Estimating the exact number of grains of sand on Earth and the number of stars in the universe is challenging due to the vast scales involved. However, I can provide you with some rough estimates that are often cited:
Grains of Sand on Earth: It is difficult to determine an exact number, as the quantity of sand on Earth can vary depending on factors like the definition of "sand" and the range of locations considered. Nevertheless, an often-cited estimate suggests that there are roughly 7.5 x 10^18 (7.5 quintillion) grains of sand on Earth.
Stars in the Universe: The number of stars in the universe is even more challenging to estimate accurately. There are an estimated 100 to 400 billion stars in our Milky Way galaxy alone. Based on observations of the visible universe and statistical modeling, astronomers estimate that there could be anywhere between 100 billion to 2 trillion galaxies in the observable universe. Assuming an average of 100 billion stars per galaxy, this would yield a rough estimate of around 10^22 (10 sextillion) stars in the observable universe.
It's important to note that these estimates are based on current knowledge and are subject to change as our understanding of the universe improves and new observations are made. The vastness of both grains of sand on Earth and stars in the universe highlights the immense scales that exist in the natural world.