New stars form in large clouds of gas and dust. A giant molecular cloud where many stars form is called a stellar nursery, and the Orion Nebula is one of the best-studied stellar nurseries. It lies in the constellation Orion, about 1500 light-years away, and it is a place where stars are born. A protostar is a baby star: a hot core that forms as a clump in a cloud collapses, before fusion begins. Let's begin with the birth of a star. A star is a giant ball of gas, and hydrogen is by far the most abundant element in most stars. A protostar needs a core temperature of about 15 million Kelvin before hydrogen fusion begins and it enters the main sequence. A star that is fusing hydrogen to helium in its core is called a main sequence star.

Let's see how a star stays alive and how long it lives. A star's life is a balance of gravity pulling inward and pressure pushing outward. The star is in hydrostatic equilibrium as long as the two are equal. Fusion releases energy that heats a star and keeps it from collapsing under gravity, and a star's mass decides how fast it uses its fuel: massive stars burn out in millions of years while low-mass stars shine much longer. Our Sun is an average-sized star. A medium-sized star, up to about 7 times the Sun's mass, becomes a red giant, then its core fuses helium into carbon, and a dying red giant blows away its outer layers, making an expanding cloud of dust and gas called a planetary nebula. A star like the Sun ends as a white dwarf, a roughly Earth-sized stellar cinder that cools over billions of years. A teaspoon of white dwarf material would weigh several tons, more than a pickup truck.

Now let's see how some stars end, and what is left behind. A supernova is a huge explosion at the end of a massive star's life. In a massive star, fusion of silicon into iron is the last step of nonexplosive element production, because iron is the most tightly bound and most stable nucleus. By the time silicon fuses into iron, a massive star runs out of fuel in a matter of days. The core then collapses, rebounds, and sends a shock wave outward in a supernova. Astronomers estimate that on average one supernova explosion occurs in the Milky Way every 25 to 100 years, though none has been observable in our galaxy since the invention of the telescope. Chinese astronomers called the supernova of 1006 a "guest star", and a faint remnant is now found in the constellation Lupus.

Let's end with what is left of a massive star. Type II, Ib and Ic supernovas come from the collapse of a massive star's core, and in less than a second a neutron star or black hole forms and the rest of the star is blown away at speeds above 50 million kilometers per hour. Stars between about 8 and 20 times the Sun's mass end as neutron stars, and those neutron stars pack more mass than the Sun into a sphere about as wide as Manhattan Island is long, while heavier stars produce stellar-mass black holes. In 1967 graduate student Jocelyn Bell discovered pulsars, and her first was a source of rapid, sharp, intense and extremely regular pulses of radio in the constellation Vulpecula. A pulsar is a rapidly rotating neutron star, and its bright hot spots spin in and out of view like the beams of a lighthouse.

Words to know

• protostar: baby star
• supernova: exploding star
• pulsar: spinning neutron star
• equilibrium: a balance