PHOTO-2024-11-13-15-44-19.jpeg

# Stars and Black Holes Stars are huge celestial bodies, mostly made of hydrogen and helium, that produce light and heat from the nuclear forges inside their cores. They are the building blocks of galaxies, and billions exist in the universe. Astronomers estimate that there are approximately 300 b...

# Stars and Black Holes Stars are huge celestial bodies, mostly made of hydrogen and helium, that produce light and heat from the nuclear forges inside their cores. They are the building blocks of galaxies, and billions exist in the universe. Astronomers estimate that there are approximately 300 billion stars in our Milky Way galaxy alone. ## A Star is Born The life cycle of a star spans billions of years. The more massive the star, the shorter its lifespan. Star birth occurs inside hydrogen-based dust clouds called nebulae. Over thousands of years, gravity causes pockets of dense matter inside these nebulae to collapse under their own weight, forming a protostar. ## Protostar to Main Sequence Millions of years later, when the core temperature climbs to about 15 million degrees Celsius, nuclear fusion begins. This ignites the core and marks the start of the next and longest stage of a star's life, known as the main sequence. ## Main Sequence Stars Most stars in our galaxy, including the Sun, are categorized as main-sequence stars. These stars exist in a stable state of nuclear fusion, converting hydrogen to helium and radiating x-rays. This energy emission keeps the star hot and shining brightly.