Question about lead-acid batteries

i am rithy now i study at university

I have just bought a sealed lead-acid battery (6V 4.5Ah , 24HR) unfortunately I don't have any experience about this kind of battery.
first I want to know what is the meaning of last number (24HR) does it relate to charging time and current charging limitation ?
after that , would you please tell me , will there be any danger or problem , if I use an normal regulator such as 7809 , a 5.6 ohm (2W) resistor and a diode to charge it ?(the charging current would be almost 410m)

In short, I mean, do I have to use special charger which produce constant current at accurate voltage or I can use the famous and simple circuit that I mentioned it.

Please excuse me if my English isn't well

Thanks in advance