Adding asterisk to required fields in Bootstrap 3

My HTML has a class called .required that is assigned to required fields.

Here is the HTML:

<form action="/accounts/register/" method="post" role="form" class="form-horizontal">
    <input type="hidden" name="csrfmiddlewaretoken" value="brGfMU16YyyG2QEcpLqhb3Zh8AvkYkJt" />
    <div class="form-group required">
       <label class="col-md-2 control-label">Username</label>
       <div class="col-md-4">
         <input class="form-control" id="id_username" maxlength="30" name="username" placeholder="Username" required="required" title="" type="text" />
       </div>
    </div>
    <div class="form-group required"><label class="col-md-2 control-label">E-mail</label><div class="col-md-4"><input class="form-control" id="id_email" name="email" placeholder="E-mail" required="required" title="" type="email" /></div></div>
    <div class="form-group required"><label class="col-md-2 control-label">Password</label><div class="col-md-4"><input class="form-control" id="id_password1" name="password1" placeholder="Password" required="required" title="" type="password" /></div></div>
    <div class="form-group required"><label class="col-md-2 control-label">Password (again)</label><div class="col-md-4"><input class="form-control" id="id_password2" name="password2" placeholder="Password (again)" required="required" title="" type="password" /></div></div>
    <div class="form-group required"><label class="col-md-2 control-label">first name</label><div class="col-md-4"><input class="form-control" id="id_first_name" maxlength="30" name="first_name" placeholder="first name" required="required" title="" type="text" /></div></div>
    <div class="form-group required"><label class="col-md-2 control-label">last name</label><div class="col-md-4"><input class="form-control" id="id_last_name" maxlength="30" name="last_name" placeholder="last name" required="required" title="" type="text" /></div></div>
    <div class="form-group required"><label class="col-md-2 control-label">&#160;</label><div class="col-md-4"><div class="checkbox"><label><input class="" id="id_tos" name="tos" required="required" type="checkbox" /> I have read and agree to the Terms of Service</label></div></div></div>

    <div class="form-group">
        <div class="col-sm-offset-2 col-sm-10">
           <button type="submit" class="btn btn-primary">
              <span class="glyphicon glyphicon-star"></span> Sign Me Up!
           </button>
        </div>
    </div>
</form>

I added the following to my CSS;

.form-group .required .control-label:after {
  content:"*";color:red;
}

Still that does not give a red * around the required fields. What am I missing here? Isn’t there a direct way in Bootstrap 3 to introduce * to required fields?

EDIT

The * in terms and conditions does not appear immediately to a checkbox. How to fix this?