Star Formation and Types of Stars

Star Formation

• Stars form from the collapse of massive gas clouds (nebulas).
• Stellar nebulas contain the raw materials for star formation, primarily hydrogen.
• The collapse creates a protostar, a sphere of gas under immense gravitational pressure.
• Protostars accumulate gas until nuclear fusion begins in the core.
• Nuclear fusion creates outward pressure, balancing inward gravitational pressure.
• Stars that don't achieve nuclear fusion become brown dwarfs.

Types of Stars

• Brown dwarfs: Failed stars, not massive enough for nuclear fusion. They cool and diminish over time, having a mass of roughly 12-18 times Jupiter's mass.

• T Tauri stars: Newly formed stars that have just entered the main sequence. They are younger than 10 million years and are not giant stars.

• Main Sequence stars: The majority of a star's life cycle is spent in this phase where nuclear fusion balances gravity.

  • Small stars (red dwarfs) can survive hundreds of billions, even trillions of years.
  • Large stars can only remain in the main sequence for 10 million years.
  • The sun is an average star, living about 10 billion years.

• Giant stars: Stars with masses eight or more times that of the Sun, living shorter lifespans. They burn through fuel at a faster rate. Lifespan: 10 million to 100 million years.

• Supergiants: Massive stars (>8 solar masses). They fuse progressively heavier elements until iron.

  • Iron fusion cannot sustain nuclear balance and, leads to a supernova.
  • Explosions create all elements heavier than iron, along with a neutron star or a black hole.
  • Stars 8-19 times the Sun's mass become neutron stars.

• Neutron stars: Incredibly dense remnants of supernova explosions, not undergoing nuclear fusion.

• White dwarfs: Dead cores of non-giant stars, extremely dense, dim, and cool until they become black dwarfs.

• Black dwarfs: The theoretical end state of white dwarfs when they completely cool but are not observed yet.

  • The universe is not old enough for black dwarfs to be formed from white dwarfs.

Stellar Nebulae

• Stellar nebulae: Massive clouds where multiple stars can form.
• Planetary nebulae: Formed from the death of lower-mass stars by the shedding of outer atmospheric layers.
• Crab nebula: A well-known example, produced from a supernova 1000 years ago.

Protostars

• Protostars are the early stages of star development that form from stellar nebulas.
• Protostars must collect enough gas to initiate nuclear fusion, otherwise, they become brown dwarfs (failed stars).

Star Composition

• Hydrogen and helium are the main components of stars, especially those that have been in the main sequence for a long time.
• Stars formed in the past and in younger stellar nebulas include heavier elements like oxygen, magnesium, carbon, and other light metals.
• Star compositions, and specifically the presence of heavier elements, are related to their ages.