What is different between * and auto on XAML?

I use Xamarin Forms.
But I don't know about XAML yet.

1) What is different between * and auto on XAML?
2) I wrote code like this. but Image viewed on emul6.0 not viewed on emul7.0. have I wrong?

 <ContentPage.Content>
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="10" />
                <RowDefinition Height="35" />
                <RowDefinition Height="*" />
                <RowDefinition Height="40" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="10" />
                <RowDefinition Height="60" />
                <RowDefinition Height="60" />
                <RowDefinition Height="40" />
                <RowDefinition Height="60" />
            </Grid.RowDefinitions>
            <Label Text="Setting" 
                   TextColor="White" 
                   BackgroundColor="#B1244F" 
                   HorizontalOptions="Fill" 
                   VerticalOptions="Fill"
                   HorizontalTextAlignment="Center" 
                   VerticalTextAlignment="Center" 
                   Grid.Row="1"></Label>

            <ContentView 
                     Grid.Row="2" 
                     Margin="10"
                     Padding="40"
                     HorizontalOptions="Fill"
                     VerticalOptions="CenterAndExpand">
                <Image x:Name="imgLogo"   />
            </ContentView>


            <Label Text="Login" 
                   TextColor="Gray" 
                   HorizontalTextAlignment="Center" 
                   Grid.Row="4"></Label>



            <Frame HeightRequest="30" WidthRequest="30" CornerRadius="15" 
                   HorizontalOptions="Center" 
                   VerticalOptions="Center" Margin="0" Padding="0" 
                       BackgroundColor="#B1244F" Grid.Row="3" >
                <Label Text="1" TextColor="White" HorizontalOptions="Center" VerticalOptions="Center" 
                        HorizontalTextAlignment="Center" VerticalTextAlignment="Center"></Label>
            </Frame >

            <BoxView Color="WhiteSmoke"
             WidthRequest="300"
             VerticalOptions="Fill" 
             HorizontalOptions="Center" 
                     Margin="5"
             Grid.Row="6" ></BoxView>


            <BoxView Color="WhiteSmoke"
             WidthRequest="300"
             VerticalOptions="Fill" 
             HorizontalOptions="Center" 
                     Margin="5"
             Grid.Row="7" ></BoxView>


            <!--
            <Frame Grid.Row="7"
            CornerRadius="5"
                    Margin="5"
            BackgroundColor="WhiteSmoke">

                <Label
                WidthRequest="40"
                HeightRequest="40"
                VerticalTextAlignment="Center"
                HorizontalTextAlignment="Center"
                Text="U">   
                </Label>
            </Frame>-->

            <Button Text="Next" 
                    BackgroundColor="#B1244F"
                    BorderRadius="0"
                    TextColor="White"
                    HorizontalOptions="Fill"
                    VerticalOptions="Fill"
                    Grid.Row="9"/>

        </Grid>
    </ContentPage.Content>