How is a star formed?

Gravity is the engine that runs most of the universe! If we start with a cloud of cold dense hydrogen (mostly), it will start to draw together due to the self-gravity of the cloud. (Where did that cloud come from? From a previous stellar explosion. Where did that previous star come from? We "chicken and the egg" back to the Big Bang!)

Now we employ some simple physics - conservation of energy! As the potential energy {energy of position} decreases for the molecules in the cloud (since they are coming together), the kinetic energy {energy of motion} increases. This kinetic energy is measured as the temperature of the cloud. As the temperature rises, there is radiation given off .. it will start to glow. Here is the sequence of a star like SOL :

• 1000K - at this point, the star is about 10 times the size it will spend most of its life and is glowing red
• 10 million K - nuclear fusion begins in the core (about a billion years have passed since the cloud started to contract)
• Stable phase - for about 10 billion years, the Sun will stay about the same size, and give off about the same energy
  • Hydrogen is fusing into Helium in the core - radiation pressure outward holds off the gravity collapse inward
  • Some of the helium in the initial cloud has formed the core (since it is "heavier")
    • at about 1/10th the radius is the "hydrogen burning zone" - 6000 million tons/sec of Hydrogen to Helium conversion!
  • If the star was 10-100 solar masses, this phase would only be around 10 million years! - Bigger burns faster!
• Unstable - when Hydrogen burns away, and only Helium left, less radiation - thus gravity starts to squeeze the star inward
  • Temperature ramps up to 100 million K as the helium fuses to higher elements - by 1 billion K, carbon and oxygen are fusing
  • Fusion reactions in bigger stars stop at Lead (Fe) - beyond that is not "energetically advantageous"
    • What about higher elements than Lead? - Created by other nuclear processes .. up to lead = "star stuff!"

What is the destiny of a given star? - Is it better to burn out or fade away?

The eventual end result of a star's life is determined by the mass of the star. Bigger stars burn out much faster (and more spectacularly). Smaller stars will have longer lives, and have calmer "deaths". Based on the starting mass:

• 1 Solar Mass (our Sun)
  • Hydrogen/Helium phase
  • star expands - becomes Red Giant
  • Helium stops burning - star contracts - blows out the outer layers - regular nebula (stuff for another solar system?)
• 10 solar masses (bluer stars)
  • Hydrogen to Helium - to large Red Giant (supergiant)
  • core collapses violently enough to create supernova - leftover core = neutron star or pulsar (very dense and spinning)
• 30-50 solar masses
  • Same as above .. up to Lead in the fusion cycles
  • So large that when it contracts .. may go past the neutron star density .. no end to collapse? ... = Black Hole?

The H-R Diagram - how to classify the stars

The mass of a star is the most important characteristic used to distinguish stars from each other. The mass will determine all the other characteristics : temperature, luminosity, size, and life span. The Hertzsprung-Russell (H-R) diagram is used to illustrate the connection between the Absolute Magnitude and the Luminosity, and the Temperature and Spectral class of the star. Our Sun (SOL) has a luminosity of 1 (since the scale is relative to our Sun), and a temperature of about 7000 degrees - this puts it along the Main Sequence, in the lower middle of the image below.

Astronomers now realize that everything which appears to distinguish one star from another - temperature, luminosity, size, life span -- is determined almost entirely by one factor: the star's mass. The main sequence along the HR diagram is not a singular evolutionary path, as many had thought, but a portrait of the sky at one moment in time of stars with varying masses. Below is a version of the Hertzsprung-Russell diagram, which shows how the size, color, luminosity, spectral class, and absolute magnitude of stars relate. Each dot on this diagram represents a star in the sky whose absolute magnitude and spectral class have been determined. Notice that the data appear to clump naturally into groups: main-sequence stars, giants, supergiants, and white dwarfs.

There is a fascinating story of a woman that helped catalog over 500,000 stars here. This would be an interesting story to find more about!