}, {}, {}, {}},
 TicksStyle -> Directive[{Opacity[1]}]
]