text-overflow:ellipsis;

HTML

<header>
    <h1>Long text is so long oh my is long indeeuinijnjnjnijndklkjkjjlkjlklkjlklklkmkjnjnuhihuuhiuh</h1>
</header>

CSS

header {
    border:1px solid red;
    width:150px;
    height:100px;
}
h1 {
    overflow:hidden;
    white-space:nowrap;
    -ms-text-overflow:ellipsis;
    text-overflow:ellipsis;
    width:150px;
    height:1.2em;
}
h1:hover {
    overflow:visible;
}