The astable mode is also called oscillator mode, because it uses the 555 as an oscillator within an electronic circuit. Astable mode creates a square wave signal.

The duty cycle in a 555 integrated circuit (IC) is the percentage of time that the output is high for each cycle of the square wave. For example, if the total cycle time is 1 s and the output is high for the first 0.4 s of each cycle, the duty cycle is 40%.

With an astable circuit, the duty cycle must always be greater than 50%. In other words, the duration for which the output is high must always be more than the duration during which the output is low.

The explanation for this is pretty simple: For the duty cycle to be 50%, the capacitor would have to charge and discharge through the same resistance. The only way to accomplish that would be to omit R1 altogether, so that the capacitor charged and discharged through R2 only.

But the problem with that is that you would end up connecting pin 7 directly to Vcc. With no resistance between pin 7 and the voltage source, the current flowing through pin 7 would exceed the maximum that can be handled by the circuitry inside the 555, and the chip would be damaged.

There's a clever way around this limitation: Place a diode across R2. This diode bypasses R2 when the capacitor is charged. That way, the capacitor charges through R1 and discharges through R2.

When a diode is used in this way, you have complete control over the duration of both the charge and discharge time. If R1 and R2 have the same value, the capacitor takes the same amount of time to charge as it does to discharge, so the duty cycle will be 50%. If R2 is smaller than R1, the duty cycle is less than 50% because the capacitor discharges faster than it charges.

If you use this approach, you must adjust the formulas for calculating the time intervals as follows:

T = 0.7 (R1 + R2) C1
Thigh = 0.7 R1 C1
Tlow = 0.7 R2 C1