## How many stars are born and die each day? (Beginner)

I am a science teacher who would like to know how many stars are born and how may die each day.

We usually talk of star formation in terms of the gas mass that is converted into stars each year. We call this the star formation rate. In the Milky Way right now, the star formation rate is about 3 solar masses per year (i.e. three times the mass of the Sun's worth of star is produced each year). The stars formed can either be more or less massive than the Sun, though less massive stars are more numerous. So roughly if we assume that on average the stars formed have the same mass as the Sun, then the Milky Way produces about 3 new stars per year. People often approximate this by saying there is about 1 new star per year.

Now what about the rate at which stars die? In typical galaxies like the Milky Way, a massive star should end its life as a supernova about every 100 years. Less massive stars (like the Sun) end their lives as planetary nebulae, leading to the formation of white dwarfs. There are about one of these per year.

Therefore we get on average about one new star per year, and one star dying each year as a planetary nebula in the Milky Way. These rates are different in different types of galaxies, but you can say that this is roughly the average over all galaxies in the Universe. We estimate at about 100 billion the number of galaxies in the observable Universe, therefore there are about 100 billion stars being born and dying each year, which corresponds to about 275 million per day, in the whole observable Universe.

#### Amelie Saintonge

Amelie is working on ways to detect the signals of galaxies from radio maps.

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