The Problem lies within the definition of `\textsuperscript`

that uses math mode.

```
\DeclareRobustCommand*\textsuperscript[1]{%
\@textsuperscript{\selectfont#1}}
\def\@textsuperscript#1{%
{\m@th\ensuremath{^{\mbox{\fontsize\sf@size\z@#1}}}}}
```

A possible workaround is redefining the actual command so that it will not get in the way with your equations, this needs `graphicx`

:

```
\renewcommand{\textsuperscript}[1]{\raisebox{0.8ex}{\scalebox{0.66}{#1}}}
```

Or this needs `relsize`

```
\renewcommand{\textsuperscript}[1]{\raisebox{0.8ex}{\smaller{#1}}}
```

I admit that this is just an easy workaround emulating the typrsetting of superscripts. It might be possible, that modern typography defines super/subscript in a certain way.

This also works with various enlargements of text sizes. This MWE also emulates a textsubscript:

```
\documentclass[12pt,oneside,letterpaper,titlepage]{article}
\DeclareMathSizes{12}{20}{14}{10}
%% Solution 1
\usepackage{relsize} %smaller
\renewcommand{\textsuperscript}[1]{\raisebox{0.8ex}{\smaller{#1}}}
\newcommand{\textsubscript}[1]{\raisebox{-0.4ex}{\smaller{#1}}}
%% Solution 2
%%\usepackage{graphicx} %scalebox
%%\renewcommand{\textsuperscript}[1]{\raisebox{0.8ex}{\scalebox{0.66}{#1}}}
%%\newcommand{\textsubscript}[1]{\raisebox{-0.4ex}{\scalebox{0.66}{#1}}}
\begin{document}
...modulation by Ca\textsuperscript{2+}...
...soluted in water H\textsubscript{2}O...\\
\tiny Ca\textsuperscript{2+}
\scriptsize Ca\textsuperscript{2+}
\footnotesize Ca\textsuperscript{2+}
\small Ca\textsuperscript{2+}
\normalsize Ca\textsuperscript{2+}
\large Ca\textsuperscript{2+}
\Large Ca\textsuperscript{2+}
\LARGE Ca\textsuperscript{2+}
\huge Ca\textsuperscript{2+}
\Huge Ca\textsuperscript{2+}
\normalsize
\begin{equation}
\sigma^2 = iI - \frac{I^2}{N}
\end{equation}
\end{document}
```

Further reading relsize and graphicx.

For chemistry typesetting (I guessed that on the calcium bit) there are some packages available, like bpchem or mhchem, that handle formulas quite well.

## Best Answer

The text asterisk is raised, while the math asterisk is used for denoting an operation.

You can use

`\mbox{*}`

or`\text{*}`

(if you load`amsmath`

).However, in general

`$Q^{*}$`

is preferred.You can get the behavior automatically:

I've added

`\ast`

that is the usual math asterisk for a comparison.I'd recommend using the first form.