Question

lives of stars
guide for reading
how does the life of a star begin?
what determines how long a star will live?
what happens to a star when it runs out of fuel?
in 1967, jocelyn bell found an object in space that gave off regular pulses of radio waves. stars like the one bell discovered are called pulsars, short for pulsating radio sources.
each star is born, goes through its life cycle, and dies. astronomers cant watch a single star for billions of years so they study many stars in different stages of their lives and see how they differ from each other.
a nebula is a large amount of gas and dust spread out in an immense volume. gravity can pull some of the gas and dust in a nebula together. the contracting cloud is then called a protostar. a protostar is the earliest stage of a stars life. a star is born when the contracting gas and dust becomes so hot that nuclear fusion starts.
how long a star lives depends on how much mass it has. small stars use their fuel more slowly than large stars, so they have much longer lives.
when a star begins to run out of hydrogen fuel, the star becomes a red giant or supergiant. when a star runs out of fuel, it becomes a white dwarf, a neutron star, or a black hole.
when small or medium stars use up their fuel, their outer layers expand. at this stage they are called red giants. eventually, the outer parts grow bigger and drift into space. the blue - white hot core of the star that is left behind is a white dwarf.
a dying giant or supergiant star can suddenly explode. the explosion is called a supernova. after the star explodes, some of the materials from the star are left behind. this forms a neutron star. neutron stars are even smaller and denser than white dwarfs.
the most massive stars become black holes when they die. after a large star explodes, a large amount of mass may remain. the gravity of the mass is so strong that gas is pulled inward, pulling more gas into a smaller and smaller space. eventually, the gravity becomes so strong that nothing can escape, not even light.
astronomers have discovered several objects that were very bright, but also very far away. these distant bright objects looked almost like stars and were given the name quasi - stellar objects, or quasars. quasars are distant galaxies with giant black holes at their centers.

Explanation:

The text describes the life - cycle of stars. It starts with the birth of a star from a nebula through the formation of a protostar and the start of nuclear fusion. It then discusses what determines a star's lifespan (mass), and what happens when a star runs out of fuel (becoming a red giant/supergiant first and then a white dwarf, neutron star, or black hole depending on mass). It also mentions pulsars and quasars.

Answer:

The life - cycle of stars begins with a nebula. Gravity pulls gas and dust in a nebula together to form a protostar. A star is born when nuclear fusion starts in the contracting gas and dust. The mass of a star determines how long it will live. Small stars use fuel more slowly and live longer. When a star runs out of hydrogen fuel, it can become a red giant or supergiant. When it runs out of all fuel, it can become a white dwarf, neutron star, or black hole depending on its mass. Small or medium stars become red giants as their outer layers expand before becoming white dwarfs. Dying giant or supergiant stars can explode in a supernova and form neutron stars. The most massive stars become black holes. Quasars are distant galaxies with giant black holes at their centers.