Red Giant stars

When a main sequence star less than eight times the Sun’s mass runs out of hydrogen in its core, it starts to collapse because the energy produced by fusion is the only force fighting gravity’s tendency to pull matter together.

• But squeezing the core also increases its temperature and pressure, so much so that its helium starts to fuse into carbon, which also releases energy. Hydrogen fusion begins moving into the star’s outer layers, causing them to expand.
• The result is a red giant, which would appear more orange than red. Eventually, the red giant becomes unstable and begins pulsating, periodically expanding and ejecting some of its atmosphere. Eventually, all of its outer layers blow away, creating an expanding cloud of dust and gas called a planetary nebula. The Sun will become a red giant in about 5 billion years.

White Dwarf star

• After a red giant has shed all its atmosphere, only the core remains. Scientists call this kind of stellar remnant a white dwarf. A white dwarf is usually Earth-size but hundreds of thousands of times more massive. A teaspoon of its material would weigh more than a pickup truck.
• A white dwarf produces no new heat of its own, so it gradually cools over billions of years. Despite the name, white dwarfs can emit visible light that ranges from blue white to red.
• Scientists sometimes find that white dwarfs are surrounded by dusty disks of material, debris, and even planets – leftovers from the original star’s red giant phase.

In about 10 billion years, after its time as a red giant, the Sun will become a white dwarf.

Neutron Stars

Neutron stars are stellar remnants that pack more mass than the Sun into a sphere about as wide as New York City’s Manhattan Island is long.

• A neutron star forms when a main sequence star with between about eight and 20 times the Sun’s mass runs out of hydrogen in its core. (Heavier stars produce stellar-mass black holes.) The star starts fusing helium to carbon, like lower-mass stars. But then, when the core runs out of helium, it shrinks, heats up, and starts converting its carbon into neon, which releases energy. This process continues as the star converts neon into oxygen, oxygen into silicon, and finally silicon into iron.
• These processes produce energy that keep the core from collapsing, but each new fuel buys it less and less time. By the time silicon fuses into iron, the star runs out of fuel in a matter of days. The next step would be fusing iron into some heavier element, but doing so requires energy instead of releasing it.
• The core collapses and then rebounds back to its original size, creating a shock wave that travels through the star’s outer layers. The result is a huge explosion called a supernova. The remnant core is a superdense neutron star.
• Pulsars: These are a type of rapidly rotating neutron star. Bright X-ray hot spots form on the surfaces of these objects. As they rotate, the spots spin in and out of view like the beams of a lighthouse. Some pulsars spin faster than blender blades.
• Magnetars: All neutron stars have strong magnetic fields. But a magnetar’s can be 10 trillion times stronger than a refrigerator magnet’s and up to a thousand times stronger than a typical neutron star’s.