Stars are continuously born and die. How many stars die in our Milky Way each year?
When you look up at the night sky, you see constellations similar to what the ancient Greeks observed. However, in reality, new stars are continuously being born while others die. This is also the fate of our Sun in about 5 billion years. How rapidly does the night sky change, and how many stars die each year in our galaxy—the Milky Way?
According to James De Buizer, a researcher at the Search for Extraterrestrial Intelligence Institute (SETI), this is quite complex.
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First, it is essential to clarify what it means for a star to die. Stars are massive balls of hot gas held together by nuclear fusion, which converts hydrogen into helium in their cores. Stars die when nuclear fusion ceases. There are two ways this can happen, and how a star dies depends on its mass.
For stars with smaller masses, nuclear fusion ends when all the hydrogen in the star’s core is converted into helium. Without the heat and pressure from fusion pushing outward, the star will collapse under its own gravity. During this collapse, the core pressure becomes so intense that the remaining helium begins to fuse into carbon, releasing energy. The outer atmosphere of the star swells and turns red, creating what is known as a red giant.
Eventually, the star sheds this swollen atmosphere, leaving behind a dense object known as a white dwarf. Approximately 97% of the stars in the Milky Way, including our Sun, are destined to become white dwarfs, De Buizer stated.
Astronomers can observe white dwarfs because they emit distinctive light signals. They use this information, along with star formation rates and the total number of stars, to calculate how many stars die each year. It is estimated that one white dwarf forms approximately every two years, researcher De Buizer mentioned.
Stars with masses eight times greater than that of the Sun will have a different death. These massive stars only make up about 3% of the stars in the Milky Way, but their impact is significant.
Eric Borowski, a postdoctoral researcher in astrophysics at Louisiana State University, stated: “These are truly violent and energetic events that I think some people might describe as a death.”
Borowski explained that this type of star fuses increasingly heavier elements in its core, ultimately becoming so heavy that it can no longer resist the gravitational pull. The result is a massive explosion, known as a supernova. According to NASA, the core of the star remains as a neutron star or black hole.
The last recorded observation of a supernova explosion in the Milky Way was in 1604, however, astronomers estimate that supernovae occur about once or twice per century in the galaxy.
So why have we not observed any since this explosion over 400 years ago in our galaxy? In fact, astronomers’ estimates are complicated by the shape of the Milky Way and dense clouds of gas and dust.
Scientist De Buizer noted: “There could be supernovae exploding on the other side of the galactic center, but there is too much in between for us to see them.”
In total, with a white dwarf forming every 2 years or longer, coupled with a few supernovae occurring every 100 years, there are nearly 53 stars that die each century in the Milky Way, or one star every 1.9 years.
Researcher De Buizer explained that understanding the death stages of stars is how astronomers piece together their life cycles.
